Ramos, Idalia University of Puerto Rico at Humacao Citation: For tireless work on behalf of physics students, especially Hispanic women, and for enthusiasm for research that has inspired generations of many Puerto Rican students to enter physics graduate programs.
Ratcliff, William Davis National Institute of Standards and Technology Citation: For seminal neutron scattering studies of the magnetic order and spin dynamics in multiferroic materials. Read, Jocelyn Samantha California State University, Fullerton Citation: For contributions to the understanding of extreme matter within neutron stars, including its effects on gravitational-wave observations, and for the inclusive recruiting and mentoring of next generation gravitational-wave scientists.
Restrepo, Juan M Oregon State University Citation: For advancing the understanding of wave dynamics and uncertainty quantification in the climate system. Nominated by: Topical Group on Physics of Climate. Rignanese, Gian-Marco UCLouvain Citation: For original efforts developing free license software in the field of electronic structure calculations, and high-throughput calculations in a broad range of materials types.
Robey, Harry Francis Lawrence Livermore National Laboratory Citation: For significant advances in the understanding of complex hydrodynamics in inertial confinement fusion and high energy density plasmas, and for leadership in the design and execution of experiments on the National Ignition Facility. Romanenko, Alexander Fermi National Accelerator Laboratory Citation: For groundbreaking contributions to understanding radio frequency power losses in superconducting radio frequency cavities for particle accelerators.
Romanov, Dmitri A Temple University Citation: For seminal contributions to our understanding of the interaction of ultra-intense, ultra-fast optical radiation with atoms and molecules for femtosecond laser filamentation-based spectroscopy, nonlinear optics, and coherent control. Roth, Connie Barbara Emory University Citation: For exceptional contributions to the understanding of glass transition and aging phenomena in polymer films and blends. Ruskai, Mary Beth University of Vermont Citation: For pioneering contributions to the mathematical theory of quantum information, including the identification and solution of additivity problems and a proof of strong subadditivity of entropy, and for tirelessly building bridges between the field of quantum information and the broader mathematical community.
Sabella, Mel S Chicago State University Citation: For contributions to research in the field of introductory physics education courses designed to leverage the strengths of underserved and diverse student populations and engage them as co-investigators, and for demonstrating the utility of nontraditional measures of success in physics education. Salahuddin, Sayeef University of California, Berkeley Citation: For pioneering the physics of negative capacitance and its translation to overcome the Boltzmann Tyranny in microelectronics. Saleh, Omar A.
University of California, Santa Barbara Citation: For outstanding contributions to single-molecule biophysics, including development of magnetic tweezer instrumentation and its use in elucidating electrostatic and self-avoidance contributions to biopolymer structure, as well as mechanisms of motion of ring-shaped ATPases along DNA. Santangelo, Christian Syracuse University Citation: For seminal theoretical contributions exploiting geometry and topology to understand the elasticity of soft materials. Sathyaprakash, B. Pennsylvania State University Citation: For leadership in and wide-ranging contributions to gravitational wave science.
Schmidt, Frank CERN Citation: For groundbreaking work in furthering the understanding of nonlinear particle motion in accelerators through experiments and simulations. Schroeder-Turk, Gerd E Murdoch University Citation: For contributions to geometrical principles in soft matter physics, particularly bicontinuous phases.
Sethna, James Patarasp Cornell University Citation: For seminal and wide-ranging contributions to information geometry, "sloppy models," crackling noise, fracture, and emergent self-similarity. Shaevitz, Joshua W Princeton University Citation: For fundamental contributions to the understanding of the mechanics and dynamics of biological systems, from single molecules to cell collectives to behaving animals, through the development of new techniques for precision measurement.
Shawhan, Peter Sven University of Maryland Citation: For the development of techniques and algorithms to search LIGO data for transient signals, and for realizing the important future scientific implications of gravitational wave observations by looking for other signals developed by electromagnetic observations.
Shumlak, Uri University of Washington Citation: For pioneering investigations of sheared flow stabilization of magnetohydrodynamics modes in the Z-pinch. Silva, Carlos Georgia Institute of Technology Citation: For the groundbreaking development of ultrafast laser techniques for probing the transient photophysics of electro-optical and excitonic materials leading to novel and unique insights into charge-separation and carrier generation in organic photovoltaic systems. Souza, Ivo S Ikerbasque Foundation and University of the Basque Country, Spain Citation: For developing the theory of geometric phases in electronic structure and its implementation in practical computational algorithms.
Sudbo, Asle Norwegian University of Science and Technology Citation: For pioneering contributions to the theory of vortex matter in strongly fluctuating superconductors, superfluids, and multicomponent condensates. Surrow, Bernd Temple University Citation: For developing the methodology and fundamental measurements for determining the spin structure and dynamics of the proton using W-boson and jet production in high-energy polarized proton collisions, and for developing a future electron-ion collider facility.
Syage, Jack A ImmunogenX Citation: For the development of time-resolved methods for studying chemical dynamics in molecular clusters, state-specific, angle-velocity resolved direct imaging, and for pioneering the commercial development of atmospheric pressure photoionization for mainstream mass spectrometric chemical analysis. Tchernyshyov, Oleg V Johns Hopkins University Citation: For seminal advances in magnetic solitons and the development of collective coordinate formalism of dynamics of magnetic solitons for ferromagnetic thin wires, skyrmion crystals and extended domain walls.
Tkatchenko, Alexandre University of Luxembourg Citation: For the development of a novel framework for modeling and understanding van der Waals interactions in molecules and materials. Toney, Michael F SLAC National Accelerator Laboratory Citation: For many contributions to the development of in situ synchrotron X-ray scattering and spectroscopy methods for studies of organic materials, photovoltaics, and electrochemical interfaces related to energy materials systems.
Valentine, Megan T. University of California, Santa Barbara Citation: For pioneering research in the development of microrheology and the applications of biomechanics at multiple length scales to diverse biological systems. Van de Water, Richard G Los Alamos National Laboratory Citation: For outstanding contributions to solar-neutrino and short-baseline accelerator-neutrino physics experiments that have shed new light on neutrino properties and have provided evidence for physics beyond the Standard Model.
Vavilov, Maxim G University of Wisconsin-Madison Citation: For important contributions to several areas of quantum information, including the development of novel qubit manipulation and readout methods for superconducting qubits, and new insight into decoherence processes in semiconducting qubits. Vinokurov, Nikolay Budker Institute of Nuclear Physics Citation: For pioneering theoretical and experimental work in the field of free electron lasers and undulators for synchrotron radiation sources and free electron lasers.
Vishveshwara, Smitha University of Illinois, Urbana-Champaign Citation: For pioneering theory of quantum dynamics in nonequilibrium systems and novel phenomena in cold Bose gases. Vlahovska, Petia M. Northwestern University Citation: For pioneering work on problems in interfacial flows and soft matter, including the fluid-structure interaction in Stokes flow, the mechanics of biomembranes, and electrohydrodynamics.
Vlassopoulos, Dimitris FORTH and University of Crete Citation: For seminal contributions to understanding the rheology of complex polymeric architectures and recognizing the need for carefully controlled polymers in these contexts. Vogler, Tracy John Sandia National Laboratories Citation: For landmark contributions to the basic understanding of shock propagation in metals, ceramics, and granular materials, for sustained service to the APS Topical Group on Shock Compression of Condensed Matter, and for leadership in the science community.
Vogt, Bryan The Pennsylvania State University Citation: For insightful contributions to the understanding of polymer thin films and process-structure relationships of self-assembled polymers. Volovich, Anastasia Brown University Citation: For introducing original perspectives on quantum field theory calculations and uncovering deep mathematical structures in supersymmetric gauge theories, leading to novel and powerful methods of scattering amplitudes evaluation. Waters, Sarah L Oxford University Citation: For exposing the intricate fluid mechanics of biomedical systems and impactfully analyzing them with elegant mathematics.
Nominated by: American Physical Society. Weiner, Neal New York University Citation: For contributions to new models of dark matter and the understanding of their implications for dark forces and multi-state dark sectors, and for connecting new models to dark matter detection strategies. Wenhui, Duan Tsinghua University Citation: For discoveries of novel physical phenomena in two-dimensional electronics and advanced functional materials using computational and theoretical approaches, and for the first-principles prediction of new quantum materials.
White, Anne Elisabeth Massachusetts Institute of Technology Citation: For outstanding contributions and leadership in understanding turbulent electron heat transport in magnetically confined fusion plasmas via diagnostic development, novel experimentation, and validation of nonlinear gyrokinetic codes. White, Marion M Argonne National Laboratory Citation: For tireless efforts to increase the participation of women and minorities in physics, especially through one-on-one mentoring and educating minorities in elementary school through college about opportunities in the field.
Wilke, Claus University of Texas at Austin Citation: For discovering that biophysical constraints are a primary driver of protein sequence evolution. Williams, David A University of California, Santa Cruz Citation: For contributions to the study of gamma rays from extragalactic sources such as gamma-ray bursts and blazars, for using gamma-ray data to test cosmological models of the extragalactic background light, and for leadership in the development of past, present, and future ground-based gamma-ray telescopes.
Wong, Chee Wei University of California, Los Angeles Citation: For contributions in mesoscopic optical physics, including photonic crystals and laser frequency microcombs. Xia, Jing University of California, Irvine Citation: For developing novel optical probes of unconventional superconductors and magnetic materials, and for transport studies of topological phases. Xing, Huili Grace Cornell University Citation: For pioneering contributions in polar wide-bandgap semiconductors, 2D crystal semiconductors, and layered crystals. Xu, Hongqi Peking University Citation: For outstanding contributions to nanophysics and quantum transport in semiconductor systems.
Xu, Ting University of California, Berkeley Citation: For the design and realization of hybrid polymers that open new and efficient paths to functional nanocomposites by elucidating the physics that control the rate and perfection of self-assembly. Zhang, Jing Shanxi University Citation: For contributions to the fields of continuous-variable quantum information and quantum gases, especially for his pioneering experiments to realize spin-orbit coupling in degenerate Fermi gases.
Zhang, Xin Boston University Citation: For research and education using microelectromechanical systems and metamaterials to address a wide range of important problems in areas ranging from energy to healthcare to homeland security. Zhou, Mingfei Fudan University Citation: For outstanding contributions to the development and application of infrared spectroscopic techniques for the elucidation of the structure, chemical bonding, and reactivity of transient new molecules and clusters.
Zhou, Ye Lawrence Livermore National Laboratory Citation: For seminal contributions to understanding the evolution of turbulent interfaces from the weakly nonlinear to fully turbulent regimes relevant to the micro-scales of laser experiments, and the inertial confinement fusion to the mega-scales of supernova explosions, space physics, and astrophysics. Zou, Xiaoqin University of Missouri Citation: For outstanding contributions to developing novel physics-based algorithms for modeling protein interactions with applications to structure-based drug design.
Nominated by: Forum on Education Ahuja, Rajeev Uppsala University Citation: For seminal contributions to the design and understanding of energy storage materials and computational studies of condensed matter under high pressure. Nominated by: Forum on Industrial and Applied Physics Analytis, James G University of California, Berkeley Citation: For elucidating the fundamental properties of topological materials, quantum spin liquids, and strange metals.
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Nominated by: Division of Condensed Matter Physics Bacca, Sonia Johannes Gutenberg University, Mainz Citation: For first-principles calculations of the electromagnetic response of nuclei, leading to insights into the microscopic origin of the giant dipole resonance, nuclear polarizability corrections in muonic atoms, and the role of three-nucleon forces in electromagnetic reactions. Nominated by: Topical Group on Few-Body Systems Balasubramanian, Vijay University of Pennsylvania Citation: For fundamental contributions clarifying the black hole information puzzle and black hole thermodynamics through work on the duality of quantum gravity and quantum field theory, and on black hole microscopics in theories of quantum gravity.
Nominated by: Division of Particles and Fields Bao, Jiming University of Houston Citation: For the discovery of photoacoustic laser streaming, for seminal contributions to the understanding of basic electronic and optical properties of nanostructured materials, and the development of new nanomaterials for applications in solar energy conversions and optoelectronic devices.
Nominated by: Division of Materials Physics Bazin, Daniel Michigan State University Citation: For groundbreaking work developing nuclear reaction mechanisms for the study of rare isotopes, and for the conception and application of innovative technology to enable novel experiments. Nominated by: Division of Laser Science Bertoldi, Katia Harvard University Citation: For blending photonics, nonlinear mechanics, origami, and robotics through theory and experiment. Nominated by: Topical Group on Soft Matter Bhattacharya, Anand Argonne National Laboratory Citation: For elucidating the magnetic and transport properties of novel oxide heterostructures and for the discovery of the spin Seebeck effect in paramagnetic and antiferromagnetic insulators.
Nominated by: Division of Materials Physics Biegert, Jens ICFO - The Institute of Photonic Sciences Citation: For the development of intense few-cycle mid-infrared sources for the generation of water-window high-order harmonics, and their use in fundamental space-time imaging of the dynamics of molecular structure. Nominated by: Topical Group on Hadronic Physics Bose, Tulika University of Wisconsin-Madison Citation: For leadership coordinating the CMS physics program and trigger system, and for contributions to the development of high level triggers and searches for heavy vector bosons and vector-like quarks.
Nominated by: Forum on Industrial and Applied Physics Bruder, Christoph University of Basel Citation: For quantum theory of many-body coherent phenomena in mesoscopic electron systems, cold atoms, and nanomechanical systems. Nominated by: Topical Group on Soft Matter Budil, Kimberly Susan Lawrence Livermore National Laboratory Citation: For extraordinary leadership in developing national security partnerships between laboratories, academia, and governments, and for promoting diversity in science.
Nominated by: Forum on Physics and Society Buttery, Richard J General Atomics Citation: For pioneering contributions to the understanding of magnetohydrodynamics stability in tokamak plasmas, including the physics of tearing modes and magnetic field errors, and for outstanding scientific leadership of national and international fusion research. Nominated by: Division of Plasma Physics Castillo, Luciano Purdue University Citation: For demonstrating the importance of the initial conditions of scaling arguments in turbulent boundary layers, and for demonstrating the importance of turbulence in wind energy, and for mentoring and creating new opportunities for under-represented minorities in fluid dynamics.
Nominated by: Division of Polymer Physics Cheben, Pavel National Research Council of Canada Citation: For field-opening contributions to subwavelength integrated photonics, and the experimental and theoretical investigations of metamaterial nanostructures in optical waveguides, including metamaterial Bloch waveguides and on-chip metasurfaces in the telecom and mid-infrared frequencies. Nominated by: Division of Laser Science Chen, Xi Tsinghua University Citation: For the development of high energy resolution scanning tunneling spectroscopy and its applications to iron-based superconductors and other quantum materials.
Nominated by: Division of Gravitational Physics Cifarelli, Luisa University of Bologna Citation: For leadership in high energy physics and tireless efforts to strengthen international collaboration in physics. Nominated by: Forum on International Physics Connolly, Amy L Ohio State University Citation: For contributions to experimental and theoretical studies of ultrahigh energy neutrinos, and to searches for these neutrinos using radio techniques.
Nominated by: Division of Astrophysics Corsi, Alessandra Texas Tech University Citation: For major contributions to the discovery of both gravitational wave sources and their electromagnetic counterparts. Nominated by: Topical Group on Statistical and Nonlinear Physics Cui, Wei Tsinghua University Citation: For multiwavelength contributions to observations of black hole phenomena, including the study of jets related to both stellar mass and super massive black holes, the elucidation of the acceleration mechanisms in active galactic nuclei, and the relation of X-ray quasi-periodic oscillations to Lense-Thirring precession.
12222 American Physical Society fellows announced
Nominated by: Division of Astrophysics Dasgupta, Mahananda Australian National University Nominated by: Division of Nuclear Physics Deserno, Markus Carnegie Mellon University Citation: For pioneering contributions to the theory and simulation of biological membranes and proteins, and their interactions, leading to improved understanding of cellular mechanics and self-organization.
Nominated by: Division of Biological Physics Dickerson, James H Consumer Reports Citation: For longstanding contributions to physics diversity through mentoring, outreach, championing the APS Bridge Program, and helping launch the Fisk-Vanderbilt Bridge model, as well as leadership to assure quality science underpins Consumer Reports' product evaluations. Nominated by: Forum on Physics and Society Dogic, Zvonimir University of California, Santa Barbara Citation: For experiments on equilibrium self-assembled systems and active liquid crystals, and for the bottom-up engineering of biomimetic systems with life-like properties.
Nominated by: Topical Group on Soft Matter Du, Shengwang Hong Kong University of Science and Technology Citation: For significant contributions to photon-atom quantum interaction, including generation and manipulation of narrowband biphotons, and for the realization of efficient quantum memory, observation of optical precursors, and demonstration of nontraditional quantum heat engines. Nominated by: Division of Nuclear Physics Evans, Matthew J Massachusetts Institute of Technology Citation: For critical contributions to the development of advanced gravitational-wave detectors, as well as for developing techniques to enable further improvements in detector sensitivity, and for leading community efforts to design future large-scale ground-based detectors.
Nominated by: Division of Gravitational Physics Falk, Michael Lawrence Johns Hopkins University Citation: For fundamental advances in our understanding of the mechanical response of amorphous solids through the use of innovative computational methods and theories that reveal the connection between local rearrangements and large scale response. Nominated by: Topical Group on Statistical and Nonlinear Physics Ferlaino, Francesca University of Innsbruck Citation: For ground-breaking experiments on dipolar quantum gases of erbium atoms, including the attainment of quantum degeneracy of bosons and fermions, studies on quantum-chaotical scattering, the formation of quantum droplets, and investigations on the roton spectrum.
Nominated by: Division of Nuclear Physics Galbiati, Cristiano Princeton University Citation: For the measurement of Berillium-7 and pep solar neutrinos and for the development of the liquid argon technology for the background-free exploration of dark matter at the Gran Sasso underground laboratory. Nominated by: Division of Fluid Dynamics Garaud, Pascale University of California, Santa Cruz Citation: For fundamental contributions to the understanding of astrophysical double diffusion, especially the formation of layers and staircases.
Nominated by: Division of Fluid Dynamics Geraci, Andrew Albert Northwestern University Citation: For developing new precision measurement techniques to search for weakly coupled interactions of mesoscopic range and demonstrating the precision sensing capability of optically levitated nanoparticles. Nominated by: Division of Plasma Physics Gershtein, Yuri Rutgers University Citation: For important contributions to searches for physics beyond the Standard Model at both the Tevatron and the Large Hadron Collider, and for developing innovative techniques for precision photon measurement that directly contributed to the Higgs boson discovery.
Nominated by: Division of Particles and Fields Ghez, Andrea M University of California, Los Angeles Citation: For the advancement of diffraction-limited observing techniques and pathbreaking measurements that established the existence of a supermassive black hole at the center of the Milky Way Galaxy, and made possible a variety of other discoveries. Nominated by: Division of Particles and Fields Hagen, Stephen J University of Florida Citation: For significant experimental work on and elucidation of protein folding and bacterial gene regulation, and for exceptional service to and on behalf of the Division of Biological Physics and the American Physical Society.
Nominated by: Division of Biological Physics Haller, George ETH Zurich Citation: For numerous contributions to nonlinear dynamics as applied to fluid flows, including stochastic transport, Lagrangian methods for coherent vortices and structure identification, and applications to geophysical transport processes, mixing, and suspension flows. Nominated by: Forum on Industrial and Applied Physics Hansen, Stephanie B Sandia National Laboratories Citation: For contributions to the fundamental modeling of nonequilibrium atoms and radiation in extreme environments, and for the advancement of spectroscopic analysis to increase understanding of diverse laboratory and astrophysical plasmas.
Nominated by: Division of Plasma Physics Hanson, Ronald Delft University of Technology Citation: For pioneering experiments in quantum information science and quantum networking, including the first loophole-free Bell test. Nominated by: Division of Quantum Information Haule, Kristjan Rutgers University Citation: For pioneering quantitative first-principles investigation of correlated electron physics in broad classes of materials, including iron pnictides, heavy fermion, and transition metal compounds.
Nominated by: Division of Particles and Fields Howes, Gregory Gershom University of Iowa Citation: For fundamental contributions to understanding of turbulence in weakly collisional, magnetized plasmas, especially the nature of energy dissipation mechanisms. A correct treatment of the heat capacity of an electron gas uses Fermi-Dirac, rather than Boltzmann, statistics [ 12 ]. A publication [ 12 ] arrives at a value of 3. Values for enthalpies of formation under the Electron Convention are higher more positive for positive ions and lower less positive for negative ions than the corresponding values expressed in the Ion Convention used here.
Problems arise when users unknowingly mix inconsistent values for enthalpies of formation in the same equation. The Table lists several commonly-used compilations and shows which convention is used in each. When sufficient information is available. See discussion. Indicates treatment of integrated heat capacity terms for molecules and ions. The following more detailed discussion of these issues is intended to present the question of how the electron is treated in a thermochemical equation in a tutorial manner, in the hope that some of the confusion will be dispelled.
This discussion is also intended to justify the choice of the usual mass spectrometrists' convention for use in these tables.
The relationships between the various quantities that must be considered are shown in the thermochemical cycles:. This discussion will be concerned with the standard temperature, K, but the arguments can obviously be extended to any other temperature. In both conventions, at 0 K the enthalpy of formation of the electron is zero and the enthalpies of formation of the ions are exactly equal to the 0 K enthalpy of formation of the molecule M plus the energy difference between M and the corresponding ion:. At absolute zero, there is no difference in values derived using the two conventions.
The enthalpy changes of reaction at K are related to the 0 K ionization energy and electron affinity through the relationships:. In the present discussion, merely for the sake of focusing our attention on the treatment of C , the integrated heat capacity of the electron, let us temporarily make this assumption in order to simplify the equations.
In the Electron Convention , the electron is treated like a standard chemical element. Therefore, using the normal procedure for treating the thermochemistry of an element, its enthalpy of formation is constrained to be zero at all temperatures, but the integrated heat capacity is not taken to be zero. Therefore, the expressions for the enthalpies of formation of positive and negative ions reduce to:. Thus, for the purposes of deriving enthalpies of formation of ions from ionization energy or electron affinity data, it does not matter what value is chosen for the integrated heat capacity of the electron, C , since it does not appear in the final expression for the enthalpy of formation.
However, the expressions relating the enthalpies of reaction at finite temperatures to the zero degree quantities, IE a and EA, do explicitly include the integrated heat capacity of the electron:. As mentioned above , it is recommended that the value for the integrated heat capacity term for the electron derived using Fermi-Dirac statistics 3.
In preparing the current edition of this data collection for distribution via the WebBook, the possibility was considered of using the Electron Convention. However, the decision was made to retain the use of the Ion Convention. The reasons for this decision were:. That is, it appears that considerable confusion could result from an attempt to change the presentation of data here. Therefore, in this database, the use of the "Ion Convention" is retained. It is recommended that in all literature dealing with thermochemistry of ions in the gas phase, clear signposts should be provided to indicate which convention is being used.
This section provides brief descriptions of some of the factors relevant to the interpretation and evaluation of ionization energy and appearance energy data. More detailed discussions of the ionization process are available in many books and reviews, notably in the Introduction to "Energetics of Gaseous Ions" [ 3 ]. Ionization of a molecule by photoionization or by energetic electrons sometimes called "electron impact" is governed by the Franck-Condon principle, which states that the most probable ionizing transition will be that in which the positions and momenta of the nuclei are unchanged.
Thus, when the equilibrium geometries of an ion and its corresponding neutral species are closely similar, the energy dependence of the onset of ionization will be a sharp step function leading to the ion vibrational ground state. These situations are illustrated for hypothetical diatomic species in Fig. In evaluating ionization energy data, the shapes of photoelectron bands are useful indicators as to which of the situations pictured in Fig.
A sharp onset indicates that the equilibrium geometries of ion and neutral are quite similar, and that photoionization or electron impact determinations of the ionization threshold are likely to be free of complications. When an ionization process proceeds according to the second situation pictured in the figure, the onset of the photoelectron band is observed approximately at the adiabatic ionization energy; adiabatic ionization energies derived from observation of the onsets of photoelectron bands are usually in excellent agreement with adiabatic ionization energies obtained from optical spectroscopy analyses of Rydberg series or from the most reliable threshold determinations.
When the equilibrium geometry of the ion is very different from that of the corresponding neutral molecule and the lowest vibrational level is not populated in ionization by photon absorption or electron ionization, it has been shown that values for the adiabatic ionization energies can be obtained by determining the equilibrium constant for charge transfer to another molecule of known ionization energy:.
In such determinations, the ions are at thermal equilibrium with their surroundings, and one measures the thermochemical properties of the ions in their equilibrium geometries. As derived in the discussion of thermochemistry of ions at finite temperatures , this difference is likely to be quite close to the difference in the adiabatic ionization energies:. Below the potential energy curves are hypothetical probabilities for ionization as a function of energy for cases a , b , and c , and, at bottom, shapes of observed photoelectron bands for the three corresponding cases.
In addition to the problem of accurately detecting an ionization onset when there is a large change of molecular geometry in the ionization process, there is the problem of "hot bands". This term is applied to the observation of ionization at energies below the adiabatic ionization energy when there is a significant population of vibrationally excited molecules in the neutral species.
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The appearance energy sometimes called the appearance potential is the minimum energy required to form a particular fragment ion from a precursor neutral molecule:. In those discussions, we specified that thermodynamic information can be obtained from appearance energy values only when there is no potential barrier in the reaction coordinate for the fragmentation reaction, and no significant " kinetic shift " associated with the determination except for those experiments specifically designed to determine the kinetic shift.
The meanings of these terms is given here. It is well known that some ionic decomposition processes occur via pathways involving the surmounting of a barrier on the potential surface. This is especially true of rearrangements, in contrast to simple bond cleavage fragmentations. Surmounting such a barrier requires an activation energy greater than the enthalpy of reaction for the process. For this reason, the determination of thermochemical information from fragmentation threshold energies is always subject to the uncertainty that the enthalpy of reaction may not be the same as the observed onset energy.
In some instances, the existence of a potential barrier for a fragmentation process may be deduced from experimental observations, such as fragment ion kinetic energies, or the shapes of metastable peaks. The "kinetic shift" is the term applied to experimentally-observed ionization onsets which are higher than the thermodynamic onset energy due to the fact that the apparatus samples the fragmenting ions at a certain time usually around l0 -5 s after ionization has occurred, when ions undergoing a slow fragmentation process have not yet had time to dissociate.
One approach for getting around this problem is an analysis based on the determination of the so-called rate-energy curve for a given fragmentation; in this approach, the rate constant of the dissociating ion is determined as a function of energy. The information is derived from an analysis of the data from a technique which is, moreover, capable of delivering very accurate thermochemical information for fragmentation processes, photoion-photoelectron coincidence spectroscopy PIPECO.
In addition, recent studies have approached this problem by sampling the fragmenting ions as a function of time using time-resolved dissociation techniques. The interpretation of optical spectra to give ionization energies is a highly developed field with an extensive theoretical foundation. To summarize briefly, an atom with one excited electron in an orbital of high principal quantum number can, in first approximation be considered to be hydrogen-like. The excited electron is subjected to the central force field of the nucleus screened by the remaining core electrons.
The energy levels of this system can be described by the Rydberg formula:. Although highly accurate ionization energies can be derived for atoms from Rydberg series, there are some possible complications such as the existence of terms and series which are perturbed by configuration interaction, or spin-orbit splitting of terms. The identification of molecular Rydberg series is often not completely straightforward. Criteria that are used include the requirement that the transitions be strong and gradually decrease in intensity with increasing energy, and, evidently, that the series fit the Rydberg formula.
Although some highly accurate molecular ionization energy data are derived from optical spectroscopic studies, especially for diatomic molecules, the evaluation of molecular Rydberg series is not always straightforward, and reported spectroscopic ionization energies of polyatomic species may disagree with values derived from ionization onset determinations or the onsets of photoelectron bands due to complications in the analysis of vibrational and rotational structure [ 14 ].
In order to determine the enthalpy changes of these reactions, values for the entropy changes of reaction must be obtained, through measurements of the equilibrium constant as a function of temperature Van't Hoff plot or through statistical mechanical estimations.
In practice, many studies have been published in which measurements were made at a single temperature, with the usually small entropy change for the reaction estimated from statistical mechanical considerations. More recently, studies have appeared which include determinations of the equilibrium constant as a function of temperature e. This, in part may be due to a problem described [ 15 ] as pertaining to accurate determination of the reactive neutral pressure in pulsed high pressure mass spectrometers.
The pressure in the source of such an instrument has often been in the transition regime between molecular and viscous flow. This can cause appreciable fractionation of neutral species in the bath gas, based on the diffusivity of the compound, which is related to the square root of the mass. Thus, both relative and absolute neutral pressures may have been poorly characterized in such experiments in the past. These could vary from instrument to instrument, based on design, and from experiment to experiment, based on varying operating conditions.
Other possible contributing factors are clustering of neutral molecules to the ions at low temperatures, and pyrolysis of the ions at high temperatures. Therefore, at the present time, it appears that the most reliable values for entropy changes associated with such ion-molecule equilibria can be obtained by judiciously examining experimentally-determined entropy changes in conjunction with ab initio calculations of those quantities.
Thermochemical scales derived from equilibrium constant determinations are, of course, scales of relative thermochemical values. Absolute values for thermochemical quantities can be assigned if a reliable value for the quantity in question ionization energy, electron affinity, proton affinity, acidity, etc. The evaluation of thermochemical scales derived from equilibrium constant determinations presents special challenges, since the data for different molecules are all interrelated, so the scale must be evaluated as a whole, not molecule-by-molecule.
In addition, trends in data for homologous series or compounds of a particular structural type must make sense. The major uncertainty in data derived from equilibrium constant determinations, aside from the question of whether thermodynamic equilibrium is actually attained, is in knowing the temperature accurately. In the recent re-evaluation of the extensive scales of proton affinity and gas basicity, it was found that for the large body of data evaluated in the publication [ 8 ] the thermochemical scales particularly the end of the scale representing high gas phase basicities had been significantly constricted because operating temperatures of the instruments used in the experiments had been underestimated.
For this reason, users will find that certain cited values for proton affinities and gas basicities given here are significantly different from those listed earlier [ 8 , 9 ], although for all species for which sufficient information was available to do a complete evaluation, internal consistency is maintained. The results were mainly derived from interlocking ladders of enthalpy changes for the proton transfer reactions:.
A thermochemical ladder of relative ionization energies determined in this way [ 16 ] closely reproduces the equivalent scale of spectroscopic ionization energies, thus demonstrating the reliability of the approach for deriving information on relative ionization energies. The most useful application of this approach for ionization energy data has proved to be the determination of ionization energies for species which undergo a large change of geometry upon ionization, and which therefore exhibit very gradual onsets of ionization as a function of energy.
For example, the only reliable information about the adiabatic ionization energies of n-alkanes and of alkyl hydrazines comes from thermochemical ladders established through equilibrium constant determinations. Since the electron transfer occurs in a long-lived collision complex which endures for many vibrational periods, the ionic configuration corresponding to the equilibrium geometry of the ion the geometry corresponding to the adiabatic transition is accessed.
Other equilibrium studies have been concerned with hydride or halide transfer reactions:. Studies of hydride transfer and halide transfer equilibria have led to quantitative information about the relative enthalpies of formation of alkyl carbocations. These data were used to supplement information from appearance potential determinations in evaluating enthalpies of formation of alkyl carbocations. The auxiliary thermochemical information required for citation of ion enthalpies of formation — enthalpies of formation of relevant neutral species - is available mostly for species at K.
However, strictly speaking, ionization energies , appearance energies , and electron affinities are quantities which correspond to processes occurring at 0 K. To correctly derive enthalpies of formation of ions from these kinds of data requires explicit treatment of the differences in thermochemical values at 0 K and at higher temperatures. This section discusses the principles involved in such a correct treatment, describes the simplifications which are often made in the literature, and specifies how data are treated here. Actually, however, the ionization energy and electron affinity are the equivalents of the spectroscopic transitions between the lowest rotational and vibrational levels of the ground state of the molecule and the lowest rotational and vibrational levels of the electronic ground state of the ion; that is, they are equal to the difference between the enthalpies of formation of the molecule and the corresponding ion at absolute zero.
As derived in the discussion on Thermochemical Conventions , the enthalpy changes of reaction at temperature T are related to the adiabatic ionization energy and electron affinity by the expressions:. Thus the expressions for the enthalpies of formation of the ions at temperature T become:. Thus, for a correct treatment of the enthalpy of formation of an ion at any temperature other than absolute zero, the integrated heat capacities of the ion, B or D , and its precursor neutral molecule, A , or more specifically the differences between those heat capacities must be taken into account.
However, it is a common practice to derive " K heats of formation" of positive ions by simply adding the 0 K value for the ionization energy to the K enthalpy of formation of the molecule. However, even this contribution will usually be sufficiently small that a significant error will not be introduced if it is ignored. For example, the lowest ionization energy of ethylene corresponds to removal of an electron from the C-C pi bond, which leads to a lowering of the frequency of the symmetric C-C stretch from to cm -1 and a reduction in the frequency of the twisting around the C—C bond from to cm Although these differences in vibrational frequencies are significant, the predicted effect on the K enthalpy of ionization is to raise it above the value for the adiabatic ionization potential by only 0.
Although this analysis was concerned with heat capacities of positive ions, the same reasoning — and the conclusions — should apply equally well to negative ions, that is, to changes in the electron affinity with temperature. Therefore we conclude that for most species, the simplifying assumption that the adiabatic ionization energy or electron affinity and the corresponding enthalpies of ionization at an elevated temperature are approximately the same:.
At higher temperatures, it is possible that larger errors would result from this simplification. In this database, most of the cited values for enthalpies of formation of molecular ions correspond to K, and were obtained by simply adding the value for the adiabatic ionization energy to the K enthalpy of formation of the neutral species; that is, the assumption discussed above was usually made. A rigorous treatment would require calculating exact values for integrated heat capacities and from complete sets of vibrational frequencies for the molecule and the ion. Vibrational frequencies for most of the ions are not available, and the correction would simply cancel out if one made the often-used assumption that the vibrational frequencies of the ion and its neutral counterpart are the same.
Whenever the original authors of a paper carried out such a complete analysis a routine procedure only for photoelectron-photoion coincidence studies , the results of that analysis are included here, and both 0 K and K values for the ion enthalpy of formation are given; only in these cases is the temperature specified. For the most common experimental techniques energy selected electron impact, photoionization techniques, etc.
This matter has been discussed at length by Traeger and McLaughlin [ 17 ]. At onset the products of the unimolecular decomposition will be formed with zero translational energy with respect to the center of mass provided that the fragmentation does not involve a reverse energy barrier and a center of mass translational energy the same as that of the precursor molecule.
In principle, if the observational time scale of the experiment and the sensitivity of the ion detector are great enough, then the observed appearance energy approaches that for products having 0 K internal energy i. In effect, this equation corrects the observed threshold energy for the fragmentation process to an effective 0 K value by adding the thermal rotational and vibrational energy contained in RX to the onset energy.
Most enthalpies of formation of fragment ions are derived making the simplifying assumption that the last two terms of this equation will cancel one another. That is, values for enthalpies of formation of fragment ions at K derived from appearance potential data are more often obtained by simply using an observed onset energy and K enthalpies of formation of relevant neutral species.
As mentioned elsewhere, this database displays evaluated values for enthalpies of formation for only a very few much-studied fragment ions at the present time; as evaluations are made, additional values will be added. However, in many cases, sufficient information is given in the WebBook databases to allow users to derive such values as needed. In the database, techniques are identified by acronyms, shown in square brackets in the discussion, or in some cases, after the appropriate headings.
A problem in defining an experimental technique for the purposes of assigning acronyms in the database is that acronyms used in the literature have evolved over the years as experimental techniques evolve, so that for a collection like this one with experimental results originating over a year time period, a problem of internal consistency arises. Because of this problem, we have elected to use fewer, more broadly-defined acronyms, rather than attempt to maintain a detailed breakdown of experimental techniques, which may differ only in subtle details, in the assignment of acronyms.
Even using broadly-defined acronyms, there are certain studies for which it is difficult to pigeonhole the experimental technique; for example, the borderline between "photoionization" and "laser spectroscopy" is sometimes not easily defined, and the distinction is sometimes made more on the basis of the focus of the study than on the actual details of the experiments. In the case of time-resolved photodissociation results, measured appearance energies often differ significantly from those determined using other techniques since slow fragmentation processes are detected.
The identification of a Rydberg series in an atomic or molecular spectrum leads to a value for the ionization energy. In cases where the analysis of the spectrum is straightforward, the spectroscopic ionization energy values are highly accurate. The determination of atomic ionization energies through optical spectroscopy is a highly developed field which has been extensively reviewed.
A large fraction of atomic ionization energies listed here are from expert evaluations of atomic spectra. In the evaluation of ionization energies of atoms and diatomic molecules, spectroscopic ionization energies have been chosen as "selected values" where they are available. For polyatomic species, a value derived from an analysis of the optical spectrum has been given great weight, unless several determinations from other highly reliable techniques are in conflict with the spectroscopic value.
The most widely-used technique for determination of ionization and appearance energies involves a direct determination of the minimum energy required to form a parent or fragment ion from a neutral species, or to detach an electron from a negative ion. In these approaches, ionization may be effected by photoionization, by interaction with energetic electrons "electron ionization" or, in older literature, "electron impact" , or by interaction with excited atoms Penning ionization or other chemi-ionization reactions; the measurement involves a determination of the minimum energy "threshold" required to form an ion.
The resulting ions or the ejected electrons, or both, are detected using various mass spectrometric techniques. Over the years, the most widely-used techniques for the determination of ionization and appearance energies have involved the use of mass spectrometers in which ionization is effected by an electron beam. The energy of the beam is varied, and the abundance of the resulting ion s is monitored; the "onset" of the ion on the energy scale must be detected.
A problem with this approach that had to be dealt with before accurate data could be obtained was that standard electron beams had a large energy spread, so the nominal energy expected from the applied electrode potentials was not a good indication of the actual energy of electrons in the beam.
However, by the s, techniques were developed [ 20 ] to narrow the energy range of the electron beams through the use of so-called "electron monochromators", in which the energy of the electron beam is narrowly defined by passing the beam through electron energy selectors of various designs. Other laboratories have utilized a so-called fast-beam apparatus to determine accurate ionization cross sections as a function of electron energy. Modern results obtained using electron beams with well-defined energies are in excellent agreement with analogous results derived from determinations of photoionization thresholds.
Early on, there were other, somewhat less rigorous, approaches to solving this problem of energy spread in electron beams, leading to what Rosenstock et al [ 3 ] call "quasi-monoenergetic" beams the "Retarding Potential Difference" [ 21 ] and "Energy Distribution Difference" [ 22 ] methods. Measurements made using the more careful approaches can be distinguished from the non-quantitative measurements by the cited error limits or where there are no error limits given by the number of significant figures displayed. Most of the quantitative measurements made during the past 20 years have been made with well-defined electron energies.
In the mids, the problem of exactly defining the energy of the ionizing agent was approached in some laboratories by replacing the electron beam by photons, whose energy could be well defined. In classical photoionization mass spectrometry, monochromators were used with standard light sources, and the ion abundance was determined as a function of photon energy. That is, the approach to determining a threshold energy was exactly analogous to that used in electron ionization experiments.
In such experiments, as with the electron ionization techniques , one must be able to detect the onset of ionization. According to the Franck-Condon Principle , if the configuration of the ion is different from that of the precursor neutral molecule, the onset of ionization as a function of energy will be gradual, and the exact onset may be difficult to pinpoint accurately. Modern photoionization experiments often utilize laser or synchrotron light sources, and may have other distinctive features designed to provide more detailed information.
For example, studies are published examining pressure- or time-dependencies of ionic photodissociation processes. The latter are assigned a separate acronym here TRPI , rather than being categorized with other "photoionization" experiments, since observed onsets may differ markedly from onsets measured on the conventional time-scale, and it is useful to be able to recognize through the acronym why this is so. The electron affinity of a species can be determined by finding the threshold for electron photodetachment, via irradiation of a trapped negative ion by variable frequency light.
This is denoted by the acronyms [PD] for photodetachment, using a continuous frequency light source and a monochromator, or [LPD] for use of a variable frequency laser as the light source. Two complementary photoionization methods are often used for studying dissociation processes where a " kinetic shift " exists that is, where experimentally-observed ionization onsets are higher than the thermodynamic onset energy due to the fact that the apparatus samples the fragmenting ions at a certain time when ions undergoing a slow fragmentation process have not yet had time to dissociate.
These approaches, both of which examine the dissociation process as a function of time, are time-resolved photodissociation the acronym TRPD is used in the literature and time-resolved photoionization mass spectrometry the acronym TPIMS is used in the literature. In time-resolved photodissociation experiments, parent ions are formed by electron impact, thermalized, and photoexcited by a monochromatic pulsed laser; the dissociation is followed as a function of time in an ion cyclotron resonance spectrometer ICR.
In time-resolved photoionization mass spectrometry, ions are formed by a pulsed VUV light source in an ion trap, and are ejected after a given delay time into a quadrupole mass filter. While both approaches give time-resolved information about dissociation processes, they are complementary rather than effectively the same since in the former technique, all parent ions are excited to the same energy, while in the latter, parent ions are excited to a range of internal energies extending from zero up to the maximum available energy. In recent years, spectroscopic studies using laser techniques have provided highly accurate ionization energy values.
For example, ionization energies for molecules have been determined using multiphoton ionization or resonance-enhanced multiphoton ionization REMPI of vibrationally-cooled species in a molecular beam. In these studies, the cooled beam of molecules is raised to a specific excited state by irradiation with a tunable laser; while this excitation energy is held constant, a second independently tunable laser is used to ionize the beam of excited molecules, with the photon energy being tuned through the ionization onset. The excitation laser is then tuned to a different transition, and the ionization scan is repeated.
In this way, the entire Franck-Condon accessible region of the intermediate electronic state is mapped out, insuring that the molecular geometry corresponding to the adiabatic ionization energy is accessed. Since every intermediate vibronic state leads to an independent value of the ionization threshold, the experiment contains an internal consistency check. It is also possible to determine the energy change associated with an ionization process by effecting ionization with a photon of well-defined energy and measuring the energy of the ejected electrons:.
The most widely-used technique of this type is conventional photoelectron spectroscopy in which the photon sources are usually the helium or neon resonance lines In cases where the equilibrium geometry of the ion and the corresponding neutral are the same or are similar, it is found that the observed onset of the first photoelectron band is usually a reliable indicator of the adiabatic ionization energy.
This situation is easily recognized by the sharp onset of the photoelectron band. Most photoelectron spectroscopy studies are carried out with the goal of elucidating the spectroscopy of the system through determinations of vertical ionization energies leading to the ground state and excited state ions, and therefore, little attention is usually given to determinations of adiabatic ionization energy values. In many instances, figures showing the photoelectron spectra are displayed, and onsets of photoelectron curves which correspond approximately to the adiabatic ionization energies can be estimated from the figure.
Such values are reported here, but are surrounded by parentheses to indicate that these are approximate values not selected by the original authors. For negative ions, if a beam of ions is irradiated by a laser beam with photons in excess of the energy required to detach the electron, then analysis of the translational energy of the detached electrons allows for determination of the electron affinity. This technique [LPES] can provide electron affinities precise to micro-electron volts. The method often provides information on the vibrational states of the neutral and ionic species as well.
However, the assignment of the threshold can be complicated by these states. The precision is commonly better than 0. Highly accurate determinations of ionization energies come from a family of techniques in which laser ionization e. In so-called "threshold photoelectron spectroscopy" or "zero-kinetic energy spectroscopy" the acronym "ZEKE" is often used in the literature ions with a well-defined energy are formed, and only those electrons which correspond to essentially zero energy of ejection are detected.
In some cases, mass analysis of the positive ion is included mass-ZEKE. The acronym [ZEKE] is used in the negative ion data base here. For the purposes of studying the thermochemistry of ionic fragmentation processes, a powerful variation of the Threshold Electron Detection approach is used which involves the simultaneous detection of a single zero-kinetic energy electron and the corresponding single positive ion. In the technique known as photoion-photoelectron coincidence or sometimes, photoelectron-photoion coincidence — PEPICO , ejected electrons which originated with "zero" kinetic energy are matched with their corresponding positive ions.
The ions can be detected at differing times after the ionization event for the determination of the time dependence of the dissociation process. The complete interpretation of such data requires a modeling of the dissociation using statistical theories of unimolecular decomposition i. As pointed out by Dannacher [ 23 ], in spite of its great strengths, this technique has not been widely utilized, possibly because of the intricate instrumentation required, the complexity of the data analysis, and the fact that each determination requires the investment of a great amount of time on the part of the experimentalist.
In the older literature, Penning Ionization — ionization by collisions with a beam of metastable neutral rare gas atoms with known excitation energy or energies — was often used as an ionization mechanism. In these experiments, metastable atoms employed include mixtures of He 2 3 S Because of the presence of two metastable states in a given atom beam, the Penning electron spectrum consists of a shifted superposition of two spectra, each formed by one of the species. The energy of the ejected electrons was analyzed.
Some recent studies have examined systems where other chemi-ionization reactions such as:. The plot of cross section as a function of recombination energy provides values for ionization and appearance energies.
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Collision of a neutral species with an energetic particle of low ionization potential, such as an alkali atom, can result in electron transfer, giving an alkali cation and an anion. The electron affinity of the neutral species is equal to the translational energy of the alkali atom less its ionization energy.
Determinations of electron affinities by this method have the advantage that one obtains values for the true electron affinity: electron attachment to a neutral species, rather than detachment from an anion. Certain anions can be produced by this technique which are not accessible via electron impact due to low energy exit channels, e.
The onset energies of fragment ions can also provide useful thermochemical information, if the thermochemistry of the coproduced neutral species is known. Normally this technique results in a determination of the adiabatic electron affinity, but for a sufficiently fast beam of neutral species, the onset corresponds to the vertical attachment energy of the electron, which, in contrast to detachment methods, is smaller than the adiabatic value.
Negative ions with electron affinities of a few tenths of an electron volt or less can undergo detachment of the electron in a strong electric field. Based on the strength of the field, the dipole moment of the neutral species produced, and the rate of loss of the electron, the electron affinity can be derived. This often is used to access information on negative ions where the electron is in a non-valence state, such as dipole-bound anions.
Thermal hysteresis in a phase change material makes it possible to build a radiative thermal memristor, a fast thermal circuit element whose response is dependent on its history. Measurements of spin and valley relaxation and decoherence times in quantum dots of graphene encapsulated by edges and back gates may lead to implementations of qubits. Volume , Issue 2 12 July On the Cover Spatial propagation of local actuation can be studied in the Miura-ori tube, a common unit cell in tessellated origami.
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