Description of Research Contributions

His work contributed significantly to the conceptual basis of chemical dynamics, molecular radiationless transitions, photoselective chemistry, laser chemistry and ultrafast dynamics in complex molecular systems. His studies of condensed phase and biophysical electron transfer dynamics established the quantum theory of charge transfer, with a wealth of chemical applications, provided structure-dynamics-function relations for biophysical dynamics in photosynthesis. His work also significantly contributed to the novel fields of molecular electronics and nanoscience.

The work on molecular radiationless transitions began in the mid 1960s. At that time only few experimental data on lifetimes and quantum yields were available, which seemed to be difficult to reconcile with the basic concepts of quantum mechanics. The Bixon-Jortner model (1968) provided the conceptual basis for the understanding of radiationless transitions in excited electronic and vibrational states of isolated, collision-free, large molecules, establishing for the first time the intramolecular nature of these general phenomena. This seminal model rests on near-resonance coupling between states accessible for excitation and a background vibronic quasicontinuum, on the introduction of molecular eigenstates, on the dynamics of wavepackets of such eigenstates, on finite-time evolution and practical irreversibility in a bound level structure. The ubiquity of this model encompasses coupling and relaxation both of interstate-type between different electronic states (internal conversion and intersystem crossing) and of intrastate-type within a single electronic configuration (intramolecular vibrational energy redistribution). The Bixon-Jortner theory provided in 1968-70 spectroscopic and dynamic predictions for line broadening and intramolecular relaxation in a bound level structure of an isolated large molecule, which were subsequently confirmed by experiments in the 1980s on laser spectroscopy of jet-cooled large molecules. The extension of the radiationless transitions theory by Mordechai Bixon, Abraham Nitzan, Shaul Mukamel and Joshua Jortner (1969-1978) elucidated interstate and intrastate coupling and relaxation in sparse, intermediate and dense intramolecular level structure, providing a unified description of energetic- spectroscopic-dynamics relations. In the course of the development of the theory (1968-1978) a set of concepts was advanced, which remain the working tools of both experimentalists and theoreticians, e.g., molecular ladder diagrams, doorway states and their influence on the system's dynamics, the intramolecular quasicontinuum, the molecular eigenstates and their wavepacket dynamics, and the dynamic manifestations of the sparse, intermediate and statistical limit level structure.

Another central concept, which constitutes a cornerstone of interstate dynamics involves the Englman-Jortner (1970) energy gap law, predicting the exponential dependence of the radiationless rate on the energy gap between the electronic states. The usefulness and universality of the energy gap law for electronic-vibrational relaxation was established for a variety of intramolecular as well as condensed phase radiationless processes, which involve intersystem crossing and internal conversion in isolated large molecules and in solvated large molecules, relaxation of complexes, vibrational relaxation in condensed phase, as well as electron transfer in isolated supermolecules and in solvated molecules.

The new concept of the intramolecular vibronic quasicontinuum inducing statistical limit relaxation was introduced in 1968 in the theory of intramolecular radiationless transitions and, in particular, nonadiabatic relaxation and vibrational energy redistribution. This became a central concept for the understanding of high-order molecular multiphoton excitation and dissociation processes, which were discovered by V.S. Lethokov in Moscow and N. Bloembergen at Harvard, among others, in the mid 1970s. Regarding indirect molecular dissociation the Beswick-Jortner theory (1977) of vibrational predissociation of van der Waals molecules provided the first description of cluster dynamics. These concepts laid foundations for the important areas of photoselective chemistry and laser chemistry.

Significant applications of the theory of isolated molecule relaxation in the statistical limit pertain to ultrafast (femtosecond) intramolecular radiationless transitions from high intravalence and Rydberg excitations of isolated large aromatic and heterocyclic molecules, with the fastest time scales of vibrational nuclear motion, setting up an upper limit for the radiationless rate, which is central for intramolecular ultrafast dynamics. Other implications of the theory pertain to vibrational mode selectivity due to mediated coupling, as well as chemical applications, e.g., long-range electron transfer in isolated supermolecules, opening new areas of intramolecular chemistry.

Jortner and his colleagues discovered rich dynamics in the intermediate level structure, predicting the occurrence of molecular wavepacket dynamics and quantum beats. Joshua Jortner and Steve Berry (1968) and Mordechai Bixon, Joe Dothan and Joshua Jortner (1969) predicted that in the intermediate level structure a wavepacket of molecular eigenstates can be coherently excited. The time evolution of such a wavepacket was predicted in 1968-9 to exhibit interference effects in its radiative and nonradiative decay, which were referred to as molecular quantum beats. These theoretical predictions were confirmed in 1981-2 when J. Kommandeur in Groningen, J.D. McDonald in Illinois and A.H. Zewail in Caltech reported on the experimental observation of molecular quantum beats for interstate and intrastate intermediate level structure, coherently excited by ns laser pulses. This novel dynamic information allowed for the direct observation of molecular eigenstates, providing overwhelming direct evidence for the novel concepts and theories advanced by Jortner and his colleagues. The advent of femtosecond laser pulses during the last decade resulted in the interrogation of vibrational coherence effects, which provide significant novel information from diatomics to biophysical systems on the level structure and nuclear dynamics on the time scale of vibrational nuclear motion. The 1969 theory of the Jortner school pioneered the concept of coherent vibrational wavepacket dynamics in femtosecond chemistry.

Moving from the world of intramolecular dynamics to the dynamics of large finite systems, Jortner pioneered since 1980 the novel research area of cluster chemical physics. His theoretical and experimental work with Uzi Even and Aviv Amirav on rare-gas-large aromatic molecules (1980-85) pioneered the exploration of microscopic solvation. His work on the borderline between molecular, surface and condensed matter systems addressed size effects in atomic, molecular and metal clusters, elucidated the quantification of energetic, quantum, electronic, spectroscopic, electrodynamic and dynamic attributes in terms of a theory of cluster size effects and scaling laws (1992). Jortner's work provided an incisive analysis of the merging from microscopic to macroscopic behavior of matter. Predictions of novel dynamic phenomena, e.g., collective excitations of clusters (1993), cluster impact chemistry (1993-1995), and cluster Coulomb explosion (1997-2007). This work on structure, spectroscopy and dynamics of clusters provided the foundation for the understanding of optical-electronic-nuclear response of nanostructures. Recent theoretical and computational work with Isidore Last (2001-2007) provided predictions on nuclear fusion driven by cluster Coulomb explosion in ultraintense laser fields, which was subsequently confirmed by experiment. This work established the transition from cluster dynamics towards nuclear reactions.

Jortner's theoretical work (1974-1980) addressed the dynamics of complex systems in the condensed phase, establishing the unifying features of intramolecular and condensed phase relaxation, e.g., vibrational relaxation, spin conversion, atom transfer and electron transfer. Kestner, Logan and Jortner (1974) addressed the analogy between thermal electron transfer in solution and the description of intramolecular relaxation in the statistical limit, with the condensed phase vibrational quasicontinuum being isomorphous with the intramolecular vibronic quasicontinuum. While until that time, theoretical studies addressed electron transfer kinetics, this work pioneered the concepts of electron transfer microscopic dynamics. In the area of condensed phase electron transfer, significant contributions were made by Kestner, Logan and Jortner (1974), Jortner and Ulstrup (1975), Jortner (1976) and Bixon and Jortner (1982, 1989) to the formulation of the quantum theory of electron transfer incorporating electronic interactions and nuclear coupling. These elucidated long-range electron transfer pertaining to electronic direct exchange and superexchange interactions, established the central role of intramolecular high frequency vibrational excitations induced by electron transfer and established low-temperature tunneling effects and low-temperature activationless electron transfer. Ultrafast electron transfer in the condensed phase transcends the limitations imposed by solvent dynamics, being dominated by the electronic coupling and nuclear Franck-Condon factors, in analogy to intramolecular dynamics, pushing electron transfer times towards the femtosecond time domain (Bixon and Jortner, 1993, 1999). This work had an audible impact on the development of the novel field of molecular electronics.

The unification of the theory of electron transfer in the condensed phase and in the biophysical protein medium was established by Jortner in 1976 providing the basis for temperature independent nuclear tunneling in electron transfer. The quantum theory was applied to the primary charge separation and quinone reduction in the bacterial photosynthetic reaction center. Prior to the availability of structural information on this system two significant results of the electron transfer theory were obtained (Jortner, 1980). Firstly, activationless electron transfer, its central role in kinetic optimization and ubiquity in the photosynthetic reaction center, was invoked to account for the non-Arrhenius weak temperature dependence of the rates for primary electron transfer and quinone reduction between room and cryogenic temperatures. Secondly, the exponential dependence of the rate on the donor-acceptor separation was used to infer on structure-dynamics relations. Subsequent work by Mordechai Bixon and Joshua Jortner (1993, 1995) elucidated the mechanism of the ultrafast primary charge separation process in the bacterial photosynthesic reaction center, introducing a parallel sequential-superexchange mechanism at finite temperatures, which insures the energetic stability and efficiency of this process. The theory also elucidates the cumulative energetic and electronic contributions to symmetry breaking effects for the unidirectionality of the primary electron transfer across one branch of the bacterial photosynthetic reaction center. These studies provide structure-dynamics- function relations for ultrafast biophysical and chemical dynamics.

Recent novel applications of electron transfer theory pertained to charge migration in DNA (1998-2001), which establish the concepts of energetic control and elucidated the interrelationship between off-resonance, superexchange induced short-range unistep charge transfer and resonance coupled long-range multistep hole transport. Energetic control in conjunction with the new mechanism of thermally induced hopping lay the foundations for 'chemistry at a distance' in DNA, which is of fundamental interest in the context of radiation damage and repair and in the novel research area of dynamics, response and function of nanostructures and biosensors in the novel area of nanoscience.

A sampling of other subjects in the areas of condensed phase energetics and dynamics in physical and theoretical chemistry, to which Joshua Jortner made significant contributions which include the following:

The dynamics of elementary excitations and transport in molecular solids. These studies, together with Sang il Choi, Robert Silbey, and Stuart A. Rice, provided the first a-priori calculations of electron, hole and exciton band structure of molecular crystals, the first calculation of triplet Davydov splitting, the first studies of electronic energy delocalization in polymers, and the theory of electronic states in disordered molecular crystals. This work elucidated the relation between band-like and hopping transport for neutral and charged excitations and had a major impact on the spectroscopy and the chemical processes of exciton formation in molecular solids and biopolymers. This work laid the foundation for recent studies by Jortner and Bixon (1998-2006) of charge transfer and transport in DNA.

Excess electrons in fluids. Jortner pioneered the establishment of the theoretical framework for the solvated electron energetics, spectroscopy and dynamics in polar solvents, advancing (in 1957-1959) the physical picture, which was referred by N.F. Mott as "Jortner's large polaron". His subsequent studies, together with Neil R. Kestner and Stuart A. Rice, of electron localization in liquid He and in other liquid and supercritical rare gases provided the basis for the distinction between localized and quasifree excess electron states in dense fluids. The pioneering development of pseudpotential models for excess electron bubble states in liquid He provide the energetics, spectroscopy, transport and localization dynamics in liquid He and in quantum clusters. The work with Michael Rosenblit on electron localization in He clusters (2000-2006) provided the basis for the exploration of superfluidity in finite boson systems.

Electron-atom interactions. The work on the electronic structure of condensed phase was supplemented by contributions to the electron-atom pseudopotential. Jortner developed, together with Neil R. Kestner, the first electron-He atom pseudopotential, which provided the conceptual basis for the understanding of electron localization via bubble formation in liquid He and in large He clusters.

Electronic excitations in condensed rare gases. Together with Baruch Raz and Ernst Eckart Koch this work documented the nature of extravalence impurity excitations in solid and liquid rare gases, providing the first evidence for Rydberg molecular excitations and Wannier excitations in condensed phases. Joshua Jortner and Stuart A. Rice pioneered the theory of exciton states in liquids, addressing their energetics, lineshape and relaxation and providing, together with Baruch Raz, the first evidence that such states existed. His exploration, together with Lothar Meyer and Stuart A. Rice, of elementary excitations in condensed rare gases provided the first luminescence spectrum evidencing energy transfer in normal and superfluid liquid Helium. These studies were extended to obtain the first evidence for excimer formations in liquid and solid rare gases. This work was crucial in the evolution of chemical lasers. Subsequent studies with Baruch Raz, Ori Cheshnovsky and Klaus Schwentner pioneered the exploration of electronic energy transfer involving excimers in condensed rare gases.

The theory of electronic structure and electronic transport in disordered media. These included the first consistent experimental and theoretical analysis, together with Uzi Even, of the metal-nonmetal transition in low-density near-critical fluids such as expanded Hg. Theoretical work, together with Morrell H. Cohen, addresses response, dynamics and metal-nonmetal transition in high-density solvated electron systems such as metal-ammonia solutions, which manifest electronic transport in microscopically inhomogeneous systems. The pioneering concepts were recently confirmed by large scale quantum simulations.

Joshua Jortner is one of a small group of scientists who has extended the boundaries of chemistry by leading the exploration of dynamical phenomena in complex microscopic and macroscopic systems and by providing links between the interpretation of these phenomena. His work has had, and continues to have, great influence on the further development of chemistry and many other related areas of science.

Joshua Jortner Festschrift, The Journal of Physical Chemistry

Perspectives in Chemistry (Parts A and B). A Tribute to Joshua Jortner, Israel Journal of Chemistry

J. Jortner, Unended Quest in Science, Israel Journal of Chemistry

J. Jortner, Reflections on Physical Chemistry: Science and Scientists. Annu. Rev. Phys. Chem.

MAIN SCIENTIFIC PUBLICATIONS

Intramolecular Radiationless Transition.

J. Chem. Phys.

Radiationless Transitions and Molecular Quantum Beats.

J. Chem. Phys.

The Energy Gap Law for Radiationless Transitions in Large Molecules.

Mol. Phys.

Resonance Fluorescence from Large Molecules.

J. Chem. Phys.

Preparation and Decay of Excited Molecular States.

In: The World of Quantum Chemistry, eds. R. Daudel and B. Pullman. Reidel Co.: Dordrecht, 1974, p. 145-209.

Thermal Electron Transfer Reactions in Polar Solvents.

J. Phys. Chem.

Temperature Dependent Activation Energy for Electron Transfer Between Biological Molecules.

J. Chem. Phys.

Radiationless Transitions.

In: MTP International Review of Science, eds. A.D. Buckingham and C.A. Coulson. Butterworth: London, 1976, Vol. 13, pp. 327-388.

Photoselective Chemistry.

In: Advances in Chemical Physics, 1981, Vol. 47, p. 1-114.

Intermolecular Dynamics of van der Waals Molecules.

In: Advances in Chemical Physics, Part 1, eds. J. Jortner, R.D. Levine and S.A. Rice. Wiley Interscience, New York, 1981, Vol. 47, pp. 363-506.

Dynamics of the Primary Events in Bacterial Photosynthesis.

J. Am. Chem. Soc.

Unidirectionality of Charge Separation in Reaction Centers of Photosynthetic Bacteria.

Biochim. Biophys. Acta

Cluster Size Effects.

Z. Physik D

Molecular Dynamics in Femtochemistry and Femtobiology, Ultrafast Reaction Dynamics at Atomic Scale Resolution. Nobel Symposium 101, ed. V. Sundström. Imperial College Press, London, 1997, pp. 349-385.

On Dynamics. From Large Molecules to Biomolecules.

Spiers Memorial Lecture. Faraday Division Royal Society of Chemistry.

Faraday Discus.

Charge Transfer and Transport in DNA.

Proc. Natl. Acad. Sci. USA

Ultrafast Processes in Chemistry and Biology.

Phil. Trans. Roy. Soc. London A

Nuclear Fusion Driven by Coulomb Explosion of Heteronuclear Clusters.

Phys. Rev. Lett.

Beyond the Rayleigh Instability Limit for Multicharged Finite Systems.

Proceed. Natl. Acad. Sciences USA

Ultracold Large Finite Systems.

Adv. Chem. Phys.

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