Richard K. Preston

Bio: Richard K. Preston is an academic researcher from Yale University. The author has contributed to research in topics: Surface hopping & Semiclassical physics. The author has an hindex of 14, co-authored 15 publications receiving 2692 citations.

Trajectory Surface Hopping Approach to Nonadiabatic Molecular Collisions: The Reaction of H+ with D2

Abstract: An extension of the classical trajectory approach is proposed that may be useful in treating many types of nonadiabatic molecular collisions. Nuclei are assumed to move classically on a single potential energy surface until an avoided surface crossing or other region of large nonadiabatic coupling is reached. At such points the trajectory is split into two branches, each of which follows a different potential surface. The validity of this model as applied to the HD2+ system is assessed by numerical integration of the appropriate semiclassical equations. A 3d “trajectory surface hopping” treatment of the reaction of H+ with D2 at a collision energy of 4 eV is reported. The excellent agreement with experiment is an encouraging indication of the potential usefulness of this approach.

Effects of surface crossing in chemical reactions - The H3 system

Abstract: Triatomic hydrogen positive ion surface crossing effects in chemical reactions based on potential energy surfaces calculation using diatomics-in- molecules approach

Quantum versus classical dynamics in the treatment of multiple photon excitation of the anharmonic oscillator

Abstract: The response of a one‐dimensional anharmonic Morse oscillator to an intense electromagnetic field has been investigated using both a quasiclassical and quantum mechanical description of the oscillator. The anharmonic nature of the Morse potential reduces the coherence of the quantum excitation process after only a few quanta have been absorbed. The classical and quantum behavior of averaged quantities such as the energy absorbed and the oscillator displacement as a function of time are in good agreement; however, the classical description cannot reproduce the multiphoton resonances. We are led to the conclusion that classical mechanics provides an adequate description of the response of a molecule in an intense laser field provided that multiphoton resonances do not individually play a fundamental role in the process.

Theoretical treatment of quenching in O(1D) + N2 collisions

Abstract: It is maintained that quenching of O(1D) by collision with N2 proceeds by formation of a collision complex on the lowest singlet potential surface. Once a collision complex is formed, even the weak spin−orbit interaction in O atom can induce quenching with essentially unit probability (at thermal energies) because the intersection of the singlet [O(1D) + N2] and triplet [O(3P) + N2] potential surfaces is crossed many times during the life of the complex. Rather crude, but qualitatively reasonable potential surfaces for O(1D) + N2 are constructed and classical trajectory calculations carried out to show that the cross section for complex formation is indeed appreciable, ∼40 A2 at thermal energy; a statistical model is used to determine the quenching probability of the collision complex. Values obtained for the magnitude of the thermal rate constant for quenching, and the fraction of the exoergicity which appears as vibrational excitation of N2, are both in good agreement with experimental results.

Molecular beam and trajectory studies of reactions of H+ with H2

Abstract: We have employed two complementary techniques, molecular beam experiments and ``trajectory surface hopping'' theory, to investigate proton‐hydrogen molecule scattering at relative energies between 1 and 7 eV. Absolute cross sections, product translational energy distributions, and product velocity contour diagrams have been obtained from both theory and experiment for the H+ + D2 and D+ + HD isotope arrangements. Agreement is excellent, particularly for a theory which involves no empirical information and no adjustable parameters. By combining results from experiment and theory we have been able to develop a fairly complete, reliable, and simple picture of the dynamics of this elementary collision process. For relative energies below about 3 eV the reaction involves a short‐lived collision intermediate, whereas above 4 eV it proceeds by a predominantly direct, impulsive mechanism. Above 5.5 eV momentum transfer is well represented by a hard‐sphere collision model involving only short‐range repulsive force...

Molecular dynamics with electronic transitions

Abstract: A method is proposed for carrying out molecular dynamics simulations of processes that involve electronic transitions. The time dependent electronic Schrodinger equation is solved self‐consistently with the classical mechanical equations of motion of the atoms. At each integration time step a decision is made whether to switch electronic states, according to probabilistic ‘‘fewest switches’’ algorithm. If a switch occurs, the component of velocity in the direction of the nonadiabatic coupling vector is adjusted to conserve energy. The procedure allows electronic transitions to occur anywhere among any number of coupled states, governed by the quantum mechanical probabilities. The method is tested against accurate quantal calculations for three one‐dimensional, two‐state models, two of which have been specifically designed to challenge any such mixed classical–quantal dynamical theory. Although there are some discrepancies, initial indications are encouraging. The model should be applicable to a wide variety of gas‐phase and condensed‐phase phenomena occurring even down to thermal energies.

Multimode molecular dynamics beyond the Born-Oppenheimer approximation

Abstract: Mise au point. Theorie des effets de couplage vibronique multimodes. Probleme a 2 etats. Couplage vibronique mettant en jeu des modes et des etats degeneres. Effets du couplage vibronique multimodes en spectroscopie. Comportement statistique des niveaux d'energie vibroniques. Intersections coniques et evolution temporelle de la fluorescence

Trajectory Surface Hopping Approach to Nonadiabatic Molecular Collisions: The Reaction of H+ with D2

Abstract: An extension of the classical trajectory approach is proposed that may be useful in treating many types of nonadiabatic molecular collisions. Nuclei are assumed to move classically on a single potential energy surface until an avoided surface crossing or other region of large nonadiabatic coupling is reached. At such points the trajectory is split into two branches, each of which follows a different potential surface. The validity of this model as applied to the HD2+ system is assessed by numerical integration of the appropriate semiclassical equations. A 3d “trajectory surface hopping” treatment of the reaction of H+ with D2 at a collision energy of 4 eV is reported. The excellent agreement with experiment is an encouraging indication of the potential usefulness of this approach.

Contemporary Issues in Electron Transfer Research

Abstract: This is an overview of some of the important, challenging, and problematic issues in contemporary electron transfer research. After a qualitative discussion of electron transfer, its time and distance scales, energy curves, and basic parabolic energy models are introduced to define the electron transfer process. Application of transition state theory leads to the standard Marcus formulation of electron transfer rate constants. Electron transfer in solution is coupled to solvent polarization effects, and relaxation processes can contribute to and even control electron transfer. The inverted region, in which electron transfer rate constants decrease with increasing exoergicity, is one of the most striking phenomena in electron transfer chemistry. It is predicted by both semiclassical and quantum mechanical models, with the latter appropriate if there are coupled high- or medium-frequency vibrations. The intramolecular reorganizational energy has different contributions from different vibrational modes, whic...

Proton transfer in solution: Molecular dynamics with quantum transitions

Abstract: We apply ‘‘molecular dynamics with quantum transitions’’ (MDQT), a surface‐hopping method previously used only for electronic transitions, to proton transfer in solution, where the quantum particle is an atom. We use full classical mechanical molecular dynamics for the heavy atom degrees of freedom, including the solvent molecules, and treat the hydrogen motion quantum mechanically. We identify new obstacles that arise in this application of MDQT and present methods for overcoming them. We implement these new methods to demonstrate that application of MDQT to proton transfer in solution is computationally feasible and appears capable of accurately incorporating quantum mechanical phenomena such as tunneling and isotope effects. As an initial application of the method, we employ a model used previously by Azzouz and Borgis to represent the proton transfer reaction AH–B■A−–H+B in liquid methyl chloride, where the AH–B complex corresponds to a typical phenol–amine complex. We have chosen this model, in part, because it exhibits both adiabatic and diabatic behavior, thereby offering a stringent test of the theory. MDQT proves capable of treating both limits, as well as the intermediate regime. Up to four quantum states were included in this simulation, and the method can easily be extended to include additional excited states, so it can be applied to a wide range of processes, such as photoassisted tunneling. In addition, this method is not perturbative, so trajectories can be continued after the barrier is crossed to follow the subsequent dynamics.