Tamar Seideman

Calculation of the cumulative reaction probability via a discrete variable representation with absorbing boundary conditions

A new method is suggested for the calculation of the microcanonical cumulative reaction probability via flux autocorrelation relations. The Hamiltonian and the flux operators are computed in a discrete variable representation (DVR) and a well‐behaved representation for the Green’s operator, G(E+), is obtained by imposing absorbing boundary conditions (ABC). Applications to a one‐dimensional‐model problem and to the collinear H+H2 reaction show that the DVR‐ABC scheme provides a very efficient method for the direct calculation of the microcanonical probability, circumventing the need to compute the state‐to‐state dynamics. Our results indicate that the cumulative reaction probability can be calculated to a high accuracy using a rather small number of DVR points, confined to the vicinity of the transition state. Only limited information regarding the potential‐energy surface is therefore required, suggesting that this method would be applicable also to higher dimensionality problems, for which the complete ...

Quantum mechanical reaction probabilities via a discrete variable representation-absorbing boundary condition Green's function

The use of a discrete variable representation (DVR) and absorbing boundary conditions (ABC) to construct the outgoing Green’s function G(E+)≡lime→0(E+ie−H)−1, and its subsequent use to determine the cumulative reaction probability for a chemical reaction, has been extended beyond our previous work [J. Chem. Phys. 96, 4412 (1992)] in several significant ways. In particular, the present paper gives a more thorough derivation and analysis of the DVR‐ABC approach, shows how the same DVR‐ABC Green’s function can be used to obtain state‐to‐state (as well as cumulative) reaction probabilities, derives a DVR for the exact, multidimensional Watson Hamiltonian (referenced to a transition state), and presents illustrative calculations for the three‐dimensional H+H2 reaction with zero total angular momentum.

Full‐dimensional quantum mechanical calculation of the rate constant for the H2+OH→H2O+H reaction

The cumulative reaction probability (CRP) (the Boltzmann average of which is the thermal rate constant) has been calculated for the reaction H2+OH↔H2O+H in its full (six) dimensionality for total angular momentum J=0. The calculation, which should be the (numerically) exact result for the assumed potential energy surface, was carried out by a direct procedure that avoids having to solve the complete state‐to‐state reactive scattering problem. Higher angular momenta (J≳0) were taken into account approximately to obtain the thermal rate constant k(T) over the range 300<T<700 K; the result is significantly larger than the experimental values (a factor of ∼4 at 300 K), indicating that a more accurate potential energy surface is needed in order to provide a quantitative description of this reaction.

Discerning vibronic molecular dynamics using time-resolved photoelectron spectroscopy

Dynamic processes at the molecular level occur on ultrafast time scales and are often associated with structural as well as electronic changes. These can in principle be studied by time-resolved scattering and spectroscopic methods, respectively. In polyatomic molecules, however, excitation results in the rapid mixing of vibrational and electronic motions, which induces both charge redistribution and energy flow in the molecule. This ‘vibronic’ or ‘non-adiabatic’ coupling is a key step in photochemical and photobiological processes and underlies many of the concepts of molecular electronics, but it obscures the notion of distinct and readily observable vibrational and electronic states. Here we report time-resolved photoelectron spectroscopy measurements that distinguish vibrational dynamics from the coupled electronic population dynamics, associated with the photo-induced internal conversion, in a linear unsaturated hydrocarbon chain. The vibrational resolution of our photoelectron spectra allows for a direct observation of the underlying nuclear dynamics, demonstrating that it is possible to obtain detailed insights into ultrafast non-adiabatic processes.

Rotational excitation and molecular alignment in intense laser fields

Rotational excitation and spatial alignment in moderate intensity radiation fields are studied numerically and analytically, using time‐dependent quantum mechanics. Substantial rotational excitation is found under conditions typically used in time‐resolved spectroscopy experiments. The broad rotational wave packet excited by the laser pulse is well defined in the conjugate angle space, peaking along the field polarization direction. Both the rotational excitation and the consequent spatial alignment can be controlled by the choice of field parameters. Fragment angular distributions following weak field photodissociation of the rotational wave packet are computed as a probe of the degree of alignment. In the limit of rapid photodissociation the angular distribution is peaked in the forward direction, reflecting the anisotropy of the aligned state. Potential applications of the effect demonstrated range from reaction dynamics of aligned molecules and laser‐control to material deposition and laser‐assisted isotope separation.

Nonadiabatic Alignment by Intense Pulses. Concepts, Theory, and Directions

We review the theory of intense laser alignment of molecules and present a survey of the many recent developments in this rapidly evolving field. Starting with a qualitative discussion that emphasizes the physical mechanism responsible for laser alignment, we proceed with a detailed exposition of the underlying theory, focusing on aspects that have not been presented in the past. Application of the theory in several pedagogical illustrations is then followed by a review of the recent experimental and theoretical advances in the field. We conclude with a discussion of new directions, future opportunities, and areas where we expect intense laser alignment to play a role in the future. Throughout we emphasize the recent evolution of the method of nonadiabatic alignment from isolated diatomic molecules to complex media.

Quantum mechanical calculations of the rate constant for the H2+OH→H+H2O reaction: Full‐dimensional results and comparison to reduced dimensionality models

The cumulative reaction probability is calculated for the H2+OH→H+H2O reaction in its full (six) dimensionality for total angular momentum J=0. The calculation, which should give the (numerically) exact result for the assumed potential energy surface, yields the cumulative reaction probability directly, without having to solve the complete state‐to‐state reactive scattering problem. Higher angular momenta (J≳0) were taken into account approximately to obtain the thermal rate constant k(T) over the range 300°<T<700°. The result deviates significantly from the experimental rate constant, suggesting that the potential energy surface needs to be improved. A systematic series of reduced dimensionality calculations is carried out in order to characterize the behavior and reliability of these more approximate treatments; a comparison of the full dimensional results with previous reduced dimensionality calculations is also made.

Theory of photoinduced surface reactions of admolecules

The absorption of photons by an adsorbate/substrate complex may induce a wide range of physical and chemical processes, such as desorption, dissociation and reactions. Although several of these processes have analogs in the gas phase, the presence of the surface opens new reaction pathways that are not available in the gas phase. These unique pathways can be used to control reactivity, product selectivity and yield, or to explore new reactions. Stimulated by the surge in experimental studies of surface photochemistry, various theoretical models have been recently developed to elucidate observations and explore new opportunities. In this review, we survey recent advances in the theoretical characterization of photoinduced chemical and physical processes occurring on solid surfaces. Our discussions are focused on two prototypical processes. The adsorbate photodissociation on insulator surfaces provides an ideal probe of the nonelectronic interaction with the substrate. Photochemical processes on conductors, on the other hand, highlight the excitation and relaxation processes induced by substrate hot carriers. The issues addressed here include excitation and relaxation mechanisms, the role played by internal modes of the adsorbate and energy transfer between the admolecule and the substrate. Both classical and quantum models are used in describing these processes.

Observation of multiple vibrational modes in ultrahigh vacuum tip-enhanced Raman spectroscopy combined with molecular-resolution scanning tunneling microscopy

Multiple vibrational modes have been observed for copper phthalocyanine (CuPc) adlayers on Ag(111) using ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS). Several important new experimental features are introduced in this work that significantly advance the state-of-the-art in UHV-TERS. These include (1) concurrent sub-nm molecular resolution STM imaging using Ag tips with laser illumination of the tip-sample junction, (2) laser focusing and Raman collection optics that are external to the UHV-STM that has two cryoshrouds for future low temperature experiments, and (3) all sample preparation steps are carried out in UHV to minimize contamination and maximize spatial resolution. Using this apparatus we have been able to demonstrate a TERS enhancement factor of 7.1 × 10(5). Further, density-functional theory calculations have been carried out that allow quantitative identification of eight different vibrational modes in the TER spectra. The combination of molecular-resolution UHV-STM imaging with the detailed chemical information content of UHV-TERS allows the interactions between large polyatomic molecular adsorbates and specific binding sites on solid surfaces to be probed with unprecedented spatial and spectroscopic resolution.

Advances in intense femtosecond laser filamentation in air

This is a review of some recent development in femtosecond filamentation science with emphasis on our collective work. Previously reviewed work in the field will not be discussed. We thus start with a very brief description of the fundamental physics of single filamentation of powerful femtosecond laser pulses in air. Intensity clamping is emphasized. One consequence is that the peak intensity inside one or more filaments would not increase significantly even if one focuses the pulse at very high peak power even up to the peta-watt level. Another is that the clamped intensity is independent of pressure. One interesting outcome of the high intensity inside a filament is filament fusion which comes from the nonlinear change of index of refraction inside the filament leading to cross beam focusing. Because of the high intensity inside the filament, one can envisage nonlinear phenomena taking place inside a filament such as a new type of Raman red shift and the generation of very broad band supercontinuum into the infrared through four-wave-mixing. This is what we call by filamentation nonlinear optics. It includes also terahertz generation from inside the filament. The latter is discussed separately because of its special importance to those working in the field of safety and security in recent years. When the filamenting pulse is linearly polarized, the isotropic nature of air becomes birefringent both electronically (instantaneous) and through molecular wave packet rotation and revival (delayed). Such birefringence is discussed in detailed. Because, in principle, a filament can be projected to a long distance in air, applications to pollution measurement as well as other atmospheric science could be earned out. We call this filamentation atmospheric science. Thus, the following subjects are discussed briefly, namely, lightning control, rain making, remote measurement of electric field, microwave guidance and remote sensing of pollutants. A discussion on the higher order Kerr effect on the physics of filamentation is also given. This is a new hot subject of current debate. This review ends on giving our view of the prospect of progress of this field of filamentation in the future. We believe it hinges upon the development of the laser technology based upon the physical understanding of filamentation and on the reduction in price of the laser system.