Molecular surface scattering potential energy surfaces

In Chapter VI, Ohm and Deumens present their electron nuclear dynamics (END) time-dependent, nonadiabatic, theoretical, and computational approach to the study of molecular processes. This approach stresses the analysis of such processes in terms of dynamical, time-evolving states rather than stationary molecular states. Thus, rovibrational and scattering states are reduced to less prominent roles as is the case in most modem wavepacket treatments of molecular reaction dynamics. Unlike most theoretical methods, END also relegates electronic stationary states, potential energy surfaces, adiabatic and diabatic descriptions, and nonadiabatic coupling terms to the background in favor of a dynamic, time-evolving description of all electrons. 

The F + H2 — HF + FI reaction is one of the most studied chemical reactions in science, and interest in this reaction dates back to the discovery of the chemical laser.79 In the early 1970s, a collinear quantum scattering treatment of the reaction predicted the existence of isolated resonances.80 Subsequent theoretical investigations, using various dynamical approximations on several different potential energy surfaces (PESs), essentially all confirmed this prediction. The term resonance in this context refers to a transient metastable species produced as the reaction occurs. Transient intermediates are well known in many kinds of atomic and molecular processes, as well as in nuclear and particle physics.81 What makes reactive resonances unique is that they are not necessarily associated with trapping... 

Experiments have also played a critical role in the development of potential energy surfaces and reaction dynamics. In the earliest days of quantum chemistry, experimentally determined thermal rate constants were available to test and improve dynamical theories. Much more detailed information can now be obtained by experimental measurement. Today experimentalists routinely use molecular beam and laser techniques to examine how reaction cross-sections depend upon collision energies, the states of the reactants and products, and scattering angles. 

The most basic information that is needed for constructing a global potential energy surface for gas phase MD simulations is the structures and vibrational frequencies. The earliest information about gas-phase RDX molecular structures was obtained from theoretical calculations [54-58]. In 1984 Karpowicz and Brill [59] reported Fourier transform infrared spectra for vapor-phase (and for the a - and p -phase) RDX in 1984, however, their data precluded a complete description of the molecular conformations and vibrational spectroscopy. More recently, Shishkov et al. [60] presented a more complete description based on electron-scattering data and molecular modeling. They concluded that the data were best reproduced by RDX in the chair conformation with all the nitro groups in axial positions. 

Potential energy surfaces can be built starting from experimental data (e.g., bond strengths, geometries, infrared and fluoresence spectra, molecular beam scattering cross sections, viscosity, diffusion coefficients, line broadening... 

It seems, therefore, with the current renewal of theoretical interest in atomic and molecular collision problems, reactive scattering, and predissociation phenomena, that it is worthwhile to examine the VB theory as a useful model that is capable of yielding accurate potential energy surfaces. 

From time to time the broad front of advance in any field is pierced by significantly greater and more important developments in some subareas. Recent developments in laser technology, mass spectrometry, and molecular beam studies have made that the case for the properties and reactions of excited states of simple atoms and molecules. This volume of the Advances in Chemical Physics is, therefore, devoted to a collection of contributions that are relevant to aspects of the physics and chemistry of excited species. The articles cover topics as diverse as theoretical estimation of potential energy, surface properties, and upper atmosphere chemistry, but all are tied together by the common denominator of the need to understand the properties of the excited states of molecules. It is hoped that this and succeeding volumes will supplement the rather broadly scattered literature, and provide an introduction for both the interested student and the working scientist. 

The first block, INTERACTION, is devoted to the calculation of electronic energies determining the potential energy surface (PES) on which the nuclear morion takes place. The second l)lock, DYNAMICS, is devoted to the integration of the scattering equations to determine the outcome of the molecular process. The third block, OBSER WBLES. is devoted to the reconstruction of the ol)serr al)le properties of the beam from the calculated dynamical quantities. All these blocks reejuiro not only different skills and expertise but also specialized computer software and hardware. 

In light of previous experimental and theoretical work on the F f H2 reaction, it can be seen why an experisient of this complexity is necessary in order to observe dynamic resonances in this reaction. The energetics for this reaction and its isotopic variants are displayed in Figure 1. Chemical laser (11) and infrared chemiluminescence (12) studies have shown that the HF product vibrational distribution is hi ly inverted, with most of the population in v=2 and v°°3. A previous crossed molecular beam study of the F + D2 reaction showed predominantly back-scattered DF product (13). These observations were combined with the temperature dependence of the rate constants from an early kinetics experiment (14) in the derivation of the semiempirical Muckerman 5 (M5) potential energy surface (15) using classical trajectory methods. Although an ab initio surface has been calculated (16), H5 has been the most widely used surface for the F H2 reaction over the last several years. 

The study of alkali atom reactions with halogen-containing molecules comprises much of the history of reactive scattering in molecular beams. The broad features of the reaction dynamics and their relation to the electronic structure of the potential energy surface are well understood.2 The reaction is initiated by an electron jump transition in which the valence electron of the alkali atom M is transferred to the halogen-containing molecule RX. Subsequent interaction of the alkali ion and the molecule anion, in the exit valley of the potential surface, leads to an alkali halide product molecule MX. 

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