Vibrationally-inelastic processes

The scattered vibrational population distribution is remarkable. First of all, only a small fraction of the prepared population remains in the initial vibrational state, indicating that the survival probability is at most a few percent. At this low incidence energy, similar experiments carried out with NO(r = 2) scattering from Au(lll) were unable to detect vibrationally-inelastic processes, that is the vibrational survival probability is near unity.33... 

In the previous section we noticed that all the scattering information can be obtained from the transition operator T which satisfies Lippmann-Schwinger equation (7). In order to obtain cross sections for vibrationally inelastic processes we project the equation (7) onto electronic space spanned by plane waves and a two-dimensional space for nuclear dynamics defined by a vibrational ground state and its first excited state /j. The resulting equation follows ... 

QM1 and QM2 indicates that the coupled channel expansion may not have converged for these vibrationally inelastic processes 78 within this level of uncertainty, however, there is good agreement between the semiclassical and quantum calculations. Also shown in Fig. 12 is a phase space distribution... 

Figure Bl.25.6. Energy spectrum of electrons coming off a surface irradiated with a primary electron beam. Electrons have lost energy to vibrations and electronic transitions (loss electrons), to collective excitations of the electron sea (plasmons) and to all kinds of inelastic process (secondary electrons). The element-specific Auger electrons appear as small peaks on an intense background and are more visible in a derivative spectrum.

The width of the peaks in LETS depends upon the sharpness of the onset of the inelastic process, which in turn depends upon the thermal distribution of electron energies about EP. Thus, the IETS line width depends strongly on temperature and as shown by (3) [75]. Because of this, vibrational IETS provides infrared-quality resolution only when performed below 5 K. Electronic transitions are usually much broader than vibrational transitions therefore, electronic IETS is usually performed at liquid nitrogen temperature and slightly above (>77 K). An example of a system showing both vibrational and electronic IETS is presented in Fig. 5 [19]. 

The tunnel current which flows from one metal to the other when a potential difference is applied across the junction is mainly due to elastic tunneling. However, if the adsorbed molecules on the junction have a characteristic vibrational mode of energy hv, then an inelastic process can occur when ev hv. 

Thus far the discussion has centered on elastic tunneling, but consideration of inelastic processes may offer additional analytical opportunities. An energy scale of the relevant phenomena is presented in Table 2. Inelastic tunneling was first observed in metal-oxide-metal junctions. It was immediately developed as a technique for photon-free vibrational spectroscopy (lETS) where the tunneling electrons dissipate energy by coupling to vibra-... 

Figure 3 Inelastic and elastic cross sections for electron impact excitation of the water molecule the data are from the review by Mark et al. [19]. The total interaction cross section ctt was determined from the sum of cross sections for all elastic and inelastic processes. Inelastic channels include the vibrational modes Cvi (the bending mode with threshold 0.198 eV), cTv2 (the sum of two stretching modes with thresholds 0.453 and 0.466 eV), and CvS (a lump sum of other vibrational excitation modes including higher hormonics and combinational modes with an assigned threshold of 1 eV). The electronic excitations and <7 2 have threshold energies of 7.5 and 13.3 eV. Ionization cross sections are those of Djuric et al. (O), and Bolarizadah and Rudd ( ). (From Ref 19.)...

Vibrational excitation is the dominant inelastic process in H++H2 scattering for collision energies above approximately 10 eV in the labora-... 

Another type of structure may result from localized resonance states formed by either traps or impurities in the film39 (see Fig. 6B). In this case, the electrons are localized at the trap and due to the high charge concentration strong electron-vibration interactions exist that result in inelastic processes in the film. While these traps are observed clearly in LEET,39 in the LEPS experiments they are manifest by reduction in the transmission probability and sometimes by charging effects, but cannot be observed as modulation on the amplitude of the spectra. 

During the past decade, the study of photoinitiated reactive and inelastic processes within weakly bound gaseous complexes has evolved into an active area of research in the field of chemical physics. Such specialized microscopic environments offer a number of unique opportunities which enable scientists to examine regiospecific interactions at a level of detail and precision that invites rigorous comparisons between experiment and theory. Specifically, many issues that lie at the heart of physical chemistry, such as reaction probabilities, chemical branching ratios, rates and dynamics of elementary chemical processes, curve crossings, caging, recombination, vibrational redistribution and predissociation, etc., can be studied at the state-to-state level and in real time. 

The inelastic processes - spontaneous Raman scattering (usually simply called Raman scattering), nonlinear Raman processes, and fluorescence - permit determination of species densities as well as temperature, and also allow one, in principle, to determine the temperature for particular species whether or not in thermal equilibrium. In Table II, we categorize these inelastic processes by the type of the information that they yield, and indicate the types of combustion sources that can be probed as well as an estimate of the status of the method. The work that we concentrate upon here is that indicated in these first two categories, viz., temperature and major species densities determined from vibrational Raman scattering data. The other methods - fluorescence and nonlinear processes such as coherent anti-Stokes Raman spectroscopy - are discussed in detail elsewhere (5). 

We define vibrational dephasing processes as those that change the phase of the oscillator in the bath without altering its vibrational amplitude. Since the contribution of inelastic (T,) processes is explicitly excluded, dephasing is described by the correlation function [see Eq. (11)]... 

Vibrational predissociation is of course intimately related to vibrationally and rotationally inelastic collisions. Both processes involve coupling between the same initial and final state channels, but they differ in Chat the inelastic processes must occur at energies above the appropriate internal motion excitation threshold and are observed in collision experiments, while predissociation (usually) occurs at energies below this threshold and is observed spectroscopically. While the present paper focusses most attention on the phenomenon of predissociation, the nature of the information contained in these two types of experiments will be compared. 

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