Collision-induced processes

In this chapter, we have dealt with radiative transfer of atoms and molecules between different energy states rather extensively. However, transitions can also be induced by collisions. Extensive information on the static and dynamic properties of atoms and molecules can be obtained from collision physics. Although this book is centred on the spectroscopy of atoms and molecules, the importance of collisional physics should be clearly pointed out. For studies of these aspects the reader is referred to [4.32-37]. 

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For most carbonyl compounds, kp is approximately independent of vibrational excitation energy (Eypp,), whereas kfjR usually increases with Evpp,. Therefore, p becomes smaller and ip becomes shorter as Evpb increases. Typically, highly sensitive technique of fluorescence excitation spectroscopy permits measurement of Op over an extensive range of Evib- Hence the rates of radiative transitions as well as the rates of radiationless transitions of SVLs and SRVLs can be readily determined (135). For simple carbonyl compounds with small amounts of vibrational energy in the Sp state, collision-induced processes can become important at pressures above a few torr, since the lifetimes (tp) are comparable to the mean collision times (tu) at these pressures. Rate data reported for a number of aliphatic carbonyls are summarized in Table 2. 

Of particular interest (although not a true collision-induced process) has been the effect of added NO on the yield of CO. The H-atom transfer reaction... 

A collision-induced process appears to be involved in product formation below 325 nm (87 kcal/mol), but the exact nature of the effect is still unknown. An internal rearrangement to HCOH, rather than a collision-induced intersystem crossing, seems likely. 

We have observed another collision induced process, the collision induced desorption of molecular methane (ref. 9). The simultaneous occurrence of this process, which is roughly an order of magnitude more probable than dissociation, necessitates that the flux of methane incident on the surface... 

This computational routine for predicting state-to-state probabilities has been shown to give quantitative agreement with experiment for a wide variety of collision-induced processes. These include rotational transfer (RT) [24, 26, 36, 38—40], i.e., state change within a vibrational manifold, vibration-rotation transfer (VRT) [41, 42], i.e., transitions between vibrational states and electronic energy transfer [43], i.e., transitions between discrete levels of different electronic states. An example is given in Fig. 6 in which the calculated data were computed using the... 

Fig. 8 Collision-induced rotational transfer in collisions between Ar and N2(0 10) squares), O2(0 12) circles) and OF1(0 3) triangles). Collision conditions are Maxwell-Boltzmann distribution of velocities at 300 K. Note that there is a Aj = 2n selection rule in collision-induced processes for homonuclear diatomic molecules...

One can distinguish between two fundamental types of collision-induced processes ... 

The intermolecular nonradiative transition can be caused by the direct collisional quenching (or transfer) and/or by the so-called collision-induced radiationless transition. The collision-induced process is particularly important in the spin-forbidden transition and is not well understood. 

Recently Ladanyi has extended the work on N2 and CO2 by examining the induced terms given by a site-site DID model. This model allows for the distribution of polarizable matter within the molecule (c.f the discussion of section 2) by representing the molecular polarizability by an isotropic point polarizability on each interaction site and taking all orders of intra-molecular DID interactions into account. For COo her results are appreciably different from Frenkel and McTague s. In particular there is an enhanced projection of the induced terms along and a better timescale separation between the allowed and collision-induced processes so that the total spectrum resembles the reorientational spectrum more closely. 

Figure 3.2 Cluster-surface collision process space diagram illustrating the dependence of the Cluster-surface collision-induced process on the cohesive energies of the system (R = ( (Coh) V( (Coh) " )) and reduced cluster ion energy ( ]dn /- ci/ (Coh) )- Reprinted with permission from Harbich (2000) Copyright 2000 from Springer-Verlag.

Elementary collision-induced processes of NH radicals can be of a chemical nature or of a physical nature (change of quantum state (excitation or quenching)). Only the product analysis gives a definite answer for the interaction pathway. Many of the investigations concerning elementary processes of NH were done in the gas phase, which is significantly different from the kinetic situation in condensed media. Therefore, the two situations are described in separate sections. 

The first of these studies not only reported the potential use of E/N dependences, but also presented details on the use of collision-induced processes in a quadrupole ion trap to differentiate monoterpenes. Although it was possible to identify specific monoterpenes based on collision-induced dissociation, the identification of several monoterpenes in a complex mixture would still be difficult. Misztal et al. also investigated the effect of varying E/N over a limited range of approximately 100-140 Td in a bid to extend the selectivity of PTR-MS this was specifically attempted in order to discriminate individual monoterpenes. By using a detailed analysis of fragmentation pathways the researchers have proposed that PTR-MS could be used to determine individual monoterpenes in a complex chemical mixture. 

The area of laser-induced chemistry is one which has generated an enormous amount of interest in the last few years, both experimentally and theoretically. In this chapter, we shall briefly review our approach to the subject and then discuss an aspect of our most recent work how incident radiation of various frequencies affects collision-induced processes. 

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