Rotational excitation inelastic case

As in Chapter 9 we discuss first the elastic limit (no exit channel excitation) in Section 10.1 and subsequently the more interesting inelastic case in Section 10.2. In Section 10.3 we consider the decay of long-lived resonance states and the impact of exit channel dynamics on the product distributions. A simple approximation, the so-called impulsive model, which is frequently employed to analyze experimental distributions in the absence of a PES, is discussed critically in Section 10.4. The chapter ends with a more qualitative assessment of thermal broadening of rotational state distributions in Section 10.5... 

Some work on collision-induced absorption in atom-molecule collisions has been done, however, results for this case are not very extensive, as yet. Moreover, the best manner for analyzing the light scattering problem in which, as in this case, other inelastic channels are open (vibrational and rotational excitation) is far from clear. 

The energy analysis of these inelastically scattered electrons is carried out by a cylindrical sector identical to the monochromator. The electrons are finally detected by a channeltron electron multiplier and the signal is amplified, counted and recorded outside of the vacuum chamber. A typical specularly reflected beam has an intensity of 10 to 10 electrons per second in the elastic channel and a full width at half maximum between 7 and 10 meV (60-80 cm l 1 meV = 8.065 cm-- -). Scattering into inelastic channels is between 10 and 1000 electrons per second. In our case the spectrometer is rotatable so that possible angular effects can also be studied. This becomes important for the study of vibrational excitation by short range "impact" scattering (8, 9, 10). 

The interaction of an electron with a molecule is described as a collision or impact, although the electron is so small that there is no collision in the usual sense of the word. The collision process may be termed elastic (the electron is merely deflected), inelastic (energy is transferred from the electron to the molecule), and superelastic (energy is transferred from the molecule to the electron). Electron-impact ionization is an example of an inelastic collision. The energy imparted to a molecule during an inelastic collision can lead to rotational, electronic, and vibrational excitation with or separate from ionization. Further, multiple-electron excitation can occur followed by autoionization, and the latter process has been shown to lead to a substantial fraction of total ionized species in many cases (S. Meyerson et al., 1963). Thus, an electron of energy 20 eV may lead to any of the above excitations of a molecule. The gas pressures used in a mass spectrometer and the density of electrons in the electron-beam are such that multiple electron-molecule interactions leading to ionization are improbable. 

Similar problems are encountered in a description of elastic or rotationally inelastic collisions of the electrons with molecules that have permanent dipole moment. However in this case K is never zero because k0 and ki have different norms due to an energy transfer to the vibrational excitation. 

It must be pointed out that considering only elastic collisions in deriving an electron temperature of several e.v. is unrealistic for molecular gases of interest to the chemist. Inelastic collisions leading to excitation of rotational, vibrational, and low-lying electronic states in molecules can enter the picture in major ways. Understanding in detail the way in which an electron energy distribution is established in most real cases of... 

The selective population of electronic fine structure states is observed in many other molecular processes, like chemical reactions, inelastic collisions and surface scattering. The basic origin for the selectivity is the same in all these cases. It requires some orientation of the unpaired lobe in the product and a well defined rotational motion during the break up of the complex. If too many different initial rotations are present, the A-doublet selectivity is smeared out and a statistical population is obtained. In most cases, the A-doublet selectivity is indeed considerably lower than expected from the degree of electron alignment. This suggests, in the view of these results, that still considerable out-of-plane motion is present in the excited complex. 

At the high level of final state resolution provided by such experiments we can discern quantal interference effects. The more prominent feature for inelastic excitation is a rotational rainbow that arises by a mechanism similar to the intense scattering of the final velocity into certain directions (Section 2.2.5). Here too, the rainbow arises from different trajectories scattered into the same final state except that the state is specified not only by the direction of v but also by the rotational state of the molecule, NO in the case of Figure 10.9. This is a stereodynamic effect because the final state is determined not only by the impact parameter but also by the angle of approach, as shown for scattering by a hard ellipsoid in Figure 10.10. 

Because they lend themselves to studies using both photochemical and chemical activation, bimolecular reactions of vibrationally excited hydrogen halides have been more throughly investigated than any other family of reactions. The rate constants in Table 1.3 have been obtained by the laser-induced vibrational fluorescence technique and correspond to the sum of rate constants for reactive and inelastic processes. The main problem is to establish the atomic concentrations accurately. This is usually accomplished by gas-phase titration in a discharge-flow system, although photolysis methods have also been employed. To find the ratio of reaction to non-reactlve relaxation, product concentrations have to be observed. This has been done in relatively few cases. Some systems have also been studied using the infrared chemiluminescence depletion technique (see Section 1.5.1). These experiments supply relative rate data for removal from several vibrational levels, and, in favorable cases, also provide some information about the rotational-state dependence of these rates. 

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