Collision cross section vibrational

The results of the present calculations that the zero-point vibrational energy of the reactants can pass smoothly into that of the intermediate complexes is entirely consistent with the basic postulate of Eyring s theory that activated complexes are created from the reactants in equilibrium states. It is easy to show that the vibrationally adiabatic model, coupled with the assumption that collision cross sections are the same for all vibrational levels, leads to the conclusion that there is a Boltzmann distribution between the vibrational levels in the activated state. Thus, consider the situation represented by the energy diagram shown in Fig. 6 two levels are shown for the initial and activated states— the ground level and the nth vibrationally excited... 

Fig. 13. Electron energy dependence of collision cross section for representative electron impact processes with molecular chlorine. Note threshold energy for endothermic processes (vibrational excitation, ionization, etc.). Qa, Qd, Qe, Qi, Qm. and Qv, correspond to electron attachment (reaction R18 in Table 4), dissociation (R19), electronic excitation (R12), momentum transfer (R20), ionization (R16) and vibrational excitation (not shown in Table 4), respectively. After [44].

Fig. 2. The pathways for collision-induced intramolecular vibrational energy transfer from the 6 level of benzene. The figures entered are the cross-sections relative to the hard sphere collision cross-section. Collision partner He.

Considerable experimental effort has been aimed at elucidating the collision-free unimolecular dynamics of excited molecules. Processes of interest include the dynamics of highly excited vibrational states, which have been reached by multiphoton absorption, and the various electronic relaxation processes that can occur in electronically excited states of moderate to large molecules, etc. The idealized collision-free limit is approached either by extrapolating data to the limit of zero pressure or by performing experiments in molecular beams. Alternatively, estimates of expected collisional effects are made by using collision cross-sections that are computed from hard-sphere collision rates. These estimates are then utilized to determine whether the experiments are performed in the collision-free domain. 

Figure 5 (Kurachi and Nakamura, 1990) presents a survey of electron collision cross sections of CF4. In addition to the momentum-transfer cross section q , it shows the vibrational-excitation cross sections q T, and q (for two different vibrational modes), the (total) electronic-excitation cross section q, the dissociation cross section q j , the electron-attachment cross section qg, and the (total) ionization cross section 9,. Each of the cross sections is a function of the electron kinetic energy and reflects the physics of the collision process, which is being clarified by theory. The cross sections designated as total can be discussed in greater detail in terms of different contributions, which are designated as partial cross sections.

Molecules in high vibrational levels n) of the electronic ground state generally show much larger collision cross sections. If the kinetic energy Ekin of the collision partners exceeds the energy difference (Ed - Ey) between the dissociation... 

Investigations of collision processes with molecules in these selectively populated levels f) yields the dependence of collision cross sections on the vibrational energy for collision-induced dissociation as well as for energy transfer into other bound levels of the molecule. Since the knowledge of this dependence is essential for a detailed understanding of the collision dynamics, a large number of theoretical and experimental papers on this subject have been published. For more information the reader is referred to some review articles and the literature given therein [1039, 1045, 1051-1053]. 

The expanded beam from a HeNe laser at A = 3.39 p,m with lOmW power is sent through a methane cell T = 300 K, / = 0.1 mbar, beam diameter 1 cm). The absorbing CH4 transition is from the vibrational ground state (r 00) to an excited vibrational level with r 20 p,s. Give the ratios of Doppler width to transit-time width to natural width to pressure-broadened linewidth for a collision cross section CTj = 10 cm. ... 

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