Collisional deexcitation

Figure 12. A series of time-resolved spectra of HC1 emission taken after initiation of a chain reaction by laser photolysis of Cl2 in the presence of C2H6. At the earliest time delay shown here HC1 is highly excited, and relaxes by collisional deexcitation at later times. Reproduced with permission from Ref. 86.

Figure 15. Time-dependent behavior of OH(t> = 1-5) observed following production of the radical by the 0( D) + H2S reaction. The data, taken at 50- s intervals, clearly show the effects of vibrational cascade by collisional deexcitation of the initially produced inverted distribution. Reproduced with permission from Ref. 45.

One of the main problems met in Laser Induced Fluorescence measurements is the excited population dependence on the quenching due to collisional deexcitation. The saturation mode proposed to avoid this dependence is very difficult to achieve U ) (2 ) particularly with molecular species and moreover the very strong laser pulses required may cancel the non-perturbing characteristic of the method. Therefore precise knowledge of the quenching is necessary in some experimental circumstances. 

It is necessary to distinguish between the effective lifetime, tc, which allows for collisional deexcitation, and the collision-free radiative lifetime of the excited species, r0. These two lifetimes are simply related through the reactive collision frequency (kn) ... 

Due to vibrational anharmonicity, this transfer is resonant only for the K = 1 exchange, which has been considered in a previous section, but it remains, in the liquid, faster by several orders of magnitude than V-T relaxation for diatomics. Relaxation of highly excited I2 and Br2 close to the vibrational dissociation limit has been observed in the dense gas (at liquid densities). These indirect measurements of T, were correlated with the gas diffusion coefficient and should hence be more reasonably accounted for in the framework of an isolated binary interaction model. This interesting exjjerimental system raises the question of the influence of the change in molecular dimensions in higher excited states, due to anharmonicity, on the efficiency of collisional deexcitation. This question could jjerhaps be answered by more precise direct relaxation measurements. 

If the density is sufficiently high, some collisional deexcitation may occur and cooling is reduced. In the two-level approach one has ... 

For a given 7 and given metallicity, Te increases with density in regions where n is larger than a critical density for collisional deexcitation of the most important cooling lines (around 5 102 - 103 cm-3). 

If large density contrasts occur in ionized nebulae, the use of forbidden lines for abundance determinations may induce some bias if collisional deexcitation is important. These... 

Near the surface, where [M] is at its maximum, the second reaction predominates because a reasonable proportion of the excited H202 can collisionally deexcite before it falls back to two OH radicals. In the lower stratosphere, M] is almost an order of magnitude lower than at the surface, and a larger fraction of H202 falls apart, so that the first reaction is favored somewhat over the second one. 

For collisions at thermal energies the collision time Tcoii = d/vis long compared to the time for an electronic transition. The interaction V(A, B) or y(M, B) can then be described by a potential. Assume that the two potential curves V(A/, B) and V Ak, B) cross at the energy E Rq) (Fig. 8.11). If the relative kinetic energy of the collision partners is sufficiently high to reach the crossing point, the collision pair may jump over to the other potential curve [999]. In Fig. 8.11, for instance, a collision A/ -h B can lead to electronic excitation /) -> k) if kin > E2, while for a collisional deexcitation A ) /) only the kinetic energy E kin > E is required. 

If the relative kinetic energy of the collision partners is sufficiently high to reach the crossing point, the collision pair may jump over to the other potential curve [13.37]. In Fig.13.11, for instance, a collision Aj+B can lead to electronic excitation i) — k), if > Ej, while for a collisional deexcitation k) — i) only the kinetic energy E j > Ej is required. 

The primary photodissociation probably occurs from high vibrational levels of the electronic ground state reached by rapid intersystem crossing directly from the originally excited Bg state. No evidence was found for collisional deexcitation, and the dissociation via reaction (4) occurred with a quantum yield close to unity. Some molecular dissociation Into N2 and H2 can not be excluded, because this process Is symmetry-allowed for C/S-N2H2 [1, 12]. The successive fission of the NH bonds is predicted from ab Initio SCF Cl studies to be the preferred pathway in the photolysis of C/S-N2H2 [13, 14]. The photolytic decomposition into N2 and H2 was already observed earlier in a few qualitative experiments at very low pressures (< 0.1 Torr) [9]. 

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