Collisional deactivation of vibrationally

Hippier H, Troe J and Wendelken H J 1983 Collisional deactivation of vibrationally highly excited polyatomic molecules. II. Direct observations for excited toluene J. Chem. Phys. 78 6709... 

The chemical-activation step is between one and two orders of magnitude faster than the subsequent collisional deactivation of vibrationally excited O2. Finally, the population of individual vibrational levels v" of O2 is probed tluough LIF in the Schiunann-Runge band Oi X E") after exciting the oxygen... 

A somewhat more systematic study has been made of collisional deactivation of vibrationally excited ions. Some diatomic and triatomic systems that have been investigated are included in Table II. Vibrational-to-rotational transfer has been demonstrated95 for vibrationally excited H2+ colliding with helium ... 

Up to this point we have discussed collisional deactivation of vibration-ally excited ions formed by ionization or as products of exoergic particle-transfer ion-molecule reactions. A somewhat different situation prevails with larger vibrationally excited ions, such as those formed as intermediates in ion-molecule association reactions. Reactions in which such excited intermediates are formed generally demonstrate a third-order dependence of the rate on the concentrations of the reactants at relatively low pressures. The general reaction mechanism may be represented as... 

Schwarzer D, Troe J, Votsmeier M, Zerezke M. Collisional deactivation of vibrationally highly excited azulene in compressed liquids and supercritical fluids. J Chem Phys 1996 105 3121-3131. 

Schwarzer, S., J. Troe, and M. Zerezke 1997, The role of local density in the collisional deactivation of vibrationally highly excited azulene in supercritical fluids . J. Chem. Phys. 107, 8380. 

The excited singlet states of alkanes show a weak fluorescence with wavelength of maximum around 200-230 nm and a quantum yield of (f) = 0.001-0.02 (Rothman et al. 1973). The lifetimes of the excited states are generally around 1 ns (Hermann et al. 1985). In gas phase, the lifetime decreases with the decreasing pressure probably due to decreased collisional deactivation of vibrationally excited Si molecules. In liquid phase, a characteristic temperature dependence of the fluorescence lifetime was detected (Flamigni et al. 1982 Dellonte et al. 1984 Wickramaaratchi et al. 1985). 

Miller L A and Barker J R 1996 Collisional deactivation of highly vibrationally excited pyrazine J. Chem. Phys. 105 1383-91... 

Michaels C A, Mullin A S, Park J, Chou J Z and Flynn G W 1998 The collisional deactivation of highly vibrationally excited pyrazine by a bath of carbon dioxide excitation of the infrared inactive (10°0), (02°0), and (02 0) bath vibrational modes J. Chem. Phys. 108 2744-55... 

Photofragmentation in the liquid phase. Phorodissociation reactions in liquid phase occur at much reduced quantum yields because of the possibility of recombination within the solvent cage. Furthermore the product formation and distribution also differ because of collisional deactivation of initially produced vibrationally excited hot molecules. Where CT absorption produces radical ions, the solvent may react with the ionic species. 

An excited ionic species that has been widely studied from the standpoint of collisional deactivation is vibrationally excited H. The collisional deactivation process,... 

The possibility of deactivation of vibrationally excited molecules by spontaneous radiation is always present for infrared-active vibrational modes, but this is usually much slower than collisional deactivation and plays no significant role (this is obviously not the case for infrared gas lasers). CO is a particular exception in possessing an infrared-active vibration of high frequency (2144 cm-1). The probability of spontaneous emission depends on the cube of the frequency, so that the radiative life decreases as the third power of the frequency, and is, of course, independent of both pressure and temperature the collisional life, in contrast, increases exponentially with the frequency. Reference to the vibrational relaxation times given in Table 2, where CO has the highest vibrational frequency and shortest radiative lifetime of the polar molecules listed, shows that most vibrational relaxation times are much shorter than the 3 x 104 /isec radiative lifetime of CO. For CO itself radiative deactivation only becomes important at lower temperatures, where collisional deactivation is very slow indeed, and the specific heat contribution of vibrational energy is infinitesimal. Radiative processes do play an important role in reactions in the upper atmosphere, where collision rates are extremely slow. 

Observable effects in the quenching of fluorescence are usually the result of competition between radiation and bimolecular collisional deactivation of electronic energy, since vibrational relaxation is normally so rapid, especially in condensed phases, that emission derives almost entirely from the ground vibrational level of the upper electronic state. The simplest excitation-deactivation scheme, which does not allow for intramolecular radiationless... 

Another interesting result obtained in experiments on these ratios of rate constants is the lack of the dependence of the ratio on the nature of the gas collisionally deactivating the vibrationally excited ozone isotopomer [46]. Different mechanisms have been postulated for the collisional deactivation, including the energy transfer mechanism used here and most commonly used elsewhere, and a chaperon mechanism in which the third-body collision partner forms collision complex with the Q or with the Q2 prior to the recombination step. Recently the chaperon mechanism was revisited for ozone formation, analyzing pressure and temperature dependent data on the recombination rate [47]. Since the ratios of rate... 

The view that electronic states of different multiplicity need not be considered cannot easily be ruled out, since both deactivation of vibrationally excited carbenes and intersystem crossing between singlet and triplet states are brought about by collision with other molecules. The difficulty is not restricted to reactions in the gas phase in solution, collisional deactivation and collision-induced intersystem crossing can still be expected to compete with collisions leading to chemical reaction. However, the parallelism between the variation in stereospecificity in the gas-phase addition of methylene to the 2-butenes with the pressure of inert gas (Frey, 1959, I960 Anet et al., 1960 Bader and Generosa,... 

The application of both criteria to gas-phase reactions is complicated further by the formation of vibrationally excited products. Both the insertion and addition reactions of methylene are exothermic by approximately 93 kcal. mole (based on recent estimates of AH (CH2) = 94 kcal.mole" ). Vibrationally excited alkanes and alkenes may dissociate into free radicals, and excited cyclopropanes may undergo structural and geometrical isomerizations unless collisionally stabilized . The occurrence of hot molecule reactions excludes any reasonable estimation of singlet and triplet methylene fractions. The data presented in the following paragraphs have been taken from experiments at high-pressures", which are thought to ensure complete collisional deactivation of excited reaction products. 

With infrared lasers the molecules are generally excited into higher vibrational levels of the electronic ground state. Assuming cross sections of 10 —10 cm for the collisional deactivation of the vibrationally excited molecules, the equipartition of energy takes only about 10 s at pressures around 1 mbar. Since the spontaneous lifetimes of these excited vibrational levels are typically around 10 —I0 s, it follows that at pressures above 1 mbar, the excitation energy absorbed from the laser beam will be almost completely transferred into thermal energy, which implies that rjk 0. 

J Benzler, S Linkersdorfer, K Luther. Density dependence of the collisional deactivation of highly vibrationally excited cycloheptatriene in compressed gases, supercritical fluids, and liquids. J Chem Phys 106 4992, 1997. 

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