Relaxation, collisional quenching

It appears that direct population of high vibrational levels of the ground state is not primarily responsible for the initial distribution of vibrational energy in the NOg molecule formed. Evidence has been presented that vibrationally excited NOg is mainly produced by radiation or collisional quenching of electronically excited NOg. Data on the effects on vibrational relaxation rates of Ng, NOg and COg have also been reported. ... 

These data have important implications for the nature of relaxation from the thermal levels in the vapor. The high levels play an important role in condensed phase at 300°K where thermal activation to this fast leak from the electronic state is efficient, and presumably a similar situation is present in vapor experiments. In the vapor, however, the leak is severely limited by the lower rate of thermal activation. This small leak in the vapor plausibly accounts for the small but observable chemical relaxation from the thermal levels, and for the small but persistent pressure sensitivity of the thermal levels, and finally for the small temperature quenching of fluorescence from the thermal levels. (Robinson has proposed another explanation of the collisional quenching of fluorescence. 

It is clear that, by changing the experimental conditions and/or detection wavelength, limiting values can be found for all of the quantities mentioned above from measurements of the fluorescence decay time. The effects of collisional and spontaneous processes can be separated by conventional Stem—Volmer analysis [36]. The concentration, [M], of quenching molecules is varied and the reciprocal of the observed lifetime is plotted against the concentration of M. The quenching rate coefficient is thus obtained from the slope and the intercept gives the rate coefficient for the spontaneous relaxation processes, which is usually the natural lifetime of the excited state. In cases where the experiment cannot be carried out under collision-free conditions, this is the only way to measure the natural lifetime from observation of the fluorescence decay. 

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... 

Collisional Electronic Relaxation Processes, a singlet excited state (S ) of an aldehyde or ketone can be electronically quenched by a collision partner molecule,... 

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