Fluorescence collisional effects

Crosley D R 1981 Collisional effects on laser-induced fluorescence Opt. Eng. 20 511-21... 

Fluorescence and collisional excitation, arising primarily from the metastability of the 23S level (see Fig. 4.9), in which consequently a high population accumulates which can cause additional emission from lines such as X 4471, X 5876 by either collisional excitation or radiative transfer effects following absorption of higher lines in the 23S — n3P series. The singlet line X 6678 can also be enhanced by collisional excitation from 23S. The collisional effects can be calculated from the known electron temperature and density, and are quite small at... 

The excited molecular systems may be roughly divided into a few groups characterized by the level-coupling schemes and the nature of the initially prepared state. Each of them shows a specific form of the fluorescence decay under collision-free conditions. The collisional effects in each group of molecules are also quite similar. 

In the case of the stepwise line fluorescence, the effect is divided into Stokes and anti-Stokes stepwise fluorescence depending on the wavelength (energy) relationships. A thermally assisted process may take place, if after radiation excitation, further collisional excitation occurs. 

To probe this question, fluorescence intensities were reexamined at pressures as low as 7 x 10 torr. " Still no hint of any collisional effects could be found. The fluorescence yield appeared constant from 7 x 10 ... 

It is important to notice that a change in lifetime is not a necessary result of a change in fluorescence intensity. For instance, the Ca2+ probe Fluo-3 displays a large increase in intensity on binding Ca2+, but there is no change in lifetime. This is because the Ca-free form of the probe is effectively nonfluorescent, and its emission does not contribute to the lifetime measurement. In order to obtain a change in lifetime, the probe must display detectable emission from both the free and cation-bound forms. Then the lifetime reflects the fraction of the probe complexed with cations. Of course, this consideration does not apply to collisional quenching, when the intensity decay of the entire ensemble of fluorophores is decreased by diffusive encounters with the quencher. 

This equation is a good approximation to the description of the relaxa-tional spectral shifts occurring with variations of xR and xF, which are brought about by temperature changes and effects of collisional fluorescence quenchers. Using this equation, xR can be easily determined if xF, v0, and vs are known for the system (the chromophore and its environment) under study. The last two values may be obtained not only from time-resolved spectra but also from steady-state spectra at the lowest (v0)and highest (vE) temperatures. The latter measurement is difficult to achieve with such labile... 

The effective lifetimes of all these excited states are determined by radiative as well as collisional deactivation, and which contribution is the more significant depends on pressure and transition probability. The simultaneous recording of the absorption and fluorescence spectra yields information about the ratio of radiative to collisioninduced nonradiative decays. This ratio is proportional to the quotient of total fluorescence from the excited level to total absorbed laser light. Such experiments have been started by Ronn oif... 

Silver ions cause strong quenching of protein fluorescence by at least two distinct mechanisms collisional quenching and energy transfer to Ag+-mercaptide absorption bands.415 The effect was studied in detail for both sulfhydryl and non-sulfhydryl proteins and had a number of practical applications including the determination of SH groups and as a probe of binding sites. 

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... 

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