Electron quenching

A S state of BaO by seven visible Ar laser lines, Sakurai etal. studied rotational and vibrational transitions induced by collisions with He, Ar and Nj. From measurements of the pressure-dependent lifetime (r = o = 12 3 /tsec), the authors found for the electronic quenching cross section of the A 2 state by He the value 6 = 0.25. ... 

The electron- and hole-trapping dynamics in the case of WS2 are elucidated by electron-quenching studies, specifically by the comparison of polarized emission kinetics in the presence and absence of an adsorbed electron acceptor, 2,2 -bipyridine [68]. In the absence of an electron acceptor, WS exhibits emission decay kinetics similar to those observed in the M0S2 case. The polarized emission decays with 28-ps, 330-ps, and about 3-ns components. For carrier-quenching studies to resolve the dynamics of electron trapping, it is necessary that the electron acceptor quenches only conduction-band (not trapped) electrons. It is therefore first necessary to determine that electron transfer occurs only from the conduction band. The decay of the unpolarized emission (when both the electron and the hole are trapped) is unaffected by the presence of the 2,2 -bipyridine, indicating that electron transfer docs not take place from trap states in the WS2 case. Comparison of the polarized emission kinetics in the presence and absence of the electron acceptor indicates that electron transfer does occur from the conduction band. Specifically, this comparison reveals that the presence of 2,2 -bipyridine significantly shortens the slower decay component of the polarized... 

In an earlier work Basco, Callear, and Norrish22 measured approximate quenching constants. There appears to be an error in Table 4 of ref. 22 which lists the radius of electronic quenching. If we assume that the quenching constants are proportional to the electronic radius (as opposed to the square of the radius) and use a proportionality constant from the data quoted in ref. 78, we can obtain reasonable values. Apparently this was done in ref. 78 when referring to the data of ref. 22. Furthermore, values of Basco, Callear, and Norrish are too low, because correction for the NO self-quenching was not made. As the NO pressures were 2-5 torr, the self-quenching was more effective than emission. 

In addition to electronic quenching of the A iBl state, S02 also produces vibrational relaxation in the relatively long lived 3 3 Bt state, as evidenced by the growth of banded emission at the expense of continuum in the 4500 A region when S02 is added. A partial analysis of absorption bands in the 3-% transition has been given230. This transition has also been observed in emission in shock heated S02231 231. ... 

Laser-induced fluorescence is a sensitive, spatially resolved technique for the detection and measurement of a variety of flame radicals. In order to obtain accurate number densities from such measurements, the observed excited state population must be related to total species population therefore the population distribution produced by the exciting laser radiation must be accurately predicted. At high laser intensities, the fluorescence signal saturates (1, 2, 3 ) and the population distribution in molecules becomes independent of laser intensity and much less dependent on the quenching atmosphere (4). Even at saturation, however, the steady state distribution is dependent on the ratio of the electronic quenching to rotational relaxation rates (4, 5, 6, 7). When steady state is not established, the distribution is a complicated function of state-to-state transfer rates. 

Figure 1. Four-level molecular model. QiS is the collisional-transfer rate constant from level i to level j, TV is the sum of the electronic quenching and spontaneous emission rate constants, W,t is the absorption rate constant, and Wlt is the stimulated emission rate constant. WIt and WtI are proportional to the laser power PL. The dashed vertical line separates levels le and 2e, which are treated as an isolated system, from those levels not affected directly by the laser radiation.

The fluorescence spectrum is found to be markedly non-Boltz-mann and sharply peaked at the directly excited level throughout the laser pulse. This is due to two effects the competition between electronic quenching and rotational relaxation processes (4) and the short length of the laser pulse. Because the pulse is so short, steady state is not established throughout the upper rotational levels. The peaks of the fluorescence pulses from levels which are not directly excited by the laser lag the laser pulse peaks by one to four nanoseconds, depending on the energy gap between the given level and the directly excited level. 

The extremely fast quenching of C-0 An. by C-O is probably rotational relaxation of the highfy rotationaily excited C O photofragment. The slower process for C Oo some unknown combination of vibrational and electronic quenching and reaction. Likewise, contributions of vibrational and electronic relaxation to the observed quenching by Ar, 0-, and N2 are not determined. 

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