Collisional relaxation of singlet O2(b1Σg+) in neat gas investigated by laser-induced grating technique

Systematic experimental and theoretical studies of the feasibility to apply the diffraction at laser-induced gratings (LIGs) for getting information on parameters of collisional rotational, electronic and vibrational relaxation in molecular gases have been accomplished for the first time, using as an example the process of collisional relaxation of optically excited metastable O-2 (b(1)Sigma(g)(+)) molecules in neat gas. Temporal profiles of LIG signals have been recorded at densities 0.2-160 Amagat and room temperature within a broad range of time intervals (0.1-1000 mus) after ns-scale laser excitation at various rovibronic transitions of the b(1)Sigma(g)(+) (nu' = 0) <-- X(3)Sigma(g)(-) (nu = 0) band Of O-2. The experimental data have been analyzed within the framework of a model of a three-stage rotational (R-T), electronic (E E,V,T + V-V and E-V,T + V-V) and vibrational (V-T) collisional relaxation. The results of the analysis are presented demonstrating temporal and amplitude variations of distinct contributions to the LIG signal within a large range of gas densities. These variations were described using a set of characteristic optical and relaxation parameters Of O-2. The parameters were derived from the experimental data and compared with the values known from the literature. Reasonable agreement was found between all the values provided by the LIG technique, and, in particular, the time constants of electronic and vibrational relaxation processes involved, and those previously obtained by other methods. (C) 2003 Elsevier Science B.V. All rights reserved.