Two-color resonant four-wave mixing spectroscopy of highly predissociated levels in the A-tilde {sup 2}A{sub 1} state of CH{sub 3}S

We report results of two-color resonant four-wave mixing experiments on highly predissociated levels of the methylthio (or thiomethoxy) radical CH{sub 3}S in its first excited electronic state A-tilde{sup 2}A{sub 1}. Following photolysis of jet-cooled dimethyl disulfide at 248 nm, the spectra were measured with a hole-burning scheme in which the probe laser excited specific rotational transitions in band 3{sup 3}. The spectral simplification afforded by the two-color method allows accurate determination of line positions and homogeneous linewidths, which are reported for the C-S stretching states 3{sup v}(v=3-7) and combination states 1{sup 1}3{sup v}(v=0-2), 2{sup 1}3{sup v}(v=3-6), and 1{sup 1}2{sup 1}3{sup v}(v=0,1) involving the symmetric CH{sub 3} stretching ({nu}{sub 1}) mode and the CH{sub 3} umbrella ({nu}{sub 2}) mode. The spectra show pronounced mode specificity, as the homogeneous linewidth of levels with similar energies varies up to two orders of magnitude; {nu}{sub 3} is clearly a promoting mode for dissociation. Derived vibrational wave numbers {omega}{sub 1}{sup '}, {omega}{sub 2}{sup '}, and {omega}{sub 3}{sup '} of the A-tilde state agree satisfactorily with ab initio predictions.