Two-color resonant four-wave mixing spectroscopy of highly predissociated levels in the A 2A1 state of CH3S.

We report results of two-color resonant four-wave mixing experiments on highly predissociated levels of the methylthio (or thiomethoxy) radical CH3S in its first excited electronic state A 2A1. 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(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 3v(v=3-7) and combination states 1(1)3v(v=0-2), 2(1)3v(v=3-6), and 1(1)2(1)3v(v=0,1) involving the symmetric CH3 stretching (nu1) mode and the CH3 umbrella (nu2) mode. The spectra show pronounced mode specificity, as the homogeneous linewidth of levels with similar energies varies up to two orders of magnitude; nu3 is clearly a promoting mode for dissociation. Derived vibrational wave numbers omega1', omega2', and omega3' of the A state agree satisfactorily with ab initio predictions.