Highly predissociative levels of CH3S (A 2A1) detected with degenerate four-wave mixing

Photodissociation of the CH3O and CH3S radical molecules: an ab initio electronic structure study

The electronic states and the spin-orbit couplings between them involved in the photodissociation process of the radical molecules CH₃X, CH₃X → CH₃ + X (X = O, S), taking place after the Ã(²A₁) ← X[combining tilde](²E) transition, have been investigated using highly correlated ab initio techniques. A two-dimensional representation of both the potential-energy surfaces (PESs) and the couplings is generated. This description includes the C-X dissociative mode and the CH₃ umbrella mode. Spin-orbit effects are found to play a relevant role in the shape of the excited state potential-energy surfaces, particularly in the CH₃S case where the spin-orbit couplings are more than twice more intense than in CH₃O. The potential surfaces and couplings reported here for the present set of electronic states allow for the first complete description of the above photodissociation process. The different photodissociation mechanisms are analyzed and discussed in light of the results obtained.

A density-functional study of the adsorption of methane-thiol on the (111) surfaces of the Ni-group metals: II. Vibrational spectroscopy

The vibrational eigenstates of methane-thiol (CH(3)SH) and methane-thiolate (CH(3)S) in the gas phase and in dense monolayers adsorbed on the (111) surfaces of the Ni-group metals have been investigated within the framework of density-functional theory using generalized response and force-constant techniques. For isolated CH(3)SH good agreement of eigenfrequencies and intensities with the measured infrared spectra is achieved. For the CH(3)S radical, experimental information from laser-induced fluorescence spectroscopy is available only for selected eigenmodes. The theoretical predictions show reasonable agreement for the C-H deformation and C-S stretching modes, but predict much higher C-H stretching frequencies in better agreement with estimates based on the vibrational fine structure of the photoemission spectra. For methane-thiol monolayers on Ni(111) and Pt(111) the calculations predict stronger red-shifts of the S-H and C-S stretching modes than reported from high-resolution electron energy loss spectroscopy (HREELS) on condensed multilayers which average over the first layer adsorbed on the metal and further physisorbed molecular layers. For methane-thiolate monolayers the calculations predict modest blue-shifts of the C-H stretching and rocking modes and for the asymmetric C-H deformation modes. Red-shifts are predicted for the symmetric C-H deformation and for the C-S stretching modes. Reasonable agreement with HREELS is achieved. The increased differences between symmetric and asymmetric C-H stretching and deformation modes induced by the adsorption is a consequence of the strongly tilted adsorption geometries.

Two-color resonant four-wave mixing spectroscopy of highly predissociated levels in the ÃA12 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.