ELECTRONIC SPECTROSCOPY OF YTTRIUM MONOSULFIDE - MOLECULAR-BEAM STUDIES AND DENSITY-FUNCTIONAL CALCULATIONS

The yttrium monosulfide molecule (YS) has been investigated using the techniques of molecular beam fluorescence spectroscopy and density fun ctional theory. Fluorescence spectra in the region of the electronic o rgin of the B2SIGMA+ <--X2SIGMA+ system (nu00 = 14 826.07 cm-1) were r ecorded using a ring dye laser, the experimental resolution being 120 MHz. The B2SIGMA+ <-- X2SIGMA+ (0,0) band, and a cold band of a hither to unreported 4PI+/-1/2 <-- X2SIGMA+ system (nu(upsilon'0) = 14 926.02 cm-1) have been rotationally analysed. (The +/- 1/2 notation implies that the state has either 4PI1/2 or 4PI-1/2 symmetry.) Improved molecu lar rotational constants were obtained for the upsilon = 0 levels of t he X2SIGMA+ and B2SIGMA+ states (r0(X) = 2.27191(17) angstrom, ro(B) = 2.32202(19) angstrom, gamma0(B) = -0.15150(14) cm-1 (2sigma error bou nds)). The magnetic hyperfine and spin rotation parameters determined for the X2SIGMA+ state were found to be in good agreement with previou s work. An accurate bond length has been derived for the upper vibrati onal level of the 14 926.0 cm-1 band (r(upsilon') = 2.49510(16) angstr om). The upsilon = 1 level of the B2SIGMA+ state is found to be strong ly perturbed by another vibrational level of the 4PI+/-1/2 state. The spin-forbidden 4PI+/-1/2 <-- X2SIGMA+ transition gains intensity via s pin-orbit mixing between the 4PI state and the B2SIGMA+ State. Radiati ve lifetimes of the observed bands were measured by digitizing the flu orescence decay traces obtained upon excitation with a pulsed dye lase r. The results of the density functional treatment provide broad confi rmation of the experimental measurements. A molecular orbital descript ion of the bonding in the YS molecule is presented.