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 functional theory. Fluorescence spectra in the region of the electronic orgin of the B 2 ∑ +  ← X 2 ∑ + system (ν 00  = 14 826.07 cm −1 ) were recorded using a ring dye laser, the experimental resolution being 120 MHz. The B 2 Σ +  ← X 2 Σ + (0,0) band, and a cold band of a hitherto unreported 4 Π ±1/2  ← X 2 Σ + system (ν υ′0  = 14 926.02 cm −1 ) have been rotationally analysed. (The ± 1/2 notation implies that the state has either 4 Π 1/2 or 4 Π −1/2 symmetry.) Improved molecular rotational constants were obtained for the ν = 0 levels of the X 2 Σ + and B 2 Σ + states (r 0 (X) = 2.27191(17) Å, r 0 (B) = 2.32202(19) Å, γ 0 (B) = −0.15150(14) cm −1 (2σ error bounds)). The magnetic hyperfine and spin rotation parameters determined for the X 2 Σ + state were found to be in good agreement with previous work. An accurate bond length has been derived for the upper vibrational level of the 14 926.0 cm −1 band (r υ′  = 2.49510(16) Å). The ν = 1 level of the B 2 Σ + state is found to be strongly perturbed by another vibrational level of the 4 Π ±1/2 state. The spin-forbidden 4 Π ±1/2  ← X 2 Σ + transition gains intensity via spin-orbit mixing between the 4 Π state and the B 2 Σ + state. Radiative lifetimes of the observed bands were measured by digitizing the fluorescence decay traces obtained upon excitation with a pulsed dye laser. The results of the density functional treatment provide broad confirmation of the experimental measurements. A molecular orbital description of the bonding in the YS molecule is presented.