IN THE NAME OF GOD,

Jet-cooled, laser-induced phosphorescence excitation spectra (LIP) of thioacetaldehyde CH3CHS, CH3CDS, CD3CHS and CD3CDS have been observed over the region 15800- 17300 cm" ^ in a continuous pyrolysis jet. The vibronic band structure of the singlet-triplet n- * n * transition were attributed to the strong coupling of the methyl torsion and aldehydic hydrogen wagging modes. The vibronic peaks have been assigned in terms of two upper electronic state (T^) vibrations; the methyl torsion mode v^g, and the aldehydic hydrogen wagging mode v^^. The electronic origin O^a ^ is unequivocally assigned as follows: CH3CHS (16294.9 cm"' '), CH3CDS (16360.9 cm"' '), CD3CHS (16299.7 cm" ^), and CD3CDS (16367.2 cm"' '). To obtain structural and dynamical information about the two electronic states, potential surfaces V(e,a) for the