Near-Ultraviolet Photodissociation of Thiophenol

H(D) Rydberg atom photofragment translational spectroscopy has been used to investigate the dynamics of H(D) atom loss C6H5SH(C6H5SD) following excitation at many wavelengths λphot in the range of 225−290 nm. The C6H5S cofragments are formed in both their ground (X2B1) and first excited (2B2) electronic states, in a distribution of vibrational levels that spreads and shifts to higher internal energies as λphot is reduced. Excitation at λphot > 275 nm populates levels of the first 1ππ* state, which decay by tunnelling to the dissociative 1πσ* state potential energy surface (PES). S−H torsional motion is identified as a coupling mode facilitating population transfer at the conical intersection (CI) between the diabatic 1ππ* and 1πσ* PESs. At shorter λphot, the 1πσ* state is deduced to be populated either directly or by efficient vibronic coupling from higher 1ππ* states. Flux evolving on the 1πσ* PES samples a second CI, at longer R S−H, between the diabatic 1πσ* and ground (1ππ) PESs, where the electronic branching between ground and excited state C6H5S fragments is determined. The C6H5S(X2B1) and C6H5S(2B2) products are deduced to be formed in levels with, respectively, a′ and a′′ vibrational symmetry−behavior that reflects both Franck−Condon effects (both in the initial photoexcitation step and in the subsequent in-plane forces acting during dissociation) and the effects of the out-of-plane coupling mode(s), ν11 and ν16a, at the 1πσ*/1ππ CI. The vibrational state assignments enabled by the high-energy resolution of the present data allow new and improved estimations of the bond dissociation energies, D 0(C6H5S−H) ≤ 28030 ± 100 cm−1 and D 0(C6H5S−D) ≤ 28610 ± 100 cm−1, and of the energy separation between the X2B1 and 2B2 states of the C6H5S radical, T 00 = 2800 ± 40 cm−1. Similarities, and differences, between the measured energy disposals accompanying UV photoinduced X−H (X = S, O) bond fission in thiophenol and phenol are discussed.