Ultraviolet photodissociation of methanethiol (CHSH): revealing an S(D) atom elimination channel.

We report time-sliced velocity map imaging studies of the methyl (CH) and electronically excited sulfur (S(D)) fragments formed following the photoexcitation of jet-cooled CHSH molecules in the 2A'' ← X̃ A' absorption band (. at wavelengths in the range 190 ≤ ≤ 210 nm). Analyses of images of CH fragments in their = 0, 1 and 2 vibrational levels confirm the perpendicular parent transition dipole moment and prompt bond fission and show that the ground state SH(X) partners are formed with an inverted vibrational population distribution, peaking at = 2 at the shortest excitation wavelengths investigated. Most of the photolysis photon energy above that required to break the C-S bond is partitioned into product translational energy. Primary S(D) products are observed on excitation at ≤ 204 nm and their relative yield is deduced to increase quite steeply with decreasing wavelength, but quantum yield estimates are beyond the scope of the present work. Image analysis reveals that the CH partners are formed with a highly inverted vibrational population distribution, largely concentrated in the bending mode. A possible formation mechanism for the S(D) + CH products is suggested, based on frustrated C-S bond extension on the initially populated 2A'' potential energy surface (PES) and re-collision between the embryonic CH and SH moieties in the extended region of conical intersection between the 2A'' and 1A'' PESs to the target products. Cutting edge electronic structure calculations along with complementary molecular dynamics studies should help validate or overturn this envisaged mechanism.