Ultraviolet photodissociation of methanethiol (CH3SH): Revealing an S(1D) atom elimination channel

Abstract

We report time-sliced velocity map imaging studies of the methyl (CH₃) and electronically excited sulfur (S(¹D)) fragments formed following photoexcitation of jet-cooled CH₃SH molecules in the 2¹A”−X¹A’absorption band (i.e. at wavelengths in the range 190 ≤ λ ≤ 210 nm). Analyses of images of CH₃ fragments in their v₂ = 0, 1 and 2 vibrational levels confirm the perpendicular parent transition dipole moment, prompt bond fission, and show that the ground state SH(X) partners are formed with an inverted vibrational population distribution, peaking at v = 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, however. Primary S(¹D) products are observed when exciting at λ ≤ 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 CH₄ partners are formed with a highly inverted vibrational population distribution, largely concentrated in the asymmetric stretch mode (υ₄). A possible formation mechanism for the S(¹D) + CH₄ products is suggested, based on frustrated C–S bond extension on the initially populated 2¹A” potential energy surface (PES) and re-collision between the embryonic CH₃ and SH moieties in the extended region of conical intersection between the 2¹A” and 1¹A” PESs en route to the target products. Cutting edge electronic structure calculations along with complementary ab initio molecular dynamics studies should help validate or overturn this envisaged mechanism.