Dynamics of multi-channel dissociation of tetrahydrofuran photoexcited at 193 nm: distributions of kinetic energy, angular anisotropies and branching ratios

Abstract

We investigated the photodissociation dynamics of tetrahydrofuran (c-C₄H₈O) at 193.3 nm in a molecular-beam apparatus using photofragment-translational spectroscopy and direct vacuum-ultraviolet (VUV) photoionization. Five dissociation channels leading to products with m/z ratios appropriate for CH₂CH₂CH₂ + H₂CO, CH₂CHCH₂ + CH₂OH, H + CH₂CH₂ + CH₂CHO, CH₂CH₂ + CH₃ + HCO and CH₂CH₂ + CH₂CO + H₂ were identified; their branching ratios were determined to be 0.40, 0.25, 0.04 0.29 and 0.02, respectively. Secondary dissociations from nascent products CH₂CH₂CH₂CHO to CH₂CH₂ + CH₂CHO and from CH₂CH₂O to CH₃ + HCO and likely to CH₂CO + H₂ were observed. We measured distributions of product kinetic energy, average kinetic-energy release, and fractions in translation for each dissociation channel. The formation of CH₂CHCH₂ + CH₂OH indicates that hydrogen migration occurs before complete fragmentation. All photofragments have nearly isotropic angular distributions, with |β| values less than 0.05. The photodissociation of tetrahydrofuran into five channels is proposed to proceed mainly on the ground state potential-energy surface following ring opening and efficient internal conversions.