Product pair correlation in CH₃OH photodissociation at 157 nm: the OH + CH₃ channel

The OH + CH₃ product channel for the photodissociation of CH₃OH at 157 nm was investigated using the velocity map imaging technique with the detection of CH₃ radical products via (2+1) resonance-enhanced multiphoton ionization (REMPI). Images were measured for the CH₃ formed in the ground and excited states (v₂ = 0, 1, 2, and 3) of the umbrella vibrational mode and correlated OH vibrational state distributions were also determined. We find that the vibrational distribution of the OH fragment in the OH + CH₃ channel is clearly inverted. Anisotropic distributions for the CH₃ (v₂ = 0, 1, 2, and 3) products were also determined, which is indicative of a fast dissociation process for the C–O bond cleavage. A slower CH₃ product channel was also observed, that is assigned to a two-step photodissociation process, in which the first step is the production of a CH₃O(X ²E) radical via the cleavage of the O–H bond in CH₃OH, followed by probe laser photodissociation of the nascent CH₃O radicals yielding CH₃(X ²A₁, v = 0) products.