State-to-state photodissociation dynamics in formic acid

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Photofragment mapping and 18O isotopic labelling have been used to probe the character of the transition state for the predissociation channels HCO2H(Ã1A″)→ HCO(X 2A′)+ OH(X 2Π) in jet-cooled, vibronically state selected formic acid, and the nature of the vibronic states initially accessed in the Franck–Condon region. The dynamics on the excited potential-energy surface are compared with those in the isoelectronic molecule HONO(Ã1A″). Almost all the available energy above the dissociation limit is concentrated into fragment recoil, regardless of the initial vibronic state selection: the experimental data indicate a substantial, late exit barrier and trajectories funnelled through a near collinear HO—C—O structure at the transition state.