Near-UV photodissociation dynamics of formic acid

(Rydberg) atom photofragment translational spectroscopy has been used to study the photodissociation dynamics of jet-cooled formic acid molecules following excitation to their first excited singlet (S-1) state at numerous wavelengths in the range 216-241 nn. Analysis of the resulting I-I-atom time-of-flight spectra indicates contributions from three H-atom formation channels, which we identify as the primary C-H and O-H bond fission processes and the secondary photolysis of HCO((X) over tilde) fragments resulting from primary C-O bond fission. It also allows determination of the bond dissociation energies: D-0(H-CO2H) approximate to 30000 cm(-1) and D-0(HCOO-H) = 39 080 +/- 100 cm(-1). The former bond fission is deduced to occur after intersystem crossing to the neighbouring (a) over tilde(3) A '' state, and to involve passage over (or tunnelling through) a barrier in the C-H dissociation coordinate on the triplet potential-energy surface. O-H bond fission, in contrast, is shown to occur predominantly on the S-1 surface but,it, too, must overcome an activation barrier, the magnitude of which we can estimate at ca. 5400 cm(-1), measured relative to the asymptotic products H + HCOO((X) over tilde and/or (A) over tilde). The latter assignment affords a refined value for the 0 K heat of formation of the formyloxyl radical: Delta(0)H(0) degrees (HCOO) = -119.5 +/- 3 kJ mol(-1).