Photofragment excitation spectroscopy of the formyl (HCO/DCO) radical: Linewidths and predissociation rates of the Ã(A″) state

Photofragment excitation (PHOFEX) spectra of the jet-cooled formyl (HCO and DCO) radical have been measured by monitoring laser-induced fluorescence of the CO fragment. The following Ã(A″)←X̃(A′) vibronic transitions were measured: (ν₁,ν₂,ν₃) ←(0,0,0) where for HCO ν₂ = 6-16 with ν₁, ν₃ = 0; ν₂ = 7-12 with ν₁ = 1, ν₃ = 0; ν₂ = 9-12 with ν₁ = 0, ν₃ = 1 and for DCO ν₂ = 14-18 with ν₁,ν₃ = 0. The PHOFEX technique provides a powerful method for discriminating against strong transitions so as to allow assignment and measurement of many weaker Franck-Condon bands. The linewidths of the diffuse transitions lie in the 16-150 cm^-1 (FWHM) range; they increase strongly with K′ (the projection of the total angular momentum onto the a axis) and decrease slightly with ν′₂ (the number of HCO/DCO bending quanta). The linewidths are interpreted as radiationless transition rates and yield upper Ã-state lifetimes ranging from 70 to 700 fs. These data are in excellent agreement with a dynamical model of HCO/DCO curve crossing developed by R. N. Dixon [Mol. Phys. 54, 333 (1985)].