Photofragment excitation spectroscopy of the formyl (HCO/DCO) radical: Linewidths and predissociation rates of the A-tilde(Ascript) 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`)←X∼(A') vibronic transitions were measured: (v₁,v₂,v₃)←(0,0,0) where for HCO v₂=6-16 with v₁,v₃=0; v₂=7-12 with v₁=1, v₃=0; v₂=9-12 with v₁=0, v₃=1 and for DCO v₂=14-18 with v₁,v₃=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 v^'₂ (the number of HCO/DCO bending quanta). The linewidths are interpreted as radiationless transition rates and yield upper Östate 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)].