Photofragment excitation spectroscopy of the formyl (HCO/DCO) radical: Linewidths and predissociation rates of the A(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: (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 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)].