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 approximately A (A") <-- approximately X(A') vibronic transitions were measured: (upsilon-1, upsilon-2, upsilon-3) <-- (0,0,0) where for HCO upsilon-2 = 6-16 with upsilon-1,upsilon-3 = 0; upsilon-2 = 7-12 with upsilon-1 = 1, upsilon-3 = 0; upsilon-2 = 9-12 with upsilon-1 = 0, upsilon-3 = 1 and for DCO upsilon-2 = 14-18 with upsilon-1,upsilon-3 = 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 alpha-axis) and decrease slightly with upsilon-2' (the number of HCO/DCO bending quanta). The linewidths are interpreted as radiationless transition rates and yield upper approximately A-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)].