Photofragmentation dynamics of formaldehyde: CO(v,J) distributions as a function of initial rovibronic state and isotopic substitution

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Abstract
Complete rotational distributions have been obtained for the CO produced following excitation of H2CO, HDCO, and D2CO near the S1 origin. The CO was detected by vacuum ultraviolet laser‐induced fluorescence. The distributions show a remarkable amount of rotational excitation, peaking at J=42, 49, and 53 for H2CO, HDCO, and D2CO, respectively, with widths of 20–25 J units (FWHM). CO(v=1) from H2CO photolysis has nearly the same rotational distribution as CO(v=0). The population of CO(v=1) is 14%±5% as large as the population of CO(v=0), in good agreement with earlier measurements. Increased angular momentum of H2CO is only partially transferred to CO, giving slightly wider rotational distributions without changing the peak value. The rotational distributions are highly nonthermal, showing that energy randomization does not occur during the dissociation event. An approximate range of product impact parameters has been determined. The impact parameters are too large to be accounted for by forces along the directions of the C–H bonds. The hydrogen appears to be most strongly repelled by the charge distribution a fraction of an Å outside the carbon atom of the CO. The distribution of impact parameters and the internal energy of the hydrogen fragment apparently do not change significantly upon isotopic substitution. The absence of population in CO(JJ≳25) as the long‐lived intermediate in formaldehyde photodissociation.