Photodissociation of ketene: Vibrationally excited CH2(a 1A1)

Ketene (CH2CO) cooled in a supersonic free jet is photodissociated by a tunable pulsed laser in the energy range from 1460 to 2900 cm−1 above the threshold for singlet methylene CH2(ã 1A1) production. By scanning the 1CH2 probe laser wavelength, the CH2 laser-induced fluorescence (LIF) excitation spectrum is recorded and the 1CH2(0,1,0), (0,2,0), and (1,0,0) product state rotational distributions determined. As observed previously for the (0,0,0) state, the rotational state distributions of vibrationally excited methylene are in good agreement with phase space theory (PST) for excess energies less than 200 cm−1 above the appearance threshold of each vibrational state probed. For higher excess energies, 1CH2(0,1,0) and (0,2,0) rotational distributions like those for (0,0,0) are substantially colder than the statistical distribution given by PST. The quantum yields of vibrationally excited states are determined at several excess energies above the singlet threshold. These values are larger than predicted by PST and match values predicted by variational Rice–Ramsperger–Kassel–Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method.