PHOTODISSOCIATION OF KETENE - VIBRATIONALLY EXCITED CH2((A)OVER-TILDE(1)A(1))

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) ab ove the threshold for singlet methylene CH2((a) over tilde (1)A(1)) pr oduction. By scanning the (CH2)-C-1 probe laser wavelength, the CH2 la ser-induced fluorescence (LIF) excitation spectrum is recorded and the (CH2)-C-1(0, 1,0), (0,2,0), and (1,0,0) product state rotational dist ributions determined. As observed previously for the (0,0,0) state, th e rotational state distributions of vibrationally excited methylene ar e in good agreement with phase space theory (PST) for excess energies less than 200 cm(-1) above the appearance threshold of each vibrationa l state probed. For higher excess energies, (CH2)-C-1(0,1,0) and (0,2, 0) rotational distributions like those for (0,0,0) are substantially c older than the statistical distribution given by PST. The quantum yiel ds of vibrationally excited states an determined at several excess ene rgies above the singlet threshold. These values are larger than predic ted by PST and match values predicted by variational Rice-Ramsperger-K assel-Marcus (RRKM) theory and by the separate statistical ensembles ( SSE) method.