PHOTODISSOCIATION OF KETENE - CH2((A)OVER-TILDE (1)A(1))(0,0,0) ROTATIONAL STATE DISTRIBUTIONS

Ketene (CH2CO) cooled in a supersonic free jet is photodissociated by a tunable pulsed laser in the energy range from the threshold for prod uction of CH2((a) over tilde (1)A(1))(0,0,0)+CO((X) over tilde (1) Sig ma(+)) to 2900 cm(-1) above. By scanning the (CH2)-C-1 probe laser wav elength, the CH2 laser-induced fluorescence spectrum is recorded and t he (CH2)-C-1 product state distributions determined. The appearance th resholds and yield curves of individual (CH2)-C-1 rotational states ar e obtained by scanning the photolysis laser frequency with a fixed (CH 2)-C-1 probe laser frequency. The yield curves, or photofragment excit ation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotationa l term values. PHOFEX yield curves combined with the distribution data demonstrate that the ketene absorption cross section is constant with in 10% over the 0-1000 cm(-1) range of the PHOFEX spectra. PHOFEX curv es up to 200 cm(-1) and rotational state distributions for singlet met hylene at 50, 100, and 170 cm(-1) above its threshold are in good agre ement with phase space theory (PST) calculations. The lowest energy me thylene rotational states of + and - symmetry are formed in the ratio predicted by phase space theory at all energies. States of mixed singl et/triplet parentage are populated with equal probability for triplet characters ranging from 30% to 70%. For energies greater than 300 cm(- 1) above the threshold, the (CH2)-C-1 rotational distribution is subst antially colder than the statistical distribution given by PST. These differences increase with excess energy above the singlet threshold. I n this energy range, the dynamics of motion between the transition sta te and separated product fragments is in neither the adiabatic nor the statistical limit though the data do suggest that motion may well be adiabatic for molecules fragmenting to yield the lowest energy rotatio nal states of ortho methylene.