Photodissociation of ketene: CH₂(ã ¹A₁)(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 production of CH₂(ã ¹A₁)(0,0,0)+CO(X̃ ¹Σ⁺) to 2900 cm⁻¹ above. By scanning the ¹CH₂ probe laser wavelength, the CH₂ laser‐induced fluorescence spectrum is recorded and the ¹CH₂ product state distributions determined. The appearance thresholds and yield curves of individual ¹CH₂ rotational states are obtained by scanning the photolysis laser frequency with a fixed ¹CH₂ probe laser frequency. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. PHOFEX yield curves combined with the distribution data demonstrate that the ketene absorption cross section is constant within 10% over the 0–1000 cm⁻¹ range of the PHOFEX spectra. PHOFEX curves up to 200 cm⁻¹ and rotational state distributions for singlet methylene at 50, 100, and 170 cm⁻¹ above its threshold are in good agreement with phase space theory (PST) calculations. The lowest energy methylene rotational states of + and − symmetry are formed in the ratio predicted by phase space theory at all energies. States of mixed singlet/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 ¹CH₂ rotational distribution is substantially colder than the statistical distribution given by PST. These differences increase with excess energy above the singlet threshold. In this energy range, the dynamics of motion between the transition state 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 rotational states of ortho methylene.