I. Garciamoreno et al., PHOTODISSOCIATION OF KETENE - CH2((A)OVER-TILDE (1)A(1))(0,0,0) ROTATIONAL STATE DISTRIBUTIONS, The Journal of chemical physics, 100(12), 1994, pp. 8890-8901
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.