Ketene photodissociation in the wavelength range 193–215 nm: The H atom production channel

Abstract

The speed and angular distributions of H atom products arising in the photodissociation of jet-cooled ketene (CH2CO) molecules following excitation at 193.3, 203.3, 209, and 213.3 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. The observed product energy disposal is interpreted in terms of one photon absorption to the B11 electronically excited state, internal conversion to high lying vibrational levels of the ground state and subsequent unimolecular decay to yield the observed H (+HCCO) products. H atoms resulting from secondary photolysis of H containing primary products (most probably singlet CH2 radicals) are evident in the measured spectra, especially at high photolysis laser pulse energies. The kinetic energy distributions of the primary H+HCCO products span all energetically accessible product internal energies, peaking at ∼1170 cm−1 in the case of parent excitation at 213.3 nm, and rising to ∼1450 cm−1 (when exciting at 193.3 nm). These distributions are reproduced, qualitatively, by the statistical adiabatic product distribution (SAPD) method proposed recently by Cole and Balint-Kurti (J. Chem. Phys., preceding paper). This method is based on the use of a quantum mechanical, J conserving, Rice–Ramsperger–Kassel–Marcus (RRKM) treatment and provides a prediction of the product quantum state distributions and the total kinetic energy release spectra. Accurate, quadratic configuration interaction, intrinsic reaction coordinates have been computed for both the lowest singlet (S0) and triplet (T1) potential energy surfaces of CH2CO. Quantum mechanical SAPD calculations have been performed using both surfaces; the results favor the conclusion that the dissociation occurs on the S0 surface. This conclusion is further supported by comparison of the calculated and previously measured CO product vibrational quantum state distributions arising from photodissociation at 193.3 nm. The variational RRKM method has also been used to compute the branching ratios for forming H+HCCO and CH2+CO products on both the S0 and T1 surfaces. Different aspects of the SAPD model, such as the inclusion of quantum mechanical tunneling, the attractiveness of the long-range interfragment potential and the assumed adiabaticity of the fragmentation, have been varied in order to shed light on the nature of the dissociation process and the possible origins of the differences between the model calculations and the experimental results. It is found that the agreement between the quantum mechanical statistical model predictions and the experimentally observed total kinetic energy release spectra for the H atom dissociation channel can be greatly improved if the contribution of lower fragment relative orbital angular momenta is increased over that required by the use of a purely statistical model. This finding is equivalent to the conclusion that the dissociation is not entirely statistical, but that the dynamics of the break-up process plays some role. In particular the initial geometry of the parent molecule may restrict the body-fixed angles into which the final products can scatter and, through this, may restrict the relative orbital angular momenta to be on average smaller than that predicted by a purely statistical theory.