Depolarization of rotational angular momentum in CN(A2Π, v = 4) + Ar collisions.

Angular momentum depolarization and population transfer in CN(A(2)Π, v = 4, j, F(1)e) + Ar collisions have been investigated both experimentally and theoretically. Ground-state CN(X(2)Σ(+)) molecules were generated by pulsed 266-nm laser photolysis of ICN in a thermal (nominally 298 K) bath of the Ar collision partner at a range of pressures. The translationally thermalized CN(X) radicals were optically pumped to selected unique CN(A(2)Π, v = 4, j = 2.5, 3.5, 6.5, 11.5, 13.5, and 18.5, F(1)e) levels on the A-X (4,0) band by a pulsed tunable dye laser. The prepared level was monitored in a collinear geometry by cw frequency-modulated (FM) spectroscopy in stimulated emission on the CN(A-X) (4,2) band. The FM lineshapes for co- and counter-rotating circular pump and probe polarizations were analyzed to extract the time dependence of the population and (to a good approximation) orientation (tensor rank K = 1 polarization). The corresponding parallel and perpendicular linear polarizations yielded population and alignment (K = 2). The combined population and polarization measurements at each Ar pressure were fitted to a 3-level kinetic model, the minimum complexity necessary to reproduce the qualitative features of the data. Rate constants were extracted for the total loss of population and of elastic depolarization of ranks K = 1 and 2. Elastic depolarization is concluded to be a relatively minor process in this system. Complementary full quantum scattering (QS) calculations were carried out on the best previous and a new set of ab initio potential energy surfaces for CN(A)-Ar. Collision-energy-dependent elastic tensor and depolarization cross sections for ranks K = 1 and 2 were computed for CN(A(2)Π, v = 4, j = 1.5-10.5, F(1)e) rotational/fine-structure levels. In addition, integral cross sections for rotationally inelastic transitions out of these levels were computed and summed to yield total population transfer cross sections. These quantities were integrated over a thermal collision-energy distribution to yield the corresponding rate constants. A complete master-equation simulation using the QS results for the selected initial level j = 6.5 gave close, but not perfect, agreement with the near-exponential experimental population decays, and successfully reproduced the observed multimodal character of the polarization decays. On average, the QS population removal rate constants were consistently 10%-15% higher than those derived from the 3-level fit to the experimental data. The QS and experimental depolarization rate constants agree within the experimental uncertainties at low j, but the QS predictions decline more rapidly with j than the observations. In addition to providing a sensitive test of the achievable level of agreement between state-of-the art experiment and theory, these results highlight the importance of multiple collisions in contributing to phenomenological depolarization using any method sensitive to both polarized and unpolarized molecules in the observed level.

The Na(+)-H(2) cation complex: Rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations.

The rotationally resolved infrared spectrum of the Na(+)-H(2) cation complex is recorded in the H-H stretch region (4067-4118 cm(-1)) by monitoring the production of Na(+) photofragments. Altogether 42 lines are identified, 40 of which are assigned to K(a)=1-1 transitions (associated with complexes containing ortho-H(2)) and two tentatively assigned to K(a)=0-0 transitions (associated with complexes containing para-H(2)). The K(a)=1-1 subband lines were fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The band origin is estimated as 4094.6 cm(-1), a shift of -66.6 cm(-1) with respect to the Q(1)(0) transition of the free H(2) molecule. The results demonstrate that Na(+)-H(2) has a T-shaped equilibrium configuration with the Na(+) ion attached to a slightly perturbed H(2) molecule but that large-amplitude vibrational motions significantly influence the rotational constants derived from the asymmetric rigid rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.493 A, increasing slightly (by 0.002 A) when the H(2) subunit is vibrationally excited. A new three-dimensional potential energy surface is developed to describe the Na(+)-H(2) complex. Ab initio points calculated using the CCSD(T) method and aug-cc-pVQZ basis set augmented by bond functions are fitted using a reproducing kernel Hilbert space method [Ho et al., J. Chem. Phys. 104, 2584 (1996)] to give an analytical representation of the potential energy surface. Ensuing variational calculations of the rovibrational energy levels demonstrate that the potential energy surface correctly predicts the frequency of the nu(HH) transition (to within 2.9 cm(-1)) and the dissociation energies [842 cm(-1) for Na(+)-H(2)(para) and 888 cm(-1) for Na(+)-H(2)(ortho)]. The B and C rotational constants are slightly underestimated (by 1.7%), while the vibrationally averaged intermolecular separation is overestimated by 0.02 A.

Characterization of the methoxy carbonyl radical formed via photolysis of methyl chloroformate at 193.3 nm.

This study investigates two features of interest in recent work on the photolytic production of the methoxy carbonyl radical and its subsequent unimolecular dissociation channels. Earlier studies used methyl chloroformate as a photolytic precursor for the CH3OCO, methoxy carbonyl (or methoxy formyl) radical, which is an intermediate in many reactions that are relevant to combustion and atmospheric chemistry. That work evidenced two competing C-Cl bond fission channels, tentatively assigning them as producing ground- and excited-state methoxy carbonyl radicals. In this study, we measure the photofragment angular distributions for each C-Cl bond fission channel and the spin-orbit state of the Cl atoms produced. The data shows bond fission leading to the production of ground-state methoxy carbonyl radicals with a high kinetic energy release and an angular distribution characterized by an anisotropy parameter, beta, of between 0.37 and 0.64. The bond fission that leads to the production of excited-state radicals, with a low kinetic energy release, has an angular distribution best described by a negative anisotropy parameter. The very different angular distributions suggest that two different excited states of methyl chloroformate lead to the formation of ground- and excited-state methoxy carbonyl products. Moreover, with these measurements we were able to refine the product branching fractions to 82% of the C-Cl bond fission resulting in ground-state radicals and 18% resulting in excited-state radicals. The maximum kinetic energy release of 12 kcal/mol measured for the channel producing excited-state radicals suggests that the adiabatic excitation energy of the radical is less than or equal to 55 kcal/mol, which is lower than the 67.8 kcal/mol calculated by UCCSD(T) methods in this study. The low-lying excited states of methylchloroformate are also considered here to understand the observed angular distributions. Finally, the mechanism for the unimolecular dissociation of the methoxy carbonyl radical to CH3 + CO2, which can occur through a transition state with either cis or, with a much higher barrier, trans geometry, was investigated with natural bond orbital computations. The results suggest donation of electron density from the nonbonding C radical orbital to the sigma* orbital of the breaking C-O bond accounts for the additional stability of the cis transition state.