Photodissociation of Ozone in the Hartley Band: Potential Energy Surfaces, Nonadiabatic Couplings, and Singlet/Triplet Branching Ratio

The lowest five 1A\u27states of ozone, involved in the photodissociation with UV light, are analyzed on the basis of multireference configuration interaction electronic structure calculations with emphasis on the various avoided crossings in different regions of coordinate space. Global diabatic potential energy surfaces are constructed for the lowest four states termed X, A, B, and R. In addition, the off-diagonal potentials that couple the initially excited state B with states R and A are constructed to reflect results from additional electronic structure calculations, including the calculation of nonadiabatic coupling matrix elements. The A/X and A/R couplings are also considered, although in a less ambitious manner. The photodissociation dynamics are studied by means of trajectory surface hopping (TSH) calculations with the branching ratio between the singlet, O(1D)+O2(1Δg), and triplet, O(3P)+O2(3Σ-g), channels being the main focus. The semiclassical branching ratio agrees well with quantum mechanical results except for wavelengths close to the threshold of the singlet channel. The calculated O(1D) quantum yield is approximately 0.90-0.95 across the main part of the Hartley band, in good agreement with experimental data. TSH calculations including all four states show that transitions B→A are relatively unimportant and subsequent transitions A→X/R to the triplet channel are negligible

State-Selective Studies of T→R, V Energy Transfer: The H+CO System

Collisional energy transfer from H atoms to CO(v=0, J≈2) has been studied at a collision energy of 1.58±0.07 eV by photolyzing H2S at 222 nm in a nozzle expansion with CO and probing the CO(v , J ) levels using tunable VUV laser-induced fluorescence. The ratio CO(v =1)/CO(v =0) is found to be 0.1±0.008. The rotational distribution of CO(v =0) peaks at J gradually; population is still observed at J \u3e45. The rotational distribution of CO(v =1) is broad and peaks near J =20. The experimental results are compared to quasiclassical trajectory calculations performed both on the H+CO surface of Bowman, Bittman, and Harding (BBH) and on the surface of Murrell and Rodriguez (MR). The experimental rotational distributions, particularly those for CO(v =1), show that the BBH surface is a better model than the MR surface. The most significant difference between the two surfaces appears to be that for energetically accessible regions of configuration space the derivative of the potential with respect to the CO distance is appreciable only in the HCO valley for the BBH surface, but is large for all H atom approaches in the MR potential. Because the H-CO geometry is bent in this valley, vibrational excitation on the BBH surface is accompanied by appreciable rotational excitation, as observed experimentally

Production of O\u3csub\u3e2\u3c/sub\u3e Herzberg States in the Deep UV Photodissociation of Ozone

High-resolution imaging experiments combined with new electronic structure and dynamics calculations strongly indicate that the O(3P)+O2 products with very low kinetic energy release (Etr2: A\u27 3Δu(v=0, 1, 2) and A 3Σ+u(v=0, 1). This interpretation contradicts the earlier assignment to very high (v≥26) vibrational states of O2(3Σ-g)

State-to-State Rotational Relaxation Rate Constants for CO+Ne from IR-IR Double-Resonance Experiments: Comparing Theory to Experiment

IR-IR double-resonance experiments were used to study the state-to-state rotational relaxation of CO with Ne as a collision partner. Rotational levels in the range Ji=2-9 were excited and collisional energy transfer of population to the levels Jf=2-8 was monitored. The resulting data set was analyzed by fitting to numerical solutions of the master equation. State-to-state rate constant matrices were generated using fitting law functions. Fitting laws based on the modifed exponential gap (MEG) and statistical power exponential gap (SPEG) models were used; the MEG model performed better than the SPEG model. A rate constant matrix was also generated from scattering calculations that employed the ab initio potential energy surface of McBane and Cybulski [J. Chem. Phys. 110, 11 734 (1999)]. This theoretical rate constant matrix yielded kinetic simulations that agreed with the data nearly as well as the fitted MEG model and was unique in its ability to reproduce both the rotational energy transfer and pressure broadening data for Ne-CO. The theoretical rate coefficients varied more slowly with the energy gap than coefficients from either of the fitting laws

Ultraviolet Photodissociation of OCS: Product Energy and Angular Distributions

The ultraviolet photodissociation of carbonyl sulfide (OCS) was studied using three-dimensional potential energy surfaces and both quantum mechanical dynamics calculations and classical trajectory calculations including surface hopping. The transition dipole moment functions used in an earlier study [J. A. Schmidt, M. S. Johnson, G. C. McBane, and R. Schinke, J. Chem. Phys. 137, 054313 (2012)] were improved with more extensive treatment of excited electronic states. The new functions indicate a much larger contribution from the 1 1A state (1Σ- in linear OCS) than was found in the previous work. The new transition dipole functions yield absorption spectra that agree with experimental data just as well as the earlier ones. The previously reported potential energy surfaces were also empirically modified in the region far from linearity. The resulting product state distributions Pv, j, angular anisotrophy parameters β(j), and carbon monoxide rotational alignment parameters A0(2)(j) agree reasonably well with the experimental results, while those computed from the earlier transition dipole and potential energy functions do not. The higher-j peak in the bimodal rotational distribution is shown to arise from nonadiabatic transitions from state 2 1A\u27 to the OCS ground state late in the dissociation

The 157 nm Photodissociation of OCS

The photodissociation of OCS at 157 nm has been investigated by using tunable vacuum ultraviolet radiation to probe the CO and S photoproducts. Sulfur is produced almost entirely in the 1S state, while CO is produced in its ground electronic state and in vibrational levels from v=0-3 in the appropriate ratio (v=0):(v=1):(v=2):(v=3) = (1.0):(1.0):(0.5):(0.3). The rotational distribution for each vibrational level is found to be near Boltzmann, with temperatures that decrease from 1350 K for v=0 to 780 K for v=3. Measurements of the CO Doppler profiles demonstrate that the dissociation takes place from a transition of predominantly parallel character (β=1.8±0.2) and that the CO velocity and angular momentum vectors are perpendicular to one another