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(1)A(") state ((1)Σ(-) 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 anisotropy parameters ß(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(1)A(') to the OCS ground state late in the dissociation.

The ultraviolet spectrum of OCS from first principles: electronic transitions, vibrational structure and temperature dependence.

Global three dimensional potential energy surfaces and transition dipole moment functions are calculated for the lowest singlet and triplet states of carbonyl sulfide at the multireference configuration interaction level of theory. The first ultraviolet absorption band is then studied by means of quantum mechanical wave packet propagation. Excitation of the repulsive 2 (1)A' state gives the main contribution to the cross section. Excitation of the repulsive 1 (1)A" state is about a factor of 20 weaker at the absorption peak (E(ph) ≈ 45,000 cm(-1)) but becomes comparable to the 2 (1)A' state absorption with decreasing energy (35,000 cm(-1)) and eventually exceeds it. Direct excitation of the repulsive triplet states is negligible except at photon energies E(ph) < 38,000 cm(-1). The main structure observed in the cross section is caused by excitation of the bound 2 (3)A" state, which is nearly degenerate with the 2 (1)A' state in the Franck-Condon region. The structure observed in the low energy tail of the spectrum is caused by excitation of quasi-bound bending vibrational states of the 2 (1)A' and 1 (1)A" electronic states. The absorption cross sections agree well with experimental data and the temperature dependence of the cross section is well reproduced.

Communication: Multi-state analysis of the OCS ultraviolet absorption including vibrational structure.

The first absorption band of OCS (carbonyl sulfide) is analyzed using potential energy surfaces and transition dipole moment functions of the lowest four singlet and the lowest four triplet states. Excitation of the 2 (1)A' state is predominant except at very low photon energies. It is shown that the vibrational structures in the center of the band are due to excitation of the 2 (3)A'' triplet state, whereas the structures at very low energies are caused by bending excitation in the potential wells of states 2 (1)A' and 1 (1)A''.

Photodissociation of N2O: triplet states and triplet channel.

The role of triplet states in the UV photodissociation of N(2)O is investigated by means of quantum mechanical wave packet calculations. Global potential energy surfaces are calculated for the lowest two (3)A' and the lowest two (3)A'' states at the multi-reference configuration interaction level of electronic structure theory using the augmented valence quadruple zeta atomic basis set. Because of extremely small transition dipole moments with the ground electronic state, excitation of the triplet states has only a marginal effect on the far red tail of the absorption cross section. The calculations do not show any hint of an increased absorption around 280 nm as claimed by early experimental studies. The peak observed in several electron energy loss spectra at 5.4 eV is unambiguously attributed to the lowest triplet state 1(3)A'. Excitation of the 2(1)A' state and subsequent transition to the repulsive branch of the 2(3)A'' state at intermediate NN-O separations, promoted by spin-orbit coupling, is identified as the main pathway to the N(2)((1)Σ(g)(+))+O((3)P) triplet channel. The yield, determined in two-state wave packet calculations employing calculated spin-orbit matrix elements, is 0.002 as compared to 0.005 ± 0.002 measured by Nishida et al. [J. Phys. Chem. A 108, 2451 (2004)].

Photodissociation of N2O: energy partitioning.

The energy partitioning in the UV photodissociation of N(2)O is investigated by means of quantum mechanical wave packet and classical trajectory calculations using recently calculated potential energy surfaces. Vibrational excitation of N(2) is weak at the onset of the absorption spectrum, but becomes stronger with increasing photon energy. Since the NNO equilibrium angles in the ground and the excited state differ by about 70°, the molecule experiences an extraordinarily large torque during fragmentation producing N(2) in very high rotational states. The vibrational and rotational distributions obtained from the quantum mechanical and the classical calculations agree remarkably well. The shape of the rotational distributions is semi-quantitatively explained by a two-dimensional version of the reflection principle. The calculated rotational distribution for excitation with λ = 204 nm and the translational energy distribution for 193 nm agree well with experimental results, except for the tails of the experimental distributions corresponding to excitation of the highest rotational states. Inclusion of nonadiabatic transitions from the excited to the ground electronic state at relatively large N(2)-O separations, studied by trajectory surface hopping, improves the agreement at high j.

Photodissociation of N2O: potential energy surfaces and absorption spectrum.

The ultraviolet photodissociation of N(2)O is studied by wave packet calculations using global three-dimensional potential energy surfaces for the two lowest (1)A' states. The incorporation of all internal degrees of freedom in the dynamics calculations is essential for a realistic treatment. The room-temperature absorption cross section is well reproduced, including the weak vibrational structures. Classical periodic orbits show that the latter can be attributed to large-amplitude NN stretch motion combined with strong excitation of the bend. Weakening of the NN bond toward the N + NO channel is the necessary prerequisite. The temperature dependence of the calculated cross section is significant, as expected for a dipole-forbidden transition of a linear molecule; but it is not as strong as observed experimentally [G. S. Selwyn and H. S. Johnston, J. Chem. Phys. 74, 3791 (1981)]. This shortcoming is due to an apparent underestimation of the (0,1,0) hot band absorption. On the other hand, the calculations yield reasonable predictions of the ratios of bending-state resolved absorption cross sections, σ(0, 1, 0)∕σ(0, 0, 0) and σ(0, 2, 0)∕σ(0, 0, 0), measured at 204 nm [H. Kawamata et al. J. Chem. Phys. 125, 133312 (2006)].

Perceived psychosocial benefits associated with perceived restorative potential of wilderness river-rafting trips.

Analysis of the restorative experiences and psychosocial benefits of wilderness river rafting trips of varying difficulty with 186 Canadian participants of different ages supported the restorative potential of natural settings for all age groups as measured by the Perceived Restorativeness Scale. The two-factor structure (General Restorativeness and Coherence) was confirmed. Significant associations were found between scores on the General Restorative subscale and perceived psychosocial benefits (relaxation, nature appreciation or kinship, and physical fitness or achievement) and positive affect. However, the findings associated with the Coherence subscale were not conclusive.

Communication: Photodissociation of N(2)O-frustrated NN bond breaking causes diffuse vibrational structures.

The photodissociation of N(2)O is studied by wave packet calculations using a global three-dimensional potential energy surface for the first excited (1)A(') state. It is shown that the weak vibrational structures of the absorption cross section are caused by large-amplitude NN stretch motion, combined with strong excitation of the bend as well as the O-NN stretch. Weakening of the NN bond toward the N+NO channel is the necessary prerequisite.

Photodissociation of ozone in the Hartley band: Potential energy surfaces, nonadiabatic couplings, and singlet/triplet branching ratio.

The lowest five (1)A(') states 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((1)D)+O(2)((1)Delta(g)), and triplet, O((3)P)+O(2)((3)Sigma(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((1)D) 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.

Towards quantum mechanical description of the unconventional mass-dependent isotope effect in ozone: resonance recombination in the strong collision approximation.

The dependence of ozone recombination rate on the masses of oxygen isotopes is examined in the strong collision approximation by means of quantum mechanical calculations of resonance spectra of several rotating isotopomers. The measured DeltaZPE effect and its temperature dependence can be reconstructed from partial widths of narrow nonoverlapping resonances. The effect is attributed to substantial contributions of highly rotationally excited states to recombination.

Relaxation of NH(a(1)Delta, v = 1) in collisions with H((2)S): an experimental and theoretical study.

Collisions of electronically and vibrationally excited NH(a(1)Delta, v = 1) with H atoms were investigated by experimental, quantum mechanical (QM) wavepacket, and quasiclassical trajectory (QCT) methods. The NH(a(1)Delta, v = 1) total loss rate constant, corresponding to the sum of the NH vibrational relaxation, N((2)D)+H(2) formation, and electronic quenching to NH(X(3)Sigma(-)), was measured at room temperature. Most of the calculations were performed within the Born-Oppenheimer approximation, neglecting electronic quenching due to Renner-Teller coupling because QCT calculations showed that for the loss of NH(a(1)Delta, v = 1) the contribution of quenching is negligible. The QM study included Coriolis couplings, and the QCT study counted only trajectories ending close to a vibrational quantum level of the product diatom. The collisions are dominated by long-lived intermediate complexes, and QM probabilities and cross sections thus exhibit pronounced resonances. QM and QCT cross sections and rate coefficients of the various processes are in very good agreement. The measured rate constant is (9.1 +/- 3.3) x 10(-11) cm(3) s(-1), compared with (14.4 +/- 0.5) x 10(-11) and (15.6 +/- 1.6) x 10(-11) cm(3) s(-1), as obtained from QM and QCT calculations, respectively. The reason for the theoretical overestimation is unknown.

Production of O2 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((3)P)+O(2) products with very low kinetic energy release (E(tr)<0.2 eV) formed in the deep UV (226 nm) photodissociation of ozone reflect excitation of the Herzberg states of O(2): A(')(3)Delta(u)(v=0,1,2) and A (3)Sigma(u)(+)(v=0,1). This interpretation contradicts the earlier assignment to very high (v> or =26) vibrational states of O(2)((3)Sigma(g)(-)).

Quantum mechanical study of vibrational energy transfer in Ar-O3 collisions: influence of symmetry.

The vibrational energy transfer in Ar-O(3) collisions is investigated within the breathing sphere approximation. Ozone wave functions are calculated with a simplified potential energy surface and used in the close coupling scattering equations. Inelastic transition probabilities are determined for all bound states of O(3). Energy transfer is studied in one asymmetric, (16)O(16)O(18)O, and two symmetric isotopomers, (16)O(16)O(16)O and (16)O(18)O(16)O. Two measures of the energy transfer are considered: Microcanonical deactivation for a fixed collision energy and thermal vibrational relaxation described by the master equation at a fixed temperature. In either case, the energy transfer is symmetry independent near the dissociation threshold and the sensitivity to symmetry grows as the ozone energy decreases.

Theoretical study of the D-->C emission spectrum of NO(2).

The 3 (2)A(')(D)-->1 (2)A(")(C) emission spectrum of NO(2) has been calculated by means of exact dynamics calculations and an accurate potential energy surface for the C state. The potential energy surface has been obtained by electronic structure calculations employing the internally contracted multireference configuration interaction method plus Davidson correction and the augmented correlation consistent polarized quadruple zeta basis set. The calculated spectrum, based on energies as well as intensities, agrees well with the measured one. Despite the two asymmetric C(s) potential wells of the C potential energy surface, the spectrum is best described by a C(2v) assignment in terms of symmetric stretch, bending, and antisymmetric stretch quantum numbers. The barrier separating the two wells is merely of the order of 500 cm(-1) with the consequence that only the two lowest states, (0,0,0) and (0,0,1), show a tunneling splitting. Essential for the correct assignment of the spectrum is the pronounced negative anharmonicity of the antisymmetric stretch mode. Excitation of the symmetric stretch mode is not directly seen in the main part of the spectrum.

Comment on "Theory of photodissociation of ozone in the Hartley continuum; effect of vibrational excitation and O(1D) atom velocity distribution" by E. Baloïtcha and G. G. Balint-Kurti, Phys. Chem. Chem. Phys., 2005, 7, 3829.

In this Comment we present quantum mechanical absorption spectra of the Hartley band originating from the four vibrationally excited levels of the ground electronic state. The calculations are performed using the diabatic B-state potential energy surface and the transition dipole moment vector constructed from the ab initio data of the title paper. The calculated spectra are multimodal (for the symmetric stretch pre-excitation) and strongly structured (for the symmetric stretch and bending pre-excitations). These results agree with the previous theoretical analysis and with the predictions of a simple model based on the reflection principle, but contradict the findings of Baloïtcha amd Balint-Kurti thus questioning the accuracy of their calculations.

New theoretical investigations of the photodissociation of ozone in the Hartley, Huggins, Chappuis, and Wulf bands.

We review recent theoretical studies of the photodissociation of ozone in the wavelength region from 200 nm to 1100 nm comprising four major absorption bands: Hartley and Huggins (near ultraviolet), Chappuis (visible), and Wulf (near infrared). The quantum mechanical dynamics calculations use global potential energy surfaces obtained from new high-level electronic structure calculations. Altogether nine electronic states are taken into account in the theoretical descriptions: four 1A', two 1A'', one 3A' and two 3A'' states. Of particular interest is the analysis of diffuse vibrational structures, which are prominent in all absorption bands, and their dynamical origin and assignment. Another focus is the effect of non-adiabatic coupling on lifetimes in the excited states and on the population of the specific electronic product channels.

Theoretical investigation of exchange and recombination reactions in O(3P)+NO(2Pi) collisions.

We present a detailed dynamical study of the kinetics of O(3P)+NO(2Pi) collisions including O atom exchange reactions and the recombination of NO2. The classical trajectory calculations are performed on the lowest 2A' and 2A" potential energy surfaces, which were calculated by ab initio methods. The calculated room temperature exchange reaction rate coefficient, kex, is in very good agreement with the measured one. The high-pressure recombination rate coefficient, which is given by the formation rate coefficient and to a good approximation equals 2kex, overestimates the experimental data by merely 20%. The pressure dependence of the recombination rate, kr, is described within the strong-collision model by assigning a stabilization probability to each individual trajectory. The measured falloff curve is well reproduced over five orders of magnitude by a single parameter, i.e., the strong-collision stabilization frequency. The calculations also yield the correct temperature dependence, kr proportional, T-1.5, of the low-pressure recombination rate coefficient. The dependence of the rate coefficients on the oxygen isotopes are investigated by incorporating the difference of the zero-point energies between the reactant and product NO radicals, DeltaZPE, into the potential energy surface. Similar isotope effects as for ozone are predicted for both the exchange reaction and the recombination. Finally, we estimate that the chaperon mechanism is not important for the recombination of NO2, which is in accord with the overall T-1.4 dependence of the measured recombination rate even in the low temperature range.

Exploring Renner-Teller induced quenching in the reaction H(2S)+NH(a 1Delta): a combined experimental and theoretical study.

Experimental rate coefficients for the removal of NH(a (1)Delta) and ND(a (1)Delta) in collisions with H and D atoms are presented; all four isotope combinations are considered: NH+H, NH+D, ND+H, and ND+D. The experiments were performed in a quasistatic laser-flash photolysis/laser-induced fluorescence system at low pressures. NH(a (1)Delta) and ND(a (1)Delta) were generated by photolysis of HN(3) and DN(3), respectively. The total removal rate coefficients at room temperature are in the range of (3-5)x10(13) cm(3) mol(-1) s(-1). For two isotope combinations, NH+H and NH+D, quenching rate coefficients for the production of NH(X (3)Sigma(-)) or ND(X (3)Sigma(-)) were also determined; they are in the range of 1 x 10(13) cm(3) mol(-1) s(-1). The quenching rate coefficients directly reflect the strength of the Renner-Teller coupling between the (2)A(") and (2)A(') electronic states near linearity and so can be used to test theoretical models for describing this nonadiabatic process. The title reaction was modeled with a simple surface-hopping approach including a single parameter, which was adjusted to reproduce the quenching rate for NH+H; the same parameter value was used for all isotope combinations. The agreement with the measured total removal rate is good for all but one isotope combination. However, the quenching rates for the NH+D combination are only in fair (factor of 2) agreement with the corresponding measured data.

Spin-orbit mechanism of predissociation in the Wulf band of ozone.

Previously calculated resonance widths of the ground vibrational levels in the electronic states 1 (3)A" ((3)A(2)) and 1 (3)A' ((3)B(2)), which belong to the Wulf band system of ozone, are significantly smaller than observed experimentally. We demonstrate that predissociation is drastically enhanced by spin-orbit coupling between 1 (3)A"/X (1)A' and 1 (3)A'/1 (3)A". Multistate quantum mechanical calculations using ab initio spin-orbit coupling matrix elements give linewidths of optically bright components of the right order of magnitude.