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ABSTRACT: The angular distribution of products from the ultraviolet photodissociation of nitrous oxide yielding O((1)D) and N(2)(X Σ(g)(+)(1)) was investigated using classical trajectory calculations. The calculations modeled absorption only to the 2(1)A(') electronic state but used surface-hopping techniques to model nonadiabatic transitions to the ground electronic state late in the dissociation. Observed values of the anisotropy parameter β, which decrease as the product N(2) rotational quantum number j increases, could be well reproduced. The relatively low observed β values arise principally from nonaxial recoil due to the very strong bending forces present in the excited state. In the main part of the product rotational distribution near 203 nm, an unusual dynamical effect produces the decrease in β with increasing j; nonaxial recoil effects remain approximately constant while higher j product molecules arise from parent molecules that had their transition dipole moments aligned more closely along the molecular axis. In both low and high j tails of the rotational distribution, the variations in β with j are caused by changes in the extent of nonaxial recoil. In the high-j tail, additional torque present on the ground state potential energy surface following nonadiabatic transitions causes both the additional rotational excitation and the lower β values.
The Journal of chemical physics 01/2012; 136(4):044314. · 3.09 Impact Factor
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ABSTRACT: Product state properties from the photodissociation of ozone in the ultraviolet Hartley band are investigated by trajectory surface-hopping calculations. The diabatic B and R state potential energy and coupling surfaces of Schinke and McBane [J. Chem. Phys. 132, 044305 (2010)] are employed. The properties computed include rotational and vibrational distributions in both the singlet and triplet channels, the total internal energy distribution in the triplet channel, and the photodissociation anisotropy parameter β in the singlet channel. A method for computing β from trajectories computed in internal Jacobi coordinates is described. In the singlet channel, the vibrational distribution is in good agreement with the experimental results. The observed increase in β with increasing photolysis wavelength is reproduced by the calculations and is attributed to the effects of the bending potential on the B state late in the fragmentation. The computed β values are too high with respect to experiment, and the peaks j(max) of the singlet-channel rotational distributions are too low; these discrepancies are attributed to a too steep bending potential at long O-O distances. In the triplet channel, the main part of the internal energy distribution is described well by the calculations, although the detailed structures observed in the experiment are not reproduced. The experimental rotational distributions are well reproduced, although the maxima appear at slightly too high j. The triplet state product energy distributions are shown to depend largely on the distribution of hopping points onto the R state surface. A Landau-Zener model constructed as a function of the O(2) bond distance provides a good physical description of the two-state dynamics. The high internal energy O(2) products that cannot be attributed to the excitation of the Herzberg states remain unexplained.
The Journal of chemical physics 10/2010; 133(14):144312. · 3.09 Impact Factor
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ABSTRACT: Four potential energy surfaces are of current interest for the Ne-CO interaction. Two are high-level fully ab initio surfaces obtained a decade ago using symmetry-adapted perturbation theory and supermolecule coupled-cluster methods. The other two are very recent exchange-Coulomb (XC) model potential energy surfaces constructed by using ab initio Heitler-London interaction energies and literature long range dispersion and induction energies, followed by the determination of a small number of adjustable parameters to reproduce a selected subset of pure rotational transition frequencies for the (20)Ne-(12)C(16)O van der Waals cluster. Testing of the four potential energy surfaces against a wide range of available experimental microwave, millimeter-wave, and mid-infrared Ne-CO transition frequencies indicated that the XC potential energy surfaces gave results that were generally far superior to the earlier fully ab initio surfaces. In this paper, two XC model surfaces and the two fully ab initio surfaces are tested for their abilities to reproduce experiment for a wide range of nonspectroscopic Ne-CO gas mixture properties. The properties considered here are relative integral cross sections and the angle dependence of rotational state-to-state differential cross sections, rotational relaxation rate constants for CO(v=2) in Ne-CO mixtures at T=296 K, pressure broadening of two pure rotational lines and of the rovibrational lines in the CO fundamental and first overtone transitions at 300 K, and the temperature and, where appropriate, mole fraction dependencies of the interaction second virial coefficient, the binary diffusion coefficient, the interaction viscosity, the mixture shear viscosity and thermal conductivity coefficients, and the thermal diffusion factor. The XC model potential energy surfaces give results that lie within or very nearly within the experimental uncertainties for all properties considered, while the coupled-cluster ab initio surface gives results that agree similarly well for all but one of the properties considered. When the present comparisons are combined with the ability to give accurate spectroscopic transition frequencies for the Ne-CO van der Waals complex, only the XC potential energy surfaces give results that agree well with all extant experimental data for the Ne-CO interaction.
The Journal of chemical physics 01/2010; 132(2):024308. · 3.09 Impact Factor
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ABSTRACT: A hierarchical family of five three-dimensional potential energy surfaces has been developed for the benchmark He-CO system. Four surfaces were obtained at the coupled cluster singles and doubles level of theory with a perturbational estimate of triple excitations, CCSD(T), and range in quality from the doubly augmented double-zeta basis set to the complete basis set (CBS) limit. The fifth corresponds to an approximate CCSDT/CBS surface (CCSD with iterative triples/CBS, denoted CBS+corr). The CBS limit results were obtained by pointwise basis set extrapolations of the individual counterpoise-corrected interaction energies. For each surface, over 1000 interaction energies were accurately interpolated using a reproducing kernel Hilbert space approach with an R-6+R-7 asymptotic form. In each case, both three-dimensional and effective two-dimensional surfaces were developed. In standard Jacobi coordinates, the final CBS+corr surface has a global minimum at rCO=2.1322a0,R=6.418a0, and gamma=70.84 degrees with a well depth of -22.34 cm-1. The other four surfaces have well depths ranging from -14.83 cm-1 [CCSD(T)/d-aug-cc-pVDZ] to -22.02 cm-1 [CCSD(T)/CBS]. For each of these surfaces the infrared spectrum has been accurately calculated and compared to experiment, as well as to previous theoretical and empirical surfaces. The final CBS+corr surface exhibits root-mean-square and maximum errors compared to experiment (4He) of just 0.03 and 0.04 cm-1, respectively, for all 42 transitions and is the most accurate ab initio surface to date for this system. Other quantities investigated include the interaction second virial coefficient, the integral cross sections, and thermal rate coefficients for rotational relaxation of CO by He, and rate coefficients for CO vibrational relaxation by He. All the observable quantities showed a smooth convergence with respect to the quality of the underlying interaction surface.
The Journal of Chemical Physics 09/2005; 123(8):084314. · 3.33 Impact Factor
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ABSTRACT: 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 modified 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.
The Journal of Chemical Physics 05/2004; 120(16):7483-9. · 3.33 Impact Factor
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ABSTRACT: An extensive data set of 54 time-resolved pump-probe measurements was used to examine CO + He rotational energy transfer within the CO v = 2 rotational manifold. Rotational levels in the range Ji = 2-9 were excited and collisional energy transfer of population to the levels Jf = 1-10 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 and ab initio theoretical calculations that employed the SAPT potential energy surface of Heijmen et al. [J. Chem. Phys. 107, 9921 (1997)]. Fitting laws based on the modified exponential gap (MEG), statistical power exponential gap (SPEG), and energy corrected sudden with exponential power (ECS-EP) models all yielded acceptable simulations of the kinetic data, as did the theoretical rate constants. However, the latter were unique in their ability to reproduce both our kinetic data and the pressure broadening coefficients for CO + He. These results provide an impressive demonstration of the quality of the symmetry adapted perturbation theory (SAPT) potential energy surface.
The Journal of Chemical Physics 03/2004; 120(5):2285-95. · 3.33 Impact Factor
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Journal of Chemical Physics, v.132, 044305-1-044305-16 (2010).
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The Journal of Chemical Physics. 124:9901.
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