A hierarchical family of three-dimensional potential energy surfaces for He-CO

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.     

Product angular distributions in the ultraviolet photodissociation of N2O

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.     

Tunable infrared generation in KTA and applications

Noncollinear difference frequency mixing of dye laser and Nd:YAG second harmonic (fundamental) radiation from a commercial laser system is employed for the generation of 2.7–5.3 μm (1.6–1.7 μm) radiations in a flux-grown KTiOAsO₁ crystal. The generated radiation is used to scan the methane absorption in the fundamental (v ₃) and its first overtone (2v ₃) band at pressure 90 torr in a laboratory made single pass gas cell of length 33 cm.     

CO Blocking of D2 Dissociative Adsorption on Ru(0001)

The influence of pre‐adsorbed CO on the dissociative adsorption of D₂ on Ru(0001) is studied by molecular‐beam techniques. We determine the initial dissociation probability of D₂ as a function of its kinetic energy for various CO pre‐coverages between 0.00 and 0.67 monolayers (ML) at a surface temperature of 180 K. The results indicate that CO blocks D₂ dissociation and perturbs the local surface reactivity up to the nearest‐neighbour Ru atoms. Non‐activated sticking and dissociation become less important with increasing CO coverage, and vanish at θ_CO≈0.33 ML. In addition, at high D₂ kinetic energy (>35 kJ mol^?1) the site‐blocking capability of CO decreases rapidly. These observations are attributed to a CO‐induced activation barrier for D₂ dissociation in the vicinity of CO molecules.     

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.     

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'.     

Communication: Mapping water collisions for interstellar space conditions

We report a joint experimental and theoretical study that directly tests the quality of the potential energy surfaces used to calculate energy changing cross sections of water in collision with helium and molecular hydrogen, at conditions relevant for astrophysics. Fully state-to-state differential cross sections are measured for H(2)O-He and H(2)O-H(2) collisions at 429 and 575?cm(-1) collision energy, respectively. We compare these differential cross sections with theoretical ones for H(2)O+H(2) derived from state-of-the-art potential energy surfaces [P. Valiron et al., J. Chem. Phys. 129, 134306 (2008)] and quantum scattering calculations. This detailed comparison forms a stringent test of the validity of astrophysics calculations for energy changing rates in water. The agreement between theory and experiment is striking for most of the state-to-state differential cross sections measured.