High-resolution spectroscopy and structure of osmium tetroxide. A benchmark study on 192OsO4.

Osmium tetroxide (OsO(4)) is a heavy tetrahedral molecule that constitutes a benchmark for quantum chemistry calculations. Its favorable spin statistics (due to the zero nuclear spin of oxygen atoms) is such that only A(1) and A(2) (T(d) symmetry) rovibrational levels are allowed, leading to a dense but quite easily resolvable spectrum. We reinvestigate here the ν(1)/ν(3) stretching fundamental (940-980 cm(-1)) dyad region and perform new assignments and effective Hamiltonian parameter fits for the main isotopologue ((192)OsO(4)). We also investigate the ν(2)/ν(4) bending fundamental dyad (300-360 cm(-1)) for the first time and perform a preliminary analysis. New experimental data have been obtained at 0.001 cm(-1) resolution using an isotopically pure (192)OsO(4) sample and the Synchrotron SOLEIL light source. Assignments and analyses were performed using SPVIEW and XTDS software, respectively. We provide precise effective Hamiltonian parameters, including the band centers for all of the fundamental levels and rotational constants for the ground state and for all four fundamental levels. We discuss isotopic shifts, estimate the equilibrium rotational constant B(e), and derive a precise value for the equilibrium bond length r(e)(Os-O) = 1.70919(16) Å. We also performed experiments to measure for the first time the IR integrated intensities for the ν(2)/ν(4) bending fundamental dyad. These new data are compared to current ab initio predictions.

Direct determination of state-to-state rotational energy transfer rate constants via a Raman-Raman double resonance technique: ortho-acetylene in v(2)=1 at 155 K.

A new technique for the direct determination of state-to-state rotational energy transfer rate constants in the gas phase is presented. It is based on two sequential stimulated Raman processes: the first one prepares the sample in a single rotational state of an excited vibrational level, and the second one, using the high resolution quasi-continuous stimulated Raman-loss technique, monitors the transfer of population to other rotational states of the same vibrational level as a function of the delay between the pump and the probe stages. The technique is applied to the odd-J rotational states of v(2)=1 acetylene at 155 K. The experimental layout, data acquisition, retrieval procedures, and numerical treatment are described. The quantity and quality of the data are high enough to allow a direct determination of the state-to-state rate constant matrix from a fit of the experimental data, with the only conditions of detailed balance and of a closed number of states. The matrix obtained from this direct fit is also compared with those obtained using some common fitting and scaling laws.

Linewidths of C2H2 perturbed by H2: experiments and calculations from an ab initio potential.

In this work we present a theoretical and experimental study of the acetylene-hydrogen system. A potential surface considering rigid monomers has been obtained by ab initio quantum chemistry methods. This 4-dimensional potential is further employed to compute, using the close-coupling approach and the coupled-states approximation, pressure broadening coefficients of C2H2 isotropic Raman Q lines over a temperature range of 77 to 2000 K. Experimental data for the acetylene nu2 Raman lines broadened by molecular hydrogen are obtained using stimulated Raman spectroscopy. The comparison of theoretical values with experimental data at 143 K is promising. Approximations to increase the computational efficiency are proposed.

Q-branch linewidths of N2 perturbed by H2: experiments and quantum calculations from an ab initio potential.

In this work the authors present an experimental and theoretical study about the Q-branch lines' broadening coefficients of N2 perturbed by H2. Experimental values for these parameters have been obtained at 440 and 580 K, and quantum calculations have been performed using a new ab initio potential energy surface, obtained by quantum chemistry methods. The results of these calculations are compared to experimental data obtained previously at 77 and 298 K [L. Gomez et al., Mol. Phys. 104, 1869 (2006)] and to the present measurements. A satisfactory agreement is obtained for the whole range of temperatures used in the experiments.