S. Montero

Raman intensities of ethane and deuterated derivatives

Experimental gas phase Raman scattering differential cross sections and depolarization ratios of ethane and its deuterated derivatives are reported. A consistent interpretation of these data is accomplished in the frame of the bond polarizability model. The bond polarizability derivatives ∂αCH/∂rCH here obtained are, within the estimated uncertainties, the same as for other hydrocarbon molecules not only for the mean polarizability but for the anisotropy as well. On the other hand, the C–C bond properties are proved to be essentially the same in ethane and in diamond. In ethane isotopic species with synclinal and antiperiplanar conformers the experimental intensities are not always in the ratio 2:1. A satisfactory explanation of this apparent anomaly is achieved on the basis of the calculated Raman tensors of homologous normal modes which are quite different in some cases.

Raman intensities of Fermi diads. I. Overtones in resonance with nondegenerate fundamentals

Expressions for the vibrational transition moments of the molecular polarizability involved in Fermi resonances of the type 2νs:νr (qs=single, doubly, or triply degenerate mode; qr=nondegenerate mode) are reported as a function of the observed splittings ‖X‖ and of the cubic force constant krss. Fermi resonances in H2O, CO2, CH4, and CD4 are interpreted in terms of bond polarizability parameters.

Experimental and numerical investigation of an axisymmetric supersonic jet

A comprehensive study of a steady axisymmetric supersonic jet of CO 2 , including experiment, theory, and numerical calculation, is presented. The experimental part, based on high-sensitivity Raman spectroscopy mapping, provides absolute density and rotational temperature maps covering the significant regions of the jet: the zone of silence, barrel shock, Mach disk, and subsonic region beyond the Mach disk. The interpretation is based on the quasi-gasdynamic (QGD) system of equations, and its generalization (QGDR) considering the translational–rotational breakdown of thermal equilibrium. QGD and QGDR systems of equations are solved numerically in terms of a finite-difference algorithm with the steady state attained as the limit of a time-evolving process. Numerical results show a good global agreement with experiment, and provide information on those quantities not measured in the experiment, like velocity field, Mach numbers, and pressures. According to the calculation the subsonic part of the jet, downstream of the Mach disk, encloses a low-velocity recirculation vortex ring.

Absolute Raman intensities, force constants, and electro-optical parameters of CH2Cl2, CD2Cl2 and CHDCl2

The absolute Raman intensities and depolarization ratios of all the fundamental bands of CH2Cl2, CD2Cl2 and CHDCl2 have been measured in the gas phase. These experimental data have been included in a refinement of the vibrational potential function of the dichloromethanes. A detailed description is made of the use of the bond polarizability approach. Explicit expressions for the derivatives of the molecular polarizability with respect to the symmetry coordinates are given in terms of the zeroth- and first-order electro-optical parameters of the theory. Good agreement is found between experimental and calculated intensities. The refined electro-optical parameters compare quite well with those obtained from methane, chloroform and carbon tetrachloride. The derivatives of the molecular polarizability with respect to the normal coordinates are also given.

Inelastic collisions in para-H2: Translation-rotation state-to-state rate coefficients and cross sections at low temperature and energy

We report an experimental determination of the k(00-->02) rate coefficient for inelastic H(2):H(2) collisions in the temperature range from 2 to 110 K based on Raman spectroscopy data in supersonic expansions of para-H(2). For this purpose a more accurate method for inverting the master equation of rotational populations is presented. The procedure permits us to reduce the measured k(00-->02) rate coefficient to the corresponding sigma(00-->02) cross section in the range of precollisional energy from 360 to 600 cm(-1). Numerical calculations of sigma(00-->02) carried out in the frame of the coupled channel method are also reported for different intermolecular potentials of H(2). A good agreement is found between the experimental cross section and the numerical one derived from Diep and Johnsons potential [J. Chem. Phys. 112, 4465 (2000)].

Anharmonic Raman intensities of overtones, combination and difference bands

The influence of anharmonicity up to the cubic force constants on the Raman tensors of binary overtone, combination, and difference bands is investigated. Formulas for the absolute differential Raman scattering cross sections of these bands are reported for all possible cases of degeneracy as a function of the invariants of the anharmonic Raman tensors. The evaluation of anharmonic Raman tensors as a function of Hoy, Mills, and Strey’s L tensor, cubic force constants, and bond polarizability parameters is illustrated for the tetrahedral AB4 molecule.

Gas phase Raman intensities of C2H2, C2HD and C2D2

Depolarization ratios and gas phase Raman scattering differential cross sections of all Raman active fundamental vibrations of C2H2, C2HD and C2D2 are reported in absolute units. These quantities may be easily interpreted in terms of bond polarizability parameters. The anisotropy of the molecular polarizability of C2H2 determined from the bond polarizability parameters is consistent with values reported by previous authors. The derivative of the CH bond polarizability tensor with respect to the stretching of the CH bond is shown to have the same value in acetylene as in methane, within experimental accuracy.

Raman spectroscopy of supersonic jets of CO2: Density, condensation, and translational, rotational, and vibrational temperatures

The links between translational, rotational, and vibrational temperatures of supersonic molecular jets, and their density and degree of condensation, are discussed in terms of quantitative Raman scattering experimental data. Such links are established as the result of energy and momentum conservation laws, and of the collisional regime in the jet. Four representative supersonic expansions of CO2, generated at different stagnation pressures are analyzed, showing the potential of quantitative Raman spectroscopy for a complete characterization of the jet.

Vibrational–torsional coupling. High‐resolution stimulated Raman spectrum of the ν3 band of ethane (12C2H6)

We report a stimulated Raman spectrum of the Q branch of the ν3 band of ethane recorded with a resolution of about 0.0055 cm−1 and an absolute wave‐number accuracy of the order of 0.001 cm−1. This spectrum is analyzed in terms of a vibrational–torsional coupling Hamiltonian that allows for the calculation of the observed wave numbers within the experimental uncertainty. The following spectroscopic parameters (in cm−1) have been obtained: ν30=994.1108(10), B3A1s−B0=−0.006 165(3), B3E3d−B0=−0.006 170(2), B3E3s−B0 =−0.006 184(2), B3A3d−B0=−0.006 257(2), A3A1s−A0=−0.003 037(15), A3E3d−A0 =−0.003 040(12), A3E3s−A0=−0.003 054(12), and A3A3d−A0=−0.003 065(12). A strong dependence of the threefold torsional potential barrier with respect to the q3 normal mode, ∂V3/∂q3=−275.9±1.4 cm−1, is deduced from the present analysis.

The torsional Raman spectra of C2H6 and C2D6

The torsional Raman spectra of C2H6 and C2D6 have been experimentally investigated with an improved conventional Raman spectrometer. Wave numbers and absolute cross sections of all observed bands have been satisfactorily interpreted within the experimental accuracy by means of a simple, pure‐torsional Hamiltonian and a model for the torsional dependence of the molecular polarizability. The effective threefold potential barrier parameters for C2H6 have been taken from the recent literature, whilst for C2D6 the values V3eff (C2D6)=990±2 cm−1 and V6eff (C2D6)=7.9±1 cm−1 were obtained.