From light-scattering measurements to polarizability derivatives in vibrational Raman spectroscopy: the 2ν5 overtone of SF6

The room-temperature isotropic spectrum of SF6 was recorded at the frequency of the 2ν5 overtone by running high-sensitivity incoherent Raman experiments for two independent polarizations of the incident beam and for gas densities varying from 2 to 27 amagat. Weak yet observable pressure effects were found. A transparent analysis of the Raman cross-section problem along with the first-ever prediction of the value of the mean polarizability second derivative ∂(2)α/∂q5(2) are made and the hitherto underestimated role of the hot bands of SF6 is brought to the wider public. The emergence of an analytic hotband factor is shown whose magnitude is dramatically increased with the order of the overtone and the gas temperature and all the more so upon considering low-frequency molecular vibrations. Our formulas, which in the harmonic approximation are exact, are still applicable to real situations provided certain conditions are fulfilled. For nondegenerated modes, generalization to higher order overtones is made, an issue addressing the much challenging problem of the IR-allowed second overtone bands. The content of this paper is also an invitation towards ab initio derivative-calculations for sulfur hexafluoride, especially given the today's needs in interpreting spectra of significance for greenhouse atmospheric issues.

Multiproperty empirical isotropic interatomic potentials for CH4-inert gas mixtures

An approximate empirical isotropic interatomic potentials for CH₄–inert gas mixtures are developed by simultaneously fitting the Exponential-Spline-Morse-Spline-van der Waals (ESMSV) potential form to viscosity, thermal conductivity, thermal diffusion factors, diffusion coefficient, interaction second pressure virial coefficient and scattering cross-section data. Quantum mechanical lineshapes of collision-induced absorption (CIA) at different temperatures for CH₄–He and at T = 87 K for CH₄–Ar are computed using theoretical values for overlap, octopole and hexadecapole mechanisms and interaction potential as input. Also, the quantum mechanical lineshapes of collision-induced light scattering (CILS) for the mixtures CH₄–Ar and CH₄–Xe at room temperature are calculated. The spectra of scattering consist essentially of an intense, purely translational component which includes scattering due to free pairs and bound dimers, and the other is due to the induced rotational scattering. These spectra have been interpreted by means of pair-polarizability terms, which arise from a long-range dipole-induced-dipole (DID) with small dispersion corrections and a short-range interaction mechanism involving higher-order dipole–quadrupole A and dipole–octopole E multipole polarizabilities. Good agreement between computed and experimental lineshapes of both absorption and scattering is obtained when the models of potential, interaction-induced dipole and polarizability components are used.

Anisotropic and Isotropic Light Scattering Data of Neon Gas Simultaneously Fitted by an Isotropic Ground State Interatomic Potential

The binary anisotropic and isotropic collision-induced light scattering spectra by gaseous room temperature neon, over an extended frequency domain have been used for deriving the empirical multi-parameter Morse–Morse–Morse–Spline van der Waals interatomic potential. These new data of scattering are accurate enough to permit for the first time a reliable determination of the isotropic potential as a function of the interatomic separation. The potential parameters describing the location and depth of the attractive well are given by ε = 41.07 K and r m = 3.087 Å. Comparison with other potentials based on bulk properties and ab initio calculations are presented; the temperature dependence of pressure second virial coefficient, self diffusion, thermal conductivity and viscosity are also discussed for the proposed potentials. In addition, our empirical potential was used to solve the nuclear Schrödinger equation for comparison with and predication of spectroscopic properties of the dimer. The results show that it is the most accurate potential reported to date for this system.

Contributions of Multipolar Polarizabilities to the Isotropic and Anisotropic Light Scattering Induced by Molecular Interactions in Gaseous Sulfur Hexafluoride

The binary isotropic and anisotropic collision-induced light scattering spectra of the gaseous sulfur hexafluoride at room temperature are analyzed in terms of recent intermolecular potential and interaction-induced pair polarizability models, using quantum line-shapes computations. The spectra at relatively low frequencies are determined largely by the effect of bound and free transitions. At intermediate frequencies the spectra are sensitive to both the attractive part of the potential and the short-range part of the polarizability trace and anisotropy. The high frequency wings are discussed in terms of the collision-induced rotational Raman effect and estimates for the dipole–octopole polarizability E, is obtained and checked with recent ab initio theoretical value. Absolute zeroth moment of both spectra have been measured and compared with theoretical calculations using different models of the intermolecular potentials. Also, the temperature dependence of the pressure second virial coefficient, viscosity and thermal conductivity are also discussed for the proposed potentials. The results show that (M3SV) is the most accurate potential yet reported for this system.

Lineshape and Moment Analysis in Isotropic and Anisotropic Interaction-Induced Light Scattering Spectra of Gaseous Helium

Calibrated Raman spectra of the helium gas diatom at room temperature (294.5 K) have recently been measured over a wide range of intensities, with polarizations of the incident beam parallel and perpendicular to the direction of observation. From the resulting spectra, the isotropic and anisotropic components are separated for the helium diatom. For direct comparison with the measurements, quantum mechanical lineshapes are computed using recent model of the interatomic potential and models of the diatom polarizability tensor invariants as input. New empirical trace and anisotropy parameters, consistent with the spectroscopic measurements, are thus obtained for the helium gas. We compare these where possible with the existing computations based on classical physics, utilizing the first few even moments of the isotropic and anisotropic spectra. Good agreement with ab initio results in the literature is obtained and profiles calculated with these models are in excellent agreement with experiments.

Empirical Models of the Pair-Polarizability Trace and Anisotropy from Polarized and Depolarized Interaction-Induced Light Scattering Spectra for Gaseous Krypton

A method based on classical physics, utilizing the first few even moments of the polarized and depolarized interaction-induced light scattering spectra at room temperature to derive an empirical model for the pair-polarizability trace and anisotropy of interacting atoms, with adjustable free parameters, is described and applied to the spectra of krypton. Good agreement with ab initio and analytical results in the literature is obtained and profiles calculated with our empirical models are in excellent agreement with experiments. Theoretical calculations using these models are compared with the experimental values of the second dielectric constant and second virial Kerr coefficient for this gas. The results show excellent agreement with the experiments.

Exact low-order classical moments in collision-induced bands by linear rotors: CO2-CO2

Exact and general analytic expressions are reported for the integrated intensity and the width of collision-induced absorption (CIA) and collision-induced scattering (CIS) bands by gases of centrosymmetric linear molecules. These expressions provide significant insight and allow assignment of partial second moments to the degrees of freedom of the colliding molecules. The expressions are applied to ambient CO(2), whose collisional spectra are reputed to be useful probes for terrestrial and planetary atmospheres. Compelling evidence of the substantial role of hitherto missing polarization and polarizability mechanisms is provided and is in remarkable agreement with experimental observation. Our findings allow the long-overdue simple interpretation of CIA and CIS by CO(2)-CO(2) without the need to resort to short-range interactions to offset the discrepancies between theory and experiment.

On a singularity-free pair-polarizability anisotropy model for atomic gases

We present a collision-induced light scattering spectrum of Ne(2) and analyze it, together with an induced spectrum of Ar(2), in terms of a model for the pair-polarizability anisotropy beta of an atomic gas. This model, which is in essence a slight modification of one introduced by Ceccherini et al. [J. Chem. Phys. 111, 6316 (1999)], is shown to describe the measured spectra much farther into the wings than does the original model. The agreement of our measurements with quantum-mechanical spectra from the most reliable, now available, ab initio computation methods for beta produces evidence in support of the data and counters criticism of our experiment.

The frequency-dependent dipole polarizability of the mercury dimer from four-component relativistic density-functional theory

The frequency-dependent dipole polarizability of Hg(2) is calculated using response theory within four-component relativistic density-functional theory [using the local-density approximation (LDA) and the hybrid functional B3LYP] including corrections for the basis-set superposition error. The anisotropic component of the polarizability tensor agrees well with the values obtained from collision-induced Raman spectroscopy carried out at a wavelength of 488 nm. The values obtained from the two density functionals agree closely with the experimentally derived anisotropy component of the dipole polarizability, despite their rather large differences in the dimer potential-energy curves (LDA is strongly overbinding while B3LYP is purely repulsive). The first two refractivity virial coefficients for the generalized Clausius-Mossotti function are derived.

Hyper-Rayleigh spectral intensities of gaseous Kr–Xe mixture

Binary collision-induced hyper-Rayleigh (CIHR) spectra of Kr-Xe gaseous system are computed quantum mechanically and classically within the frequency range up to 380 cm(-1). The intensities are expressed in absolute units. The details of the theory developed for the CIHR spectra are given and the properties of the profiles as well as the depolarization ratio frequency dependence are discussed. The contributions to the spectra related to the vector b10(r) and the septor b30(r) components of the hyperpolarizability tensor are evaluated.

Induced anisotropy of Ar2: A state-of-the-art semiempirical model

We report a semiempirical induced anisotropy model for Ar2 that optimally reproduces highly accurate experimental induced scattering band shapes over a large domain of frequency detunings. We believe that our model, which was derived (after an extensive compilation of the data) by using a systematic methodology and a novel reliable multiparameter equation solver (presented here), is the best induced anisotropy ever proposed. The applicability of our methodology to any induced band shape and its economical implementation make it a promising device for accessing various incremental properties for weakly interacting systems of an arbitrary complexity.

Ground-state properties and static dipole polarizabilities of the alkali dimers from K2n to Fr2n(n=0,+1) from scalar relativistic pseudopotential coupled cluster and density functional studies

The newly adjusted energy-consistent nine-valence-electron pseudopotentials for K to Fr are used to calculate spectroscopic properties for the neutral and positively charged alkali dimers using coupled cluster and density functional theory. For the neutral dimers the static dipole polarizability was calculated. The coupled cluster results are all in excellent agreement with experimental values. The density functionals used can give quite different spectroscopic properties especially for the dipole polarizability, with the Perdew-Wang PW91 functional performing best.

Double incoherent Raman scattering induced by molecular interactions in gases

A new Raman band is observed in two gases, CF4 and CH4. It is attributable to a double incoherent Raman scattering that takes place in pairs of interacting molecules. Spectral scattering intensities are given on an absolute scale. Zero order spectral moments are measured and are found higher than their long-range interaction value.

Empirical Interatomic Potential from Depolarized Interaction – Induced Light Scattering Spectra for Mercury

Depolarized interaction-induced light scattering spectra of mercury between 793 K and 1073 K have been used for deriving the empirical multiparameter M3SV interatomic potential. Absolute zeroth and second moments have been measured and compared with theoretical calculations using various models for the interatomic potential. The results show that it is the most accurate potential yet reported for this system.