M.S.A. El-Kader

Contributions of multipolar polarizabilities to the isotropic and anisotropic light scattering induced by molecular interactions in gaseous methane

The binary isotropic and anisotropic collision-induced light scattering spectra of gaseous methane at room temperature are analyzed in terms of a recent ab initio intermolecular potential and interaction-induced pair polarizability trace and anisotropy models, using quantum line-shapes computations. The translational spectra at relatively low frequencies are determined largely by the effects 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-quadrupole polarizability A and the dipole-octopole polarizability E are obtained and checked with recent ab initio theoretical values.

Empirical models of the pair-polarizability trace and anisotropy from polarized and depolarized interaction-induced light scattering spectra for gaseous krypton

Abstract 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.

Empirical pair polarizability anisotropy and interatomic potential for monatomic gas mixture of Kr and Xe

Quantum mechanical lineshapes of collision-induced light scattering (CILS) at room temperature are computed for gaseous binary mixtures of krypton with xenon using theoretical induced pair polarizability anisotropy as input. Comparison with measured anisotropic spectrum of light scattering shows good agreement, in spite of the uncertainty in its spectral moments. Empirical model of the pair polarizability anisotropy which reproduce the experimental spectrum and the first few spectral moments more closely than the fundamental theory are also given. Good agreement between computed and experinental lineshapes of scattering is obtained when potential models which are constructed from the thermophysical and total scattering cross sections are used.

Lineshapes of collision-induced absorption (CIA) and of collision-induced scattering (CIS) for monatomic gas mixtures of Ne-Ar

Quantum mechanical lineshapes of collision-induced absorption (CIA) at different temperatures and of collision-induced light scattering (CIS) at room temperature are computed for gaseous binary mixtures of neon with argon using theoretical induced dipole moment and pair-polarizability trace and anisotropy as input. Comparison with measured spectra of isotropic and anisotropic light scattering shows satisfactory agreement, for which the uncertainty in measurement of its spectral moments is seen to be large. Empirical models of the dipole moment and pair-polarizability trace and anisotropy which reproduce the experimental spectra and the first three spectral moments more closely than the fundamental theory are also given. Good agreement between computed and experinental lineshapes of both absorption and scattering is obtained when potential models constructed from the thermophysical, transport and spectroscopic properties are used.

Spectral line-shapes and moment analysis in isotropic and anisotropic light scattering spectra for gaseous argon

For direct comparison with the measurements of isotropic and anisotropic collision-induced light scattering spectra of argon gas at room temperature performed by Le Duffs group, quantum mechanical lineshapes are computed using different models 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 argon gas. These are compared, where possible, with existing computations based on classical physics, utilizing the first few even moments of the isotropic and anisotropic light scattering spectra. Some discrepancies with experiment, specially for the second Kerr coefficient, are found, but overall the agreement is satisfactory and the profiles calculated with these models coincide with experiments.

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

Abstract 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.

An approximate multi-property empirical isotropic interatomic pair potential for the krypton dimer

An approximate empirical isotropic interatomic potential for krypton interaction is developed by simultaneously fitting the Morse-Morse-Morse-Spline-van der Waals potential form to the pressure second virial coefficient, viscosity, thermal conductivity and depolarized interaction-induced light scattering data. Absolute zeroth and second moments of the two-and three-body spectra, the pressure third virial coefficient and isotopic thermal diffusion factor 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. The use of the new potential in lattice sum calculations yields good results for several properties of solid krypton.

Empirical interatomic potential from polarized and depolarized interaction-induced light scattering spectra for gaseous helium

Polarized and depolarized interaction-induced light scattering spectra at room temperature (294.5±1) K have been used for deriving the empirical multiparameter 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 interatomic potential of the gaseous helium as a function of the interatomic separation. The potential parameters describing the location and depth of the attractive well are given by ε = 11.2±0.015 K, rm = 2.96±0.01 Å. Comparison with other He2 potentials based on bulk properties and ab initio calculations are presented; the temperature dependence of the pressure second virial coefficient and viscosity are also discussed for the proposed potential.

Spectral line shapes of collision-induced light scattering (CILS) and collision-induced absorption (CIA) using isotropic intermolecular potential for H2–Ar

ABSTRACT Quantum mechanical line shapes of collision-induced light scattering at room temperature (295 K) and collision-induced absorption at T = 195 K are computed for gaseous mixtures of molecular hydrogen and argon using theoretical values for pair-polarisability trace and anisotropy and induced dipole moments as input. Comparison with other theoretical spectra of isotropic and anisotropic light scattering and measured spectra of absorption shows satisfactory agreement, for which the uncertainty in measurement of its spectral moments is seen to be large. Ab initio models of the trace and anisotropy polarisability which reproduce the recent spectra of scattering are given. Empirical model of the dipole moment which reproduce the experimental spectra and the first three spectral moments more closely than the fundamental theory are also given. Good agreement between computed and/or experimental line shapes of both absorption and scattering is obtained when the potential model which is constructed from the transport and thermo-physical properties is used.

Theoretical Calculation of the Rototranslational Collision-Induced Absorption (CIA) Spectra in O2–O2 Pairs

Abstract Quantum mechanical lineshapes of collision-induced absorption (CIA) at room temperature are computed for gaseous molecular oxygen using theoretical values for induced dipole moments and new isotropic interatomic potential as input. Comparison with measured spectra of the rototranslational collision-induced absorption shows good agreement over the full range of frequencies. Empirical models of the dipole moment which reproduce the experimental spectra and the first two spectral moments more closely than the fundamental theory are also given. The quality of the present potential has been checked by comparing between calculated and experimental thermo-physical and transport properties over a wide temperature range, which are found to be in good agreement.