Massimo Moraldi

Collision-induced infrared spectra of H2-He pairs involving 0-1 vibrational transitions and temperatures from 18 to 7000 K

Theoretical estimates are given of the absorption spectra of collisional pairs of hydrogen and helium (H2-He). The range of temperatures considered is from 18 to 7000 K. The resulting spectra, which can be computed in seconds even on small computers, are in close agreement with presently known laboratory measurements and extrapolate these dependably to higher as well as to lower temperatures, and to higher frequencies. This work is of interest for the modeling of the atmospheres of the outer planets and certain cool stars that contain neutral molecular hydrogen and helium in the near-IR region of the spectrum. 38 references.

INTERACTION-INDUCED POLARIZABILITY INVARIANTS AND THE INTERATOMIC POTENTIAL OF THE MERCURY DIATOM

Analytical models of the invariants (trace and anisotropy) of the diatom polarizability tensor are formulated that differ from existing models by the suppression of the classical long-range components at close range (“damping”). Such damping is imperative in diatomic systems where the long-range polarizability terms are relatively large and/or the collision energies are high so that substantial penetration of the electronic shells of the colliding atoms takes place, e.g., for collision-induced light scattering by mercury vapor at high temperatures. When a small number of physically meaningful parameters of the polarizability models are properly adjusted, the binary trace and anisotropy collision-induced spectra of mercury vapor are closely reproduced from theory in all details. An improved model of the Hg–Hg interaction potential, which reproduces the existing bound mercury dimer states as well as the viscosity data, includes a similar damping term for suppression of the dispersion part at close range.

Interatomic potentials of group IIB atoms (ground state)

Excitation and fluorescence spectra of group IIB atoms (Hg, Cd, Zn) have been extensively measured in the last years. From these measurements dependable values for potential parameters like well-depth and vibrational frequency for the ground electronic state have been derived with a high degree of accuracy. A determination of the equilibrium distance for Hg is also available. For Cd and Zn, the equilibrium distances are not known, though. Here we propose to use low-density viscosity measurements together with the spectroscopic data to give an estimate of these distances.

Two-body depolarized cils spectra of krypton and xenon at 295 K

Abstract We have experimentally determined the two-body depolarized CILS spectra of krypton and xenon at room temperature between 2 and 120 cm −1 . Comparison of the first three even experimental moments of the spectra with theoretical calculations shows, as in argon, the necessity of introducing a short-range negative contribution to the induced pair polarizability.

Experimental and theoretical analysis of the rotational Raman spectrum of hydrogen molecules in clathrate hydrates

The Raman spectra of H(2) and HD molecules in simple hydrogen and binary hydrogen-tetrahydrofuran clathrate hydrates have been measured at temperatures as low as 20 K. The rotational bands of trapped molecules in simple and binary hydrates have been analyzed, and the contributions originating from hydrogen molecules in the large cages have been separated from those in the small cages. A theoretical model, consisting in rigid cages enclosing interacting hydrogen molecules, has been exploited to calculate, on the basis of quantum mechanics, the Raman intensity of the rotational transitions for up to two interacting molecules in one cage. A comparison with experiment leads to a clear interpretation of sidebands appearing in the Raman rotational lines. The quantitative agreement between theory and experiment obtained in some cases clarifies the importance of the choice of the interaction potential, and of the proton disorder in the clathrate crystal.

Polarizabilities of the rare-gas homonuclear diatoms

We present a simple, analytical model of the interaction-induced polarizability of the homonuclear rare-gas diatoms which contains only one free parameter. Electrostatic and exchange contributions to the pair polarizability are derived under the assumption that the electronic properties of each atom can be simulated by a single effective electron whose average distance from the nucleus is determined by the atomic polarizability. The dipole-induced dipole contribution at short internuclear distances is damped as appropriate for any realistic model. The model permits the reproduction of the available measurements of the interaction-induced, binary depolarized Raman spectra of the rare gases.

Depolarized Rayleigh scattering in normal and para‐hydrogen

An expression for the shape of pressure‐broadened rotational lines is obtained by perturbation techniques which account for the details of binary collisions. This expression is used for a numerical computation of the depolarized far wing of the hydrogen Q0(1) line, a Raman line at zero frequency shift, at moderate gas density (1–100 amagat). Agreement of the computed profile with an extraction of this line wing from recent measurements of the depolarized Rayleigh spectra of para‐ and normal hydrogen is reported in absolute intensity units. The depolarized, collision induced Rayleigh spectra, a background present in these measurements, is shown to be very nearly the same in para‐ and normal hydrogen.

The role of the anisotropic interaction on collision induced absorption of systems containing linear molecules: The CO2–Ar case

Numerical calculations are presented of spectral moments of collision induced absorption (CIA) coefficient of CO2–Ar mixtures at various temperatures. Three spherical components have been used for the induced dipole moment all of which arise from pure multipolar induction. The calculations have been performed with six different potentials which differ with respect to the amount of anisotropy. The comparison of experimental and numerical results shows that the potential proposed by Preston and Pack [J. Chem. Phys. 66, 2480 (1977)] is the most suitable to describe CIA spectra. In addition, it is shown that the role of the anisotropy of the potential with regard to its contribution to the spectral moments, particularly the amount of mixing of dipole components of different symmetry, cannot be neglected.

Comment on the detailed balance symmetrization procedure of classical spectra

By means of the Wigner theory we derive an expression for the first approximation with respect to ħ of the spectrum related to a general correlation function and its zero-th moment. We then show that the symmetrization of spectra with simply the application of the detailed balance term to the classical spectra is valid in general only for ω ≪ 2kT/ħ .

Far-infrared absorption by collisionally interacting nitrogen and methane molecules

Quantum line shape calculations of the rototranslational enhancement spectra of nitrogen-methane gaseous mixtures are reported. The calculations are based on a recent theoretical dipole function for interacting N(2) and CH(4) molecules, which accounts for the long-range induction mechanisms: multipolar inductions and dispersion force-induced dipoles. Multipolar induction alone was often found to approximate the actual dipole surfaces of pairs of interacting linear molecules reasonably well. However, in the case of the N(2)-CH(4) pair, the absorption spectra calculated with such a dipole function still show a substantial intensity defect at the high frequencies (>250 cm(-1)) when compared to existing measurements at temperatures from 126 to 297 K, much as was previously reported.