Lothar Frommhold

Collision-induced rototranslational absorption spectra of CH4-CH4 pairs at temperatures from 50 to 300 K

The zeroth, first, and second spectral moments of the rototranslational collision-induced absorption (RT CIA) spectra of hydrogen-helium mixtures are calculated from the fundamental theory, for temperatures from 40 to 3000 K. With the help of simple analytical functions of three parameters and the information given, the RT CIA spectra of H/sub 2/-He pairs can be generated on computers of small capacity, with rms deviations from exact quantum profiles of not more than a few percent. Such representations of the CIA spectra are of interest for work related to the atmospheres of the outer planets and cool stars. The theoretical spectra are in close agreement with existing laboratory measurements at various temperatures from about 77 to 3000 K. 28 references.

Modeling of pressure-induced far-infrared absorption spectra Molecular hydrogen pairs

Meyer et al. (1985) have calculated the accurate induced dipole moment function of H2-H2 from first principles, using highly correlated wave functions for the first time in such work. The present paper is concerned with the collision-induced translational-rotational absorption coefficient for molecular hydrogen pairs, taking into account computations on the basis of the fundamental theory considered by Meyer et al. Data have been obtained for temperatures in the range from 40 to 300 K. Criteria are developed for choosing among various model line shapes. It is found that certain models are capable of approximating the quantum profiles closely, with rms errors of only a few percent.

Theoretical collision-induced rototranslational absorption spectra for the outer planets - H2-CH4 pairs

The modeling of the collision-induced rototranslational absorption spectra for H2-CH4 molecules is described. The intermolecular potentials of the molecular complex are analyzed. The absorption coefficient is dependent on the molecular structure and the line-shape formulas for induction by hydrogen and methane are provided. Dipoles induced by an electric multipole field are examined. The measurements of the collision-induced absorption spectrum of the H2-CH4 system at 195 and 297 K are fitted to the Sutter et al. data. The relation between spectral components and H2-CH4 intermolecular potential are studied using the Hanley-Klein potential model (1972). Absorption coefficients of H2-CH4 are presented. 51 references.

The collision induced spectroscopies concerning the desymmetrization of classical line shape

Classical spectral line shapes are symmetric and easy to compute. Measured broad profiles, on the other hand, are asymmetric (principle of detailed balance) and can be reproduced from a much more involved quantum theory. Various desymmetrization procedures are known which, when applied to classical profiles, result in a line shape that is formally consistent with the asymmetry required by detailed balance. It is an interesting fact that various desymmetrization procedures known give rise to very different profiles. Little is known about which (if any) results in a simple but accurate substitute for the quantum shape. An ab initio quantum calculation is in close agreement with a measurement of the binary collision induced, translational Raman spectrum of helium at 30 amagat. To a classical line shape based on the same input, four common desymmetrization procedures are applied. The comparison shows that the Egelstaff procedure simulates the quantum profile most closely and should have broad applications in ...

New H2 collision-induced absorption and NH3 opacity and the spectra of the coolest brown dwarfs

We present new cloudy and cloudless model atmospheres for brown dwarfs using recent ab initio calculations of the line list of ammonia (NH3) and of the collision-induced absorption of molecular hydrogen (H2). We compare the new synthetic spectra with models based on an earlier description of the H2 and NH3 opacities. We find a significant improvement in fitting the nearly complete spectral energy distribution of the T7p dwarf Gliese 570D and in near-infrared color-magnitude diagrams of field brown dwarfs. We apply these new models to the identification of NH3 absorption in the H-band peak of very late T dwarfs and the new Y dwarfs and discuss the observed trend in the NH3-H spectral index. The new NH3 line list also allows a detailed study of the medium-resolution spectrum of the T9/T10 dwarf UGPS J072227.51–054031.2 where we identify several specific features caused by NH3.

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.

Rare gas diatom polarizabilities

Large scale self‐consistent field (SCF) computations of the polarizabilities of the rare gas symmetric diatoms were recently reported, the effect of electron correlation was evaluated for the He2 and Ne2 systems, and a new procedure proposed of scaling SCF results to approximate diatom polarizabilities. In this work, these new results are used to compute from an exact wave mechanical theory binary collision‐induced Raman spectra of the rare gases for comparison with recent measurements. Depolarized spectra show agreement typically within the small experimental uncertainties and over a range of intensities that may exceed 100 000:1. Computed and measured polarized spectra similarly agree, but with lesser precision. The scaling of SCF polarizabilities appears to be remarkably successful. Uncertainties arising from the use of semiempirical potentials are not always negligible.

Absolute cross sections for collision-induced scattering of light by binary pairs of argon atoms

Absolute intensities of the collision-induced Raman spectrum of argon are measured relative to certain rotational Raman lines of known intensity in hydrogen and nitrogen. Using the 4880 A laser line for excitation and a detector sensitive to both polarizations, at a frequency shift of -12 wavenumbers, the cross section times unit volume and per unit wavenumber band, for scattering of light at right angles by binary pairs of argon atoms, is found to be 2·89 × 10-53 cm6±7 per cent. Rigorous wave-mechanical computations on the basis of Placzeks polarizability theory predict a value of 2·83 × 10-53 cm6±2 per cent and reproduce the experimental spectrum from about 6 to 60 wavenumbers, if the empirical MSV III potential is used together with the point-dipole model of the anisotropy of the polarizability tensor of two interacting atoms. For comparison, computations based on the rigorous classical trajectories of the collisions are presented. If the same potential function and model of the anisotropy are employe...

A new computation of the infrared absorption by H2 pairs in the fundamental band at temperatures from 600 to 5000 K

A new computation of the absorption spectrum of H2 pairs in the fundamental band is given for temperatures from 600 to 5000 K. It is based on advanced interaction potentials and recent induced dipole components which were shown to be consistent with the existing laboratory measurements of low-temperature absorption spectra. The absorption is greater by factors of 2-3 than previous estimates and show a different band profile at the higher temperatures. 11 refs.

Collision‐induced dipoles of rare gas mixtures

New ab initio calculations of the collision‐induced dipole moment of the rare gas systems He–Ar, Ne–Ar, Ne–Kr, and Ar–Kr are obtained on the basis of a molecular Hartree–Fock treatment. With these and recent potential functions the spectral moments and line shapes of collision‐induced absorption spectra are computed. Reasonable agreement with existing measurements is observed for the first time for the system Ne–Ar and Ne–Kr.