Interaction-induced dipoles of hydrogen molecules colliding with helium atoms: A new ab initio dipole surface for high-temperature applications

Abstract

We report new ab initio results for the interaction-induced dipole moments Δμ of hydrogen molecules colliding with helium atoms. These results are needed in order to calculate collision-induced absorption spectra at high temperatures; applications include modeling the radiative profiles of very cool white dwarf stars, with temperatures from 3500 K to 9000 K. We have evaluated the dipoles based on finite-field calculations, with coupled cluster methods in MOLPRO 2006 and aug-cc-pV5Z (spdfg) basis sets for both the H and He centers. We have obtained values of Δμ for eight H(2) bond lengths ranging from 0.942 a.u. to 2.801 a.u., for 15 intermolecular separations R ranging from 2.0 a.u. to 10.0 a.u., and for 19 different relative orientations. In general, our values agree well with earlier ab initio results, for the geometrical configurations that are treated in common, but we have determined more points on the collision-induced dipole surface by an order of magnitude. These results make it possible to calculate transition probabilities for molecules in excited vibrational states, overtones, and rotational transitions with ΔJ > 4. We have cast our results in the symmetry-adapted form needed for absorption line shape calculations, by expressing Δμ as a series in the spherical harmonics of the orientation angles of the intermolecular vector and of a unit vector along the H(2) bond axis. The expansion coefficients depend on the H(2) bond length and the intermolecular distance R. For large separations R, we show that the ab initio values of the leading coefficients converge to the predictions from perturbation theory, including both classical multipole polarization and dispersion effects.

Note: Computation of collision-induced absorption by dense deuterium–helium gas mixtures

“…Two of the four measurements, the red and green traces, agree closely with each other and with the theory at almost all frequencies, much as was observed in most other cases considered previously. 1,5,10 The shapes of the remaining two spectral profiles resemble theory quite closely, but the intensities are substantially larger in one case and smaller in the other case. 12 We note that near a frequency of 3000 cm −1 , the "intercollisional dip" appears in the measurements.…”

“…4,5 From the IDSs and PESs, the collisioninduced absorption spectra have been calculated and compared to laboratory measurements at infrared and microwave frequencies. 4,5 Close agreement between theory and experiment has been observed. 1 Recently, an elaborate IDS and PES were obtained for H 2 interacting with He.…”

Comparison of the Calculated Collision-Induced Absorption Spectra by Dense Hydrogen-Helium, Deuterium-Helium, and Tritium-Helium Gas Mixtures

“…These surfaces have been obtained using quantum chemical methods [7,8]. The thus computed collisioninduced absorption spectra have been compared to laboratory measurements at infrared and microwave frequencies [7][8][9]. Close agreement between theory and experiment has been observed [1].…”

“…To compute the binary collision-induced absorption spectra from the fundamental theory, the induced dipole surfaces (IDSs) and potential energy surfaces (PESs) of the binary van der Waals complexes under consideration have to be known. These surfaces have been obtained using quantum chemical methods [7,8]. The thus computed collisioninduced absorption spectra have been compared to laboratory measurements at infrared and microwave frequencies [7][8][9].…”

Intermolecular polarizabilities in H2-rare-gas mixtures (H2–He, Ne, Ar, Kr, Xe): Insight from collisional isotropic spectral properties

“…[36][37][38][39][40] To the best of our knowledge none of the remaining pairs, with Rg = Ne, Kr, and Xe, have received much attention in reports on collisional absorption yet published. Besides, a series of articles devoted to the so-called hyper-Rayleigh collisional scattering have appeared in which a comparative study on the evolution of collisional properties (hyperpolarizabilities in this case) within the whole set of the H 2 -Rg compounds is discussed.…”