Infrared absorption spectra by H-2-He collisional complexes: The effect ofthe anisotropy of the interaction potential

As an extension of previous work which was based on the isotropic interacti on approximation, absorption spectra in the rotational and fundamental band s of H-2, induced by collisions with He, are calculated by numerical integr ation of the close-coupled Schrodinger equation to account for the anisotro py of the interaction potential. A refined quantum chemical dipole surface of interacting H-2-He pairs is also obtained with an extended grid of molec ular geometries. This dipole surface agrees generally well with previous re sults, but is smaller by about 5% in the isotropic overlap term which is si gnificant only in the fundamental band. The effects of the anisotropy of th e interaction are to reduce the peak intensities of the Q and S lines by ro ughly 10% and to increase absorption in the far wings by a similar amount. The accuracy of the dipole surface as well as that of the ab initio interac tion potential that enters the calculations of the spectra are believed to permit the prediction of absolute spectral intensities with an accuracy of about +/- 5%. Comparisons with the available measurements show very good ag reement of the shapes of the spectral profiles, but the absolute intensitie s differ by up to 10% in some cases. These remaining differences between th eory and measurements appear to be random and are generally smaller than th e differences among comparable measurements. Our results should therefore p rovide a reliable basis for predicting absorption by H-2-He pairs for tempe ratures and frequencies for which no laboratory measurements exist. This fa ct is of a special interest, for example, for the spectroscopic analyses of the atmospheres of the outer planets. (C) 2000 American Institute of Physi cs. [S0021-9606(00)01333-7].