| 3. |
- Abel, Martin, et al.
(författare)
-
Collision-induced absorption by supermolecular complexes from a new potential energy and induced dipole surface, suited for calculations up to thousands of kelvin
- 2010
-
Ingår i: 20th International Conference on Spectral Line Shapes. - Melville, NY : American Institute of Physics (AIP). - 9780735408456 ; , s. 251-257
-
Konferensbidrag (refereegranskat)abstract
- Absorption by pairs of H2 molecules is an important opacity source in the atmospheres of the outer planets, and thus of special astronomical interest. The emission spectra of cool white dwarf stars differ significantly from the expected blackbody spectra, amongst other reasons due to absorption by H2-H2, H2-He, and H2-H collisional complexes in the stellar atmospheres. To model the radiative processes in these atmospheres, which have temperatures of several thousand kelvin, one needs accurate knowledge of the induced dipole (ID) and potential energy surfaces (PES) of such collisional complexes. These come from quantum-chemical calculations with the H2 bonds stretched or compressed far from equilibrium. Laboratory measurements of collision-induced (CI) absorption exist only at much lower temperature. For H2 pairs at room temperature, the calculated spectra of the rototranslational band, the fundamental band, and the first overtone match the experimental data very well. In addition, with the newly obtained IDS it became possible to reproduce the measurements in the far blue wing of the rototranslational spectrum of H2 at 77.5 K, as well as at 300 K. Similarly good agreement between theory and measurement is seen in the fundamental band of molecular deuterium at room temperature. Furthermore, we also show the calculated absorption spectra of H2-He at 600 K and of H2-H2 at 2,000 K, for which there are no experimental data for comparison
|
|
| 5. |
- Li, Xiaoping, et al.
(författare)
-
Collision-induced dipoles and polarizabilities of pairs of hydrogen molecules : Ab initio calculations and results from spherical tensor analysis
- 2012
-
Ingår i: International Conference of Computational Methods in Sciences and Engineering 2009. - Melville, NY : American Institute of Physics (AIP). - 9780735411227 ; , s. 100-135
-
Konferensbidrag (refereegranskat)abstract
- New ab initio results are reported for the interaction-induced changes in the dipole moments and polarizabilities of pairs of hydrogen molecules, computed using finite-field coupled-cluster methods in MOLPRO 2000 and GAMESS, with an aug-cc-pV5Z (spdf) basis set. Earlier work by X. Li, C. Ahuja, J. F. Harrison, and K. L. C. Hunt, J. Chem. Phys. 126, 214302 (2007), on collision-induced polarizabilities Δα has been extended with 170 additional geometrical configurations of the H2 pairs. In calculations of Δα, we have used a "random field" technique, with up to 120 different field strengths, having components that range from 0.001 to 0.01 a.u. Numerical tests show that the pair dipoles Δμ can be obtained accurately from calculations limited to 6 values of the field in each direction, so this approach has been used to compute Δμ by X. Li, K. L. C. Hunt, F. Wang, M. Abel, and L. Frommhold, Int. J. Spectroscopy 2010, 371201 (2010). We have evaluated the collision-induced dipoles of H2 pairs for 28 combinations of bond lengths (ranging from 0.942 a.u. to 2.801 a.u.), 7 intermolecular separations R, and 17 different relative orientations. In our work on Δα, the bond lengths are fixed at 1.449 a.u. Our results agree well with the previous ab initio work of W. Meyer, A. Borysow, and L. Frommhold, Phys. Rev. A 40, 6931 (1989), and of Y. Fu, C. G. Zheng and A. Borysow, J. Quant. Spectroscopy and Rad. Transfer, 67, 303 (2000)-where those data exist-for Δμ of H2 pairs. For Δα, our results agree well with the CCSD(T) results obtained by G. Maroulis, J. Phys. Chem. A 104, 4772 (2000) for two pair orientations and fixed R. The pair polarizability anisotropies also agree well with the small-basis self-consistent field results of D. G. Bounds, Mol. Phys. 38, 2099 (1979), although the trace of the polarizability differs by factors of 2 or more from Bounds' results. We have determined the expansion coefficients for Δμ and Δα, expressed as series in the spherical harmonics of the orientation angles of the intermolecular vector and of unit vectors along the molecular axes. The leading coefficients converge at long range to the predictions from perturbation theory, derived by J. E. Bohr and K. L. C. Hunt, J. Chem. Phys. 87, 3821 (1987); T. Bancewicz, W. G.az, and S. Kielich, Chem. Phys. 128, 321 (1988); and X. Li and K. L. C. Hunt, J. Chem. Phys. 100, 7875 (1994); ibid, 9276 (1994). Based on our results for Δμ, we find excellent agreement for the binary rototranslational absorption spectrum of H2 at 297.5 K as calculated by X. Li, K. L. C. Hunt, F. Wang, M. Abel, and L. Frommhold, Int. J. Spectroscopy 2010, 371201 (2010) and as determined experimentally by G. Bachet, E. R. Cohen, P. Dore, and G. Birnbaum, Can. J. Phys. 61, 591 (1983), out to ∼1500 cm-1. We have also calculated the vibrational spectra out to 20,000 cm-1, at T = 600 K, 1000 K, and 2000 K, for which there are no experimental data. We are currently working to extend the temperature range in the calculations to 7000 K, for application in modeling the spectra of cool white dwarf stars. We have used the results for Δα to calculate collision-induced rototranslational Raman spectra for H2 pairs [M. Gustafsson, L. Frommhold, X. Li, and K. L. C. Hunt, J. Chem. Phys. 130, 164314 (2009)]. Experimental results for the Raman spectra have been reported by U. Bafile, M. Zoppi, F. Barocchi, M. S. Brown, and L. Frommhold, Phys. Rev. A 40, 1654 (1989); U. Bafile, L. Ulivi, M. Zoppi, F. Barocchi, M. Moraldi, and A. Borysow, Phys. Rev. A 42, 6916 (1990); and M. S. Brown, S.-K. Wang, and L. Frommhold, Phys. Rev. A 40, 2276 (1989). Agreement between our calculations and experiment is good for both the polarized and depolarized spectra, with the remaining discrepancies probably attributable to the difference between the static (calculated) and frequency-dependent (experimental) values of Δα.
|
|
