The absorption due to pairs of H2 molecules is an important opacity source in the atmospheres of various types of planets and cool stars, such as late stars, low-mass stars, brown dwarfs, certain white dwarfs, etc., and therefore of special astronomical interest [13]. The emission spectra of cool white dwarf stars differ signicantly from the expected blackbody spectra of the cores, due to collision-induced absorption by collisional complexes of hydrogen and helium in the stellar atmospheres. In order to model the radiative processes in these atmospheres, which have temperatures of several thousand kelvin, one needs accurate knowledge of the induced dipole and potential energy surfaces of collisional complexes such as H2-H2. These come from quantum-chemical calculations with the H2 bonds stretched or compressed far from equilibrium
length. Since no measurements of the collision-induced absorption for these high temperatures exist, one has to undertake ab initio calculations which take into account the high vibrational excitations of the hydrogen molecules. However, before one attempts to proceed to higher temperatures where no laboratory measurements exist it is good to know that the formalism is correct
and reproduces the results at temperatures where measurements exist. Therefore, in order to make sure that the calculations are reliable one compares the results of the calculations with existing laboratory measurements where
possible before proceeding to higher temperatures.
Molecular hydrogen has always played a special role in the collision-induced spectroscopies. The rotational transition frequencies of H2 are widely
separated so that translational, rotational and vibrational induced spectral bands can be studied separately. Moreover, the H2 molecule has a small anisotropy of the intermolecular interactions which may often be ignored in
first order approximations. In general hydrogen gas is a mixture of para- and ortho-hydrogen. Para-hydrogen at sufficiently low temperature is not rotationally excited and is therefore an isotropic system. However, the anisotropy can be turned on and of by raising and lowering the temperature, because the ratio of para- to ortho-hydrogen depends on the temperature. What is even more, roughly 90% of all the known matter in the universe is hydrogen, in the ionized, atomic or molecular states, which makes hydrogen one of the most important species in astrophysics. The hydrogen molecule is non-polar, and
some of the most important spectra in the near and far infrared and microwave region are collision-induced, due to H2-H2 complexes.
At the temperature of 297.5K measurements of the collision-induced absorption spectra of H2-H2 gas are reported in the frequency range from 1900
to 2260cm^{-1} [9]. The gas densities for these measurements ranged from 51 to 610 amagat. These measurements were compared with ab initio calculations
of the absorption. For these calculations the isotropic potential approximation
was used. In contrast to previous ab initio calculations [9] agreement between calculations and measured spectra is now observed over the full frequency range considered. A major difference to the earlier calculations is that in this work new dipole and potential energy surfaces were used.
Furthermore, measurements exist of the fundamental band and first and second overtone of H2 in dense hydrogen gas. They have been compared with ab initio calculations based on the new method. Over the full range of
frequencies considered the agreement between calculations and measurements is remarkable. This work demonstrates that the new method is capable of reproducing the measured spectra where those exist with high accuracy, and
predicts reliable opacities where no laboratory measurements exist. / text
| Identifer | oai:union.ndltd.org:UTEXAS/oai:repositories.lib.utexas.edu:2152/ETD-UT-2009-08-235 |
| Date | 2009 August 1900 |
| Creators | Abel, Martin Andreas |
| Source Sets | University of Texas |
| Language | English |
| Detected Language | English |
| Type | thesis |
| Format | application/pdf |
Page generated in 0.0017 seconds