Physical Adsorption of Hydrogen on Interstellar Graphite Grain Surfaces
We review existing single-particle theories concerning parameters of importance which determine the kinetics of hydrogen molecule formation and ejection from cold (T gr≲20 K) graphite grain surfaces. The nature of the single-particle quantum states of low mass gas atoms and molecules in a periodic surface lattice potential is considered. Contributions to the physical adsorption potential due to dynamic polarizability effects arising from the long-range collective valence-electron charge-density oscillations (plasmons) of the substrate are discussed. Short-range electron correlation effects at the surface may lead to the formation of a ‘quasimolecular state’ of adsorbed H2 with a bond length ∼3.5 Å and a reduced bond energy ∼0.075 eV. It is proposed, that one consequence of this dynamical screening of the adsorbed molecules is that they are ejected normal to the grain surface with velocities ≲20 km s−1 and not necessarily in a high vibrational state. Similar dynamical effects could be important in determining activation processes and long-range ordering in monolayer films of adsorbed H2. The astrophysical consequences of these many-body effects are discussed in the light of recent experimental and observational results.
KeywordsKcal Mole Physical Adsorption Adsorption Energy Graphite Surface Adsorption Potential
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- Hagen, D. E., Novaco, A. D., and Milford, F. J.: 1971, Proc. Symp. on Adsorption-Desorption Phenomena, Florence, Italy, Academic Press, New York, 1972.Google Scholar
- Herzberg, G.: 1950, Molecular Spectra and Molecular Structure, Vol. I, Van Nostrand, New York.Google Scholar
- Ross, S. and Oliver, J. P.: 1964, On Physical Adsorption, Interscience, New York.Google Scholar
- Van de Hulst, H. C.: 1949, Rech. Astron. Obs. Utrecht 11, Part 2.Google Scholar