Abstract
We study the interaction of atomic and molecular hydrogen with a surface of tholin, a man-made polymer considered to be an analogue of aerosol particles present in Titan's atmosphere, using thermal programmed desorption at low temperatures below 30 K. The results are fitted and analyzed using a fine-grained rate equation model that describes the diffusion, reaction, and desorption processes. We obtain the energy barriers for diffusion and desorption of atomic and molecular hydrogen. These barriers are found to be in the range of 30-60 meV, indicating that atom/molecule-surface interactions in this temperature range are dominated by weak adsorption forces. The implications of these results for the understanding of the atmospheric chemistry of Titan are discussed.
Highlights
In the last decade, the study of the interaction of hydrogen with surfaces at low temperatures has become a topic of interest in fields as different as hydrogen storage[1] and interstellar chemistry, where molecular hydrogen forms on the surfaces of dust grains.[2,3] In the latter field, there are several laboratories studying the mechanisms of reaction of molecular hydrogen in various spacelike environments
The typical cross section is expected to be of the order of a few Å2.28–30 as already mentioned in the Introduction, a much smaller value of ∼ 3 × 10−2 Å2 was found on H-loaded amorphous carbon.[12]
In a rate equation model, we cannot employ a continuous distribution of energies, and neither can we model a particular realization of surface site energies. (Note that a continuous distribution of binding energies can be obtained by direct inversion of temperature programmed desorption (TPD) traces,[10,42,43] but this method does not include the possibility of simultaneous recombination processes.) We approximate the distribution of binding energies of a certain site type α around the mean value by using 21 different binding energies as samples for each type
Summary
The study of the interaction of hydrogen with surfaces at low temperatures has become a topic of interest in fields as different as hydrogen storage[1] and interstellar chemistry, where molecular hydrogen forms on the surfaces of dust grains.[2,3] In the latter field, there are several laboratories studying the mechanisms of reaction of molecular hydrogen in various spacelike environments. Titan’s atmosphere is composed mostly of diatomic nitrogen (> 95%) and methane.[14] The dissociation of methane and nitrogen in the upper atmosphere creates radicals that eventually aggregate in macroscopic particles that form the well-known brownish haze that surrounds Titan. Information on this haze is limited because of the difficulty of obtaining data from ground observatories or space probes that would reveal its chemical structure.[15] Over the years, starting with the seminal work of Sagan and Khare,[16] analogues of those particles, called tholins, have been produced and characterized in many laboratories,[17,18,19] and they were found to reproduce the optical signature of Titan’s haze. We are interested in the addition or removal of hydrogen via the hydrogenation and abstraction of C−C and C−N double and triple bonds.[13]
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