Yechiel Frank

Interaction of Atomic and Molecular Hydrogen with Tholin Surfaces at Low Temperatures

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 Titans 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.

Influence of atomic modeling on integrated simulations of laser-produced Au plasmas.

Time-integrated x-ray emission spectra of laser-irradiated Au disks were recorded using transmission grating spectrometry, at laser intensities of 10(13) to 10(14) W/cm(2). Radiation-hydrodynamics and atomic physics calculations were used to simulate the emitted spectra. Three major plasma regions can be recognized: the heat wave, the corona, and an intermediate region connecting them. An analysis of the spectral contribution of these three plasma regions to the integrated recorded spectrum is presented. The importance of accurate atomic modeling of the intermediate plasma region, between the corona and the heat wave, is highlighted. The influence of several aspects of the atomic modeling is demonstrated, in particular multiply-excited atomic configurations and departure from local thermal equilibrium.