Joseph E. Roser

Laboratory measurements of molecular hydrogen formation on amorphous water ice

We report on an experimental study of the formation of hydrogen molecules by surface recombination of adsorbed H atoms on amorphous water ice under conditions closely simulating those encountered in astrophysical environments. Our results show that hydrogen recombination via surface reactions on icy mantles on grains is able to account for H2 formation in dense cloud environments.

Formation of Molecular Hydrogen on Amorphous Water Ice: Influence of Morphology and Ultraviolet Exposure

In this paper, we report on the formation of molecular hydrogen on different types of amorphous water ice. We show that mass spectra of desorbing molecules upon formation are sensitive to the way in which ice is deposited on a cold substrate, to its thermal history, and to the action of UV photons. Implications that these results bear on H2 formation in dense quiescent clouds are presented and discussed. Subject headings: astrochemistry — dust, extinction — ISM: molecules — methods: laboratory — molecular processes The formation of the hydrogen molecule, the most abundant in the universe, is one of the fundamental processes occurring in the interstellar medium. It has been recognized that it cannot form efficiently in the gas phase, because upon formation the release of the energy excess via radiative decay is not allowed by selection rules, and that the role of dust grains as catalysts is crucial to explain its abundance.

Measurement of the Kinetic Energy of Hydrogen Molecules Desorbing from Amorphous Water Ice

A hydrogen molecule that is formed on an interstellar grain might retain some of the 4.48 eV of energy that is released in the recombination reaction of two hydrogen atoms. We set up an experiment to measure the translational (kinetic) energy of hydrogen molecules after they are formed on and are ejected from the surface of an interstellar dust grain analog. Here we report the first measurements of the kinetic energy of molecular deuterium as it leaves the surface of an amorphous water sample. The astrophysical implications of such measurements are discussed.

A Summary of Experimental Results on Molecular Hydrogen Formation on Dust Grain Analogues

We review the main laboratory results of investigations of processes of molecular hydrogen formation on surfaces. The problem of the formation of molecular hydrogen is a fundamental issue in astrophysics/astrochemistry, because of the great importance that molecular hydrogen has for the structure and evolution of our Universe. Such experiments are done using ultra-high vacuum, low temperature, and atomic/molecular beam techniques to study the formation of molecular hydrogen on dust grain analogues in conditions as close as technically feasible to the ones present in relevant ISM environments. In experiments conducted at Syracuse University, we studied H 2 formation on the three most ISM-relevant classes of surfaces: silicates, carbonaceous materials and amorphous water ice. Our experimental investigations range from the evaluation of the catalytic efficiency of the studied surfaces to the energetics of the reaction, i.e. the partition of the formation energy between the grain and the nascent molecule. Such measurements have been done by changing various parameters such as: the temperature of the interstellar dust analogue, the kinetic temperature of the atoms, the morphology of the surface and, to be completed soon, the composition of the solid. Quantitative and qualitative information on the processes of H 2 formation is then fed in theoretical models to extract results that pertain to desired ISM environments.

Dust Chemistry in the Laboratory

The presence of dust grains in space is of fundamental importance for the emergence of chemical complexity in the Interstellar Medium (ISM) because grains may catalyze chemical reactions on their surfaces enabling the formation of species that cannot be produced easily in the gas phase, such as H2 [1, 2, 3, 4, 5] and CO2 [6].