Competing Mechanisms of Molecular Hydrogen Formation in Conditions Relevant to the Interstellar Medium

Abstract

The most efficient mechanism of the formation of molecular hydrogen in the current universe is by association of hydrogen atoms on the surface of interstellar dust grains. The details of the processes of its formation and release from the grain are of great importance in the physical and chemical evolution of the space environments where it takes place. The main puzzle is still the fate of the 4.5 eV released in H 2 formation and whether it goes into internal energy (rovibrational excitation), translational kinetic energy, or heating of the grain. The modality of the release of this energy affects the dynamics of the ISM and its evolution toward star formation. We present results of the detection of the rovibrational states of the just-formed H 2 as it leaves the surface of a silicate. We find that rovibrationally excited molecules are ejected into the gas phase immediately after formation over a much wider range of grain temperatures than anticipated. Our results can be explained by the presence of two mechanisms of molecule formation that operate in partially overlapping ranges of grain temperature. A preliminary analysis of the relative importance of these two mechanisms is given. These unexpected findings, which will be complemented with experiments on the influence of factors such as silicate morphology, should be of great interest to the astrophysics and astrochemistry communities.

Sticking coefficient of hydrogen and deuterium on silicates under interstellar conditions

“…On the other hand, according to Amiaud et al (2007) and Fillion et al (2009), results for amorphous water ices, the molecular saturation of the compact np-ASW ice surface occurs close to 0.5 ML exposure of D 2 at 10 K (1 ML = 10 15 molecules cm −2 ), corresponding to a few seconds D 2 deposition time, whereas p-ASW ice surface with a wide distribution of adsorption sites requires longer exposure times and a higher saturation dose of D 2 (about 4 ML). The compact structure of our silicate sample has also been confirmed by the King and Wells experiments (described in the next later section), which indicate a gradual saturation of the silicate surface after ∼100 s irradiation of D 2 at 10 K as in the case of the np-ASW ice film (Matar et al 2010, Fig. 3) for the same amount of D 2 flux ∼9 × 10 12 molecules cm −2 s −1 (Lemaire et al 2010). The hydrogen and deuterium molecules are introduced into the UHV chamber via a triple differentially pumped molecular beam line aimed at the sample holder at an incidence angle of 62 • .…”

“…With a silicate surface at a higher temperature (36 K) and a beam temperature of 50 K, it has been shown (Lemaire et al 2010, Fig. 3) that a steady state is immediately reached as soon as the beam is aimed at the surface because of the low residence time of the molecules at high surface temperatures.…”

“…The sticking of hydrogen atoms is by far harder to solve experimentally, because (a) atoms that are stuck can react and form molecules, this process leads to some difficulties for unbiased measurements within our experimental setup, (b) at the same time it is also known that the already adsorbed molecules play a role in the sticking and the dynamic recombination (Schutte et al 1976;Govers et al 1980;Govers 2005;Lemaire et al 2010). Consequently, the sticking coefficient of hydrogen atoms cannot be directly measured as for molecules (H 2 and D 2 ).…”

CO2FORMATION IN QUIESCENT CLOUDS: AN EXPERIMENTAL STUDY OF THE CO + OH PATHWAY

“…Briefly, the experimental setup consists of an ultrahigh-vacuum chamber (base pressure ∼10 −10 mbar), containing an amorphous silicate-coated copper surface (5-400 K; Lemaire et al 2010). Molecules are dosed onto the surface via two triply differentially pumped beam lines.…”

Free-electron laser induced processes in thin molecular ice

“…In particular the formation of water and of molecular hydrogen is believed to take place under such conditions. [3][4][5][6][7][8] Inner cores of large molecular clouds in the interstellar medium (ISM) are largely opaque to radiation in the visible, ultraviolet (UV), and vacuum ultraviolet (VUV) spectral range, and only partially transparent to IR radiation. 9 Although only a low level of VUV and extreme ultraviolet (XUV) radiation is present 10 due to the interaction of cosmic rays with the constituents of the clouds, its impact on the desorption of simple and complex molecules is relevant.…”