Laboratory evidence for the non-detection of excited nascent H2 in dark clouds

There has always been a great deal of interest in the formation of H-2 as well as in the binding energy released upon its formation on the surface of dust grains. The present work aims at collecting experimental evidence for how the bond energy budget of H-2 is distributed between the reaction site and the internal energy of the molecule. So far, the non-detection of excited nascent H-2 in dense quiescent clouds could be a sign that either predictions of emission line intensities are not correct or the de-excitation of the newly formed molecules proceeds rapidly on the grain surface itself. In this Letter, we present experimental evidence that interstellar molecular hydrogen is formed and then rapidly de-excited on the surface of porous water ice mantles. In addition, although we detect ro-vibrationally excited nascent molecules desorbing from a bare non-porous (compact) water ice film, we demonstrate that the amount of excited nascent hydrogen molecules is significantly reduced no matter the morphology of the water ice substrate at 10 K (both on non-porous and on porous water ice) in a regime of high molecular coverage as is the case in dark molecular clouds.