  There has always been a great deal of interest in the formation of H2 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 H2 is distributed between the reaction site and the
internal energy of the molecule. So far, the non-detection of excited nascent
H2 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.
