Hydrogen formation in PDRs: Laboratory studies on recombinative hydrogen
desorption from carbonaceous materials.
Saoud BAOUCHE
IFA Aarhus Dk \& LAMAp/LERMA
Résumé :
The understanding of hydrogen molecule formation on bare carbonaceous
grain surfaces at the temperatures found in the photodissociation
regions (PDR) of interstellar molecular clouds has been boosted by
the discovery that hydrogen atoms with high translational energies
($>2000$K) can bind to graphite surfaces in the chemisorption state.
In the same study it was found that this state is stable at room
temperature on a laboratory time scale, and that hydrogen molecules
desorb recombinatively with a first order kinetics mechanism when
the surface is heated.
We have performed laboratory studies on the dynamics and kinetics
of hydrogen molecules formation on carbonaceous materials (graphite,
single wall carbon nanotubes, polycyclic aromatic hydrocarbons and
graphene) in the chemisorption regime. In our laser assisted associative
desorption (LAAD) experiments we find a high translational energy
distribution for the desorbing molecules, as a result of the strong
exothermicity of the recombination reaction. We will also present
temperature programmed desorption and angular distribution measurements,
including desorption from defect sites. Isotope effects have also been
studied. All results will be discussed in terms of the energy barriers
involved and will be compared to ab-initio calculations.
Scanning tunnelling microscopy images on atomic hydrogen adsorbate
structure on graphene and graphite will be compared.
