Interactions of Atomic and Molecular Hydrogen with a Diamond-like Carbon
  Surface: H2 Formation and Desorption
  The interactions of atomic and molecular hydrogen with bare interstellar dust
grain surfaces are important for understanding H2 formation at relatively high
temperatures (>20 K). We investigate the diffusion of physisorbed H atoms and
the desorption energetics of H2 molecules on an amorphous diamond-like carbon
(DLC) surface. From temperature-programmed desorption experiments with a
resonance-enhanced multiphoton ionization (REMPI) method for H2 detection, the
H2 coverage-dependent activation energies for H2 desorption are determined. The
activation energies decrease with increasing H2 coverage and are centered at 30
meV with a narrow distribution. Using a combination of photostimulated
desorption and REMPI methods, the time variations of the surface number density
of H2 following atomic and molecular hydrogen depositions are studied. From
these measurements, we show that H2 formation on a DLC surface is quite
efficient, even at 20 K. A significant kinetic isotope effect for H2 and D2
recombination reactions suggests that H-atom diffusion on a DLC surface is
mediated by quantum mechanical tunneling. In astrophysically relevant
conditions, H2 recombination due to physisorbed H-atoms is unlikely to occur at
20 K, suggesting that chemisorbed H atoms might play a role in H2 formation at
relatively high temperatures.
