  We have studied the microscopic dynamics of hydrogen adsorbed to bundles of
single walled carbon nanotubes using inelastic neutron scattering. Evidence is
obtained for much higher storage capacities in chemically treated compared to
as prepared material. This indicates an additional adsorption layer inside the
tubes. Well pronounced excitations in the H2 spectrum at low energies confirm
this conclusion. The desorption of hydrogen is monitored in real time as a
function of temperature. Hydrogen storage is highly stable below 150 K in
agreement with the harmonic evolution of the hydrogen spectrum, which indicates
a strong binding potential. Above 200 K hydrogen can be released in a
controlled way by simple heating. The excitation spectrum changes significantly
during the release. Remnants of hydrogen persist up to 400 K.
