  We present experimental data on H2 formation processes on gas-phase
polycyclic aromatic hydrocarbon (PAH) cations. This process was studied by
exposing coronene radical cations, confined in a radio-frequency ion trap, to
gas phase H atoms. Sequential attachment of up to 23 hydrogen atoms has been
observed. Exposure to atomic D instead of H allows one to distinguish
attachment from competing abstraction reactions, as the latter now leave a
unique fingerprint in the measured mass spectra. Modeling of the experimental
results using realistic cross sections and barriers for attachment and
abstraction yield a 1:2 ratio of abstraction to attachment cross sections. The
strong contribution of abstraction indicates that H2 formation on interstellar
PAH cations is an order of magnitude more relevant than previously thought.
