Spectroscopy of Polycyclic Aromatic Hydrocarbon Cations and Complexes Martin Vala, Department of Chemistry, University of Florida, Gainesville, FL 32611 The formation of the most abundant molecule in the interstellar medium (ISM), H2, has long been thought to occur on the surface of dust grains. But recently several groups have proposed that polycyclic aromatic hydrocarbons (PAHs) could act as catalysts for its formation. In the past year we have investigated a prototypical PAH, 1,2-dihydronaphthalene, in its gaseous protonated form, via infrared multiphoton dissociation spectroscopy and discovered that it dissociates along two independent pathways, one of which yields the hydrogen molecule. Extensive density functional calculations have pinpointed the most probable site of protonation and the mechanism of dehydrogenation. This work led us to propose a new mechanism for H2 formation in the ISM. The model involves hydrogenation of a PAH cation to produce one or more aliphatic hydrogen-bearing carbons on the PAH framework followed by photolytic excitation and eventual ejection of molecular hydrogen.