Gas-phase infrared photodissociation spectroscopy of cationic polyaromatichydrocarbons

Infrared spectra of gas-phase cationic naphthalene, phenanthrene, anthracen e, and pyrene are recorded in the 500-1600 cm(-1) range using multiphoton d issociation in an ion trap. Gas-phase polyaromatic hydrocarbons are photoio nized by an excimer laser and stored in a quadrupole ion trap. Subsequent i nteraction with the intense infrared radiation of a free electron laser tha t is tuned in resonance with an infrared-allowed transition of the ion lead s to sequential multiphoton absorption facilitated by rapid intramolecular vibrational redistribution. Absorption of more than 50-100 infrared photons raises the internal energy to above the dissociation threshold, leading ev entually to fragmentation of the ion. Mass selective detection of the catio nic species stored in the trap yields the infrared absorption spectrum of t he parent ion.