Infrared spectroscopy of jet-cooled cationic polyaromatic hydrocarbons: Naphthalene

We present the infrared (IR) absorption spectrum of the jet-cooled naphthal ene-Ar van der Waals cluster cation in the 400-1600 cm(-1) range, which is expected to resemble closely the IR spectrum of the bare naphthalene cation . Cluster cations are produced in their ground states via resonance-enhance d two-color ionization. Subsequent irradiation of the ions with IR light of a free-electron laser causes cluster dissociation, if this light is resona nt with IR-active transitions of the cationic cluster. Mass-selective measu rement of the dissociated fraction as a function of IR wavelength yields th e IR spectrum of the cationic duster. Comparison with previously reported A r matrix isolation spectra of naphthalene(+) shows good agreement, with mor e bands observed in the present study. The IR spectrum observed is compared to a calculated spectrum of the naphthalene cation, allowing for unambiguo us assignments of all the observed modes. The experimental method presented is generally applicable to obtain laboratory IR spectra of ionized polyaro matic hydrocarbons under astrophysically relevant conditions.