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Abstract

We present the infrared (IR) absorption spectrum of the jet-cooled naphthalene-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-enhanced two-color ionization. Subsequent irradiation of the ions with IR light of a free-electron laser causes cluster dissociation, if this light is resonant with IR-active transitions of the cationic cluster. Mass-selective measurement of the dissociated fraction as a function of IR wavelength yields the IR spectrum of the cationic cluster. Comparison with previously reported Ar matrix isolation spectra of naphthalene⁺ shows good agreement, with more bands observed in the present study. The IR spectrum observed is compared to a calculated spectrum of the naphthalene cation, allowing for unambiguous assignments of all the observed modes. The experimental method presented is generally applicable to obtain laboratory IR spectra of ionized polyaromatic hydrocarbons under astrophysically relevant conditions.