Rotationally resolved ultraviolet spectrum of the benzene–Ar complex by mass-selected resonance-enhanced two-photon ionization

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Abstract
High resolution laser excitation was combined with the technique of mass-selected two-photon ionization via a resonant intermediate state to measure rotationally resolved UV spectra of benzene–Ar van der Waals clusters. When the second laser pulse in the two color experiment is delayed by 7 ns no line broadening due to the second ionizing absorption step is observed. Spectra of three vibronic bands in the S1 ←S0 transition of benzene (h6)–Ar and benzene (d6)–Ar were measured yielding a line spectrum with a linewidth of 130 MHz. Resolution is sufficient to demonstrate that no asymmetry splitting of the rotational lines occurs and the spectrum is to a high precision that of a symmetric rotor. A detailed analysis of the rotational structure yields an accurate set of rotational constants. We find that the Ar is located on the C6 rotational axis. Its distance from the benzene ring plane is 3.582 Å in the electronic ground state and decreases by 59±3 mÅ in the electronically excited state due to the increased polarizability of the benzene molecule after electronic excitation.