Structure of the Benzene-Argon Complex in the Gas Phase at 3 Kelvin

New spectroscopic data of large molecules can be obtained when one expands the molecules in a seeded hypersonic jet [1]. The efficient cooling of the rotational temperature to a few Kelvin eliminates the spectral congestion at room temperature and allows for separation of vibrational bands. In addition, when one excites the molecules with a high resolution laser single rotational state analysis can be performed [2], which is not possible at room temperature. I t is known that molecules when expanded in a seeded beam readily form complexes with the carrier gas. The hypersonic jet thus permits a study of formation, structure and kinetic behaviour of a complex with a single bound noble gas atom as well as a molecule with many inert atoms, a matrix like environment and all stages between these two "phases". The formation of these Van der Waals complexes has been deduced from fluorescence excitation spectra where the stoichiometry of the complexes is obtained from the shift of the complex peaks relative to the bare molecule peak, using simple sum rules or the effect of the pressure of the carrier gas on the relative intensities of the peaks [3]. A method for the direct analysis of such complexes is the resonant two color photoionization together with time-of-flight mass analysis [4]. This gives a direct mass identification of each aggregate, however it provides no information on the structure of such complexes. This further information can be obtained from the rotational analysis of the complex band contour either in ionization or fluorescence