SPECTRA OF C6H6-Rgn (n=1,2) IN THE 3 MIRCON INFRARED BAND SYSTEM OF BENZENE

Benzene-noble gas complexes were one of the earliest topics of interest in spectroscopic investigation of van der Waals (vdW) complexes. Smalley et al.{\footnote{S. M. Beck, M. G. Liverman, D. L. Monts and R. E. Smalley, J. Chem. Phys. 70, 232 (1979).}} observed C$_6$H$_6$-(He)$_{1,2}$ vdW complexes in the late 1970s by means of electronic spectroscopy. A recent study on the same species was done by Hayashi and Oshima{\footnote{M. Hayashi and Y. Ohshima , Chem. Phys. 419, 131 (2013).}} at higher resolution (250 MHz). Here, we present an extensive infrared observation of C$_6$H$_6$-Rg$_n$ (n=1,2) with the rare gas being He, Ne, or Ar, in the 3 micron region. The spectra were observed using a tunable optical parametric oscillator to probe a pulsed supersonic-jet expansion from a slit nozzle.\\ Benzene monomer is known to have a complex band system in this region.{\footnote{J. Pliva and A.S. Pine, J. Mol. Spectrosc. 126, 82 (1987).}} The strongest band, centered around 3047.91 \wn, belongs mainly to the C-H stretching fundamental $\nu_{12}$ of symmetry E$_{1u}$. Other strong perpendicular bands occurring just above the main band as a result of intensity borrowing via anharmonic resonances between the fundamental $\nu_{12}$ and the combinations are $\nu_{2}+\nu_{13}+\nu_{18}$, occurring near 3079 \wn, and $\nu_{13}+\nu_{16}$ and $\nu_{3}+\nu_{10}+\nu_{18}$, both occurring near 3100 \wn. The latter two bands are separated by merely 1.45 \wn. Although data analysis and observation are presently ongoing, we observe analogous bands for C$_6$H$_6$-Rg$_n$ (n=1,2). Spectra were assigned to a symmetric top with C$_{6v}$ symmetry with the rare gas atom being located on the C$_6$ symmetry axis. Spectra of the C$_6$H$_6$-Rg$_2$ trimers are in agreement with a D$_{6h}$ symmetry structure, where the rare gas atoms are positioned above and below the plane of the Benzene monomer. Although jet conditions have resulted in excellent signal to noise for the dimer and trimer spectra, we have not been able to identify any lines which might be due to tetramers or larger clusters. We intend to pursue the search for large clusters using a cooled nozzle