Jet-cooled high resolution infrared spectroscopy of small van der waals SF6 clusters

Using a pulsed slit nozzle multipass absorption spectrometer with a tunable quantum cascade laser we investigated van der Waals clusters involving sulfur hexafluoride in the spectral range near the \nub{3} stretching vibration. Different sized homo\-complexes were generated in a planar supersonic expansion with typically 0,5 \% \chem{SF_6} diluted in 6 bar He. Firstly, several rotationally resolved parallel and perpendicular bands of \chem{(SF_6)_2}, at 934,0 and 956,1 \wn (\#1 structure) in agreement with Takami et al.\footnote{R.~D.~Urban and M.~Takami, J. Chem. Phys. 103, 9132 (1995).} but also one band at 933,6 \wn (\#2 structure) never observed previously, were analyzed in light of a recent theoretical study predicting three nearly isoenergetic isomers of D$_{2d}$, C$_{2h}$ and C$_2$ symmetry for the dimer. \footnote{T.~Vazhappily, A.~Marjolin and K.~D.~Jordan, J. Phys. Chem. B, DOI: 10.1021 / acs.jpcb.5b09419 (2015).} Furthermore, some broader bands were detected around 938 and 964 \wn and assigned to \chem{(SF_6)_3} and \chem{(SF_6)_4} clusters on the grounds of concentration effects and/or ab initio calculations. Lastly, with 0,5 \% rare gas Rg (Rg = Ne, Ar, Kr and Xe) added to the \chem{SF_6}:He gas mixture, a series of van der Waals \chem{(SF_6)_2}-Rg hetero-trimers were observed, which display a remarkable linear dependence of the vibrational shift with the polarizability of the rare gas atom provided that the initial \chem{SF_6} dimer structure is \#2 . In the same time no transitions belonging to the binary complexes \chem{SF_6}-Rg were found near the \nub{3} monomer band. This result suggests a complex thermodynamics within the pulsed supersonic expansion leading to the preponderance of \chem{(SF_6)_2}-Rg clusters over \chem{SF_6}-Rg binary systems. \begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.8]{SF6dimer-Rg.eps} \end{wrapfigure