High Resolution IR Spectroscopy of the C-H Stretch Bands of Benzene Monomer and Dimer in Helium Nanodroplets

The IR spectrum of benzene and benzene dimer embedded in superfluid helium droplets in the region of the C-H stretch vibration (3000-3110 cm-) was observed. By analyzing the pressure dependence of the spectra it was possible to assign these spectra to benzene monomer and dimer. Based on a comparison of the observed position of the dimer bands to experimental bands recorded in the gas phase and to theoretical frequency predictions the authors assign these to the tilted T-shaped benzene dimer structure. None of the other two possible structures (PD or PTS) were found. The above assignment is firm. For the monomer, we find in helium nanodroplets a splitting of the nu(8) + nu(19) combination band observed in the gas phase. We assign this to an additional Fermi resonance which is caused by the matrix shift in helium nanodroplets.