Hole-burning spectra of phenol-Arn (n = 1, 2) clusters: resolution of the isomer issue

The hole-burning (HB) spectra of phenol-Arn (PhOH-Arn) clusters with n = 1 and 2 have been measured in a molecular beam to clarify the possible existence of isomers. Two species were identified to give rise to signals in the S1-S0 spectrum recorded for the n = 1 cluster; however, one of the species was found to originate from dissociation of an n = 2 cluster. Similarly, three species were observed in the spectrum of the n = 2 cluster, and two of them were assigned to n = 3 and larger clusters. The spectral contamination from larger size clusters was quantitatively explained by the dissociation after photoexcitation. The analysis of the spectra demonstrates that only a single isomer exists in the molecular beam for both the n = 1 and the n = 2 clusters. In addition to two previously detected intermolecular modes, a third low-frequency mode, assigned to an intermolecular bending vibration, is observed for the first time in the HB spectrum of the n = 2 cluster. The assignments of the intermolecular vibrations were confirmed by ab initio MO calculations. The observation of the third intermolecular vibration suggests that the geometry of the n = 2 cluster has Cs or lower symmetry.