Mass-selective ionization-detected stimulated Raman spectroscopy of benzene trimer and higher clusters

We present the results of mass-selective, ionization-detected stimulated Raman spectroscopies on jet-cooled benzene clusters from the trimer to the octamer. The data pertain to the fundamentals of the totally symmetric ring-breathing (ν1) and C--H stretch (ν2) modes of benzene and provide information about geometrical structure and vibrational dynamics. For the trimer, the results indicate a species in which all three benzenes reside in equivalent sites. For the higher clusters the results point to geometries involving two or more inequivalent sites. The ν1-excited trimer is found to decay on a nanosecond, or longer, time scale. The ν2-excited trimer decays on a time scale of greater than 5 ps. Similar timescales characterize the decays of the ν2-excited tetramer and pentamer