Intermolecular vibrations of naphthalene trimer by ionization-detected stimulated Raman spectroscopy

摘要

We present nonlinear Raman spectra of intermolecular vibrational transitions in four naphthalene trimer isotopomers. The spectra, measured at 0.03 cm~(-1) by mass-selective ionization-loss stimulated Raman spectroscopy, reveal distinctly shaped pendular band contours, which, upon comparison to simulated pendular contours, lead to ready assignments for almost all of the observed bands. The results show clearly that the trimer has C_(3h) (or very nearly C_(3h)) symmetry with the naphthalenes arranged such that their long axes are parallel to one another. Comparison of the experimental results to the results of calculations of naphthalene-trimer intermolecular spectra performed by assuming harmonic intermolecular modes and by taking the intermolecular potential energy surface to be a pair-wise additive one reveals significant qualitative agreement. This agreement strongly suggests a relatively rigid cluster with no large amplitude intermolecular motions within 100 cm~(-1) of the zero-point level. Finally, the results highlight the dominance of librational motions in giving rise to scattering intensity in the intermolecular portion of the cluster's Raman spectrum.