Bound orbiting states of benzene–Ar and evidence for reversible intramolecular vibrational energy redistribution within the complex

We have explored the proposition that emission from bound orbiting states of aromatic van der Waals complexes occurs where the free aromatic emits. Pumping 6 1 ¯ in benzene–Ar is predicted to lead, via intramolecular vibrational energy redistribution (IVR), to excitation in van der Waals modes associated with 16 1 ¯ that are above the barrier to bound orbital motion. The 6 1 ¯ dispersed fluorescence spectrum shows a red-shaded, broad band with a maximum 21 cm−1 to the blue of the expected 6 1 0 16 1 1 ¯ transition, where the 6 1 0 16 1 1 band of free benzene occurs. The time dependence of the parent and growth bands in the 6 1 ¯ spectrum suggests that IVR from this level is reversible.