Spectroscopic identification of the onset of the vibrational quasicontinuum in large molecules

We advance a new spectroscopic criterion for the identification of the threshold of the vibrational quasicontinuum (VQ) in large molecules, which rests on the observation of an onset for a congested vibrational level structure in the electronically excited states of isolated ultracold molecules seeded in supersonic beams. The onset of the VQ in tetracene (C₁₈H₁₂), pentacene (C₂₂H₁₄) and ovalene (C₃₂H₁₄) is exhibited at the excess vibrational energies of 2000 cm^-1 -1000 cm^-1.