Rotationally resolved UV spectroscopy of weakly bound complexes: structure and van der Waals vibronic bands of phenylacetylene·Ar

The pure electronic transition S1 ← S0 and four van der Waals vibronic bands of phenylacetylene·Ar are measured with rotational resolution in a REMPI experiment. Here the isotropic Ar atom is attached to a molecule with two covalently bound π-electron systems. Based on the rotational analysis we obtain accurate structural data and a new assignment of the van der Waals vibronic bands up to 43 cm−1 excess energy. A significant shift of the Ar atom from the center of the phenyl ring towards the acetylene substituent demonstrates that the acetylenic π-electron system contributes to the van der Waals binding.