The van der Waals rovibronic spectrum of p-difluorobenzene–Ar up to 125 cm−1 intermolecular energy: Assignment and character of van der Waals modes

The van der Waals (vdW) vibronic spectrum built on the electronic 000, S1(B2)←S0(A1) origin of p-difluorobenzene–Ar is investigated with rotational resolution (ΔνUV=60 MHz). For the first time vdW vibronic bands are detected up to a van der Waals energy of 125 cm−1 and assigned by a rotational analysis of the band structure. The band origin positions of the ten detected bands display a regular behavior with moderate anharmonicities and minor influences due to Fermi resonances. Using the concept of three-dimensional Kraitchman equations and of normalized effective planar moments characteristic data on the nuclear displacements in the two different bending coordinates are deduced from the measured rotational constants. The fundamental vdW vibronic states at low energies can be described in terms of one-dimensional normal modes while vdW states at higher energies display mixed mode character.