Electronic spectrum of the anthracene–ammonia complex

Abstract

Fluorescence excitation and ultraviolet–ultraviolet hole burning spectra of jet-cooled anthracene–ammonia clusters were observed. The S1–S0 origin of the 1∶1 complex shows doublet bands, which were assigned to those in the ground and the first excited states of the internal rotation associated with the ortho and para hydrogen nuclear spin states of NH3, respectively. The ammonia molecule is located above the centre of the aromatic plane with a centre-of-mass distance of 3.6 Å, which is typical of a π hydrogen bonded aromatic complex. The rotational contour of the internal rotation excited state was analyzed with an effective Hamiltonian including the coupling between the internal and end-over-end rotations, to show that the internal rotation angular momentum is partially quenched because of the potential anisotropy for rotation around the axis that connects the centres of mass of ammonia and anthracene.