Coupling of hindered internal rotation with van der Waals vibration in aniline–H2(D2 ) complexes

Abstract

The laser-induced fluorescence excitation spectra from the van der Waals complexes of aniline with hydrogen and deuterium, formed in a free jet expansion, have been recorded in the spectral region associated to the electronic band origin S0 –S1 (000) of the free aniline molecule. For each complex the spectrum exhibits a vibrational progression involving a van der Waals vibration in the excited state. Each vibrational band of this progression is made of a doublet, the components of which are assigned to the ‘‘j=0’’ (para-H2 , ortho-D2 ) and ‘‘j=1’’ ortho-H2 , para-D2 ) Δj=0 transitions, as confirmed by nuclear spin statistics. The internal rotation of the hydrogen molecule in the complex is hindered by a barrier, the height of which is different in the ground X̃ 1 A1 state and the excited à 1B2 state. The doublet splitting (going from 3.1 to 6.2 cm−1 ) shows a systematic variation with the van der Waals vibrational quantum number, which is monotonic for D2 and presents a minimum for H2 , indicative of a strong coupling between internal rotation and the involved van der Waals motion. A quantum-mechanical treatment using the ‘‘nearly free rotor’’ picture has been done, which is able to reproduce the two spectra with very good accuracy, both for the positions and the intensities of the various bands. A detailed description of the full potential energy surface V(R, θ), including the coupling terms between the van der Waals bond length R and the hydrogen rotation angle θ in the excited state, is derived.