15min:

PH. BRECHIGNAC, S. DOUIN AND P. PARNEIX, Laboratoire de Photophysique Moléculaire, CNRS, Université Paris-Sud, F-91405 Orsay Cedex, France.

The LIF excitation spectra of the S₁ S₀ transition of the aniline-nitrogen van der Waals (vdW) complex have been recorded near the origin band region for a broad range of expansion conditions. They exhibit a double set of vibronic bands, involving excitation of vdW modes. The relative intensities of the two subsets are dependent upon the expansion conditions, and these subsets are assigned to the complexes with ortho-N₂ and para-N₂ respectively. The internal rotation and libration problem was solved assuming separability of the two internal rotational degrees of freedom.

Dispersed emission spectra have allowed to compare the frequencies of the vibrational modes in the S₁ and S₀ states. The assignment of the 55 cm^-1 mode in S₁ to the first excited librator state allows to rationalize, by the concept of "stabilization by anisotropy", both the anomalously large spectral vdW shift of the S₁ S₀ transition, and the anomalously small vdW shift of the ionization potential. On the other hand, the "helicopter" motion corresponds to a nearly free internal rotation.

These assignments differ from the recently published ones [Schaefer and Pratt, J.Chem. Phys.115, 11147 (2001)].