Resonant two-photon ionization spectra of van der Waals complexes: o-, m- and p-C6H4R2 center dot center dot center dot N-2 (R F, CH3)

The jet-cooled van der Waals (vdW) complexes o-, m- and p-C6H4R2...N-2 were studied by the one-colour resonant two-photon ionization technique (R2PI) through the S-1 <-- S-0 transition around the <(0)over bar (0)(0)> band. N-2 in o-, m- and p-C6H4F2...N-2 was found to rotate almost freely around an axis perpendicular to the benzene ring, whereas no such internal rotational features were observed for o-, m- and p-C6H4(CH3)(2)...N-2. All the vdW vibrational modes of the complexes studied were identified and assigned accordingly. For p-C6H4(CH3)(2)...N-2, Fermi resonance was observed between the CH3 internal rotational levels and the vdW stretching motion of N, against the benzene ring. For all the complexes studied (o-, m- and p-C6H4R2...N-2), irrespective of whether R = F or CH3, the <(0)over bar (0)(0)> band shifts relative to the monomer origins <(0)over bar (0)(0)> were largest for the para-substituted complexes, least for the meta-substituted complexes and in between for the ortho-substituted complexes. (C) 1997 Elsevier Science S.A