ORIENTATION DEPENDENCE IN COLLISION-INDUCED ELECTRONIC RELAXATION STUDIED THROUGH VAN-DER-WAALS COMPLEXES WITH ISOMERIC STRUCTURES

Weakly bound molecular complexes with more than one well-defined structures provide us with an unique opportunity to investigate dynamic processes induced by intermolecular interactions with specific orientations. The relative orientation of the two interacting molecules or atoms is defined by the complex structure. The effect of the orientation in the spin changing collisions glyoxal(S1)+Ar-->glyoxal(T1)+Ar and acetylene (S1)+Ar-->acetylene (T)+Ar have been studied by measuring the intersystem crossing (ISC) rates of the glyoxal (S1).Ar and acetylene (S1).Ar complexes with different isomeric structures. Results show that there is a strong orientation dependence in the ISC of glyoxal (S1) induced by interaction with the Ar atom: the Ar atom positioned in the molecular plane is much more effective than in the out-of-plane position in inducing the S1-->T1 transition of glyoxal. On the other hand, studies of acetylene (S1).Ar complexes indicate that the Ar-induced ISC rates are nearly identical for the in-plane and out-of-plane positions. Orientation dependence in the collision induced vibrational relaxation process C2H2(S1, upsilon(i))+Ar-->C2H2(S1, upsilon(f)<upsilon(i))+Ar is also studied by measuring the vibrational predissociation rates of the acetylene (S1).Ar complex isomers. The results indicate that collisions of C2H2(S1, upsilon3=3, 4) with Ar at two orthogonal orientations are equally effective in causing vibrational relaxation of C2H2.