Molecular Physics seminar

Photodissociation of small molecular clusters

by Prof. David Parker (University of Nijmegen, The Netherlands)

FA 31 ()

FA 31

Over the past years our understanding of the photodissociation of isolated diatomic and small polyatomic molecules has greatly improved, due to advances in experimental technique and to increasingly sophisticated theory .

For an increasingly larger group of diatomic molecules we can now measure and fully account for the populations and polarization of all product fragments, and describe the dissociation dynamics in terms of non-adiabatic and coherence effects, based on adiabatic/diabatic models or on fully quantum treatments. This progress makes similar studies possible for the photo-dissociation of simple van der Waals bonded molecular clusters, the first step in complexity on the way towards condensed matter.

In this talk an overview of our recent results on the van der Waals clusters O2-X, (CH3I)-X, I2-X, and NO-X will be described, where X is another molecule or an atom such as Argon. Our group has studied the photodissociation of isolated molecular oxygen in the ultraviolet and vacuum ultraviolet regions, using the velocity map imaging technique. When O2 is clustered with another molecule, the absorption cross section increases dramatically and several new dissociation channels related to charge-transfer states appear . Methyl iodide clusters are more strongly bound and on photodissociation they produce molecular iodine as a product. We find this process, the original example of ‘concerted photochemistry’, does not take place for the simple dimer , (CH3I)2. I2-Ar is probably the most studied van der Waals photodissociation system; our imaging results show evidence for dissociation of the linear-geometry cluster.

Finally, preliminary studies of threshold dissociation of the NO(A)-Ar cluster, where NO is in the excited A2Sigma+ state, show interesting rotationally resolved angular distributions.