Short review on the acetylene photochemistry in clusters: photofragment caging and reactivity

Abstract

We have investigated solvent effects on acetylene photodissociation in clusters of different sizes and compositions generated in molecular beams: pure acetylene clusters (C₂H₂) _n , 10–200, and mixed (C₂H₂) _m  · Rg _n (Rg = Ar, Xe) species where the single acetylene molecule or small cluster can be either adsorbed on the surface or embedded inside the rare gas cluster. We review our previous experiments in which the photodissociation dynamics was studied by the H-fragment time-of-flight (TOF) technique, and extend the experiments by measurements with a new velocity map imaging set up. We observe an unusual manifestation of the cage effect leading to a formation of fast hydrogen fragments. Such observation is interpreted as resulting from a two-step process: first, the initially excited molecule is quenched by the cluster cage to a vibrationally hot ground state; then the molecule dissociates upon subsequent photoabsorption from the vibrationally excited state. Thus the faster fragments result from the caging combined with the multiphoton processes. In xenon clusters with acetylene molecules adsorbed on the surface, we have also found evidence for organo-xenon molecule HXeCCH generated in the singlet excited state of a acetylene–xenon complex.

Keywords:

• acetylene clusters
• photochemistry
• molecular beams
• cage effect
• organoxenon molecule

Acknowledgements

Support by the Grant Agency of the Czech Republic via grant No. 203/09/0422 and P208/11/0161 is gratefully acknowledged, and JF acknowledges the support by the Swiss National Science Foundation project No. PZ00P2-132357/1. JK acknowledges the grant No. 238671 ICONIC within FP7-MC-ITN. We are grateful to U. Buck (Max-Planck Institute for Dynamics and Self-organization, Göttingen) for numerous fruitful discussions.