Experimental set-ups and methodologies

As is well known, very cold molecules are produced in a supersonic expansion, which allows for the formation of weakly bound van der Waals molecules (Levy, 1980). Early molecular beam studies were limited to gas-phase species, where, by the use of co-expansion with an inert gas, the amount of cooling could be enhanced, allowing the formation of van der Waals clusters. This technique was used in the early 1970s to study the reactivity of (CHal) with alkali atoms (e.g. see Gonzalez Urena et al. (1975)). The basic formulae of cluster-beam formation are summarized in Box 24.1. 

More recently, the pick-up technique in the form of a beam-gas arrangement has been developed to 

Laser Chemistry Spectroscopy, Dynamics and Applications Helmut H. Telle, Angel Gonzalez Urena Robert J. Donovan 2007 John Wiley Sons, Ltd ISBN 978-0-471-48570-4 (HB) ISBN 978-0-471-48571-1 (PB) 

CH24 LASER STUDIES OF COMPLEXES VAN DER WAALS AND CLUSTER REACTIONS 

Van der Waals clusters can be produced in a supersonic expansion of a gas into vacuum through a nozzle of small diameter. The consequence of the expansion is a sudden drop in the pressure and the temperature of the expanding gas, resulting in condensation when the expanding gas crosses the conditions of gas-liquid or gas-solid equilibrium. Because of the rapidity of the pressure and temperature changes, typically two to six orders of magnitude in a few microseconds, the nucleation process taking place under these conditions is complex and not totally understood. 

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