Femtosecond studies of the entire reaction path

The earliest studies in which changes of bonding in molecules actually in a reacting state were observed spectrophotometiically were on gas-phase reactions. One of the first systems of these to be studied (1988) was the reversible dissociation of ICN [2,e,f]  

The gas-phase dissociation behaviour of the molecule Hgl2 (Hgl2 —Hgl - -1) is more complex. It requires consideration of two coordinates representing the two independent Hg-I distances in Hgl2 and also the vibrational and rotational motions of the Hgl fragment [2,b]  

A pump-and-probe technique was again used. The different species were monitored spectrophotometrically, the probe wavelength being adjusted to match appropriate absorbance maxima. Starting with laser excitation of Hgl2 vapour, it was found possible to follow the entire dissociation sequence from the transition-state configuration to the products. 

Monitoring of the Hgl signal gives the time required for the separation into the two fragments to become effectively complete. This can be determined for various values of 

These remarkable gas-phase experiments marked the start of a new era of direct study of the nuclear motions near transition states in reacting systems. They established the feasibility of monitoring the entire course of the reaction through the transition region to products. The techniques were soon applied also to reactions in solution. 

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