Module 6 : Reaction Kinetics and Dynamics
Lecture 27 : Experimental methods in chemical kinetics
 

A strong and short pulse, called the pump, excites the molecules A in the sample cell. The possible sequence of reactions of A are given in eq (27.5) above. The beam splitter takes part of the initial beam through an alternative route so that this beam can be used to probe the time dependent reactive processes. By passing this second beam through an appropriate liquid such as H2O or CCl4 a wide distribution of frequencies (continuum generation) can be obtained. From this continuum, frequencies useful to analyse the products can be obtained by using suitable filters. A time delay between the initial pump light and the probe beam can be introduced by allowing the probe beam to travel an addition distance. If the additional distance is 1 mm, the time delay t = 1mm / c, which is 3.3ps. Different time delays are obtained by moving the prisms on a motorized stage. The difference in the probe beam resulting after its passage through the sample is analysed by the detector.

 
When one considers a duration of less that a pico second, molecules normally do not complete even one vibration. Using pulses of 10 to 100fs (A pulsed Ti 3+ sapphire laser can produce 10fs pulses), it is possible to monitor the

detailed dissociation of a molecule into different possible products. We will illustrate this using the example of NaI dissociation which was first reported by Zewail who has pioneered the work in femtochemistry. The reaction sequence may be written as

NaI NaI*

(27.6a)
NaI* [Na..I]+* (27.6b)
[Na..I] +* Na + I + translational energy Na+ + I - (27.6c)
The potential energy diagram for NaI dissociation is given in fig 27.6
 
 
Figure 27.6 The ionic(V1) and covalent (V0) potential energies of NaI as a function of the NaI distance, r.
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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