Large perturbation techniques, laser

Large perturbations using flash or laser photolysis and shock tubes require a new equilibrium situation to be set up which is far from the initial equilibrium state. These methods are generally used in gas phase studies, and small perturbations are used for solutions, though there is nothing constraining the techniques in this way. 

Another important source of perturbation of a chemical system is light, such as a laser flash. The irradiation can cause a rapid photochemical reaction, such as photohomolysis of a single bond. The reverse, thermal reaction will then regenerate the reactant(s). This method differs from the other relaxation methods mentioned above in that the relaxation process brings the system back to its initial state rather than to a new equilibrium. The amount of energy deposited with a flash is often large enough to temporarily perturb even an irreversible thermal system, which makes this technique applicable to both reversible and irreversible reactions. Flash photo-lytic methods are a subject of a later chapter and will not be dealt with here. 

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