Laser-induced fluorescence and cavity ring-down studies

Laser-induced fluorescence and cavity ring-down studies 

LIE is clearly limited to the observation of species that give rise to fluorescence (i.e. the upper state must not be strongly predissociated). A more universal method of detection is absorption spectroscopy, and this was the mainstay of the now classical flash photolysis technique. One disadvantage of using absorption techniques is the loss of sensitivity. However, as discussed in Chapter 6, sensitivity can be greatly 

As the photolysis energy increases, channels leading to electronically excited states become accessible and the products can then be observed by monitoring fluorescence from the excited states produced, i.e. 

Provided the fluorescence involves an allowed transition, this is a relatively simple method and it was used in many of the early photodissociation studies. However, with forbidden transitions, the signal will be weak and collisional quenching may become important due to the longer lifetime of the excited states. Clearly, the effect of coUisional quenching can be avoided to some extent by working at reduced pressures, but this can create other experimental difficulties and, therefore, is not always possible. 

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