Multiphoton ionization detection photofragments

The photolytic and probe pulses are colinear when they reach the sample. The photolytic pulse produces excited states and photofragments, and the probe pulse which follows closely behind must be used to analyse the concentration and/or the chemical nature of the transients. The major detection processes are known as laser-induced fluorescence (LIF) and multiphoton ionization (MPI). Transient absorptions can also be used in some cases, and this is similar to ps spectroscopy. 

Although not an essential part of REMPI spectroscopy, mass-selective detection (using either a time-of-flight or a quadrupole mass filter) of the REMPI signal confers enormous advantages at the cost of negligible loss of sensitivity. With mass-selection one can (i) simultaneously record spectra of two (or more) isotopomers (see Fig. 1.14) (ii) exploit multiphoton ionization of a selected photofragment as a detection channel, for example... 

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