Principles of laser-induced fluorescence spectroscopy

If the particle is resonantly stimulated by the laser source, then a photon of energy hi/u will be absorbed, lifting the particle to the excited state. As is well known, both stimulated and spontaneous emissions have to be considered in the temporal decay of the excited level, where the relative ratio between the two is determined by the laser intensity. It should be noted that the stimulated emission process constitutes a loss mechanism for LIF observation at right angles, as 

The absorption starts at a distinct rotational and vibrational level within the lower electronic (ground) 

The fluorescence spectral radiant power that an optical system will collect from the laser interaction volume to a detector is, to a good approximation, given by 

The total fluorescence radiant energy Qf arriving at the detector (with light frequency dispersion capability, if required) will be 

The integration is over the spectral interval response interval of the detector and over a suitable time interval, associated with the duration of the laser excitation and the actual fluorescence lifetime. Most likely 2 will be a function of time, and thus combining Equations (7.B1) and (7.B2) and taking into account this time dependence yields 

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