Laser induced Fluorescence

When the life - times of the excited states of molecules (which are produced by short laser pulses) are short compared to collision times, the excited molecules decay to their stable ground states by fluorescence (which is the process by which excess energy is irradicated to the surroundings). This techniques is of course restricted to molecules that fluoresce (rather than give away their energy by radiationless processes) and are spectroscopically well characterized. Suitable lasers should be available in the required frequency range. The Schematic diagram is shown in figure 27.4 below.

Figure 27.4 Schematic diagram for Laser Induced Fluorescence (LIF).

The fluorescence is detected using efficient photomultiplier tubes (PMTs) and photon counting techniques. Gated photon counting synchronized to a pulsed laser further enhances the signal to noise ratio. This technique can detect lower concentrations of species, even in the range of 10⁶ to 10¹⁰ molecules/cm³. Variety of reactions between atoms with molecules, radicals with molecules as well as unimolecular reactions can be studied by this method. The time resolution extends from s to ns (nanoseconds).

Examples of species detected by LIF are hydrogen halides, NO, CN, NO₂, CF₂, NO₃, CH₃O and so on. Having low lying excited states increases the fluorescing ability of molecules.

The fluorescence intensity (which is proportional to the concentration or the population of the fluorescing states) is plotted versus time to get the appropriate rate constant.