Laser-Induced Fluorescence Methods for Transient Species Detection in High-Pressure Discharges

LIF is based on a two-step process. In the first step molecules or atoms in the electronic initial state |i> are pumped (excited) to an upper electronic state |j> by absorbing one laser photon with energy hνij. In the second step |j> undergoes spontaneous radiative decay to a lower state |k>, and the corresponding hνjk photon is the spectroscopic observable called the fluorescence signal. A conceptual scheme of the LIF process is depicted in Figure 9.1. The states |i>, |j>, and |m> represent atomic levels and molecular rovibronic levels for atoms and molecules, respectively. Depopulation of level |j> is determined by the sum of radiative transitions and by collision processes whose final state is not defined here. A more general scheme allows for collisional population of another state |m> from |j>, with observation of fluorescence from |m>. The LIF scheme shown in Figure 9.1 can be described by a set of coupled rate equations* for the densities N of the three levels:ddddNt B t Ngg NNt B t Ngg N= − −   = − ρρ( ) ,( )   − +( )= − +( )A Q NNt R N A Q Ndd,.