(Ta)lif
(Ta)LIF
Instead of determining the density of a certain atomic level in the plasma directly from the absorption coefficient, one can also measure the absorption indirectly by monitoring the radiation that is emitted from the atomic level to which the atoms are pumped by the laser (in the absorption process). This technique is known as Laser Induced Fluorescence (LIF). The upper level, to which the atoms are pumped, will in general emit fluorescence radiation at several wavelengths. It is convenient to choose a wavelength different from the pump wavelength, so that stray light is no concern. In addition, the detection of fluorescence radiation is far more sensitive than absorption techniques because it is in general easier to measure a very low light level than to detect a small change in a relatively high intensity.
The LIF technique can usually only be applied to excited atomic states, because pumping from the ground state usually requires very high energetic photons. However, it is possible to use two photons instead of one (TaLIF). Of course the cross section of such a two-photon process is much lower than for a single photon process, but the alternative, generation of high-energy photons, is also inefficient.
LIF on mercury has been applied to the QL lamp to give the distribution of the density of the 63P1 level (see picture above). A major problem with this kind of measurements on a plasma contained in glass is again stray light. As pointed out before, by detecting photons with a wavelength different from the pump wavelength, stray light will not distort the measured spectrum itself. However, if there is a high stray light level, effectively the whole plasma will be illuminated by the laser, rather than only a narrow laserbeam. This causes a huge increase in detection volume and the interpretation of the measurements becomes very difficult. The only solution to this problem is a strong reduction of stray light by Brewster windows and diafragms, and by using a very weak laser beam (much weaker than the power needed for saturation). In the picture on the right, the effect of stray light is clearly visible, especially near the edges of the plasma (excited densities are expected to be low!).
Measured (relative) densities of the 63P1 level in the right half of the QL lamp