Atomic and ionic fluorescence dip spectroscopy as a tool for flame and plasma diagnostics

When two pulsed dye lasers are tuned in spatial and temporal coincidence to two connected atomic transitions in a flame or plasma, the resonance fluorescence monitored from the first excited level decreases due to the depletion of the population of that level induced by the second laser excitation step. The monitoring of such a decrease (fluorescence dip) can be shown from simple theoretical considerations to be useful for diagnostic studies and for the evaluation of some fundamental parameters of the atomic transition involved in the second-excitation step. Both steady state and transient behaviour are discussed. The information content of the fluorescence dip is similar to that of the saturated fluorescence signal. However, several distinct advantages are offered by the new technique especially when the level reached by the second excitation step is close to the ionization limit of the atom.