Stepwise excitation laser-enhanced ionization spectroscopy utilizes two independently tunable dye lasers to populate high-lying excited states of atoms in flames. Two atomic resonances are required, with the upper level of the first-step transition coinciding with the lower level of the second-step transition. Efficient population of a high-lying atomic level is achieved, from which a high rate of collisional ionization can take place. The double-resonance aspect of such excitation adds an extra dimension of spectroscopic selectivity to the measurement. A computer-controlled dual-wavelength LEI spectrometer, including a Fizeau wave-meter for wavelength verification, is used to record three-dimensional spectra—ionization signal as a function of both first- and second-step wavelengths. Examples illustrate the accuracy advantage accorded by the three-dimensional survey.
G. C. Turk, F. C. Ruegg, J. C. Travis, and J. R. Devoe, "Three-Dimensional Atomic Spectra in Flames Using Stepwise Excitation Laser-Enhanced Ionization Spectroscopy," Appl. Spectrosc. 40, 1146-1152 (1986)