Two-photon-excited fluorescence measurement of hydrogen atoms in flames.

We report the first two-photon-excited hydrogen-atom fluorescence measurements in flames made to our knowledge. The n = 3 level of the H atom was excited by 205.1-nm radiation generated by Raman shifting a 224-nm beam produced by frequency mixing. Fluorescence was observed at 656.3 nm as a result of radiative decay from n = 3 to n = 2, the Balmer-alpha transition. A novel technique, photoionization-controlled loss spectroscopy, is proposed to eliminate the quenching dependence of the fluorescence signal.