Two-photon-excited stimulated emission from atomic hydrogen in flames

The first observation to the author’s knowledge of optically excited stimulated emission in atomic hydrogen is described. The two-photon n = 1 → n = 3 transition of hydrogen atoms in low-pressure flames is excited with 205-nm radiation produced by using a beta barium borate crystal to frequency mix the fundamental and frequency-doubled radiation from a 615-nm pulsed dye laser. The resulting 656-nm n = 3 → n = 2 Balmer-α radiation is readily observable by eye as a coherent beam propagating in both the forward and reverse directions. We describe a variety of characteristics of the stimulated emission, comparing its behavior with simultaneous measurements of two-photon-excited fluorescence, and discuss the possibility that the stimulated-emission process may affect the quantum yield of fluorescence and ionization detection methods. Potential diagnostic applications of two-photon-excited stimulated-emission detection for measuring atomic hydrogen in flames are demonstrated, using simultaneous profile measurements of stimulated emission and fluorescence signals.