A measurement method of absolute hydrogen atom density in plasmas by (2+1)-photon laser-induced fluorescence spectroscopy

A technique of (2+1)-photon laser-induced fluorescence (LIF) spectroscopy has been developed for measuring hydrogen atom density in plasmas. In this method, the following two-step excitation scheme is employed to excite ground-state H atoms to the 4p state. In the first step, H atoms at the ground (1s) state are excited to the 2s state by two photons at a wavelength of 243 nm. Subsequently, the 2s state is excited to the 4p state by the third photon at 486 nm in the second step. Fluorescence emission at 486 nm (4p --> 2s, H-beta line) is detected to determine the H atom density at the ground state. Since the wavelength used in the first-step excitation is the half of the wavelength used in the second-step excitation, one tunable laser with a system for second harmonic generation can be utilized in the measurement. The absolute density was evaluated by comparing the intensity of LIF emission from H with that from Xe at a known gas pressure. The present method is suitable for diagnostics of reactive plasmas since optical dissociation of molecules and radicals can be avoided because of the low photon energy of the laser radiation. (C) 2001 American Institute of Physics.