Measurements of absolute hydrogen atom densities in plasmas by (2+1)-photon laser-induced fluorescence spectroscopy

Hydrogen atom densities in H2 and fluorocarbon (CHF3 and CF4-H2) plasmas have been measured by (2+1)-photon laser-induced fluorescence spectroscopy. In this method, the following two-step excitation scheme is used to excite ground-state H atoms to the 4p state. In the first step, H atoms are excited to the 2s state by absorbing two photons at a wavelength of 243 nm. The 2s state is excited to the 4p state by the third photon at 486 nm in the second step. Since the wavelength used in the first step excitation is the half of the wavelength used in the second-step excitation, one tunable laser having a system for second harmonic generation can be utilized for the (2+1)-photon excitation. Fluorescence emission at 486 nm (Hβ emission) is detected. A calibration method to determine the absolute H atom density has also been developed. The calibration method is based on the comparison of fluorescence emission intensity from H atoms with that from Xe at a known gas pressure. The present (2+1)-photon LIE (Laser-Induced Fluorescence) technique 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. (author)