RF-conductivity method to study Resonance Enhanced Multiphoton Ionization (REMPI).

We have developed a novel technique to detect the weak ionization that is produced by Resonance Enhanced Multiphoton Ionization (REMPI) of a small admixture of nitric oxide ( ~ milliTorr) in helium ( ~ 200Torr). In the experiment, an unfocussed beam from an excimer pumped dye laser (scanned from λ = 226.811 to 226.955 nm) was used to ionize an NO/He mixture between two plane-parallel condenser plates. The ionization was detected by observing the small phaseshift of a radio-frequency (14.7MHz) signal that is transmitted from one plate to the other. Although the experimental setup is very simple and compact, we were able to obtain the wavelength dependence of REMPI with very high spectral resolution, sufficient to resolve rotational lines. The method may be useful for detecting gaseous impurities, as a means to calibrate the wavelength scale of lasers, and perhaps to study fast kinetic processes in plasma afterglows.