Kinetics of the Opto-Galvanic Effect in Rare Gas Hollow Cathode Discharges.

The opto-galvanic effect in helium, neon, and krypton discharges within the negative glow region of a hollow cathode has been investigated. Experimental observations of the phase lag between the opto-galvanic current and the optical modulation inducing the opto-galvanic signal were made as a function of the modulation frequency. Detailed kinetic models were proposed for each of the rare gas hollow cathode discharges. These models were used to calculate the phase lag between the opto-galvanic current and the optical modulation. Allowing the unknown rate constants within these models to be variable parameters, the calculated phase lag versus frequency was fit to the measured phase lag versus frequency. These same kinetic models were then used to calculate the relative magnitude contribution of each of the ionization reactions to the opto-galvanic signal as a function of the optical modulation frequency. The resulting magnitude plots were a useful tool in developing physical insight into the details of the opto-galvanic effect in these rare gas hollow cathode discharges. In helium, we found the associative ionization reaction dominated the opto-galvanic signal production. In neon and krypton we found that the opto-galvanic signal was produced by collisional ionization and ultraviolet photoionization of metastable and resonance states. It was also found a significant contribution to the opto-galvanic signal due to a modulation of the beam electron density within the hollow cathode discharge. Measurements of cross sections were obtained for all of the pertinent ionization processes mentioned above. The cross sections for the associative ionization reaction, collisional ionization reaction and beam electron ionization reaction were found to be in agreement with the current literature values for helium. The previously unmeasured values obtained for the cross sections of collisional ionization of the metastable and resonance states in neon and krypton were 90 (+OR-) 30 (ANGSTROM)('2) and 102 (+OR-) 50 (ANGSTROM)('2) respectively. The values for the cross section of vacuum ultraviolet photoionization of the metastable and resonance states in neon and krypton were found to be 6.6 x 10(' -2) (ANGSTROM)('2) and 37 x 10('-2) (ANGSTROM)('2). These values are a factor of 7 and 29 larger than the current theoretical values in the literature.