THE OPTOGALVANIC EFFECT IN A NEON TOWNSEND DISCHARGE

The optogalvanic effect for the 1s5-2p2 transition (Paschen notation) in a low current (0.5 muA) neon Townsend discharge has been studied fo r three pressures (1, 4 and 10 Torr). The optogalvanic effect occurs b y irradiating the discharge with an expanded laser beam, in a variable plane parallel to the electrodes. By moving the discharge tube perpen dicular to the electrodes and by using a dye laser with a continuously adjustable frequency, measurements of the optogalvanic effect could b e carried out for a number of irradiation positions as a function of t he laser frequency. In calculating the optogalvanic effect, secondary ionization processes are dominant and therefore one needs to know the probabilities that resonant photons and metastables produced in the di scharge reach the cathode. The first probability has been calculated u sing Monte Carlo simulation methods, with the method developed by Lee to calculate the frequency redistribution of a resonant photon after t he interaction with an atom. The second probability has been found by solving the transport equation of the metastables. The optogalvanic ef fect measurements agree to first order with the theoretical calculatio ns. A sign reversal of the optogalvanic effect was found at a specific irradiation position for pressures of 4 and 10 Torr. This can be expl ained by the secondary ionization process. Apparently the probability of reaching the cathode and liberating an electron is identical for a resonant photon and a metastable, generated at that position. In both the measurements and the calculations of the optogalvanic effect as a function of frequency the large influence of Ne-22 is apparent.