Excitation by fast atoms at very high electric field to gas-density ratios in argon

Measurements of transient and steady-state light emission and of current transients from Ar at very high electric field to gas-density ratios E/n and low gas densities show that the excitation of the 811-nm lines of Ar is primarily by fast neutral Ar atoms. A drift tube with parallel-plane electrodes produces a spatially uniform electric field. Pulsed laser irradiation at 266 nm of the semitransparent cathode produces current pulses of {}10 mA and {}10 ns width. The time and spatial dependences of the line emission at 811±{}1 and 750±{}1 nm were measured for E/n in the range 280 townsend (Td) to 84 kTd at gas densities from 1.3×{}10²³ to 9×{}10²⁰ m^-3, where 1 Td=10^-21 V m². At E/n4 kTd the time dependence of the 811-nm emission is consistent with direct and cascade excitation due to electron collisions with Ar. At E/n>18 kTd the emission transient consists of an early peak caused by the initial electron avalanche followed by a delayed peak that moves toward the cathode with an apparent velocity of about twice that of the ions. Steady-state 811-nm emission is also observed in a region of very low electric field near the cathode where the Ar⁺ ions have insufficient energy for excitation and the fast atoms are injected from a high-field region. These observations of 811-nm emission, plus previous measurements of steady-state spatial distributions of emission, are interpreted as demonstrating that excitation of the 811-nm lines is primarily by fast Ar atoms formed by charge transfer in Ar^+-Ar collisions. For 750-nm emission the prompt portion is much larger and the delayed portion is much weaker than for 811-nm emission showing the importance of excitation by electrons even at these high E/n values. Steady-state observations of 811-nm emission when slow Ar⁺ ions are injected into a region of high electric field further confirm the model.