ELECTRONIC EXCITATION OF THE 750-NM AND 811-NM LINES OF ARGON

Measurements have been made on the electron excitation coefficients of four Ar levels: 2p1, 2p5, 2p7, and 2p9. Radiative decay of these levels to 1s levels is responsible for the strongest line features in argon spectra, at 750 nm due to radiation from 2p1 and 2p5 levels, and at 811 run due to transitions from 2p7 and 2p9 levels. We have measured excitation coefficients of these levels in the range of E/N from 50 Td to 9 kTd (1 Td = 10(-21) V m2). Here E is the electric field and N is the gas density. The optical emission intensities normalized to the local current density were placed on the absolute scale after normalizing the data for the 2p1-1s2 transition (750.4 nm) to the previous experimental and theoretical results at 50 Td. We have also made measurements of the excitation coefficients of several 4d and 6s vs E/N, in order to estimate the effect of cascading from upper levels to 2p levels. We have found that the excitations of 2p9 and 2p7 are influenced by collisional population transfer between 2p levels at low E/N (E/N < 100 Td, N > 2.7 X 10(23) m-3), by cascading from 3d and 4s levels above 100 Td, and by cascading from 4d and 6s levels at E/N > 3 kTd.