Electronic excitation of the 750- 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 nm 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 2p_1-1s₂ 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/N100 Td, N>2.7×{}10²³ 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.