Electronic excitation of the 750- and 811-nm lines of argon

Abstract

Measurements have been made on the electron excitation coefficients of four Ar levels: 2${\mathit{p}}_{1}$, 2${\mathit{p}}_{5}$, 2${\mathit{p}}_{7}$, and 2${\mathit{p}}_{9}$. 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 2${\mathit{p}}_{1}$ and 2${\mathit{p}}_{5}$ levels, and at 811 nm due to transitions from 2${\mathit{p}}_{7}$ and 2${\mathit{p}}_{9}$ levels. We have measured excitation coefficients of these levels in the range of E/N from 50 Td to 9 kTd (1 Td=${10}^{\mathrm{\ensuremath{-}}21}$ V ${\mathrm{m}}^{2}$). 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 2${\mathit{p}}_{1\mathrm{\ensuremath{-}}}$1${\mathit{s}}_{2}$ 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 2${\mathit{p}}_{9}$ and 2${\mathit{p}}_{7}$ are influenced by collisional population transfer between 2p levels at low E/N (E/N100 Td, N>2.7\ifmmode\times\else\texttimes\fi{}${10}^{23}$ ${\mathrm{m}}^{\mathrm{\ensuremath{-}}3}$), by cascading from 3d and 4s levels above 100 Td, and by cascading from 4d and 6s levels at E/N>3 kTd.