Potential curves from emission continua. IV. The upper state of the vacuum uv continua of Ar2

Abstract

Photoelectric spectra of weak pulsed discharges in argon (4 < P < 20 torr, 125 < T < 200°K) show that the red‐degraded first continuum of Ar₂ begins at 1074 Å and exhibits oscillatory structure out to 1500 Å, this structure being due to interference effects in the energy dependence of bound‐continuum transitions from high vibrational levels. A semiclassical procedure previously used for He₂ is employed to derive the upper state potential curve for 2.0 < R < 3.2 Å from the oscillatory structure and different assumed lower (repulsive) potentials. Vibrational levels in this well are computed numerically; comparison of their spacings with those observed by Tanaka and Yoshino in the absorption spectrum of Ar₂ show that the radiating state is a high vibrational level (ν = 22±2) of the $0_u^{+}$ state (dissociation limit ³P₁ + ¹S₀), populated by a collision‐induced curve crossing in a collision of Ar(³P₂) + Ar(¹S₀). Spectroscopic constants of the $0_u^{+}$ state, which depend somewhat on the lower potential assumed, are D′_e = 6000±600 cm⁻¹, ω′_e = 310±30 cm⁻¹, ω_ex′_e = 2.5±0.5 cm⁻¹, and R′_e = 2.32±0.10 Å. These constants agree well with those computed by assuming that ${\mathrm{Ar}}_2{\text{(0}}_u^{+})$ is a Rydberg state with configuration $[{\mathrm{Ar}}_2^{+}{\mathrm{\hspace{0.17em}(X\hspace{0.17em}}}^2{\Sigma }_u^{+}{\text{)]4s\hspace{0.17em}σ}}_g$ . Calculations of the position of the maximum of bound‐continuum emission from ν = 0 of the $0_u^{+}$ state confirm that it is also the upper state of the second continuum, which peaks at 1270 Å. The kinetics of formation of the $0_u^{+}$ state are briefly discussed.