Analysis of low-J perturbations in the [Btilde](000)²Σ_u⁺ electronic state of the CO₂⁺ molecular ion

Abstract

The [Btilde](000)²Σ_u ⁺ state of the CO₂ ⁺ molecular ion is known from previous work to be seriously perturbed at low J by two other vibronic states, as well as showing a number of other smaller perturbations at higher J. New spectra of ultraviolet emission bands with the [Btilde](000)²Σ_u ⁺ state as upper state, recorded with low rotational temperature to emphasize the low-J transitions, are reported and analysed here. These bands include the [Btilde](000)-[Xtilde](000)²Π_g band and longer wavelength [Btilde](000)-[Xtilde](υ₁υ₂υ₃) bands observed in the region of the Ã(000)²Π_u-[Xtilde](000)²Π_g band. These long-wavelength bands are shown to be parallel bands with ²Σ⁽⁺⁾ _g or ²Σ^(-) _g lower states. It follows that the low-J perturbers of the [Btilde](000) state are one ²Σ⁽⁺⁾ _u and one ²Σ^(-) _u vibronic state. From their energies and symmetries, using the known parameters of the òΠ_u electronic state, these states can now be identified as Ã(231)μ²Σ⁽⁺⁾ _u and Ã(151)κ²Σ^(-) _u. The parameters of the [Xtilde]²Π_g electronic state permit the identification of the lower states of the long-wavelength bands as [Xtilde](υ₁, 3, υ₃)μ²Σ⁽⁺⁾ _g or [Xtilde](υ₁, 5, υ₃)κ²Σ^(-) _g with υ₃ = 1 or 3 and with various values of υ₁. This analysis is shown to agree with various features of previously observed spectra. Fits of the relative intensities of the rotational lines give rotational temperatures of 38 K and 35 K in two different jet-expansion sources, together with values of θ parameters where tan θ is the ratio of the Ã(151)κ²Σ^(-) _u-[Xtilde](υ₁υ₂υ₃)²Σ_g and Ã(231)μ²Σ⁽⁺⁾ _u-[Xtilde](υ₁υ₂υ₃)²Σ_g transition moments. These new data can be used to improve the empirical potentials of the [Xtilde] and à electronic states, and the relative intensities should give information on the Franck-Condon overlaps of their vibronic wavefunctions.