Accurate Ritz wavelengths of parity-forbidden [], [], and [] infrared lines of astrophysical interest^*

Atomic Astrophysics, Lund Observatory, Lund University, Box 43, 221 00 Lund, Sweden e-mail: [maria;sveneric.johansson]@astro.lu.se

Received: 16 January 2007
Accepted: 6 March 2007

Abstract

Context.With new astronomical infrared spectrographs the demands of accurate atomic data in the infrared have increased. In this region there is a large amount of parity-forbidden lines, which are of importance in diagnostics of low-density astrophysical plasmas.

Aims.We present improved, experimentally determined, energy levels for the lowest even LS terms of , and , along with accurate Ritz wavelengths for parity-forbidden transitions between and within these terms.

Methods.Spectra of , and have been produced in a hollow cathode discharge lamp and acquired using high-resolution Fourier Transform (FT) spectrometry. The energy levels have been determined by using observed allowed ultraviolet transitions connecting the even terms with upper odd terms. Ritz wavelengths of parity-forbidden lines have then been determined.

Results.Energy levels of the four lowest terms (a⁶D, a⁴F, a⁴D and a⁴P) have been determined, resulting in 97 different parity-forbidden transitions with wavelengths between 0.74 and 87 μm. For the energy levels of the two lowest terms (a⁴F and b⁴F) have been determined, resulting in 24 different parity-forbidden transitions with wavelengths between 8.9 and 130 μm. Also for the energy levels of the two lowest terms (a⁶S and a⁶D) have been determined, in this case resulting in 12 different parity-forbidden transitions with wavelengths between 0.80 and 140 μm.