Accurate Ritz wavelengths of parity-forbidden [FeII], [TiII] and [CrII] infrared lines of astrophysical interest

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 Fe ii, Ti ii and Cr ii, along with

accurate Ritz wavelengths for parity-forbidden transitions between and within these terms.

Methods: Spectra of Fe ii, Ti ii and Cr ii 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 Fe ii terms (a6D, a4F, a4D and a4P) have been determined, resulting in 97 different parityforbidden

transitions with wavelengths between 0.74 and 87 µm. For Ti ii 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 Cr ii

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.