Isotope shift measurements in the 660 spectral lines of Er I covering the 340–605 nm wavelength region with a Fourier Transform Spectrometer

Isotope shift measurements in 660 spectral lines covering the 340–605 nm wavelength region of Er I were carried out using a Fourier Transform Spectrometer. The spectra were recorded using a liquid nitrogen cooled hollow cathode discharge source containing highly enriched ¹⁶⁶Er and ¹⁷⁰Er isotopes in the oxide form and two different detectors namely PMT and silicon photo diodes. Out of 660 spectral lines involving 216 even and 182 odd Er I levels, the isotope shift data were new in the 406 lines. On the basis of their level isotope shifts out of 114 unassigned even parity levels 27 levels assigned to 4f¹¹6s²6p, 72 to 4f¹¹5d6s6p and 15 to 4f¹²6s6d configuration whereas 12 each of unassigned odd parity levels assigned to 4f¹¹5d6s², and 4f¹²6s6p configurations and 16 unassigned odd parity levels assigned to 4f¹¹5d²6s configuration. Configuration mixing for 30 odd parity energy levels has been theoretically calculated applying 'Sharing Rule' to the experimentally derived level isotope shifts, which were finally compared with mixings available in the literature. - Highlights: • Isotope shifts in 660 spectral lines of Er I presented. • Isotope shifts in 406 lines reported for the first time. • Even levels assigned to 4f¹¹6s²6p, 4f¹¹5d6s6p and 4f¹²6s6d configurations. • 12 each of odd levels assigned to 4f¹¹5d6s² and 4f¹²6s6p configurations. • Configuration mixing compositions derived for 30 odd levels using 'Sharing Rule'