The first spectroscopic observation of YbS: the A0⁺–X0⁺ visible laser excitation spectrum

Abstract

The first spectroscopic observation of YbS has been made using visible laser excitation spectroscopy. Gas-phase YbS was produced in a Broida oven by the reaction of Yb metal vapour with either CS₂ or OCS. Dispersed fluorescence spectra of an Ω=0–Ω=0 electronic transition were recorded with a CCD array detector. A least-squares fit of 34 band-heads leads to the following molecular parameters for the lower state: ω_e″=365.6(2) cm⁻¹ and ω_e″x_e″=1.14(2) cm⁻¹. Determination of the parameters for the upper state of the transition was precluded by the irregular vibrational spacings, which increased with increasing v′.

Introduction

The visible absorption spectra of numerous lanthanide-containing diatomic molecules have been studied over the last twenty years. In particular, a great deal of progress has been made in probing the electronic structure of rare-earth monoxides, both experimentally 1, 2, 3, 4, 5, 6, 7, 8and theoretically 9, 10, 11, 12, 13, 14. Part of the motivation for this work lies in the fact that molecules containing a lanthanide atom offer a unique opportunity to examine both open and closed f-shell electronic configurations.

The ground state of the ytterbium atom has the closed-shell electron configuration (Xe) 4f¹⁴6s², and is therefore similar to an alkaline-earth atom, but with the addition of a filled 4f-subshell. Experimental 8, 15, 16and theoretical 11, 12, 13, 17, 18, 19work on YbO have revealed the presence of two low-lying electronic configurations, Yb²⁺(4f¹⁴)O²⁻ and Yb²⁺(4f¹³6s)O²⁻. Experimentally, the ground state of YbO has been found to have 0⁺ symmetry (in Hund's case (c) notation), and is attributed to the 4f¹⁴ configuration [15].

Although there has been no report of a spectroscopic study of YbS, a recent density functional (DF) investigation concluded that the ground state of YbS arises from the 4f¹⁴ configuration [19], the same as that found in YbO. The present work was undertaken to extend the fragmentary information available on rare-earth containing compounds and to test the validity of the conclusions in the DF study of Liu et al. [19].

While sulfur exists naturally in 4 isotopes, ${}^{32}\text{S}$ (95.0%), ${}^{33}\text{S}$ (0.75%), ${}^{34}\text{S}$ (4.2%) and ${}^{36}\text{S}$ (0.02%), ytterbium is present in nature as a mixture of 7 isotopes, 6 of which have >1% abundance. The spectroscopic resolution of the current work is not sufficient to resolve transitions of different Yb isotopomers of YbS, though several bands of the minor isotopomers Yb${}^{33}\text{S}$ and Yb${}^{34}\text{S}$ were observed.