The permanent electric dipole moments and magnetic hyperfine interaction in the A 2 P state of yttrium monosulfide T.C. Steimle * and Wilton Virgo Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287-1604, USA Received 12 March 2003; in revised form 8 May 2003 Abstract The optical Stark effect in the ð0; 0Þ A 2 P–X 2 R þ band systems of a yttrium monosulfide, YS, supersonic molecular beam sample have been analyzed to produce the magnitude of the permanent electric dipole moments, jlj, for the A 2 P 3=2 , and A 2 P 1=2 states of 5.9(2), and 6.8(1) D, respectively. Small splittings in the field free A 2 P 3=2 –X 2 R þ and A 2 P 1=2 –X 2 R þ spectra were analyzed to produce the magnetic hyperfine spectroscopic parameters a ¼36ð6Þ, c ¼ 111ð7Þ, and d ¼107ð6Þ MHz for the A 2 P state. Transition frequencies of the low rotational lines in the ð0; 0Þ A 2 P r –X 2 R þ band system were measured and analyzed to produce the first complete set of fine structure parameters for the A 2 P r state. A comparison with YO and other isovalent molecules is made. Ó 2003 Elsevier Inc. All rights reserved. Keywords: Optical Stark effect; Permanent electric dipole moments; Magnetic hyperfine spectroscopic parameters 1. Introduction The diatomic molecules composed of the transition metals of group III (Sc, Y, and La) are the simplest of the d-block open shell molecules. There exists a useful, yet incomplete spectroscopic database for these group III species that can trace the trends in bonding between the metals in the group and various ligands. Laser ab- lation combined with supersonic free jet expansion has become the standard technique for production of tran- sition metal containing molecules, which are otherwise a challenge to study due to their ephemeral nature. The interpretation of the high resolution spectra of the group III metal containing diatomics provides insight into the nature of transition metal bonding. Quantitative prop- erties that elicit the bonding of these diatomic molecules are the permanent electric dipole moment, l, and mag- netic hyperfine structure parameters. The dipole mo- ment describes the overall charge distribution of the metal–ligand bond and can be used to characterize the different electronic states by the spatial configuration of electrons therein. Magnetic hyperfine parameters de- scribe the chemically relevant valence electrons and their interaction with nuclei of non-zero spin. Measurements of the hyperfine parameters and dipole moments can directly be used to judge the accuracy of theoretical ab initio electronic wavefunctions used to predict molecular properties. Our work reports on the fine structure, magnetic hyperfine structure, and permanent electric dipole moments for the A 2 P 1=2 and A 2 P 3=2 states of YS. The only high resolution investigation of the A 2 P–X 2 R þ band system of YS has been the field free measurements of a free jet expansion performed by Kowalczyk et al. [1] which was restricted to the ð0; 0Þ A 2 P 1=2 –X 2 R þ subband. The spin–rotation and mag- netic hyperfine interactions in the X 2 R þ state have been precisely determined using molecular beam laser radio- frequency double resonance [2]. The rotational con- stants B and D for the X 2 R þ state are well determined from the analysis of the B 2 R þ –X 2 R þ band system [3,4] and the aforementioned A 2 P 1=2 –X 2 R þ subband [1]. The dipole moment for the X 2 R þ ( ¼ 6.10(6) D) and B 2 R þ ( ¼ 4.57(9) D) states have been derived from the analysis of the optical Stark effect in the ð0; 0Þ B 2 R þ –X 2 R þ band system [5]. The observed marked reduction of lðB 2 R þ Þ compared to lðX 2 R þ Þ was unexpected and prompted further experimental and theoretical investigations [4]. * Corresponding author. Fax: 1-480-965-2747. E-mail address: tsteimle@asu.edu (T.C. Steimle). 0022-2852/$ - see front matter Ó 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0022-2852(03)00165-6 Journal of Molecular Spectroscopy 221 (2003) 57–66 www.elsevier.com/locate/jms