Fourier transform emission spectroscopy and ab initio calculations on WO Ram S. Ram a, * , J. Liévin b , Peter F. Bernath a,c,d a Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA b Université Libre de Bruxelles, Service de Chimie quantique et Photophysique, CP 160/09, Av. F. D. Roosevelt 50, Bruxelles, Belgium c Department of Chemistry, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 d Department of Chemistry, University of York, Heslington, York YO10 5DD, UK article info Article history: Received 13 February 2009 In revised form 15 April 2009 Available online 3 May 2009 Keywords: Emission spectroscopy Transition metal oxide Free radical abstract Emission spectra of WO have been observed in the 4000–35 000 cm 1 region using a Fourier transform spectrometer. Molecules were produced by exciting a mixture of WCl 6 vapor and He in a microwave dis- charge lamp. A 3 R  state has been assigned as the ground state of WO based on a rotational analysis of the observed bands and ab initio calculations. After rotational analysis, a majority of strong bands have been classified into three groups. Most of the transitions belonging to the first group have an X =0 + state as the lower state while the bands in the second group have an X 00 = 1 state as the lower state. These two lower states have been assigned as X0 + and X1 spin components of the X 3 R  ground state of WO. The third group consists of additional bands interconnected by common vibrational levels involving some very low-lying states. The spectroscopic properties of the low-lying electronic states have been predicted from ab initio calculations. The details of the rotational analysis are presented and an attempt has been made to explain the experimental observations in the light of the ab initio results. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction There has been considerable interest in the study of transition metal containing species due to their importance in organic and organometallic chemistry [1,2] as well as theoretical chemistry [3]. The spectroscopic studies of these molecules provide insight into chemical bonding in simple metal-containing systems. These studies are also needed to test and advance the quality of ab initio calculations. Because of relatively high cosmic abundances of these molecules, these species are also of astrophysical importance. In general, the electronic structure is not well understood because the complexity of the electronic spectra. The spectra of these mol- ecules are usually complex due to presence of a large number of electronic states derived from several close-lying electronic config- urations. The presence of open d-shells in transition metals gives rise to states with high spin and large orbital angular momenta. The components of high multiplicity states in these molecules are further split by substantial spin–orbit interactions which limits the validity of the usual Hund’s case (a) coupling scheme. In some cases the electronic states tend towards Hund’s case (c) coupling, which creates difficulty in the electronic assignments. WO is a very good example of such complexity. Although the visible bands of WO have been known for decades the electronic states remain poorly characterized. Initial observations of WO in the visible and near infrared re- gions were reported by Gatterer and Krishnamurthy [4] and Vitt- alachar and Krishnamurthy [5]. In a latter study Gatterer et al. [6] published the spectra of WO in an atlas which also included the spectra of many other transition metal oxides. Some WO bands were also observed in a shock tube [7] but the spectra remained largely unclassified. Weltner and McLeod [8] observed the elec- tronic spectra of WO in Ne and Ar matrices while Green and Ervin [9] measured the ground state vibrational frequency of WO in the Ar and Kr. The visible and near infrared bands observed by Weltner and McLeod [8] have been classified into several transitions which were labeled using letters ranging from A to G. The absorption spectra of WO have also been investigated by Samoilova et al. [10] who maintained the letter notation proposed by Weltner and McLeod [8], and also observed some additional bands. Samoil- ova et al. [10] obtained a rotational analysis of a number of bands in the visible and provided the first rotational constants for WO. The ground state vibrational constants obtained in this work were consistent with those obtained by Green and Ervin [9] in the ma- trix isolation spectra. Kuzyakov et al. [11,12] carried out an intra- cavity electronic absorption study of WO and obtained a rotational analysis of some additional bands and proposed new vibrational assignments in the A–X transition based on isotope shifts. More recently Lorenz et al. [13,14] obtained laser-induced fluorescence measurements of WO in solid neon and extended emission observations into the near infrared. In a recent paper Kraus et al. [15] observed the spectrum of the 0-0 band of the F– X transition by cavity ringdown laser absorption spectroscopy. 0022-2852/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2009.04.010 * Corresponding author. E-mail address: rram@u.arizona.edu (R.S. Ram). Journal of Molecular Spectroscopy 256 (2009) 216–227 Contents lists available at ScienceDirect Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms