Investigating the Electronic States of Baoh by V-Type Double Resonance
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Journal of Molecular Spectroscopy 270 (2011) 44–50 Contents lists available at SciVerse ScienceDirect Journal of Molecular Spectroscopy journal homepage: www.elsevier.com/locate/jms Investigating the electronic states of BaOH by V-type double resonance spectroscopy e and ab initio calculations: Further evidence of perturbation from the A02D state ⇑ J.D. Tandy a, J.-G. Wang a, J. Liévin b, P.F. Bernath a, a Department of Chemistry, University of York, Heslington, York YO10 5DD, United Kingdom b Service de Chimie Quantique et Photophysique, Université Libre de Bruxelles, CPi 160/09, 50 av F.D. Roosevelt, B-1050 Bruxelles, Belgium article info abstract e e þ Article history: A single band belonging to the A02D À X 2R band system has been rotationally analyzed for each of the Received 19 July 2011 two isotopologues, BaOH and BaOD, using high-resolution V-type optical–optical double resonance spec- In revised form 30 August 2011 troscopy. BaOH and BaOD molecules were synthesized in a Broida-type oven. High-resolution spectra Available online 8 September 2011 e þ e þ were recorded by monitoring the dip in fluorescence of the B2R À X 2R transition excited by a sin- gle-mode ring dye laser (pump laser), whilst a single-mode Ti:Sapphire laser scanned the corresponding e e þ Keywords: A02D À X 2R transition. The observed spectra resemble a typical 2PÀ2R transition, believed to emanate High-resolution e from single or triple quanta of the bending vibration in the A02D state. Measured rotational lines have Laser excitation been assigned and rotational and fine structure parameters determined through a combined least- BaOH e 2 þ BaOD squares fit with the millimeter-wave pure rotational data of the X R state. Previous analyses of the e2 e 2 þ V-type double resonance A P À X R transitions of BaOH and BaOD yielded significantly different spin–orbit coupling constants, Ab Initio which were attributed to possible global and local perturbations arising from vibrationally-excited bands e e of the A02D state. Although the newly observed A02D state bands have not been conclusively assigned a 2 specific spin state, the derived X-doubling constants show significant P1=2 character, further indicating e e strong interactions between the A2P and A02D states of BaOH. To validate these conclusions, ab initio cal- e þ culations have been carried out to further understand the nature of the BaOH excited states. The D02R , e þ e e þ e e e þ D2R , C 2P, B2R , A2P, A02D and X 2R states have been characterized by means of multireference con- figuration interaction calculations using the MOLPRO software. Calculated vertical term energies show relatively good agreement with existing optical data. Ó 2011 Elsevier Inc. All rights reserved. e02 e 2 þ 1. Introduction A D5=2 À X R transition [3]. We recently reported high-resolution e e þ analyses of the A2Pð000ÞX 2R ð000Þ transitions of BaOH and Previous spectroscopic analyses of the excited electronic states BaOD that yielded an unexpected and considerable difference be- e of BaOH have been somewhat limited but have all shown a degree tween the two spin–orbit constants, A, for the A2P states of the e þ e þ e e of complexity and ambiguity [1–5]. The D2R À X 2R and C À X two molecules [4,5]. This difference was suggested to result from e transitions of BaOH have been observed at relatively low-resolution strong global perturbations originating from the lower-lying A02D by Pooley et al. in a supersonic expansion using laser ablation [1]. state. These perturbations were also indicated by the apparent fail- Two band systems exhibiting 2PÀ2Rþ character between 20000 ure of the pure precession and unique perturber models in both À1 e e and 22000 cm (tentatively named the C À X spectral region of BaOH and BaOD and the unrealistic r0 (OH) bond length, calculated e BaOH) were observed but unable to be assigned unambiguously. from the B values of the A2P states. e e The apparent anomalous behavior of the C 2P state was attributed A detailed examination of the A02D state is now required to fur- to either Renner–Teller effects or a perturbing 2D state in close ther understand the interactions between the low-lying electronic proximity. Kinsey-Nielsen et al. first published an analysis of the states of BaOH. Barrow and co-workers previously examined the e þ e þ e B2R À X 2R transition that showed evidence of parity dependant A02D state of the isoelectronic species BaF at low-resolution and e perturbations from interactions with the A2P state [2]. The found the vibrational levels to be interleaved with those of the e e þ e e þ e A2P À X 2R and A02D À X 2R transitions were then investigated A2P state [6]. It was also noted that the vibrational spacing in the e at low-resolution by Fernando et al. who tentatively assigned two A02D state is of the same order as the spin–orbit splitting, potentially À1 2 2 bands at 11784 and 11304 cm to a stretching sequence of the causing strong perturbations between the D5=2 and D3=2 spin com- ponents. The same group then examined six electronic excited ⇑ Corresponding author. Fax: +44 1904 432516. states of BaF via high-resolution Fourier transform spectroscopy e02 E-mail address: [email protected] (P.F. Bernath). including the A D state. The following deperturbation analysis led 0022-2852/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.jms.2011.08.009 J.D. Tandy et al. / Journal of Molecular Spectroscopy 270 (2011) 44–50 45 to large effects on the energies and spin–orbit coupling constants of to process the fluorescence signal from the PMT. Spectra were re- e þ e e 1 the 5d states (B2R ; A2P and A02D) [7,8]. Indeed, their prior investi- corded in 5 cmÀ segments at a scan speed of 3 GHz per second gations on BaH and BaD showed the corresponding 5d states formed with a data sampling interval of 50 MHz. The absolute frequency a mutually perturbing triad [9]. It is therefore quite possible that of the dye laser was calibrated by simultaneously recording the these observations may also be true in the low-lying states of BaOH emission spectrum of an I2 cell [17]. The rotational lines of the e e þ e þ and thus a high-resolution analysis of the A02D state should assist in B2R À X 2R transitions were identified by referring to the previ- verifying these predictions. ous work of Kinsey-Nielsen et al. [2]. Ab initio calculations are also useful in the investigation of small The dye laser was then used to pump specific rotational lines of e þ e þ metal-containing species [10–16]. In particular the use of complete the B2R À X 2R transition whilst the Ti:S laser scanned through e e þ active space self-consistent field (CASSCF) and multi-reference the resonant frequencies of the A02D À X 2R transition. All double e e þ configuration interaction (MRCI) approaches has proved to be ade- resonance signals corresponding to rotational A02D À X 2R transi- quate for characterizing the excited-state electronic structure of tions with an identical J00 value in the common lower electronic e þ such systems and for predicting their vertical term energies. A sim- state (i.e. the X 2R state) were observed as dips in the monitored e þ e þ ilar investigation of the excited electronic states of BaOH may B2R À X 2R fluorescence. The overlap of the two laser beams therefore yield valuable information regarding the character of was carefully adjusted to maximize any double resonance signal these states and the interactions between them. obtained. Spectra were recorded in 5 cmÀ1 segments at a scan The present paper reports our high-resolution study of two speed of 1 GHz per second with a data sampling interval of e bands lying in the A02D state spectral region of BaOH and BaOD 30 MHz. The absolute frequency of the titanium:sapphire laser using V-type optical–optical double resonance spectroscopy. Rota- was calibrated by simultaneously recording the absorption spec- tional and fine structure constants have been determined for both trum of a heated I2 cell [17]. bands through a combined least-squares fit. Tentative assignments for the two bands are discussed with reference to potential pertur- e bations with the close-lying A2P state. Calculated vertical term 3. Results and analysis energies for the six lowest-lying excited electronic states e þ e þ e e þ e e (D02R ; D2R ; C 2P; B2R ; A2P and A02D) of BaOH are presented 3.1. V-type double resonance spectroscopy and compared to existing experimental data. Analyses of the singly e þ occupied molecular orbital (SOMO) populations for each state are Previous analyses of the B2R states of BaOH and BaOD [2] were e þ e þ also discussed. first utilized to locate the B2R ð000ÞX 2R ð000Þ transitions, which were then scanned using the ring cavity dye laser. With these assignments it was possible to select specific e þ e þ 2. Experimental B2R ð000ÞX 2R ð000Þ rotational transitions to pump whilst the Ti:Sapphire laser scanned the spectral region of interest in an BaOH and BaOD molecules were generated by the reaction of Ba attempt to locate the corresponding rotational lines of an e02 e 2 þ metal vapor with H2O2 or D2O vapor respectively in a Broida-type A D À X R transition.