Further spectroscopic investigations of the high energy electronic states of SrOH: The and the transitions
Introduction
Recently we have been conducting a series of high resolution spectroscopic investigations focusing on the high energy (>20 000 cm−1) excited electronic states of CaOH and SrOH [1], [2], [3]. These states are of particular interest because the molecular geometry may differ from the linear configuration of the lower lying states. For example, extensive activity observed in the bending vibrational mode of the state of CaOH has suggested a non-linear structure for this state [4]. These geometric changes may be the result of the different atomic orbital character of the unpaired electron in the higher energy states as compared to the lower lying states and vibronic interactions between states.
For SrOH, the low-lying , , and states have been the subject of numerous detailed spectroscopic investigations [5], [6], [7], [8], [9], [10], [11], but studies of the higher energy states have been far more limited. The first observation of the higher energy electronic states of SrOH was reported by Beardah and Ellis [12], [13]. In their work they used pulsed laser excitation spectroscopy to record moderate resolution spectra of transitions originating from the 2Σ+ ground state to the , , , , and excited states in the range of 25 000–34 000 cm−1. High-resolution measurements, however, are desirable to more precisely determine rotational and fine structure parameters for each of these high-lying electronic states. In turn this allows for a more detailed understanding of the electronic and geometric structure of the molecule. To this end, we have recently applied the technique of optical–optical double-resonance (OODR) spectroscopy [14], [15] to record high resolution spectra of the transitions of SrOH [2] and SrOD [3] and the transition of CaOH [1]. Both the and states of SrOH are excellent candidates for study by this method and are of particular interest for high resolution investigations. The state lies in a congested spectral region where many excited vibronic levels of the state exist and the state does not appear to have an equivalent state in the isolelectronic molecule, SrF.
In this paper, we present a high resolution study of the and the transitions of SrOH using OODR spectroscopy. A least-squares fit that includes the assigned lines of each of these electronic transitions along with OODR data of the transition, the optical data of the transition [6] and the millimeter-wave data of the state [10] has been performed. From this fit, more precise spectroscopic molecular constants for the and states have been determined. The molecular structure and atomic orbital character of the and states will be discussed based on the observed molecular parameters. In addition, the analysis of these data suggests that the state is severely perturbed, most likely by an excited vibronic component of the state.
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Experimental
A detailed description of the experimental apparatus and method used to synthesize SrOH in the gas phase has been given previously [2]. Briefly, Sr vapor was produced by resistively heating strontium metal in a graphite crucible using a Broida-type oven. The metal vapor was then entrained in a flow of argon carrier gas (1–5 Torr) and directed into the reaction region, where ∼5 mTorr of concentrated hydrogen peroxide was introduced through a perforated ring above the crucible. A weak
Results and analysis
The moderate-resolution study of SrOH by Beardah and Ellis [13] was used to determine the approximate frequency range to scan the probe laser in this investigation of the transition of SrOH. The top panel of Fig. 1 shows an overview of the high-resolution spectrum of the transition in the 11445–11480 cm−1 range. This spectrum exhibits a Hund’s case (b) 2Σ+–Hund’s case (a) 2Π transition branch structure [17]. In the top panel of Fig. 1, the pump laser frequency was
Discussion
The low-lying electronic states of the alkaline-earth metal containing molecules are largely derived from the atomic orbital character of the unpaired electron on the metal atom [19], [20]. For example, in SrOH the state arises mainly from the 5s atomic orbital on the strontium ion, while the first and second excited electronic states and ) are correlated to the 5p and 4d Sr+ atomic orbitals. Theoretical calculations [21] based on a ligand field approach have sought to quantify
Conclusion
High resolution spectra of the and transitions of SrOH have been recorded using optical–optical double-resonance spectroscopy. The observed frequencies of the rotational lines were fitted in combination with the optical data of the [6] transition and the millimeter-wave pure rotational data of the state [10]. Rotational and fine structure constants have been determined for the and states. The dominant
Acknowledgment
Financial support for this work was provided by the Natural Sciences and Engineering Research Council (NSERC) of Canada.
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Present address: Department of Chemistry, University of York, Heslington, York YO10 5DD, UK.
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Present address: Department of Chemistry and Biochemistry, Canisius College, Buffalo, NY 14208-1098, USA.