Further spectroscopic investigations of the high energy electronic states of SrOH: The B2Σ+(000)A2Π(000) and the D2Σ+(000)A2Π(000) transitions

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Abstract

The B2Σ+ and D2Σ+ states of SrOH were investigated using optical–optical double-resonance (OODR) spectroscopy. Rotational and fine structure parameters have been determined for these two states through a combined least-squares fit of the current OODR data along with the OODR data of the C2ΠA2Π transition, the optical data of the A2ΠX2Σ+ transition and the millimeter-wave pure rotational measurements of the X2Σ+ state. The spin–rotation constant, γ, of the B2Σ+ state was found to be 0.002653 cm−1, which is two orders of magnitude smaller than that of the B2Σ+ state (−0.1447 cm−1). This small γ value suggests that this state arises from a Sr+ atomic orbital of mainly 6 character. This atomic orbital assignment is also supported by the large rotational constant observed in the B2Σ+ state and the similarity of the molecular constants to those of the D2Σ+ state of CaOH. The rotational energy levels of the D2Σ+state of SrOH were found to be largely perturbed, which prohibited the accurate determination of the spin–rotation constant in this state. This perturbation is most likely due to an interaction with a 2Σ vibronic component of the nearby C2Π state.

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 F2Π 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 X2Σ+, A2Π, and B2Σ+ 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 C2Π, D2Σ+, E2Σ+, F2Π, and B2Σ+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 C2ΠA2Π transitions of SrOH [2] and SrOD [3] and the D2Σ+A2Π transition of CaOH [1]. Both the D2Σ+ and B2Σ+ states of SrOH are excellent candidates for study by this method and are of particular interest for high resolution investigations. The D2Σ+ state lies in a congested spectral region where many excited vibronic levels of the C2Π state exist and the B2Σ+ 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 B2Σ+A2Π and the D2Σ+A2Π 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 C2ΠA2Π transition, the optical data of the A2ΠX2Σ+ transition [6] and the millimeter-wave data of the X2Σ+ state [10] has been performed. From this fit, more precise spectroscopic molecular constants for the B2Σ+ and D2Σ+ states have been determined. The molecular structure and atomic orbital character of the B2Σ+ and D2Σ+ states will be discussed based on the observed molecular parameters. In addition, the analysis of these data suggests that the D2Σ+ state is severely perturbed, most likely by an excited vibronic component of the C2Π state.

Section snippets

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 B2Σ+A2Π1/2 transition of SrOH. The top panel of Fig. 1 shows an overview of the high-resolution spectrum of the B2Σ+A2Π1/2 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 X2Σ+ state arises mainly from the 5s atomic orbital on the strontium ion, while the first and second excited electronic states (A2Π and B2Σ+) 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 B2Σ+(000)A2Π1/2(000) and D2Σ+(000)A2Π3/2(000) 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 A2Π(000)X2Σ+(000) [6] transition and the millimeter-wave pure rotational data of the X2Σ+ state [10]. Rotational and fine structure constants have been determined for the B2Σ+ and D2Σ+ 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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