The rotational spectrum of 17O2 up to the THz region

https://doi.org/10.1016/j.jqsrt.2015.08.011Get rights and content

Highlights

  • Pure rotational spectrum of the 17O2 in the electronic ground state.

  • Extension to the submillimeter-wave region: recording from 230 GHz up to 1.06 THz.

  • A joint experimental–theoretical investigation: a complete characterization of the hyperfine parameters.

Abstract

The investigation of the pure rotational spectrum of the 17O2 isotopic species of molecular oxygen has been extended with respect to previous investigations to the submillimeter-wave region, from 230 GHz up to 1.06 THz. The resulting spectroscopic parameters, which have an accuracy comparable to that of the constants obtained from an updated isotopic invariant fit involving data for three electronic states and six isotopologues [Yu et al. High resolution spectral analysis of oxygen. IV. Energy levels, partition sums, bandconstants, RKR potentials, Franck–Condon factors involving the X3Σg, a1Δg, and b1Σg+ states. J Chem Phys 2014;141:174302/1–12], permit the prediction of the pure rotational transitions up to 2 THz with an estimated uncertainty not greater than 100 kHz. In the present study, high-level quantum-chemical calculations guided, supported, and complemented the determination of the hyperfine parameters of 17O, with particular effort made in determining an accurate and reliable experimental value for the nuclear spin–rotation constant. A detailed discussion about the magnitude of the nuclear spin–rotation constant in the 17O-containing O2 species for both the ground and the first excited electronic state is presented.

Introduction

Molecular oxygen is the second most abundant component of the Earth's atmosphere. Its a1ΔgX3Σg and b1Σg+X3Σg electronic bands play a fundamental role in remote sensing of the atmosphere in relation, for example, to high-accuracy measurements of atmospheric greenhouse gases, such as CO2 and CH4 (see for example, Refs. [1], [2], [3], [4] and references therein). To this purpose, the accurate knowledge of the corresponding spectroscopic parameters is required and, recently, a lot of effort has been taken in this direction also for the rare isotopic species, as demonstrated, for example, by Refs. [1], [2], [3] and references therein. The growing interest on the rare isotopologues of molecular oxygen is related to the fact that their transitions have significant absorption strengths, thus high-precision spectroscopic parameters are required for their analysis [5]. Despite the importance of the rare O2 isotopologues for remote sensing applications, only a few high-resolution studies have actually been carried out.

Recently, Campargue and coworkers studied the very weak a1ΔgX3Σg system for the three 17O-containing isotopologues of molecular oxygen, namely 16O17O,17O18O and 17O2, by means of high-sensitivity CW-Cavity Ring Down Spectroscopy [2]. For 17O2, to derive accurate spectroscopic parameters, and in particular, the hyperfine coupling constants, the authors combined their data set with the microwave (MW) measurements from Cazzoli et al. [6], [7]. Prior to this work, the measurements carried out by Cazzoli and coworkers in the 80's were the only pure rotational investigations available for the double-substituted 17O2 in the X3Σg electronic ground state. In the last thirty years, rotational spectroscopy has seen noticeably improvements in the technology and sensitivity. For this reason, the present study intends to improve the work carried out in Refs. [6], [7] and to extend it to higher frequencies. In particular, a joint experimental–theoretical investigation of the hyperfine structure has been carried out aiming at the first complete characterization of all hyperfine parameters of 17O2: high-level quantum-chemical calculations have guided, supported and complemented the experimental determination and a critical analysis of the reliability and accuracy of the experimental nuclear spin–rotation constant in the 17O-containing species for both the ground and the first excited electronic state has been carried out.

Recently, as a part of ongoing atmospheric studies, Drouin and coworkers carried out a systematic analysis of the high-resolution spectroscopic data available for molecular oxygen with the aim of developing a Hamiltonian model capable of predicting all transition frequencies involving the three low-lying electronic states [8]. In Ref. [8] they presented a global Dunham analysis of the three low-lying electronic states that simultaneously reduced the data from all six isotopologues to an isotopically independent Hamiltonian. In a subsequent paper [9], the authors presented new pure rotational spectra for all substituted isotopologues in the vibrational v=0,1 states of the a1Δg state to demonstrate the accuracy of the predictions from Ref. [8]. This study, for example, led to the first determination of the nuclear quadrupole-coupling and nuclear spin–rotation constants for the 17O-substituted species in their a1Δg state. In the present work, the frequency predictions provided by Ref. [8] were used to guide the recording, thus providing a further confirmation of the reliability of the isotopic-invariant fit reported in Ref. [8]. More recently [10], the measurements performed in Refs. [9] and [3] have been used to update and improve the isotopically invariant Dunham fit reported in Ref. [8]. Ref. [10] provides the most accurate spectroscopic parameters for all molecular oxygen isotopologues, but unfortunately it does not report the hyperfine parameters for the 17O-containing isotopic species.

The paper is organized as follows. In the next section the effective Hamiltonian is described. Thereafter, the experimental and computational aspects of the investigation performed are addressed. Finally, our results concerning the spectroscopic parameters are reported and discussed.

Section snippets

Effective Hamiltonian

For open-shell species, the total angular momentum J is given by the coupling of the rotational angular momentum N, the electronic angular momentum L and the spin angular momentum S:J=N+L+S.The electronic ground state of the oxygen molecule is 3Σg. Hence, Λ (the quantum number associated to the component of the electronic angular momentum along the internuclear axis) = 0 and the effective Hamiltonian operator may be expressed in terms of the quantum numbers J and N corresponding to the total

Experimental details

Measurements were performed using a frequency-modulated computer-controlled spectrometer working from 65 GHz up to 1.6 THz [12], [13]. The actual frequency range considered is 230 GHz–1.06 THz. The millimeter-/submillimeter-wave sources employed, phase-locked to a rubidium frequency standard, are frequency multipliers driven by Gunn diode oscillators. While a detailed description of the spectrometer can be found in Refs. [13], [14], here we briefly summarize the relevant details. The modulation

Computational details

Quantum-chemical calculations of the spectroscopic parameters involved in the Hamiltonians above were carried out at the coupled-cluster (CC) singles and doubles (CCSD) approach augmented by a perturbative treatment of triple excitations (CCSD(T)) [15] together with the use of correlation consistent (aug-)cc-p(C)VnZ (n=D-6) basis sets [16], [17], [18], [19], [20]. Additional calculations were also carried out at the full CC singles, doubles, and triples (CCSDT) [21], and the CC singles,

Results and discussion

The new measurements together with those reported in Refs. [6], [7] were fitted using Pickett's SPFIT program [39], with each transition weighted proportionally to the inverse square of its experimental uncertainty. A total of 567 distinct frequency lines (65 are the newly detected), with uncertainties ranging from 30 to 100 kHz, were included in the fit, and led to the determination of 10 spectroscopic parameters with a root mean square deviation of 56 kHz. The results of the fit are collected

Concluding remarks

For the first time, the investigation of the pure rotational spectrum of 17O2 in its electronic ground state has been extended to the submillimeter-wave region, up to the THz frequency range. This allowed a significant reduction of the experimental uncertainty for all spectroscopic parameters with respect to the previous studies involving only the 17O2 isotopic species and obtaining an accuracy comparable to that of an isotopic invariant Dunham fit involving the rotational, vibrational, and

Supplementary materials

Supplementary data associated with this article (transition frequency values together with the corresponding observed – calculated differences) can be found in the online version at http://dx.doi.org/10.1016/j.jqsrt.2015.08.011.

Acknowledgements

This work has been supported in Bologna by ‘PRIN 2012’ funds (project “STAR: Spectroscopic and computational Techniques for Astrophysical and atmospheric Research”) and by the University of Bologna (RFO funds), and in Mainz by the Deutsche Forschungsgemeinschaft. C.P. acknowledges the COST CMTS-Action CM1405 (MOLIM: MOLecules In Motion).

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