The rotational spectrum of 17O2 up to the THz region
Introduction
Molecular oxygen is the second most abundant component of the Earth's atmosphere. Its and 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 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 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 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 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:The electronic ground state of the oxygen molecule is . 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).
References (42)
- et al.
High sensitivity CRDS of the band of oxygen near 1.27 umextended observations, quadrupole transitions, hot bands and minor isotopologues
JQSRT
(2010) - et al.
The band of 16O17O, 17O18O and 17O2 by high sensitivity CRDS near 1.27 μm
JQSRT
(2011) - et al.
The HITRAN 2012 molecular spectroscopic database
JQSRT
(2013) - et al.
Ground-based photon path measurements from solar absorption spectra of the O2 A-band
JQSRT
(2005) - et al.
Submillimeter wave spectrum of 17O2
Chem Phys Lett
(1985) - et al.
The Lamb-dip spectrum of methylcyanideprecise rotational transition frequencies and improved ground-state rotational parameters
J Mol Spectrosc
(2006) - et al.
Fifth-order perturbation comparison of electron correlation theories
Chem Phys Lett
(1989) - et al.
Gaussian basis sets for use in correlated molecular calculations. VI. Sextuple zeta correlation consistent basis sets for boron through neon
J Mol Struct (THEOCHEM)
(1996) - et al.
Higher excitations in coupled-cluster theory
J Chem Phys
(2011) - et al.
The hyperfine structure in the rotational spectra of D217O and HD17Oconfirmation of the absolute nuclear magnetic shielding scale for oxygen
J Chem Phys
(2014)