RADIO PHYSICS AND RADIO ASTRONOMY

Subject and Purpose. The work is aimed at investigating the spectra of the ground-state and a few of the excited vibrational states for the main isotopologues of the selenium dioxide molecule, SeO₂, in order to provide a reliable basis for its further search in the interstellar medium.

Method and methodology. The method is based on real-life measurements and onward analysis of the microwave rotational spectrum of the SeO₂ molecule. The measurements are carried out with an automated millimeter wave spectrometer of the Institute of Radio Astronomy of the NAS of Ukraine, (Kharkiv, Ukraine), and the submillimeter-wave spectrometer of the V. N. Karazin Kharkiv National University (Kharkiv, Ukraine).

Results. The measurements were carried out within a range between 70GHz and 500 GHz, where frequencies of about 650 rotational transitions were measured. Most of these belong to the ⁷⁶SeO₂ , ⁷⁷SeO₂ , ⁷⁸SeO₂ , ⁸⁰SeO₂ , and ⁸²SeO₂ molecules in their respective vibrational ground states. The rest of the lines can be assigned to isotopic species of the ⁷⁶SeO₂ , ⁷⁷SeO₂ , ⁷⁸SeO₂ , ⁸⁰SeO₂ , and ⁸²SeO₂ molecules in their excited vibrational state ν₂ = 1, while in the case of ⁸⁰SeO₂ in the excited state ν₂ = 2 as well. In addition, tentative assignments have been suggested for 10 transitions in the ground -state isotopic species of ⁷⁷SeO₂.

Conclusions. The transitions between states characterized by quantum numbers J up to 70 and quantum numbers K_ɑ up to 19, embracing the ground-state and first-, and second-order excited vibrational states of the ⁷⁴SeO₂ , ⁷⁶SeO₂ , ⁷⁷SeO₂ , ⁷⁸SeO₂ , ⁸⁰SeO₂ , and ⁸²SeO₂ molecules have been identified. Based on these assigned transitions, significantly improved estimates have been obtained for sets of rotational and centrifugal distortion constants, including octic ones, as well as for A-reduced Watson’s Hamiltonian in the I^r coordinate representation. The parameter sets obtained provide for reliable predictions for possible future astronomical search of the most abundant isotopic species of the SeO₂ molecule.

Key words: selenium dioxide molecule; millimeter wave spectrum; asymmetric top; isotopic substitution

Manuscript submitted 29.01.2025

Radio phys. radio astron. 2025, 30(2): 141-158

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