HIGH PRECISION ROVIBRATIONAL SPECTROSCOPY OF OH⁺

Abstract

The molecular ion OH+ has long been known to be an important component of the interstellar medium. Its relative abundance can be used to indirectly measure cosmic ray ionization rates of hydrogen, and it is the first intermediate in the interstellar formation of water. To date, only a limited number of pure rotational transitions have been observed in the laboratory making it necessary to indirectly calculate rotational levels from high-precision rovibrational spectroscopy. We have remeasured 30 transitions in the fundamental band with MHz-level precision, in order to enable the prediction of a THz spectrum of OH+. The ions were produced in a water cooled discharge of O2, H2, and He, and the rovibrational transitions were measured with the technique Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy. These values have been included in a global fit of field free data to a 3Σ− linear molecule effective Hamiltonian to determine improved spectroscopic parameters which were used to predict the pure rotational transition frequencies.

OH+and H2O+absorption toward PKS 1830–211

“…The transition frequencies of the N =1-0 transition were recorded by Bekooy et al (1985) with ∼ 1 MHz accuracy. The THz spectrum of OH + has recently been revised by Markus et al (2016) and frequencies of the N =1-0 rotational transitions are predicted with an accuracy better than 1 MHz (i.e., < 0.3 km s −1 ). The J=0-1 fine structure (FS) component near 909 GHz is the weakest FS component and thus least prone to be affected by optical depth effects.…”

Improved Ultraviolet and Infrared Oscillator Strengths for OH+

“…Recent precise laboratory measurements of the fundamental mode (Markus et al 2016) have spurred renewed efforts to improve the available line list of OH + in support of vibrational astronomical searches. We have analyzed near-UV spectra of the A 3 Π-X 3 Σ − band system, which were fit with the combined infrared and submillimeter measurements, to improve the predicted rest frequencies of the IR and UV transitions (Hodges & Bernath 2017).…”

Improving cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and Brewster-plate spoilers

“…The narrower sub-Doppler feature could allow for resolving hyperfine structure. In a recent NICE-OHVMS study of OH + , transitions with hyperfine components separated by ∼120 MHz were treated as intensityweighted averages [10]. A narrower central Lamb dip would enable resolved structure which would improve the predictions of the pure rotational spectra.…”

“…With NICE-OHVMS, we have successfully measured a number of transitions of H + 3 , HeH + , and OH + with the highest precision to date [5,[8][9][10]. However, insufficient signal-to-noise (S/N) of many transitions has limited the extent of these studies, and only the frequencies of the strongest transitions within the fundamental bands were determined with sub-MHz precision.…”