Hyperfine-Resolved Rovibrational and Rotational Spectroscopy of OH$^+$ ($X ^3\Sigma^-$)

TL;DR

This study used high-resolution IR, THz spectroscopy, and ion trapping to precisely measure and refine the hyperfine-resolved rovibrational and rotational spectra of the OH$^+$ radical cation, including first-time measurements.

Contribution

It presents the first high-accuracy measurements of the N=2 rotational transition of OH$^+$ and refines spectroscopic constants using combined experimental and literature data.

Findings

First measurement of N=2 rotational transition at 2 THz with microwave accuracy.

Refined spectroscopic constants for OH$^+$, especially in the ground vibrational state.

High-precision determination of hyperfine components affected by Zeeman splittings.

Abstract

The OH$^+$ ($X ^3\Sigma^-$) radical cation has been investigated by combining a 4 K 22-pole ion trap apparatus with high-resolution IR and THz radiation sources. Applying different types of action spectroscopic methods, the fundamental vibrational band in the 3 $\mu$m range and the spin manifold of the $N=1 \leftarrow 0$ rotational transition around 1 THz have been extended and refined. Additionally, the spin manifold of the $N=2 \leftarrow 1$ rotational transition, scattered around 2 THz, has been measured for the first time with microwave accuracy. Although all hyperfine components of the pure rotational transitions are affected by considerable Zeeman splittings, a simulation of their contours allowed us to extract the field-free center frequencies with high accuracy. A global fit combining rovibrational and pure rotational transitions from the literature with those newly obtained in this work was performed, leading to improvements in the spectroscopic constants of OH$^+$, particularly those in the ground vibrational state.