High-resolution infrared spectroscopy of OH in a cryogenic ion trap.

The protonated oxygen molecule, OH, and its helium complex, He-OH, have been investigated by vibrational action spectroscopy in a cryogenic 22-pole ion trap. For the He-OH complex, the frequencies of three vibrational bands have been determined by predissociation spectroscopy. The elusive OH has been characterized for the first time by high-resolution rovibrational spectroscopy via its ν OH-stretching band. Thirty-eight rovibrational fine structure transitions with partly resolved hyperfine satellites were measured (56 resolved lines in total). Spectroscopic parameters were determined by fitting the observed lines with an effective Hamiltonian for an asymmetric rotor in a triplet electronic ground state, X̃A, yielding a band origin at 3016.73 cm. Based on these spectroscopic parameters, the rotational spectrum is predicted, but not yet detected.