Communication: Visible line intensities of the triatomic hydrogen ion from experiment and theory

The visible spectrum of H₃⁺ is studied using high-sensitivity action spectroscopy in a cryogenic radiofrequency multipole trap. Advances are made to measure the weak ro-vibrational transitions from the lowest rotational states of H₃⁺ up to high excitation energies providing visible line intensities and, after normalisation to an infrared calibration line, the corresponding Einstein B coefficients. Ab initio predictions for the Einstein B coefficients are obtained from a highly precise dipole moment surface of H₃⁺ and found to be in excellent agreement, even in the region where states have been classified as chaotic.