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# Visible transitions from ground state${\rm{H}}_{\rm{3}}^{\rm{ + }}$measured with high-sensitivity ac...

Journal Title: Philosophical Transactions: Mathematical, Physical and Engineering Sciences , , Philosophical Transactions: Mathematical, Physical and Engineering Sciences, 370, 2012, 1978, p. 5028-5040 E-Article English The Royal Society
Summary:

We report on the recent observation of new spectral lines of cold ${\rm{H}}_{\rm{3}}^{\rm{ + }}$ ions lying well in the visible spectral region. Transitions from the two lowest ro-vibrational levels to final levels up to 16 700 cm⁻¹, almost half way to the dissociation limit, have been measured, involving up to eight vibrational quanta. The observed transitions are more than six orders of magnitude less intense than the $v_2^1$ fundamental band and yet another order of magnitude weaker than reached by previous sensitive action spectroscopy in the near-infrared region. The measurements were carried out in a cryogenic 22-pole ion trap with ${\rm{H}}_{\rm{3}}^{\rm{ + }}$ ions cooled to their lowest rotational levels by helium buffer gas. Laser-induced chemical reactions lead to the formation of ArH⁺ ions detected with single-ion sensitivity. These visible measurements, together with the previous near-infrared measurements, have helped to further develop empirically corrected calculations and have provided essential benchmarks for new ab initio calculations that now reach a spectroscopic accuracy of 0.1 cm⁻¹ on average up to the highest observed transition. Highly sensitive action spectroscopy and the attained high-accuracy predictions will enable us to find and measure transitions even further into the visible region of ${\rm{H}}_{\rm{3}}^{\rm{ + }}$ , paving the way towards the dissociation limit.

5028-5040 1364-503X