Observation of the infrared spectrum of the helium molecular ion(3He4He)+

The vibration-rotation spectrum of the helium molecular ion ${(}^3$He ${}_{}{}^4{}_{}{}^{}$He${)}^{+}$ has been observed for the first time. The Doppler-tuned ion-beam laser-resonance technique was used. Nine transitions in the fundamental band have been measured with an uncertainty of 0.0006 ${\mathrm{cm}}^{\mathrm{-}1}$ or 0.3 ppm. The accuracy exceeds that of the available theory by more than 3 orders of magnitude.