Observation of the Infrared Spectrum of the Helium-Hydride Molecular IonHe4H+

We have made the first observation of the vibrational-rotational spectrum of the electronic ground state of the helium-hydride molecular ion ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ ${\mathrm{H}}^{+}$. The Doppler-tuned ion-beam laser-spectroscopic technique was used. The frequencies of five transitions between 1700 and 1900 ${\mathrm{cm}}^{-1\mathrm{}}$ have been measured to ± 0.002 ${\mathrm{cm}}^{-1\mathrm{}}$ (±1 ppm). The experimental values deviate from current theory by typically 0.2 ${\mathrm{cm}}^{-1\mathrm{}}$, and are two orders of magnitude more precise than the theoretical values.