Observation of the infrared spectrum of the triatomic molecular ionH2D+

The infrared vibrational-rotational spectrum of the ${\mathrm{H}}_2$ ${\mathrm{D}}^{+}$ molecular ion has been observed. Nine resonances between 1800 and 2000 ${\mathrm{cm}}^{-1\mathrm{}}$ have been measured to a precision of 0.0005 ${\mathrm{cm}}^{-1\mathrm{}}$ or 0.3 ppm (at 68% confidence level). This triatomic molecular ion is the simplest asymmetric top in nature. The quantum numbers of the observed transitions have not yet been unambiguously identified. This species is of great astrophysical interest in view of its important role in the chemistry of molecular clouds in the interstellar medium.