Observation of OD- by microwave spectroscopy.

For the first time the rotational spectrum of the molecular anion OD- has been observed by microwave spectroscopy. OD- has been produced both by a positive and negative glow discharge; by investigating the Doppler shift in both cases, the unshifted frequency of the J=1<--0 rotational transition has been derived. This allowed us to improve the value of the ground-state rotational constant: B(0)=299 264.940(90) MHz. In order to assess that a negative ion has really been observed, the cation N2H+ has been produced and investigated in the same experimental conditions.