Blackbody thermometry with cold molecular ions and application to ion-based frequency standards

We have used laser spectroscopy to measure the rotational level distribution of trapped molecular HD+ ions at translational temperatures in the milli Kelvin range. The HD+ ions are loaded into an ion trap by electron-impact ionization, and sympathetically cooled using laser-cooled Be+ ions which are already stored in the trap. Under our experimental conditions, the internal (rotational) degrees of freedom turn out to be independent of the translational degrees of freedom, and an effective rotational temperature close to room temperature is found. The near absence of background-gas collisions allows the rotational temperature to be related directly to the temperature of the ambient blackbody radiation (BBR). This feature suggests the use of molecular ions for BBR thermometry, which may help to improve the accuracy of frequency standards based on trapped atomic ions. For the spectroscopic measurement of the rotational populations, we propose a nondestructive technique