Rovibrational spectroscopy of trapped molecular hydrogen ions at millikelvin temperatures

We report a high-resolution spectroscopic study of molecular ions at millikelvin temperatures. We measured several rovibrational infrared transitions in HD{sup +} molecular ions, stored in a radio-frequency trap and sympathetically cooled to {approx_equal}20 mK by laser-cooled Be{sup +} ions. We observed hyperfine splitting of the lines, in good agreement with theoretical predictions. The transitions were detected by monitoring the decrease in ion number after selective photodissociation of HD{sup +} ions in the upper vibrational state. The method described here is expected to be generally applicable.