Observation of the v′=8←v=0 vibrational overtone in cold trapped HD

We report the observation of the hitherto undetected v ′=8← v =0 vibrational overtone in trapped HD + molecular ions, sympathetically cooled by laser-cooled Be + ions. The overtone is excited using 782 nm laser radiation, after which HD + ions in v =8 are photodissociated by the 313 nm laser used for Be + cooling. The concomitant loss of HD + is detected by the method of secular excitation (Roth et al. in Phys. Rev. A. 74:040501(R), 2006 ). We furthermore present details of the experimental setup, and we show that results from spectroscopy of v ′=8← v =0 overtones in combination with accurate ab initio calculations may yield a new value for the proton–electron mass ratio with an accuracy of order 1 ppb.