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.
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