A femtosecond optical frequency comb and continuous-wave pulse-amplified laser were used to measure 12 transition frequencies of antiprotonic helium to fractional precisions of 9-16 10 ÿ9 . One of these is between two states having microsecond-scale lifetimes hitherto unaccessible to our precision laser spectroscopy method. Comparisons with three-body QED calculations yielded an antiproton-to-electron mass ratio of M p =m e 1836:152 6745. DOI: 10.1103/PhysRevLett.96.243401 PACS numbers: 36.10.ÿk, 06.20.Dk, 14.20.Dh, 32.70.Jz We report here new measurements on the transition frequencies of antiprotonic helium atoms ( pHe e ÿ ÿ p ÿ 4 He 2 ) [1] using a femtosecond optical frequency comb [2,3] in conjunction with a continuous-wave (cw) pulse-amplified laser (Fig. 1). Their experimental precision is a factor 6 -20 better than our previous best ones [4], and now approaches those of, e.g., the 1 1 s-2 1 s [5] and 1 1 s-2 1 p [6] transitions in ordinary helium. From the frequencies of 12 transitions measured to the Doppler-broadened limit at a cryogenic temperature of 10 K, we have deduced the mass and charge of the antiproton relative to both the proton and the electron with a precision of the order of the known proton-to-electron mass ratio [7].Reference [4] describes how a radio-frequency quadrupole decelerator was used to slow down the antiprotons emerging from the CERN Antiproton Decelerator to 100-keV energies. They were then stopped in a helium target of low atomic density 10 18 cm ÿ3 to produce pHe atoms which filled a volume V 100 cm 3 . Antiprotons in pHe states with high principal (n 38) and angular momentum (') quantum numbers reach the helium nucleus over a period of several microseconds. The resulting delayed annihilation time spectra (DATS), i.e., the annihilation rate versus time elapsed since pHe formation, was measured by Cherenkov counters [ Fig. 2(a)]. In all but one of the present experiments, linearly polarized laser pulses of energy density " 0:04-1 mJ=cm 2 (e.g., applied here at t 1 s) stimulated transitions with dipole moments 0.02 -0.3 D from these pHe states, to states with nanosecond-scale lifetimes against Auger emission [1] and annihilation. The resulting peak in the DATS signaled the resonant frequency.Only pulsed lasers can provide the megawatt-scale intensities needed here to induce the pHe transitions. However, fluctuations in their frequency and linewidth and the difficulty of calibrating the wide range of pHe wavelengths 264:7-726:1 nm have so far limited our experimental precision [4]. We have now circumvented these problems by basing our experiments on a cw laser whose frequency cw could be stabilized with a precision <4 10 ÿ10 against an optical comb. Its intensity was then amplified [6,8,9] by a factor 10 6 to produce a pulsed laser beam of frequency pl cw with an accuracy and resolution 1-2 orders of magnitude higher than before [4].This was done as follows: First, a Nd:YVO 4 laser (Coherent Verdi, B in Fig. 1