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Precision spectroscopy of the hydrogen molecule is a test ground of quantumelectrodynamics (QED), and may serve for determination of fundamentalconstants. Using a comb-locked cavity ring-down spectrometer, for the firsttime, we observed the Lamb-dip spectrum of the R(1) line in the overtone of HD.The line position was determined to be 217 105 182.79$\pm0.03_{stat}\pm0.08_{syst}$ MHz ($\delta\nu/\nu=4\times 10^{-10}$), which isthe most accurate transition ever measured for the hydrogen molecule. Moreover,from calculations including QED effects up to the order $m_e\alpha^6$, weobtained predictions for this R(1) line as well as for the HD dissociationenergy, which are less accurate but signaling the importance of the completetreatment of nonadiabatic effects. Provided that the theoretical calculationreaches the same accuracy, the present measurement will lead to a determinationof the proton-electron mass ratio with a precision of 1.3 parts per billion.

Toward a determination of the proton-electron mass ratio from the Lamb-dip measurement of HD