Dissociation Energy of the Hydrogen Molecule at 10-9 Accuracy

The ionization energy of ortho-H2 has been determined to be EIo(H2)/(hc)=124 357.238 062(25) cm-1 from measurements of the GK(1,1)-X(0,1) interval by Doppler-free, two-photon spectroscopy using a narrow band 179-nm laser source and the ionization energy of the GK(1,1) state by continuous-wave, near-infrared laser spectroscopy. EIo(H2) was used to derive the dissociation energy of H2, D0N=1(H2), at 35 999.582 894(25) cm-1 with a precision that is more than one order of magnitude better than all previous results. The new result challenges calculations of this quantity and represents a benchmark value for future relativistic and QED calculations of molecular energies.