Dissociation energy of the hydrogen molecule at 10$^{-9}$ accuracy

Abstract: The ionization energy of ortho-H$2$ has been determined to be $E^\mathrm{o}\mathrm{I}(\mathrm{H}2)/(hc)=124\,357.238\,062(25)$ 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. $E^\mathrm{o}\mathrm{I}$(H$2$) was used to derive the dissociation energy of H$_2$, $D^{N=1}{0}$(H$_2$), 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.