Precision Spectroscopy of thenfTriplet Rydberg States ofH2and Determination of the Triplet Ionization Potential

Several triplet Rydberg $f$ states in molecular hydrogen were observed with a resolution of about 50 MHz under stepwise excitation. Extrapolation of the energy intervals to the Rydberg series limit gives a precise value for the ionization energy for ${\mathrm{H}}_2$ relative to the lowest triplet bound state, the ${}_c{}^{}{}_{}{}^{}\mathrm{}\left(2p\right)\mathrm{}{}_{}{}^3{}_{}{}^{}\Pi _u^{}{}_{}{}^{}$ state, and an improved value for the ionization potential of the ${\mathrm{H}}_2$ molecule in the ground state. The new value for the ionization potential is in excellent agreement with ab initio calculations and confirms the Lamb shift in the ground state of ${\mathrm{H}}_2$.