PRECISION MEASUREMENT OF THE IONIZATION ENERGY OF METASTABLE He2

Predicting the energy-level structure of molecules from first principles is one of the major goals and tasks of theoretical molecular physics and chemistry. Few-electron molecules are particularly attractive systems for comparison with experimental results because numerically ``exact'' predictions of molecular properties can in principle be obtained, i.e., predictions that are only limited in accuracy by the uncertainties of fundamental constants. We present absolute-frequency measurements of transitions from rotational levels of the a $^3\Sigma_\text{u}^+\,(v''=0)$ metastable state of He${_2}$ to $n$p Rydberg states. The transition frequencies are determined by one-photon UV spectroscopy in slow molecular beams using a narrow-band laser system referenced to a frequency comb. The ionization energy of metastable He$_2$ and the rotational structure of the X$^+$ $^2\Sigma_\text{u}^+\,(v^+=0)$ ground state of He${_2}^+$ have been determined with unprecedented precision and accuracy using Rydberg-series extrapolation