Precision Measurements in Few-Electron Molecules: The Ionization Energy of Metastable <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>He</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math> and the First Rotational Interval of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msubsup><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi>He</mml:mi></mml:mrow><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>4</mml:mn></mml:mrow></mml:mmultiscripts></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:mrow></mml:math>

Abstract

Molecular helium represents a benchmark system for testing ab initio calculations on few-electron molecules. We report on the determination of the adiabatic ionization energy of the $a\text{ }{^{3}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ state of ${\mathrm{He}}_{2}$, corresponding to the energy interval between the $a\text{ }{^{3}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ (${v}^{\ensuremath{'}\ensuremath{'}}=0$, ${N}^{\ensuremath{'}\ensuremath{'}}=1$) state of ${\mathrm{He}}_{2}$ and the ${X}^{+}\text{ }{^{2}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$ (${v}^{+}=0$, ${N}^{+}=1$) state of ${\mathrm{He}}_{2}^{+}$, and of the lowest rotational interval of ${\mathrm{He}}_{2}^{+}$. These measurements rely on the excitation of metastable ${\mathrm{He}}_{2}$ molecules to high Rydberg states using frequency-comb-calibrated continuous-wave UV radiation in a counterpropagating laser-beam setup. The observed Rydberg states were extrapolated to their series limit using multichannel quantum-defect theory. The ionization energy of ${\mathrm{He}}_{2}$ ($a\text{ }{^{3}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$) and the lowest rotational interval of ${\mathrm{He}}_{2}^{+}$ (${X}^{+}\text{ }{^{2}\mathrm{\ensuremath{\Sigma}}}_{u}^{+}$) are $34\text{ }301.207\text{ }002(23)\ifmmode\pm\else\textpm\fi{}0.000\text{ }03{7}_{\text{syst}}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$ and $70.937\text{ }589(23)\ifmmode\pm\else\textpm\fi{}0.000\text{ }06{0}_{\text{syst}}\text{ }\text{ }{\mathrm{cm}}^{\ensuremath{-}1}$, respectively.

Topics & Concepts