Precision Measurements in Few-Electron Molecules: The Ionization Energy of Metastable ^{4}He_{2} and the First Rotational Interval of ^{4}He_{2}^{+}.

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 ^{3}Σ_{u}^{+} state of He_{2}, corresponding to the energy interval between the a ^{3}Σ_{u}^{+} (v^{''}=0, N^{''}=1) state of He_{2} and the X^{+} ^{2}Σ_{u}^{+} (v^{+}=0, N^{+}=1) state of He_{2}^{+}, and of the lowest rotational interval of He_{2}^{+}. These measurements rely on the excitation of metastable He_{2} molecules to high Rydberg states using frequency-comb-calibrated continuous-wave UV radiation in a counterpropagating laser-beam setup.

Fundamental vibration frequency and rotational structure of the first excited vibrational level of the molecular helium ion ( ).

The term values of the rotational levels of the first excited vibrational state of the electronic ground state of with a rotational quantum number ≤ 13 have been determined with an accuracy of 1.2 × 10 cm (∼35 MHz) by multichannel-quantum-defect-theory-assisted Rydberg spectroscopy of metastable He. Comparison of the experimental term values with the most accurate results for available in the literature [W.

Determination of the Spin-Rotation Fine Structure of He_{2}^{+}.

Measuring spin-rotation intervals in molecular cations is challenging, particularly so when the ions do not have electric-dipole-allowed rovibrational transitions. We present a method, based on an angular-momentum basis transformation, to determine the spin-rotational fine structure of molecular ions from the fine structure of high Rydberg states. The method is illustrated by the determination of the so far unknown spin-rotation fine structure of the fundamentally important He_{2}^{+} ion in the X ^{2}Σ_{u}^{+} state.

Precision measurement of the rotational energy-level structure of the three-electron molecule He.

The term values of all rotational levels of the He X  Σ (ν=0) ground vibronic state with rotational quantum number N ≤ 19 have been determined with an accuracy of 8 × 10 cm (∼25 MHz) by multichannel-quantum-defect-theory-assisted Rydberg spectroscopy of metastable He. Comparison of these term values with term values recently calculated ab initio by Tung et al. [J.

Precision Spectroscopy in Cold Molecules: The Lowest Rotational Interval of He_{2}^{+} and Metastable He_{2}.

Multistage Zeeman deceleration was used to generate a slow, dense beam of translationally cold He_{2} molecules in the metastable a ^{3}Σ_{u}^{+} state. Precision measurements of the Rydberg spectrum of these molecules at high values of the principal quantum number n have been carried out. The spin-rotational state selectivity of the Zeeman-deceleration process was exploited to reduce the spectral congestion, minimize residual Doppler shifts, resolve the Rydberg series around n=200 and assign their fine structure.