title: Precision Spectroscopy in Cold Molecules: The Lowest Rotational Interval of He_{2}^{+} and Metastable He_{2}.
abstract: 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. The ionization energy of metastable He_{2} and the lowest rotational interval of the X^{+} ^{2}Σ_{u}^{+} (ν^{+}=0) ground state of ^{4}He_{2}^{+} have been determined with unprecedented precision and accuracy by Rydberg-series extrapolation. Comparison with ab initio predictions of the rotational energy level structure of ^{4}He_{2}^{+} [W.-C. Tung, M. Pavanello, and L. Adamowicz, J. Chem. Phys. 136, 104309 (2012)] enabled us to quantify the magnitude of relativistic and quantum-electrodynamics contributions to the fundamental rotational interval of He_{2}^{+}.
authors.affiliation: Laboratory of Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Paul
authors.initials: P
authors.lastname: Jansen
authors.affiliation: Laboratory of Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Luca
authors.initials: L
authors.lastname: Semeria
authors.affiliation: Laboratory of Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Laura Esteban
authors.initials: LE
authors.lastname: Hofer
authors.affiliation: Laboratory of Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Simon
authors.initials: S
authors.lastname: Scheidegger
authors.affiliation: Laboratory of Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Josef A
authors.initials: JA
authors.lastname: Agner
authors.affiliation: Laboratory of Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Hansjürg
authors.initials: H
authors.lastname: Schmutz
authors.affiliation: Laboratory of Physical Chemistry, ETH Zurich, CH-8093 Zurich, Switzerland.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Merkt