Precision Spectroscopy in Cold Molecules: The Lowest Rotational Interval ofHe2+and MetastableHe

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}^{+}.

Precision measurement of the rotational energy-level structure of the three-electron molecule He2+

“…The adiabatic ionization energy of He 2 (a 3 Σ + u ), defined as the interval between the a 3 Σ + u (ν = 0, N = 1) state of He 2 and the X + 2 Σ + u (ν + = 0, N + = 1) state of He 2 + , is found to be 34 301.205 65(12) ± 0.0014 sys cm −1 , as already reported in Ref. [38]. Table III summarizes the main sources of systematic and statistical uncertainty in the determination of ionization thresholds and He 2 + rotational intervals.…”

“…Experimental measurements of the low-lying levels of 4 He 2 + are complicated by the fact that 4 He 2 + is apolar and has no electric-dipole-allowed vibrational and rotational transitions. Recently we have started systematic measurements of the energy-level structure of H 2 + and He 2 + by multichannel-quantum-defect-theory(MQDT)assisted Rydberg spectroscopy of H 2 [7,9,12,36] and He 2 [37,38]. In the case of He 2 + , the procedure consists of measuring high-resolution spectra of triplet (S = 1), np Rydberg states of very high principal quantum number n from the a 3 Σ + u metastable state of He 2 and extrapolating the Rydberg series to their limits using MQDT [39].…”

Multistage Zeeman decelerator for molecular-scattering studies

“…Specifically, formation of metastable He 2 molecules in the a 3 state (from here on He 2 *) is expected, as is also observed in the experiments by Motsch et al and Jansen et al that use a similar discharge source [51,52]. However, He 2 * is indistinguishable from He* in our detection system.…”

Magnetic Trapping of Cold Methyl Radicals

“…One immediate application of the trapped cold radicals is precision spectroscopy [15] for the determination of molecular parameters of the radicals precisely by resolving hyperfine structures in infrared transitions [41]. Another application is the observation of cold reactive collisions.…”

“…Since molecules with unpaired electron(s) always have a nonzero magnetic moment, it is natural to use magnetic fields for the deceleration, i.e., Zeeman deceleration [7][8][9][10][11][12], and as well as for trapping [13] of these reactive species. So far, only two diatomic molecules, He 2 [14,15] and O 2 [10][11][12], have been successfully decelerated by Zeeman deceleration, except for our preliminary work on methyl radicals (CH 3 ) [16]. The magnetic trappings of CaH [17], CaF [18], NH [19,20], and OH [21] were reported following buffer gas cooling, and optical loading as well as Stark deceleration.…”