High-resolution spectroscopy and quantum-defect model for the gerade triplet np and nf Rydberg states of He2

Abstract

Precision measurement of the rotational energy-level structure of the three-electron molecule He 2 + The data were combined with spectroscopic data on low-lying triplet np and nf Rydberg states from the literature to derive energy-and internuclear-distance-dependent eigenquantum-defect parameters of multichannel quantum-defect theory (MQDT). The MQDT calculations reproduce the experimental data within their experimental uncertainties and enabled the derivation of potential-energy curves for the lowest triplet p Rydberg states (n = 2-5) of He 2 . The eigenquantum-defect parameters describing the p -f interaction were found to be larger than 0.002 at the energies corresponding to the high-n Rydberg states, so that the p -f interaction plays an important role in the autoionization dynamics of np Rydberg states with

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

“…However, all npN + N + −1 Rydberg states with N + > 1 located above the N + − 2 ionization thresholds are subject to rapid autoionization, because of the strong coupling, indicated by the curved arrows in Fig. 2, between channels of the same value of N , but differing in N + by 2 [37,42]. The corresponding transitions are strongly broadened and thus not detectable in our high-resolution spectra.…”

“…[32]) in the X + 2 Σ + u (ν + = 0) state of He2 + (full squares). The experimental results of Sprecher et al [37] are plotted as diamonds. The dashed line represents the corrections to the energy levels of molecular hydrogen that were scaled to the relativistic correction of the fundamental vibration in 3 He 4 He + (see text).…”

“…High-n Rydberg states were not used in the fit, because the level energies of high-n Rydberg states are insensitive to adjustments of the quantum defects, a fact that is also exploited in the present analysis (see Ref. [37] for details).…”

“…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].…”

Slow and velocity-tunable beams of metastableHe2by multistage Zeeman deceleration

“…Such calculations become increasingly challenging as the number of electrons increases, and the magnitudes of the relativistic and quantum-electrodynamics corrections are not accurately known for He + 2 . In a recent high-resolution spectroscopic study of the threshold ionization spectrum of He * 2 , the level spacings between the lowest four rotational levels of He + 2 could be determined at an accuracy of 100 MHz [35]. Comparison with the latest ab-initio calculations neglecting quantum-electrodynamics corrections [36] did not reveal significant deviations from the experimental results at this level of accuracy, except perhaps for the highest rotational level observed experimentally.…”