Abstract
Photoionization spectra and Rydberg-state-resolved threshold-ionization spectra of the gerade triplet np Rydberg states of (4)He2 located in the vicinity of the X(+) (2)Σ(u)(+) (ν(+) = 0) ionization threshold were recorded from the 2sσ a (3)Σ(u)(+) metastable state. An accuracy of 0.01 cm(-1) was achieved for the experimental term values of the observed Rydberg states. 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 He2. 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 v(+) = 0. By extrapolating the experimental term values of triplet np Rydberg states of (4)He2 in the range of principal quantum number n between 87 and 110, the positions of the (v(+) = 0, N(+) = 3) and (v(+) = 0, N(+) = 5) levels of the ground state of (4)He(+)(2) were determined to lie 70.937(3) cm(-1) and 198.369(6) cm(-1), respectively, above the (v(+) = 0, N(+) = 1) ground rotational level.
Highlights
Atomic helium has the lowest polarizability1 (0.205 Å3) and the largest ionization energy2 (198310.6664(2) cm−1)of all neutral elements and is chemically very inert.3 The potential-energy well of He2 in the X 1+ g ground state is so shallow that it only supports a single quantum state (v= 0, N = 0) for 4He2 with a dissociation energy D0(4He2)= 0.00112(2) cm−1.4–6 The X+ 2+ u ground state ofHe+2 responds to the electronic configuration (1sσ g)2(1sσ u)1.corHe+2 has a covalent bond of order 1/2 with a Born-Oppenheimer binding The rovibrational energy energyDe(He+2 ) levels of= 19954.583 cm−1.7 He+2 serve as benchmark quantities for ab initio calculations of three-electron systems.7–16 In contrast to the numerous theoretical studies reprioerdteoduot.nFHoer+2th, eonmlyaianfenwatuerxaplelyrimocecnutrarlinstgudisioestohpaovme ebre,e4nHcea+2r, no electric-dipole-allowed rotational and vibrational spectra exist
By changing to the detection scheme of Rydberg-state-resolved threshold-ionization (RSR-TI) spectroscopy,32 which restricts the detection to electrons produced by field ionization of long-lived Rydberg states, the spectral congestion could be considerably reduced
By combining the data with spectroscopic data on low-lying triplet np and nf Rydberg states from the literature22–29 a new set of energy- and R-dependent eigenquantum-defect parameters of multichannel quantum-defect theory was derived using a methodology established by Ch
Summary
Atomic helium has the lowest polarizability (0.205 Å3) and the largest ionization energy (198310.6664(2) cm−1). In the experiments presented in this article, an accuracy of 0.01 cm−1 was achieved in the determination of the term values of high-np Rydberg states of 4He2 and in the extrapolation of the observed Rydberg series to their limits using an extended MQDT model. This model includes internuclear-distance- and energy-dependent eigenquantumdefect functions and considers the effects of the p -f interaction, which is known to induce perturbations in the structure of the np and nf Rydberg states of He2.23,27 Using this model, a global description of the triplet np Rydberg states of. States can be calculated within their experimental uncertainties, Born-Oppenheimer potential-energy curves of the 2–5p Rydberg states were derived, and the corresponding leading adiabatic corrections were estimated
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