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

Photoionization spectra and Rydberg-state-resolved threshold-ionization spectra of the gerade triplet np Rydberg states of ⁴He₂ located in the vicinity of the X^+ ^2Σ _u^+ (v^+=0) ionization threshold were recorded from the 2sσ a ^3Σ _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 ⁴He₂ 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 ⁴He_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.