Gerade Rydberg states and n s 3Σ+u(1u,0−u) photoionization spectra of the rare gas dimers (n=2–6)

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Abstract
Transient absorption spectra of rare gas dimer Rydberg transitions in the ultraviolet, visible, and near infrared (220≤λ≤900 nm) spectral regions have been observed in electron beam excited rare gases. The most prominent molecular bands for He2, Ne2, Ar2, Kr2, and Xe2 are assigned to the mp 3Πg←ns 3Σ+u (He:n =2, 3≤m≤10; Ne:n =3, 4≤m≤10; Ar:n =4, 5≤m≤15; Kr:n =5, 6≤m≤16; Xe:n=6, 8≤m≤11) Rydberg series of the dimer. Adiabatic ionization potentials, relative to the ns 3Σ+u state, are determined by extrapolation of the series to their limits (m→∞) to be 34 361.4±30 cm−1 (4.260±0.004 eV) for He2, 34 396.3±25 cm−1 (4.265±0.003 eV) for Ne2, 29 351.9±15 cm−1 (3.639±0.002 eV) for Ar2, 28 428.5±10 cm−1 (3.525±0.001 eV) for Kr2, and 26 664.5±250 cm−1 (3.306±0.031 eV) for Xe2. Absorption bands which are ascribed to m’p 3Σ+g←ns 3Σ+u Rydberg series of Ne2(m’=4,6,8), Ar2(5≤m’≤10), Kr2(6≤m’≤10), and Xe2(m’=8,9) are also reported, and the ns 3Σ+u ionization potentials found to be identical to the respective values quoted above. All of the observed molecular Rydberg states have an A 2Σ+1/2u ion core. The lowest vibrational quanta (ΔG1/2) for the ns 3Σ+u states are determined from vibronic structure in the Rydberg transitions to be 1723 cm−1 for He2, 539 cm−1 for Ne2, 302 cm−1 for Ar2, 172 cm−1 for Kr2, and 135 cm−1 for Xe2, and are consistent with previously reported values. The instability of the ns 3Σ+u metastable state with respect to the dimer ion core potential [D0(A 2Σ+1/2u)−D0(ns 3Σ+u)] is calculated to be 0.508±0.004 eV for He2, 0.681±0.003 eV for Ne2, 0.572±0.002 eV for Ar2, 0.560±0.001 eV for Kr2, and 0.52±0.03 eV for Xe2. Using these values and those reported in the literature for the dissociation energy of the dimer ion A 2Σ+1/2u state, D0(ns 3Σ+u) is estimated (Ne2: 0.67±0.07 eV; Ar2: 0.76±0.02 eV; Kr2: 0.62±0.02 eV; Xe2: 0.52±0.03 eV). By comparing energy level spacings, quantum defects, and absorption oscillator strengths for transitions between molecular or atomic states, the mp 3Πg Rydberg levels appear to correlate with the mp[3/2]2+1S0 atomic asymptotes. An important aspect of the experimental approach is that the diatomic excited states are studied in the vicinity of Re rather than at the van der Waals (ground state) minimum. Chemical forces (as opposed to ion‐induced dipole interactions) dominate at small R and the resulting spectra are largely free of congestion arising from the perturbations and avoided curve crossings that are prevalent at large R. Therefore, insight can be obtained into the structure and behavior of molecular Rydberg states in the region near the first ionization limit A 2Σ+1/2u[1(1/2)u].