High orbital angular momentum states in H2 and D2. III. Singlet–triplet splittings, energy levels, and ionization potentials

Spectral identification of diffuse resonances in H2 above the n = 2 dissociation limit

The resonance structure in molecular hydrogen above the n = 2 dissociation limit is experimentally investigated in a 1 XUV + 1 VIS coherent two-step laser excitation process, with subsequent ionization of H(n = 2) products. Diffuse spectral features exhibiting widths of several cm(-1) in the excitation range of 118,500-120,500 cm(-1) are probed. Information on angular momentum selection rules for parallel and crossed polarizations, combination differences, the para-ortho distinction, extrapolation from rovibrational structure in the bound region below the n = 2 threshold, and mass-selective detection of H(2)(+) parent and H(+) daughter fragments is used as input. This allows for an assignment of the diffuse resonances observed in terms of (1)Σ(g)(+), (1)Π(g), and (1)Δ(g) states, specified with vibrational and rotational quantum numbers.

High-resolution millimeter wave spectroscopy and multichannel quantum defect theory of the hyperfine structure in high Rydberg states of molecular hydrogen H2

Experimental and theoretical methodologies have been developed to determine the hyperfine structure of molecular ions from detailed studies of the Rydberg spectrum and have been tested on molecular hydrogen. The hyperfine structure in l=0-3 Rydberg states of H2 located below the X 2Sigmag+(v+=0,N+=1) ground state of ortho H2+ has been measured in the range of principal quantum number n=50-65 at sub-MHz resolution by millimeter wave spectroscopy following laser excitation to np and nd Rydberg states using a variety of single-photon and multiphoton excitation sequences. The np1(1), nd1(1), and the nf1(0-3) Rydberg states were found to be metastable and to have lifetimes of more than 5 micros beyond n=50. Members of other series, such as the nd1(2), nd1(3), and the np1(0) series, were found to have lifetimes of more than 1 mus. Local perturbations induced by low-n Rydberg states belonging to series converging on rovibrationally excited levels of H2+ reduce the lifetimes in narrow ranges of n values. The hyperfine structure is strongly dependent on the value of the orbital angular momentum l. In the penetrating s and p states at n approximately 50 the exchange interaction dominates over the hyperfine interaction and the levels can be labeled by the total electron spin angular momentum quantum number S (S=0 or 1). In the less penetrating d and f Rydberg states, the hyperfine interaction between the core nuclear and electron spins is larger than the exchange interaction and the Rydberg states are of mixed singlet and triplet character. A procedure based on the Stark effect and on the systematic analysis of selection rules and combination differences was developed to determine the orbital and the total angular momentum quantum numbers l and F and to construct an energy map of p and f Rydberg levels between n=54 and 64 with relative positions of an accuracy of better than 1 MHz. Multichannel quantum defect theory (MQDT) was extended to treat the hyperfine structure in molecular Rydberg states and was used to analyze the observed hyperfine structure of the p and f Rydberg states of H2. The frame transformation between the Born-Oppenheimer channels described by the angular momentum coupling scheme (abetaJ) and the asymptotic channels described by the (e[bbetaS+]) coupling scheme was derived and enables an elegant treatment of all intermediate coupling cases. Purely ab initio quantum defect theory reproduced the experimentally determined positions to within 40 MHz for the p levels and 13 MHz for the f levels. By slight adjustments of the quantum defect functions and their energy dependences and by consideration of the p-f interaction, of the singlet-triplet splittings of the f levels, and of the departure of the ionic levels from pure coupling case (bbetaS+), the agreement between theory and experiment could be improved to 600 kHz. By comparing the results of MQDT calculations of the hyperfine structure of f Rydberg levels with those of coupled equations calculations, the frame transformation approximation of MQDT was shown to be accurate to within 300 kHz. The extrapolated ionic hyperfine structure of the X 2Sigmag+(v+=0,N+=1) ionic level corresponds to the ab initio prediciton of Babb and Dalgarno [Phys. Rev. A 46, R5317 (1992)] within the experimental error.

Highly accurate transition frequencies in the H2 Lyman and Werner absorption bands

The transition frequencies of low-J excitation lines in the B1Σu+– X1Σg+,(ν′,0) Lyman bands have been measured for ν′ = 2–18 and in the C1Πu–X1Σg+,(ν′,0) Werner bands for ν′ = 0–4, using a narrow-band tunable extreme UV laser source at wavelengths λ = 92–105 nm in conjunction with the technique of 1 +  1′ two-photon ionization. The measurements can be divided into three categories for which varying absolute accuracies were obtained. Special focus was on the B, ν′ = 2–5 bands, where an accuracy of 0.004 cm–1 or δv/v = 4 x 10–8 is achieved. For transitions to B, ν′ ≤ 13 and C, ν′ ≤ 3 states the accuracy is 0.005 cm–1 or δv/v = 5 x 10–8. Due to a different frequency mixing scheme uncertainties for B, ν′ ≥ 13 and C, ν′ = 4 are at the level of 0.011 cm–1 or δv/v = 1.1 x 10–7. Inspection of combination differences between R(J) and P(J + 2) lines shows that the accuracies are even better than estimated in the error budget. Based on the measurements of 138 spectral lines and the known combination differences, transition frequencies of 60 P-lines could be calculated as well, so that a data base of 198 accurately calibrated lines results for the Lyman and Werner bands of H2. Key words: vacuum UV, molecular spectroscopy, hydrogen, precision metrology.

Dissociation energies of molecular hydrogen and the hydrogen molecular ion

We have obtained improved values for the dissociation energies of molecular hydrogen and its ion by using a high-resolution pulse-amplified laser to probe the second dissociation limit. The onset of the vibrational continuum is observed by state-selective detection of the atomic products of dissociation, and several auxiliary measurements link the results to the ground state. The dissociation energies are accurate to 0.010-0.026 cm(-1), improving previous measurements by a factor of 3-7. Agreement with ab initio calculations is good for H2, D2, and their ions, but not for HD and HD+.

New Scheme for Measuring the Angular Momentum Spatial Anisotropy of Vibrationally Excited H2 via the I 1Πg State

We report the spectroscopic detection of vibrationally excited molecular hydrogen using 2+1 resonantly enhanced multiphoton ionization (REMPI) via the I