Rydberg series and ionization potential of the H2 molecule

doi:10.1016/0022-2852(72)90064-1

Journal of Molecular Spectroscopy

Volume 41, Issue 3, March 1972, Pages 425-486

https://doi.org/10.1016/0022-2852(72)90064-1 Get rights and content

Abstract

The Rydberg series of ortho- and para-H₂ in the region 835-765 Å have been studied in absorption at low temperature under high resolution. In the spectrum of para-H₂ at 80°K [when practically all molecules are in the lowest rotational level (J = 0)] there are two Rydberg series converging to each vibrational level of the $X^{2} Σ_{g}^{+}$ ground state of H₂⁺. At low n, these are the npσ and npπ series, while at high n, complete l-uncoupling takes place and as a result the two series converge to the appropriate N = 0 and N = 2 levels of the H₂⁺ ion and are designated np0 and np2, respectively. Strong perturbations between these series are observed and have already been accounted for by an elegant theory of Fano; in the present paper a more elementary treatment of these perturbations is given. The perturbations arise from the coupling of the orbital motion of the Rydberg electron with the rotation of the H₂⁺ core. In addition to these rotational interactions (with Δv = 0) there are also strong interactions with the vibrations of the core which lead to perturbations whenever a Rydberg level corresponding to v + 1 is close to a Rydberg level corresponding to v (Δv = 1 perturbations). The interaction of the npλ Rydberg electron with both the rotational and the vibrational motion of the H₂⁺ core has been formulated in terms of δ_λ(r), the quantum defect and its dependence on the internuclear distance r.

The rotational interaction leads to preionization of the higher np2 levels above the N = 0 limit and this results in windows in the continuum of the np0 series, i.e., an apparent emission series. The vibrational interaction leads to preionization of the np series for v ≠ 0 above the v = 0 series limit. The observed line widths agree fairly well with those predicted.

The interaction parameters determined from the Rydberg series at high n account not only for the preionization rates but also for the deviations of the molecular constants at low n from those of the ion.

Once the Rydberg series had been deperturbed, accurate values for the series limits, i.e., for the rotational and vibrational levels of H₂⁺, were obtained. The absolute values of the limits are subject to a further correction because of a pressure shift of all Rydberg lines caused by the fairly high He pressure (40 mm) used in obtaining the spectra. This shift (to shorter wavelengths) is independent of n above n = 10 and under the conditions of the experiment (p = 40 mm, T = 80°K) amounts to 1.0₅ cm⁻¹ (corresponding to 5.7 ± 0.5 cm⁻¹/amagat). With this correction, the ionization potential of H₂ is found to be 124417.2 ± 0.4 cm⁻¹. The best theoretical value is 124417.3 cm⁻¹. Similar agreement is obtained for the dissociation energy of the H₂⁺ ion.

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¹: N.R.C.C. postdoctorate fellow 1968-70.

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Rydberg series and ionization potential of the H2 molecule

Abstract

J. Chem. Phys.

J. Chem. Phys.

Phys. Rev. Lett.

Phys. Rev. A

Phys. Rev. A

Z. Phys.

Z. Phys.

Z. Phys.

J. Chem. Phys.

J. Mol. Spectrosc.

J. Mol. Spectrosc.

J. Amer. Chem. Soc.

J. Amer. Chem. Soc.

J. Amer. Chem. Soc.

J. Mol. Spectrosc.

Nuovo Cimento

Chem. Phys. Lett.

Z. Phys.

Nuovo Cimento

Phys. Rev.

Phys. Rev. A

Rydberg series and ionization potential of the H₂ molecule