Heavy Rydberg and ion-pair states: chemistry, spectroscopy and theory

Abstract

Recent advances in our knowledge of heavy Rydberg and ion-pair states are critically reviewed, with emphasis placed on the close kinship between the two. Heavy Rydberg states are long-range vibrational states, reaching far beyond $\gg {10}^2$ Å for higher levels. Enhanced chemical reactivity and efficient energy transfer are frequently encountered. Unusual physical properties result from the large dipole moments, including laser-induced reactions and amplified spontaneous emission, and are discussed in the context of the underlying electronic structure. Heavy Rydberg states have a rich spectroscopy which is amenable to quantum defect analysis, as illustrated for a wide range of UV and VUV spectra previously analyzed in terms of Dunham coefficients. The lifetimes of heavy Rydberg states can be long, enabling them to be isolated in cryogenic matrices or as high angular momentum states in the gas phase. Heavy Rydberg and electronic Rydberg states often occupy the same energy region and this, together with the high density of heavy Rydberg vibrational levels, leads to vibronic mixing and numerous perturbations that are a fertile field for analysis by multichannel quantum defect theory and reactive scattering calculations.

Keywords:

• Rydberg states
• heavy Rydberg states
• ion-pair states
• charge transfer
• spectroscopy
• collisions
• gas-phase reactions
• quantum defect theory
• dynamics

Acknowledgments

This review is dedicated in memory of Professor David Buckingham CBE FRS FAA. The authors thank Dr Trevor Ridley for his assistance with an early draft of this review. AK wishes to express his deep gratitude to Dr Christian Jungen for his generous support and inspiration, both scientifically and personally.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1 The Born-Huang ansatz can lead to nonvanishing couplings at large internuclear distances, but in practice this is mostly a formal issue, see e.g. refs. [201, 268].

2 Ahmed Zewail was awarded the chemistry Nobel Prize for femtochemistry in 1999.

3 A similar analysis showing that stabilization occurs as a consequence of phase interference between nuclear wave functions can also be carried out using multichannel quantum defect theory, see [256].

4 We also note more recent experimental observations of heavy Rydberg tates in HD by Beyer and Merkt [269], which have not yet been matched by theoretical calculations.

Additional information

Funding

AK acknowledges funding from the EPSRC (EP/V049240/1 and EP/V006819/1), the Leverhulme Trust (RPG-2020-208), and the Swedish Collegium for Advanced Studies supported by the Erling-Persson Family Foundation and the Knut and Alice Wallenberg Foundation.