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Light–Matter Interaction
Published in Thomas C. Weinacht, Brett J. Pearson, Time-Resolved Spectroscopy, 2018
Thomas C. Weinacht, Brett J. Pearson
A nice tool for visualizing this basic physics is to consider a funnel shaped spiral wishing well, where one drops coins and watches them spiral into the center of the well (see Figure 3.1). The shape of the well approximates a Coulomb potential (1/r), and two pennies spinning around the center represent two electrons in an atom. The penny closest to the center represents a core electron, while the one near the outer edge represents a loosely bound valence electron (perhaps in a highly excited Rydberg state). For such well-separated electrons, it is reasonable to neglect their interaction and consider them to be independent particles. The application of an external field is mimicked by tilting the wishing well. If one tilts the well slowly in one direction (comparable to a strong, DC electric field), the outer penny is affected more by the tilt and will leave the well more readily (just as the outer electron in a two-electron atom is affected more by a strong DC field). If the well is tilted back and forth, similar to an atom in an oscillating electric field, the frequency of the tilting determines which of the two pennies is more affected. Tilting the well at a frequency corresponding to the orbital frequency of either penny will result in that penny responding more to the field.
Symmetry, Spin, and Statistics, 1926-1930
Published in John C.D. Brand, Lines of Light, 2017
The Dennison treatment of H2 was confirmed from an unexpected direction. In 1929 Bonhoeffer and Harteck19 discovered that the modifications of H2 exchanged at measurable rates in the presence of a catalyst, a surface that acted by reversible adsorption of hydrogen as atoms. Exposed to the catalyst at low temperature, molecules pooled down into the J = 0 level and so accumulated as an even-J modification. Adopting Heisenberg’s notation for the orbital functions of a two-electron atom (such as He), Bonhoeffer named the modifications ortho- and para-hydrogen. In absence of catalysts, para-hydrogen, the even-J modification in the ground state, was stable for weeks at room temperature but reverted gradually to the 1:3 mixture imposed by the statistical weights. Mulliken7 thought the ortho/para notation was “inadvisable” on the ground that it already applied to atoms with two outer electrons; but in the long run it was the application to states of atoms, better denoted triplet or singlet, that fell from favor and eventually died out.
Theoretical approaches for doubly-excited Rydberg states in quasi-two-electron systems: two-electron dynamics far away from the nucleus
Published in Molecular Physics, 2021
In singly-excited Rydberg states, only one electron of the quasi-two-electron atom is excited. In this case, the two electrons occupy largely different regions of phase space. They can be considered as independent and are consequently well described by a single electronic configuration (e.g. in Mg). The energies of singly-excited Rydberg states follow in most cases the Rydberg formula where Ry is the Rydberg constant and is the quantum defect, which depends on the orbital angular momentum l of the Rydberg electron. The development of multichannel quantum-defect theory (MQDT) [30–33] has offered a unified picture of the physical origin of quantum defects and of the local deviations from the Rydberg formula observed, for example, in Sr, Ba or Yb (see, e.g. Refs. [34–36], respectively). Quantum defects are understood as arising from the elastic scattering of the Rydberg electron off the core, which results in the appearance of an additional phase shift in the Rydberg-electron wave function. Inelastic scattering couples different Rydberg series together and this interaction induces local displacements of the energy levels. In MQDT, a large number of Rydberg states and series can be described by only a small number of scattering parameters, that can either be fitted to reproduce experimental data or calculated ab inito [31,33,37–39].