The consequences of improperly describing oscillator strengths beyond the electric dipole approximation

The interaction between a quantum mechanical system and plane wave light is usually modeled within the electric dipole approximation. This assumes that the intensity of the incident field is constant over the length of the system and transition probabilities are described in terms of the electric di...

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Bibliographic Details
Published in:The Journal of chemical physics 2015-12, Vol.143 (23), p.234103-234103
Main Authors: Lestrange, Patrick J, Egidi, Franco, Li, Xiaosong
Format: Article
Language:English
Subjects:
Approximation
Dipole moments
Electric dipoles
Magnetic dipoles
Markov analysis
Mathematical analysis
Multipoles
Oscillator strengths
Physics
Plane waves
Quadrupoles
Quantum mechanics
Transition probabilities
X ray absorption
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Summary:The interaction between a quantum mechanical system and plane wave light is usually modeled within the electric dipole approximation. This assumes that the intensity of the incident field is constant over the length of the system and transition probabilities are described in terms of the electric dipole transition moment. For short wavelength spectroscopies, such as X-ray absorption, the electric dipole approximation often breaks down. Higher order multipoles are then included to describe transition probabilities. The square of the magnetic dipole and electric quadrupole are often included, but this results in an origin-dependent expression for the oscillator strength. The oscillator strength can be made origin-independent if all terms through the same order in the wave vector are retained. We will show the consequences and potential pitfalls of using either of these two expressions. It is shown that the origin-dependent expression may violate the Thomas-Reiche-Kuhn sum rule and the origin-independent expression can result in negative transition probabilities.
ISSN:0021-9606
1089-7690
DOI:10.1063/1.4937410