We reexamine the invariance of two-photon transition matrix elements and corresponding two-photon Rabi frequencies under the "gauge" transformation from the length to the velocity gauge. It is shown that gauge invariance, in the most general sense, only holds at exact resonance, for both one-color as well as two-color absorption. The arguments leading to this conclusion are supported by analytic calculations which express the matrix elements in terms of hypergeometric functions, and ramified by a "master identity" which is fulfilled by off-diagonal matrix elements of the Schrödinger propagator under the transformation from the velocity to the length gauge. The study of the gauge dependence of atomic processes highlights subtle connections between the concept of asymptotic states, the gauge transformation of the wave function, and infinitesimal damping parameters for perturbations and interaction Hamiltonians that switch off the terms in the infinite past and future [of the form exp(-ε|t|)]. We include a pertinent discussion.
U. D. Jentschura, "Nonresonant Two-Photon Transitions in Length and Velocity Gauges," Physical Review A, vol. 94, no. 2, American Physical Society (APS), Aug 2016.
The definitive version is available at https://doi.org/10.1103/PhysRevA.94.022117
Keywords and Phrases
Hamiltonians; Photons; Wave functions, Analytic calculations; Asymptotic state; Damping parameters; Gauge invariance; Gauge transformation; Hypergeometric functions; Off-diagonal matrix elements; Two-photon transitions, Linear transformations
International Standard Serial Number (ISSN)
Article - Journal
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