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Journal of Physics B Atomic and Molecular Physics

On the derivation of the Kramers-Heisenberg dispersion formula from non-relativistic quantum electrodynamics

W P Healy, R. Guy Woolley

Published Apr 14, 1978• 11 citations
Publisher

Abstract

It is shown that the on-energy-shell single-particle scattering amplitudes obtained from the minimal-coupling and the multipolar forms of the Hamiltonian in non-relativistic quantum electrodynamics are equal to order e 2 ; both forms of the Hamiltonian thus lead to the same differential cross section for Kramers-Heisenberg scattering. The proof given is based directly on the relations which hold between the two transition matrices and is simpler than a previous proof which relied on sum rules derived from the canonical commutation relations. The proof is also more general in that the multipolar Hamiltonian is defined in terms of line integrals along paths which are not assumed to be straight lines but which can be chosen from a large class of possible curves.

DOI
10.1088/0022-3700/11/7/013

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It is shown that the on-energy-shell single-particle scattering amplitudes obtained from the minimal-coupling and the multipolar forms of the Hamiltonian in non-relativistic quantum electrodynamics are equal to order e 2 ; both forms of the Hamiltonian thus lead to the same differential cross section for Kramers-Heisenberg scattering. The proof given is based directly on the relations which hold between the two transition matrices and is simpler than a previous proof which relied on sum rules derived from the canonical commutation relations. The proof is also more general in that the multipolar Hamiltonian is defined in terms of line integrals along paths which are not assumed to be straight lines but which can be chosen from a large class of possible curves.
A generalization of Dirac's transformation of the off-resonance single-photon scattering amplitude that leads to the Kramers-Heisenberg dispersion formula is described. The amplitude is first calculated to order e2{e}^{2}e2 in perturbation theory with the minimal-coupling interaction Hamiltonian as perturbing energy, and is then transformed so that the effects of multipole transition moments of all orders and types (electric, magnetic, and diamagnetic) readily become manifest. No appeal is made to any multipolar expansions, however, as exact integral expressions are used for the summed series. It is shown that the same dispersion formula is obtained by using the multipolar Hamiltonian partitioned in the usual way, and this despite the fact that the unperturbed Hamiltonians are different in the two calculations.
The electromagnetic field operators in the Heisenberg picture are found in both the minimal and multipolar forms of nonrelativistic quantum electrodynamics within the electric dipole approximation. These are made up of free- and source-field terms. The free electric- and magnetic-field operators in the two formalisms differ since the creation/annihilation operators are manifestly distinct. Their explicit forms in the Heisenberg picture are given. However, the source fields are formally the same when expressed in terms of dipole moment operators at positive retarded time. The cancellations that lead to this equivalence are presented. In many applications it is necessary to express the operators at retarded time in terms of creation and annihilation operators at the initial time. Explicit expressions for the fields up to second order in terms of the transition dipole moments are given. These enable the formulas for induced dipole moments to be derived within the context of quantum electrodynamics whereas previous derivations have been based on semiclassical electrodynamics.
In this paper we use Dirac's relativity quantum mechanics to derive the well-known Kramers-Heisenberg dispersion formula for an atom with one electron. The treatment is not limited to the case of a central field, but is quite general. An expression is also obtained for the dipole moment to which is due the incoherent scattering. The formulae obtained are similar to those obtained by O. Klein for the case of a central field. We find in this way an explicit expression for f, the number of dispersion electrons for any line of the optical spectrum (being a measure of the intensity of the line), in terms of the solutions of the four wave equations of Dirac's theory. It is further shown that the sum of the number of dispersion electrons for any state of the atom is not exactly unity, but differs from it by an amount of the order of 10−4. The result Σf = 1 has been shown by London to hold exactly for the simple wave equation as originally given by Schrödinger. It is here shown that the exact relativity treatment has a very small effect.
The equations of motion and the class of Hamiltonians for electrodynamics, both classical and quantal, are considered for the interaction of radiation with bound charges within the electric-dipole approximation. It is emphasized that, despite the uniqueness of the equations of motion for physical variables, the form of the Hamiltonian is not unique and that a physical system is associated with an equivalence class of Hamiltonians. Two of the members of this class are the minimal-coupling and multipolar forms extensively used in quantum optics. It is shown that although different choices of Hamiltonians lead to different first-order equations of motion that connect canonical coordinates and momenta, they lead to the same second-order equations of motion for the physical variables and physical electromagnetic fields. In the quantum theory of electrodynamics the role played by the choice of representation for the operators and for the states is analyzed with care. The choice of representation leads to a freedom additional to that provided by canonical transformations in classical electrodyamics, although the two lead to the same operator equations of motion.
The method of Weyl transforms is used to rigorously derive path integral forms for position and momentum transition amplitudes from the time-dependent Schrödinger equation for arbitrary Hermitian Hamiltonians. It is found that all paths in phase space contribute equally in magnitude, but that each path has a different phase, equal to 1/h/ times an ’’effective action’’ taken along it. The latter is the time integral of p⋅q̇−h (p,q), h (p,q) being the Weyl transform of the Hamiltonian operator H, which differs from the classical Hamiltonian function by terms of order h/2, vanishing in the classical limit. These terms, which can be explicitly computed, are zero for relatively simple Hamiltonians, such as (1/2M)[P−eA (Q)]2+V (Q), but appear when the coupling of the position and momentum operators is stronger, such as for a relativistic spinless particle in an electromagnetic field, or when configuration space is curved. They are always zero if one opts for Weyl’s rule for forming the quantum operator corresponding to a given classical Hamiltonian. The transition amplitude between two position states is found to be expressible as a path integral in configuration space alone only in very special cases, such as when the Hamiltonian is quadratic in the momenta.