12.7: Electric Dipole Approximation
( \newcommand{\kernel}{\mathrm{null}\,}\)
In general, the wavelength of the type of electromagnetic radiation that induces, or is emitted during, transitions between different atomic energy levels is much larger than the typical size of an atom. Thus, exp(ik⋅r)=1+ik⋅r+⋯,
Now, it is readily demonstrated that [r,H0]=iℏpme
so ⟨f|p|i⟩=−imeℏ⟨f|[r,H0]|i⟩=imeωfi⟨f|r|i⟩.
wabsi→f=πϵ0ℏ2|ϵ⋅dif|2ρ(ωfi),wstmi→f=πϵ0ℏ2|ϵ⋅dif|2ρ(ωif),
Equations ([e13.97]) and ([e13.98]) give the transition rates for absorption and stimulated emission, respectively, induced by a linearly polarized plane-wave. Actually, we are more interested in the transition rates induced by unpolarized isotropic radiation. To obtain these we must average Equations ([e13.97]) and ([e13.98]) over all possible polarizations and propagation directions of the wave. To facilitate this process, we can define a set of Cartesian coordinates such that the wavevector k, which specifies the direction of wave propagation, points along the z-axis, and the vector dif, which specifies the direction of the atomic dipole moment, lies in the x-z plane. It follows that the vector ϵ, which specifies the direction of wave polarization, must lie in the x-y plane, because it has to be orthogonal to k. Thus, we can write k=(0,0,k),dif=(difsinθ,0,difcosθ),ϵ=(cosϕ,sinϕ,0),
d2if=|⟨f|ex|i⟩|2+|⟨f|ey|i⟩|2+|⟨f|ez|i⟩|2.
Contributors and Attributions
Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)