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Physics LibreTexts

12.7: Electric Dipole Approximation

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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(ikr)=1+ikr+,

can be approximated by its first term, unity. This approach is known as the electric dipole approximation. It follows that f|ϵpexp(ikr)|iϵf|p|i.

Now, it is readily demonstrated that [r,H0]=ipme

so f|p|i=imef|[r,H0]|i=imeωfif|r|i.

Thus, our previous expressions for the transition rates for radiation induced absorption and stimulated emission reduce to

wabsif=πϵ02|ϵdif|2ρ(ωfi),wstmif=πϵ02|ϵdif|2ρ(ωif),

respectively. Here, dif=f|er|i
is the effective electric dipole moment of the atom when making a transition from state i to state f.

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),

which implies that |ϵdif|2=d2ifsin2θcos2ϕ.
We must now average the previous quantity over all possible values of θ and ϕ. Thus, |ϵdif|2av=d2ifsin2θcos2ϕdΩ4π,
where dΩ=sinθdθdϕ, and the integral is taken over all solid angle. It is easily demonstrated that |ϵdif|2av=d2if3.
Here, d2if stands for

d2if=|f|ex|i|2+|f|ey|i|2+|f|ez|i|2.

Hence, the transition rates for absorption and stimulated emission induced by unpolarized isotropic radiation are wabsif=π3ϵ02d2ifρ(ωfi),wstmif=π3ϵ02d2ifρ(ωif),
respectively.

Contributors and Attributions

  • Richard Fitzpatrick (Professor of Physics, The University of Texas at Austin)


This page titled 12.7: Electric Dipole Approximation is shared under a not declared license and was authored, remixed, and/or curated by Richard Fitzpatrick.

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