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12.12: Intensity Rules

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Now, we know, from Section [s12.8], that when we take electron spin and spin-orbit coupling into account the degeneracy of the six 2P states of the hydrogen atom is broken. In fact, these states are divided into two groups with slightly different energies. There are four states characterized by the overall angular momentum quantum number j=3/2—these are called the 2P3/2 states. The remaining two states are characterized by j=1/2, and are thus called the 2P1/2 states. The energy of the 2P3/2 states is slightly higher than that of the 2P1/2 states. In fact, the energy difference is ΔE=α216E0=4.53×105eV.

Thus, the wavelength of the spectral line associated with the 2P1S transition in hydrogen is split by a relative amount Δλλ=ΔEω=4.4×106.
Note that this splitting is much greater than the natural line-width estimated in Equation ([e13.144a]), so there really are two spectral lines. How does all of this affect the rate of the 2P1S transition?

Well, we have seen that the transition rate is independent of spin, and hence of the spin quantum number ms, and is also independent of the quantum number m. It follows that the transition rate is independent of the z-component of total angular momentum quantum number mj=m+ms. However, if this is the case then the transition rate is plainly also independent of the total angular momentum quantum number j. Hence, we expect the 2P3/21S and 2P1/21S transition rates to be the same. However, there are four 2P3/2 states and only two 2P1/2 states. If these states are equally populated—which we would certainly expect to be the case in thermal equilibrium, because they have almost the same energies—and given that they decay to the 1S state at the same rate, it stands to reason that the spectral line associated with the 2P3/21S transition is twice as bright as that associated with the 2P1/21S transition.

Contributors and Attributions

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


This page titled 12.12: Intensity Rules is shared under a not declared license and was authored, remixed, and/or curated by Richard Fitzpatrick.

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