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The molecular population is :

  • A.Proportional to \((2J + 1)^{-1}\)
  • B.Proportional to \(\dfrac{(2J + 1)}{e^{-E/kT}}\)
  • C.Proportional to \((2J + 1)e^{-E/kT}\)
  • D.Proportional to \((2J + 1)^{-1}e^{E/kT}\)

Answer: C

The population of rotational level J is proportional to (2J + 1)e−E/kT.

Two factors combine. Level J has 2J + 1 degenerate states (MJ = −J ... +J), so it can hold proportionally more molecules; and each state is populated according to the Boltzmann factor e−E/kT, which falls with energy. Their product rises at first and then falls, which is why rotational spectra show a maximum in intensity at an intermediate J.

Option A omits the Boltzmann factor and inverts the degeneracy. Option B divides by the Boltzmann factor, so higher levels would be more populated, and option D both inverts the degeneracy and reverses the sign of the exponent.

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