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A plasma runs at about \( 7000 \ \mathrm{K} \) against a flame's \( 2500 \ \mathrm{K} \). The excited fraction for a two electron volt transition rises by roughly

  • A.A factor of ten
  • B.No appreciable factor
  • C.A factor of several hundred
  • D.A factor of three

Answer: C

Comparing the Boltzmann factors at the two temperatures:

\[ \begin{aligned} \frac{e^{-2.1/kT_{p}}}{e^{-2.1/kT_{f}}} \\ &\approx \frac{0.032}{6\times10^{-5}} \end{aligned} \]

which is a factor of several hundred. That exponential dependence on temperature is why emission techniques gain so enormously from a hotter source, and why absorption, which relies on the ground state, gains almost nothing.

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