What effect does Doppler broadening have on absorption cross sections?

What effect does Doppler broadening have on absorption cross sections?

• A. It broadens resonances, increasing cross sections at all energies
• B. It has no effect on cross sections
• C. It broadens resonances, yielding a lower effective absorption cross section across a wider energy range
• D. It narrows resonance peaks at all energies

Answer: (C)

Doppler broadening comes from the thermal motion of absorbing nuclei, which causes a range of Doppler shifts for photons hitting the nuclei. When you look at absorption as a function of energy, this motion spreads a sharp resonance into a broader, typically Gaussian-shaped feature. The peak value of the cross section drops because the same total absorption strength is now distributed over a wider energy interval. The overall area under the resonance is largely conserved (the integrated cross section stays roughly the same), but the distribution in energy changes. So the resonance is broadened, and the effective cross section near the center energy is reduced while the absorption is felt over a wider energy range.

Doppler broadening comes from the thermal motion of absorbing nuclei, which causes a range of Doppler shifts for photons hitting the nuclei. When you look at absorption as a function of energy, this motion spreads a sharp resonance into a broader, typically Gaussian-shaped feature. The peak value of the cross section drops because the same total absorption strength is now distributed over a wider energy interval. The overall area under the resonance is largely conserved (the integrated cross section stays roughly the same), but the distribution in energy changes. So the resonance is broadened, and the effective cross section near the center energy is reduced while the absorption is felt over a wider energy range.