21.3 Absorption Spectra and Excitation Pathways

When broadband light passes through cooler gas, atoms absorb only photons whose energies exactly equal allowed upward transitions from populated states. As a result, the transmitted spectrum shows dark lines at precisely those wavelengths. The same atoms can later emit photons at those wavelengths, so absorption and emission line positions coincide for a given element.

The matching is not a coincidence: both processes are controlled by the same pairwise energy differences. The difference is directional. Absorption moves electrons upward in energy. Emission moves electrons downward after excitation. Line positions therefore encode internal structure independent of whether the sample appears bright or dark.

Photon absorption is selective: the incoming photon must match a valid level gap. If the energy is slightly off, the photon is not absorbed by that transition. Electron impact is more flexible because a colliding electron can transfer only part of its kinetic energy, leaving with reduced speed while still exciting the atom by an allowed amount.

This distinction appears often in conceptual questions. The microscopic reason is that the electromagnetic field mode associated with one photon has fixed energy hf, while collisional transfer between massive particles can partition energy continuously subject to conservation laws and allowed atomic final states.

Because each element has unique line positions, spectra act as chemical fingerprints. In astronomy this allows composition mapping of stellar atmospheres and nebulae without direct sampling. Combined with Doppler shifts, the same lines provide radial velocity information, making atomic transitions foundational for cosmology and galactic dynamics.