Matching Photon and Atom Energies
Most recent answer: 04/07/2017
- Bill (age 55)
New York, NY USA
Great question! The answer is a little subtle.
Other than the ground state energy, the energy levels of an atom are not completely precisely defined. The excited states of the atom are not true energy eigenstates, states with single exact energy values. Energy eigenstates persist forever without physically changing. The excited states of atoms don't persist forever. They decay by spontaneously emitting one or more photons. So it's precisely the same interaction with electromagnetism that's needed to allow photon absorption that also makes the states have a small intrinsic energy width, i.e. frequency width.
Here's a site that has some quantitative discussion: http://farside.ph.utexas.edu/teaching/qmech/Quantum/node122.html
http://farside.ph.utexas.edu/teaching/qmech/Quantum/node122.html.
In addition to the intrinsic linewidth, several other effects broaden the absorption line. One is Doppler broadening, since in a gas the atoms are moving around with random thermal velocities. Another comes from collisions between the atoms, so that they aren't quite idealized isolated atoms. As a result, light whose frequency in the lab frame differs from the ideal absorption frequency by more than the intrinsic linewidth will still be absorbed by some atoms that happen to be moving with the right velocity to make the frequency almost ideal in their own frame.
Mike W.
(published on 04/07/2017)