Do atoms emit and absorb photons at the same wavelengths?

Question

I know that atoms absorb photons, which promotes electrons to higher energy levels. Then upon de-excitation a photon is released. Is it true that the lines for absorption and emission spectrums are the same i.e. the same wavelengths are both absorbed and emitted?

I believe this is true, but then how does this explain fluorescent minerals? You shine a UV light on them of some wavelength, then they will emit a characteristic color, such as red, which is not the same wavelength as the UV light. How does this work?

Answer 1

For every specific transition up where a photon of a given wavelength is absorbed there is a corresponding transition down where a photon of the same wavelength is emitted. However, there is not a "memory" that ensures that the same transitions are followed one direction or the other.

For example, in the hydrogen spectrum we see the following possible transitions:

So, for example, a hydrogen atom in the ground state could absorb a 97 nm photon and then relax back to the ground state by emitting a 97 nm photon. Or it could relax back to the ground state by emitting a 486 nm photon and then a 122 nm photon. Fluorescence is similar to that, but it also usually involves one or more non-radiative transitions.

Answer 2

I wanted to add that the absorption spectrum and the emission spectrum of an atom can be closely related, but are not the same.

When an atom absorbs a photon, it raises one of its electrons to a higher energy level (excited state). Eventually, the electron will drop back down to its lower or original energy level, releasing a photon of light in the process. The energy of this photon will determine its wavelength, and the resulting light is what is seen as the emission spectrum of the atom.

In regards to fluorescence, some materials have the ability to absorb a photon of light at a specific wavelength. Instead of immediately releasing that energy as a photon, it instead traps the energy temporarily in a form of excited state. Eventually, the energy is released as a photon of light, often at a longer wavelength than the one that was absorbed. This is why fluorescent minerals emit light that appears to be a different color than the light that was shone on them. It is known as Fluorescence spectra.