|MadSci Network: Physics|
Dear Rich, thank you for our question. You asked about the possible differences between the frequencies of light absorbed and emitted by metal atoms caused perhaps by “recoil, vibration and dissipation as heat. You mentioned metal atoms and solid metals and I think this is where the apparent contradictions in what you have read have crept in. You describe a process of absorption of a photon and it’s re-emission. This is a process which can occur with isolated atoms. When atoms are in a gaseous state at very low pressures things are a lot simpler. In this case re-emission takes place before the atom is affected by collisions and the emitted photon will usually have the same frequency as the absorbed photon. There are of course, several energy states to which an electron can be promoted and if a higher energy state is reached, in principle, the electron can drop in energy by stages emitting light of a frequency corresponding to the energy drop. The probability of this will depend on the quantum mechanical rules which apply to such transitions. ( These rules can be transgressed in heavy metals such as mercury whose strongest absorption and emission line at 254nm is not strictly allowed but “bent” by the effects of spin-orbit coupling ). At higher pressures an atom which is being excited or is ready to emit a photon can be under the influence of adjacent atoms or other gaseous components. In this case, the electron energy levels are less predictable and broadening of the absorption and emission lines occurs. The exact correspondence between absorbed and emitted light when the electron moves between the same two states is then lost and some frequency shift can occur in both directions. When you mention solid metals you mention the electrons ( I’m sure you meant electrons not atoms ! ) excited by an absorbed photon jumping the “band gap” and you asked if the emission is the same frequency as the absorption. Here we are dealing with a very different situation from a gas. In solids and liquids most electrons promoted in energy by absorbed photons lose their energy by conversion to vibrational energy, in other words the energy is converted to heat. Occasionally this process is interrupted when a relatively stable excited state is reached and while in that state the opportunity to emit a photon can be taken. This is the process of photo-luminescence ( fluorescence between singlet excited states or phosphorescence between triplet and singlet states). The emitted photon is therefore normally of lower frequency than the exciting photon. As far as I am aware though, photo-luminescence does not occur in solid metals and the absorbed energy finishes up as heat. Most metals are highly reflective of course but the process of reflection is very different from photo-luminescence. Solid metals are arrays of metallic ions which have donated electrons to a population of electrons which occupy the “conduction band” of energies. They are often described as the glue keeping the metal ions together and their behaviour has parallels with a gas. They are highly mobile and give metals their characteristic high electrical and thermal conductivity. When a photon impinges on the metal surface the conduction band electrons fall under the influence of the oscillating electrical field provided by photon. They are accelerated by this field and since accelerated electrons emit electromagnetic radiation, photons are generated. The resulting photons have the same frequency and their directions are described by the laws of reflection. This process then occurs in such a way that the reflected light appears to be made up of the self same photons which hit the surface and bounced off. This phenomenon is not confined to metals. The ions in the ionosphere behave in the same way when reflecting radio waves. Only the dimensions are different.
Try the links in the MadSci Library for more information on Physics.