Reflection and Quantization of Energy Levels

[wgreen]

From what I have been reading, the reflection of light from an object, like th eyellow color of sulfur is the same phenomenon as Rayleigh scattering. It seems that the electrons receive the incoming photon and are raised to a higher "virtual energy state." When they return to the ground state they emit a photon with the same frequency.

This seems to indicate that an atom or molecule could receive any given amount of energy and that the energy of the atom or molecule is not truly quantized. How can these explanations be reconciled?

(I have seen other descriptions from other perspectives, such as viewing the incomin photon as a wave that sets the electron cloud oscillating at the frequency of the photon, and this oscillation generates the scattered photon. Though this explanation may make sense, I am interested in an expalnation that accounts for energy states. Even in the oscillating cloud explanation, it would seem that the oscillating cloud is at ahigher energy than the non-scillating cloud.)

Thanks for your help.

 

[Parlyne]

An electron absorbing a photon can't conserve both energy and momentum. This is why the excited state must be "virtual." But, virtual states can only exist for an extremely limited period of time (something like $$\frac{\hbar}{\Delta E}$$, where $$\Delta E$$ is the amount of departure from energy conservation).

When talking about an atom or molecule absorbing light, we're talking about real states. That is, those states are ones which conserve both energy and momentum. The do this by storing some of the energy in internal structure.

 

[wgreen]

Thanks, Parlyne.

If the virtual state has a lifetime, then it seems to be "real." Why then is it called "virtual?"

 

[ZapperZ]

Er... again, there is a strong misconception here of physical processes involving SOLIDS.

There is an FAQ in the General Physics forum describing light transmission through a solid, and in it, there is a brief description of what happens to atoms when they conglomerate to form a solid. The properties of the solid is no longer dictated strictly by the properties of the invidual atoms. The collective behavior is now the more relevant aspect. One of the collective behavior that is important here is the vibrational spectrum, or phonons, that is only present in such a solid and not at the individual atom or molecule level. This phonon spectrum governs many properties of the solid, including light absorption (make it transparent or opaque), heat transport, electrical transport, etc.

This is relevant here too. As as been brought up, since atoms have "discrete" states, how come we can get almost a continuous reflection (or even transmission) of light? This is because this explanation isn't quite correct. The SOLID itself has a large range of phonon spectrum that is continuous. Often, if these modes are active, it can easily absorbe a large range of the visible EM spectrum, leaving only a smaller band to be either transmitted or reflected. That is why you can get a continuous reflected or transmitted spectrum. You don't get this from individual atoms, because we know the absorption spectrum of gasses shows discrete lines.

Moral of the story : solids behave differently than atoms. If they're the same, then why do we have two separate areas of study, atomic/molecular physics, and solid state/condensed matter physics?

Zz.

 

[marlon]

From what I have been reading, the reflection of light from an object, like th eyellow color of sulfur is the same phenomenon as Rayleigh scattering. It seems that the electrons receive the incoming photon and are raised to a higher "virtual energy state." When they return to the ground state they emit a photon with the same frequency.

First of all, in the case of both atoms and solids, electrons do NOT absorb the photons. The photon is absorbed by the entire atom or solid. For example, in the case of the atom, the specific electronic energy levels are explained by the fact that these electrons are a part of a bigger system : ie the atom. Free electrons have a different energy spectrum (continuous) than electrons from an atom (discrete).

Secondly, the example you give needs to be explained in terms of a many particle system : you don't have just one atom but a bunch of gazzilion atoms interacting together : like a solid, crystal. The incident photon is NOT aborbed by one atom in this case, but by the lattice. Which photons (ie which energies) will be absorbed, depends on the available (resonant) lattice vibrations or phonons (ie the photon phonon coupling). The phonon spectrum, ie the possible lattice vibrations that can absorb a photon, is continuous and this explains many of the continuous EM reflection spectra that we observe.

regards
marlon