# Lattice Vibrations and em waves

1. Oct 28, 2009

### manofphysics

Can EM waves falling on a solid contribute towards lattice vibrations?

If yes, then
i)when is the energy used in lattice vibrations,
ii)when is it used in excitation of electron into higher energy level, and finally,
iii)when is the energy utilized for slight vibration of the electron cloud at the frequency of em wave thus producing a scattered EM wave ?

Do these happen simulataneously? What is the order and energy conditions for each of these happening?

2. Oct 28, 2009

### kanato

Yes.
i) Lattice vibrations (phonons) can be excited by light, but usually only so-called the optical branches, which not every material has (although every material with a structure more complex than a single atom per unit cell will have optical branches). These are pretty much always in the infrared part of the spectrum
ii) To excite electrons in an insulator or semiconductor, you need light that is larger than the (direct) band gap of the material. For Si that's aboue 1.3 eV, so any visible light will do, which gives it a very dark appearance since it absorbs most visible light (1.6 eV - ~3 eV) but for diamond the band gap is above 5 eV, so no visible light is absorbed. For metals, there is no band gap, so typically any light can excite electrons, and tends to interact with plasmons [more info: http://en.wikipedia.org/wiki/Plasmon]
iii) I don't quite understand what you're asking in this one, but it might be discussed on the plasmon link above.

3. Oct 29, 2009

### manofphysics

I understood the first two points.Thanks,kanato

In (iii) , I was simply asking under what circumstances the radiation is NOT absorbed but passes right through the solid causing the electron cloud surrounding the atom to oscillate slightly at the same freq. as the incident wave, hence each atom of the lattice producing a spherical wave of the same freq. as the incident wave (scattering on the surface of a solid)

4. Nov 2, 2009

### conway

In general if there is any net charge displacement associated with a mechanical distortion of the solid, there ought to be an interaction with light of the same wavelength. In other words, the positively charged atomic lattice vibrates mechanically while the total electron cloud lags slightly behind. I can't think of any other mechanism for the overall absorption of light by a solid. I don't know why this mechanism doesn't work for glass.

5. Nov 2, 2009

### kanato

Lattice vibrations are pretty much always in the infrared spectrum. Generally the phonons that you are talking about are called IR active because they interact with light for exactly the reason you describe. It probably does work for glass, but since it is in the IR part of the spectrum it does not affect the way glass interacts with visible light.

6. Nov 9, 2009

### Rajini

few cents..
allmost all molecules have acoustics (roughly less than 12 meV or 100cm^-1 in the phonon vibrational density spectrum) and intermolecular vibrations (one can separate them by seeing the spectrum). I am writing this based on nuclear scattering technique.
For pure element..acoustic modes-sound waves is dispersed..
As far as i know Debye model gives a good explanation for acoustic and Einstein for molecular vibrations.

7. Nov 19, 2009

### newtophy

can anybody tell me how to interpret phonon band structures.any book available or any material which would help me.