why in general, when the frequency increases, the net polarization of the material decreases as each polarization mechanism stops contributing, and thus its dielectric constant decreases.
Mohammed Mesrar, you can represent your medium as a bunch of harmonic oscillators, each responding to the electromagnetic field with a Lorentzian response. The total polarizability of the medium is then given by the sum of these responses, and since each of these Lorentzians dies off pretty quickly with increasing frequency, the dielectric permittivity will also decrease correspondingly at large frequencies.
From a classical point of view, it is actually a problem of inertia. The higher the frequency, the more often the field changes its sign at a given point. At low frequencies, molecules with dipole moments can rotate to align themself with the field. At higher frequencies, this is no longer possible, but now mater can change the dipole moment locally by vibration. Again, if the frequencies become too high, even low masses like a hydrogen atom, can no longer follow. Then, at even higher frequencies, the movement of bound electrons is influenced by radiation. When the frequencies become again higher, even the inertia of an electron is too high for the electron to respond to the field variation...
The region where the dielectric function becomes unity depends certainly on the material. For lighter atoms and their molecules/solids it is somewhere in the UV. For heavier atoms with many electrons, some of excitations may be even in the X-ray region, but if I am not mistaken the oscillator strengths are not very high, so the contribution to the real part of the dielectric function is small. Overall, you find this generally in textbooks like Mark Fox "Optical properties of solids", or in Eugene Hecht "Optics". Maybe for the details on the electronic excitations the Atkins "Physical Chemistry"...
The dielectric function is definitely temperature dependent. Concerning special effects due to elasticity on polymer dielectric functions - I have never heard of any, but this certainly does not mean that they do not exist. I am in particular familiar with the dielectric functions of Polyethylene and PMMA, and both don't display any particular peculiarities compared to other organic compounds in the infrared spectral region.
- Badges
- Science topic
- Similar topics
- Chemistry
- Nanotechnology
- Nanostructures
- Thin Films