[Michael]

hi guys,

first up, I'd like this opportunity to thank everyone in this great website

back to my question , while I'm reading about the atoms and the electrons, and how the electron moves from the ground state to the excited state by absorbing energy .. a question turned up into my mind that what is the form of this energy ? is it small particles or something different ??



many thanks in advance, and sorry for my  poor English lol

[xiankai]

from the theory of QED, i think it is photons.

or rather, electromagnetic waves as they are known in classical physics.

[Michael]

thanks alot for the reply

[P]

Read up on the 'Young's double slit experiment'    It explains how light (energy) travels as a wave and hits as a particle  (it can be a little difficult to get your head round but it is quite cool) it's called wave-particle duality.

[Michael]

many many thanks thats exactly what i wanted to know. here is the link for anybody wants to read it.

http://en.wikipedia.org/wiki/Double-slit_experiment

[Assembler]

Someone correct me if I'm wrong (I'm recalling this from memory), but just to elaborate on the electromagnetic radiation:
 In the visible light and ultraviolet wavelengths it causes the electrons to go to an excited state
 In the infrared wavelength to increase the vibration level (of the molecule)
 In the microwave wavelength to increase the rotational level (of the molecule)

[Yggdrasil]

Yup.

[xiankai]

i thought molecular vibration was dependent on the size of the molecule? because the outer electrons are the entities that directly interact with the electromagnetic radiation (correct me if im wrong), therefore depending on how strongly the electrons are attracted to the molecule, there are different cut-off frequencies/wavelengths for certain modes of vibration.

[P]

Its to do with resonance.   Matter will absorb radiation  (by electronic, rotational, vibrational or whatever mode) if the frequency of that radiation hits a resonance with something  -  i.e the frequency of a carbon - chlorine bond vibrates in the UV range somewhere, so C-Cl bonds absorb in the UV range  -  hit it hard enough with enough UV and the bond will break.

Infrared can be absorbed by vibrating molecules as well as UV (if there is a change in dipole moment during vibration - but that is another story)   If you 'zoom in'  on an IR spectrum and look at the ultra fine structure of a peak,   you can see that most broad peaks are made up of lots of other peaks close together  -  there is one main frequency at the vibrational resonance of the bond and lots of other little side peaks at strict quantisable spacings due to the addition rotational and electronic additions and subtractions to the energy required to be absorbed.

So, yes, IR causes vibration of a bond - but you get rotational and electronic fine structure around the peaks (if your IR machine is of high enough resolution).

[xiankai]

from what i can infer, resonance refers to a 'cut-off frequency' for certain modes of vibration to be achieved for a specified system. (consider different bonds like C-Cl as P said)

therefore it does seem abit wrong to me say that IR/UV/microwave/light etc. cause specified modes of vibration, when it actually depends on the system being considered.

or am i missing something?

[Yggdrasil]

therefore it does seem abit wrong to me say that IR/UV/microwave/light etc. cause specified modes of vibration, when it actually depends on the system being considered.

I agree.  But, in general, energies corresponding to transitions between rotational states tend to have energies around the energy of microwave photons, transitions between vibrational states tend to have energies around those of IR photons, and transitions between electronic states tend to have energies around those of UV/visible photons.