# Infra-red spectroscopy .

1. Nov 17, 2004

### Cheman

Infra-red spectroscopy.....

I have been taught that with Infra-red spectroscopy, when different infra-red wavelengths are fired at the sample when we reach the correct frequency we can cause the atoms in the bond to resonate as they absorb the correct energy. My question is why does it have to do with energy? After all, lets take the example of a swing and talk about resonance - you have to push the swing with the correct frquency to cause it to resonate; it does not matter how much energy you supply as this will only make it resonate more. So why do we talk about the correct energy of the infra red light rather than the frequency at which a force or energy is supplied?

2. Nov 17, 2004

### Dr Transport

$$E = \hbar \omega$$, the frequency of the light corresponds to the resonant frequency of the lattice in the material.

3. Nov 18, 2004

### Cheman

But surely, if we take the swing again, that vibrate bak and forth because you exert a force on it a particular number of times a second. Now, if you fire a photon at an atom, this will give it a set amount of energy all at once, not over set periods of time - so I don't see how this could lead to resonance. Please could someone help to elaborate.

4. Nov 18, 2004

### Integral

Staff Emeritus
A photon is not a classical localized particle. A atomic ensemble is not a classical swing. So be very careful with classical analogies to a Quantum Mechanical system. An atomic resonance will occur when the incident photons have an energy which corresponds to an available quantum state of the electrons in the system.

5. Nov 25, 2004

### Dr Transport

We are talking phonons, not photons. The atoms in a lattice resonate with characteristic frequencies, i.e. lattice waves otherwise known as phonons. Look at Ziman's book on solids, or any other solid state text, Kittel does a good job.