# Absorption of light

1. Oct 23, 2011

### Gavroy

hi

i am a little bit confused right now. i just read wikipedia about IR-spectroscopy and it is said, that:
"In order for a vibrational mode in a molecule to be "IR active," it must be associated with changes in the dipole. A permanent dipole is not necessary, as the rule requires only a change in dipole moment."
okay, i heard that his has to do with the transition dipole moment. but here comes my question, now:
if it is necessary for absorption of IR-light to have a change in dipole moment, why don't you need this when it comes to electron transitions with visible light.
cause, if i refer to the hydrogen atom, where you can it is possible to have the transition from the state n=1 to n=2. the dipole moment is always zero???

and my second question is this one:

if you have a hydrogen atom in a 1s-ground state. and you have a photon that is absorbed and takes the electron to the 2- state, then my book says, this would have to be the 2p state, as there has to be a change in angular momentum. but as far as i know, photons do not have angular momentum. they only have a spin. so why do you have to consider angular momentum instead of spin in this case. and where does the angular momentum of the photon come from?

ah and sorry for my english skills, i am still practising.

2. Oct 23, 2011

### Staff: Mentor

"Spin" = "intrinsic angular momentum". The other kind of angular momentum is "orbital angular momentum". The sum of the two, over all particles in an isolated system, is conserved.

3. Oct 23, 2011

### Gavroy

okay, but this would not explain, why it is impossible to have a transition from a 1s to a 2s state (so same orbital angular momentum) and change the spin of the electron from let me say -1/2 to 1/2, or where am I wrong?

4. Oct 24, 2011

### DrDu

In IR spectroscopy, like in spectroscopy with visible light, there only has to be a transition dipole moment. Neither the initial nor the final state have to have a dipole moment.
As far as the transition from 1s to 2s is concerned: It is possible as far as angular momentum conservation is concerned, however, there is no associated change in dipole moment whence it is not observed.