# Rotational and vibrational spectroscopy

1. Mar 12, 2012

### Chemist20

Hello there!

Im studying microwave spectroscopy, and I'm stuck because I don't see why the spacing between rotational energy levels increases as J does. Can anyone explain please????

Also, about IR, I see everywhere that the selction rule is Δv=±1. Correct me if I'm wrong but I'm guessing that's only when you use IR light. If you where to use UV, where an electronic transition can take place, we could also go from l=1 v=0 to l=2 v=0 and hence Δv=±0,1. (excluding overtones of this) . Is this correct?

Thanks!!!!

2. Mar 12, 2012

### DrDu

In electronic transitions you usually observe a whole series of transitions of possible v values (so called Franck Condon progression) as the equilibrium positions of the electronic ground and excited state do not coincide.