The key ideas can

be found in the wikipedia article on rotational spectroscopy:

http://en.wikipedia.org/wiki/Rotational_spectroscopy

The P, Q, R branches correspond to delta(J) = -1, 0, +1.  You can

find details at

http://www-teach.ch.cam.ac.uk/teach/A3/A3_Keeler.pdf

http://www-teach.ch.cam.ac.uk/teach/A3/A3_Klinowski_Raman.pdf