After you find the irreducible representation of a molecule, how do you determine the number of IR active bands? For example, in methane, the pointgroup is Td , then you found that the irreducible representation is A1 + T2. How do you find the number of IR active and Raman active bands? Thanks Btw, a friend of mine said there are 2 IR active bands but I am not sure how to figure that out. Also, I know that T is triple degenerate and A is nondegenerate. But not sure how that relates to the number of IR active bands.