Mr. Lindmark,

Yes it is. The electrons are excited to upper energy states.

First of all energy is directly proportional to wavenumber.

Hence, you mean intensity not energy -> Lambert Beer (1st question)

An electronic transition corresponds to wavenumbers in the range of 15000 cm-1. Vibrational states (I2) in the range of 200 cm-1 and rotations in the range smaller than 10 cm-1. You only need to calulate the thermal energy in cm-1 and you will see which deactivation transfers the light to heat. Keyword are Franck Condon and Kasha rule.

And for the same reason IR absorbance have nothing to do with homo lumo transition (MO = molecular orbita). It is normally a transition from the lowest vibrational state 0 to the first vibrational state with the quantum number 1, from zero this is the “only” possible transition. This is not some ominous vibration of electrons, this is the vibration of the cores.

Please compare also to a Physical chemical textbook for Additional details more detailed explanation... .

The IR is a broad spectral region and it's energy is low to cause atomic excitations. Molecular energy levels are lower and denser than atomic. Therefor, IR causes transitions between molecular states depending on the wavelength and the molecule. If i remember well, IR changes mostly vibrational but also rotational states of the molecule