How do combination bands appear in vibrational spectra?

[Talita]

In vibrational spectroscopy we read peaks related to normal modes of a molecule.

But I just can't understand: we can have a combination of those normal modes of vibration, right? So how can I read a spectrum of a molecule if this molecule can absorb a frenquency that might excite a combination of normal modes and not a single normal mode?

Sorry if I made any english mistake.
Thank you (:

 

[TeethWhitener]

Molecules can absorb frequencies that excite combinations of normal modes, or overtones of normal modes. These absorption bands tend to be much weaker than excitation of a single normal mode. In fact, in the harmonic approximation, the probability of overtone excitation by one photon is exactly zero (google "spectroscopic selection rules" for a good overview of this). Of course, it's only an approximation, so that overtone/combination band excitation is possible, but quite weak. However, there exist techniques where these combinations and overtones can be observed more easily (e.g., by using high temperatures or nonlinear optical processes).