For anharmonic potentials, such as the Morse potential,
higher order transitions with Dv = ±2, ±3, . . .
are also observed. Such overtone-transitions are, however,
much weaker than the fundamental transitions with
Dv = ±1.
As the transition dipole is given by d*x you need the matrix elements of the position operator. For the Morse potential that is known (see attached file). This can also be found on wikipedia. If you know the transition dipole for the fundamental transition the overtones, sequence transitions etc. can easily be found using this equation. (Given of course that the Morse potential is a good approximation for your vibrational mode.)
http://en.wikipedia.org/wiki/Morse_potential
Mr./Miss/Mrs. Jabbari,
They are really wery weak, but well distinguishable experimentally. Please pay attention on the attachment, where the region of the overtone and combination bands has been shown, using the spectrum of C6H6 as an example.
Furthermore, when the overtone coincides with a basic frequency and there have, the last one is splitted, and in fact you have observed two very well defined bands corresponding to this basic frequency (Fermi resonance). It is shown splitting of the nsNH2, due to coinciding of the basic frequency with the dNH2 mode and the same symmetry class. As far as the nsNH2 are very intensive bands, you may determine their common origin easy, like those ofnes of the overtones and the combination modes in C6H6 as well as any other molecule, using polarized vibrational spectroscopy.
Thank you very much for your answers
Thomas La Cour Jansen and Bojidarka B. Ivanova
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