- #1
mikeph
- 1,235
- 18
Hello
If a molecule has two specific vibrational modes (eg. symmetric and antisymmetric stretching in H2O), and they occur at different frequencies, v1 and v2, then why is the absorption band associated with a combination of these two modes labelled "v1 + v2"?
My intuition is that the H2O can absorb exact frequencies which resonate with the dipole change associated with that specific oscillation. So if a v1 oscillation is stimulated by one photon, how can a v1 and v2 oscillation both be stimulated by a frequency of (v1 + v2)? Shouldn't it be the lowest common multiple of v1 and v2 which stimulates both vibrations, as only this frequency can be a harmonic of both modes? In other words, the mode v1 can only be stimulated by radiation of frequencies which are multiples of v1, but v1 is not nessecarily a factor of v1+v2!
I can see how the ENERGY of this v1+v2 photon is enough to cause a jump in energy level, but does classical physics have an explanation as well?
Thanks,
Michael
If a molecule has two specific vibrational modes (eg. symmetric and antisymmetric stretching in H2O), and they occur at different frequencies, v1 and v2, then why is the absorption band associated with a combination of these two modes labelled "v1 + v2"?
My intuition is that the H2O can absorb exact frequencies which resonate with the dipole change associated with that specific oscillation. So if a v1 oscillation is stimulated by one photon, how can a v1 and v2 oscillation both be stimulated by a frequency of (v1 + v2)? Shouldn't it be the lowest common multiple of v1 and v2 which stimulates both vibrations, as only this frequency can be a harmonic of both modes? In other words, the mode v1 can only be stimulated by radiation of frequencies which are multiples of v1, but v1 is not nessecarily a factor of v1+v2!
I can see how the ENERGY of this v1+v2 photon is enough to cause a jump in energy level, but does classical physics have an explanation as well?
Thanks,
Michael