# Does every wavenumber of IR result in a different kind of vibration?

Does every wavenumber of IR result in a different kind of vibration? If that is true, what if a molecule absorb 2 different wavenumbers (which cause different rocking and symmetrical stretching for example ) in the same time?

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You get a "mixture" of the vibrational motions. A simplified analogy: Imagine a ball that has two springs attached to either side: It will swing from left to right with a certain frequency $\omega_1$. You then attach two weaker springs to the top and bottom; now, the ball can also move up and down with another frequency $\omega_2$. If you "excite" both vibrations at the same time, your motions superpose. For springs of the same strengths, i.e. $\omega_1 = \omega_2$, this motion is elliptic, otherwise it will be more wave-like. (But keep in mind this is only a very simple model! Read on below.) – Antimon Oct 19 '12 at 10:01