Vibration analysis

Every molecule possess a small, finite number of normal modes of vibration, in which every atom moves harmonically around its rest position with the same frequency. The general vibrational motion, as excited by thermal collisions, is simply a sum of the normal mode motions, with random amplitudes and phases. But normal modes are easy to excite as pure motions. When an electromagnetic wave of the right frequency washes over a molecule, every atom feels a push-pull force, and if that force has the frequency of a normal mode, the whole molecule begins to move in that pure normal mode. This is the classical view of infrared absorption spectroscopy.