Infra-Red Spectroscopy:

Molecules could absorb energy within the infra-red region of the electromagnetic spectrum resultant in the raised vibration of covalent bonds. There are two kinds of vibration resultant either in the stretching or the bending of bonds. These vibrations take place at fixed frequencies (or energies) depending upon the bond involved. It is helpful to think of the bonds like springs and the atoms as weights to rationalize the energy needed for such vibrations. There are two issues that are influencing the frequency of vibration - the masses of the atoms and the 'stiffness' of the bond. Multiple bonds like double or triple bonds are stronger and stiffer than single bonds and thus their stretching vibrations take place at higher frequency (or energy). The stretching vibration of bonds as well depends upon the mass of the atoms.

The vibration is faster while the bond includes a light atom rather than a heavy atom. Stretching vibrations need more energy than bending vibrations.