Exposing molecules to the correct frequencies of infra-red light will result in some of that light being absorbed. Absorption of light by a molecule gives that molecule more energy. Absorption of infra-red light results in more energetic motion of the atoms in the molecule. Typical motions that infra-red light can stimulate are shown in the following movies. These movies are best viewed in "loop" mode in your movie viewer. (In FastPlayer for the Macintosh, this should be set under Movie before playing the movie)
Bond Stretching (12K)
Angle Bending (30K)