As a molecule sits on a surface, it will vibrate. Such vibrations can be studied by shining infrared light on to the surface. If the molecule has a dipole moment, that is one end of the molecule has a positive charge and the other end a negative charge, then the molecule can absorb infrared light, but only at certain fixed frequencies. Hence, an infrared spectrum of light reflected from the surface will show absorption peaks which are characteristic of the molecule and its method of bonding to the surface. This is the basis of the RAIRS technique. Vibrations can only be detected if the vibration is perpendicular to the surface. Detection of the infrared spectrum is generally acquired using the FTIR technique. Vibrations of molecules on surfaces can also be investigated using HREELS.