vibrational spectroscopy
Hold a row of balls joined by springs and give it a shake: the springs stretch, squeeze and bend, each with its own rhythm. A molecule's bonds do exactly this, jiggling endlessly even when the molecule looks still. Vibrational spectroscopy listens in on those jiggles, because a bond can only vibrate with certain allowed amounts of energy and absorbs light whose energy matches the jump between them.
Vibrational spectroscopy is the study of transitions between a molecule's quantised vibrational energy levels — the stretching and bending of its bonds. These energy gaps lie in the infrared region, so the technique is carried out chiefly by infrared spectroscopy (which detects absorption) and Raman spectroscopy (which detects scattered light shifted in energy). Each distinct way a molecule can vibrate, called a normal mode, absorbs at its own frequency, set by the masses of the atoms and the stiffness of the bonds.
It is invaluable because vibrational frequencies are sensitive to exactly which bonds and groups a molecule contains, making it a reliable way to identify materials and follow chemical reactions. The subtlety is that infrared and Raman do not see the same vibrations: a mode shows up in infrared only if it changes the dipole moment, and in Raman only if it changes the polarizability, so the two techniques complement one another and together give a fuller picture.
A water molecule has just three normal modes — a symmetric stretch, an asymmetric stretch, and a bend. Each absorbs infrared light at its own frequency, and together they are the reason water vapour is such a strong greenhouse gas: it soaks up the infrared heat the Earth tries to radiate back to space.
Each way a molecule can wobble absorbs infrared at its own frequency.
Even at absolute zero a bond does not stop vibrating — quantum mechanics forbids perfect stillness, leaving a residual zero-point energy. Vibrational spectroscopy measures transitions above that floor, not motion starting from rest.