<P> Molecular spectroscopy deals with the response (spectrum) of molecules interacting with probing signals of known energy (or frequency, according to Planck's formula). Molecules have quantized energy levels that can be analyzed by detecting the molecule's energy exchange through absorbance or emission . Spectroscopy does not generally refer to diffraction studies where particles such as neutrons, electrons, or high energy X-rays interact with a regular arrangement of molecules (as in a crystal). </P> <P> Microwave spectroscopy commonly measures changes in the rotation of molecules, and can be used to identify molecules in outer space . Infrared spectroscopy measures changes in vibration of molecules, including stretching, bending or twisting motions . It is commonly used to identify the kinds of bonds or functional groups in molecules . Changes in the arrangements of electrons yield absorption or emission lines in ultraviolet, visible or near infrared light, and result in colour . Nuclear resonance spectroscopy actually measures the environment of particular nuclei in the molecule, and can be used to characterise the numbers of atoms in different positions in a molecule . </P> <P> The study of molecules by molecular physics and theoretical chemistry is largely based on quantum mechanics and is essential for the understanding of the chemical bond . The simplest of molecules is the hydrogen molecule - ion, H, and the simplest of all the chemical bonds is the one - electron bond . H is composed of two positively charged protons and one negatively charged electron, which means that the Schrödinger equation for the system can be solved more easily due to the lack of electron--electron repulsion . With the development of fast digital computers, approximate solutions for more complicated molecules became possible and are one of the main aspects of computational chemistry . </P> <P> When trying to define rigorously whether an arrangement of atoms is sufficiently stable to be considered a molecule, IUPAC suggests that it "must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state". This definition does not depend on the nature of the interaction between the atoms, but only on the strength of the interaction . In fact, it includes weakly bound species that would not traditionally be considered molecules, such as the helium dimer, He, which has one vibrational bound state and is so loosely bound that it is only likely to be observed at very low temperatures . </P>

Where is the energy in a molecule or compound found