Molecular physics is the study of the physical properties of molecules and of the chemical bonds between atoms that bind them into molecules. Its most important experimental techniques are the various types of spectroscopy. The field is closely related to atomic physics and overlaps greatly with theoretical chemistry and physical chemistry. A chemical bond is the phenomenon of atoms being held together in molecules or crystals. ... A molecule is the smallest particle of a pure chemical substance that still retains its chemical composition and properties. ... Spectroscopy is the study of spectra, that is, the dependence of physical quantities on frequency. ... Atomic physics (or atom physics) is physics of the electron hull of atoms. ... Theoretical chemistry is the use of non-experimental reasoning to explain or predict chemical phenomena. ... Physical chemistry is the study of the physical basis of chemical systems and processes. ...

Additionally to the electronic excitation states which are known from atoms, molecules are able to rotate and to vibrate. These rotations and vibrations are quantized, there are discrete energy levels. The smallest energy differences exist between different rotational states, therefore pure rotational spectra are in the far infrared region (about 30 - 150 µm wavelength) of the electromagnetical spectrum. Vibrational spectra are in the near infrared (about 1 - 5 µm) and spectra resulting from electronic transitions are mostly in the visible and ultraviolet regions. From measuring rotational and vibrational spectra properties of molecules like the distance between the nuclei can be calculated. Image of a small dog taken in mid-infrared (thermal) light (false color) Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than visible light, but shorter than microwave radiation. ... Ultraviolet (UV) radiation is electromagnetic radiation of a wavelength shorter than that of the visible region, but longer than that of soft X-rays. ...