Atoms produce line spectra , whereas molecules produce band spectra .
The various ranges of the electromagnetic spectrum are probed as follows, in order of increasing frequency. In the radio region, nuclear magnetic resonance provides information, especially on protons, and hence on the structure of organic compounds in particular. In microwave spectroscopy , pure rotational transitions of molecules can cause absorption, and electron spin resonance can be used to look at radicals . In the visible and ultraviolet, electronic transitions are observed. Spectroscopy has a large number of applications, from probing the chemical composition, physical structure and electronic structure of materials in the laboratory to the elucidation of the composition and temperature of distant stars.