spectrum of an ion
"John Farley's laboratory has a laser beam/ion beam spectrometer. In this instrument, a laser beam is used to measure the absorption spectrum of molecular ions. Measuring the spectrum of an ion is like taking the fingerprint of the ion: it allows identification of the ion. Furthermore, the spectrum reveals the energy levels of the ion, the spacing of the atoms, and the strength of the chemical bonds. Molecular ions are important in the upper atmosphere (ionosphere), in molecular clouds in the interstellar medium and also in industrial plasmas such as those used to make computer chips."