Absorption Spectra for Fluorine. Fluorine (F) forms a single bond with itself in elemental form, resulting in the diatomic F2 molecule. F2 is a supremely reactive, poisonous, pale, yellowish brown gas. Elemental fluorine is the most chemically reactive and electronegative of all the elements. Absorption spectroscopy is a measurement of the absorption of radiation, as a function of frequency or wavelength, due to its interaction with a sample. Absorption spectroscopy is used as an analytical chemistry tool to determine the presence of a particular substance in a sample and to quantify the amount of the substance present. It is also employed in studies of molecular and atomic physics, astronomical spectroscopy and remote sensing.