Absorptionsband

An absorption band is a region of the electromagnetic spectrum in which a material absorbs radiation to a noticeable degree, producing a dip in transmitted or reflected light. In spectroscopy, absorption bands occur when photons promote molecules from a lower energy state to a higher one, typically through electronic transitions in the ultraviolet or visible range, vibrational transitions in the infrared, or rotational transitions in the microwave region.

The position, width, and intensity of an absorption band depend on molecular structure, bonding, environment, and phase. Electronic bands in UV–visible spectroscopy arise from transitions between molecular orbitals and often define the color of a compound. Infrared absorption bands correspond to fundamental vibrational motions of bonds and provide fingerprints for functional groups. In solids, bands can reflect electronic or vibronic transitions within a crystal or pigment.

Bands can be broadened by temperature, pressure, solvent interactions, and inhomogeneous environments, and they may overlap in complex spectra. Quantitative use relies on Beer-Lambert’s law, A = εlc, linking absorbance to concentration, path length, and molar extinction coefficient.

Applications include identifying substances and determining concentrations in chemistry and biochemistry. Notable examples include the chlorophyll absorption bands near 430 nm and 662 nm, which influence plant coloration, and atmospheric ozone absorption in the ultraviolet. Absorption bands also underpin color in dyes and pigments and are central to spectroscopic analysis across disciplines.