fotonabsorption

Photon absorption is the process by which an incoming photon is captured by matter, transferring its energy to the absorbing system. In atoms and molecules this energy promotes the system to a higher electronic, vibrational, or rotational state, or removes an electron entirely through ionization. In solids, absorption can excite electrons to higher bands or create excitons. The specific energies that can be absorbed are determined by the allowed transitions and the structure of the material.

Quantum mechanically, absorption arises from the interaction between the electromagnetic field and charged particles. A photon can be absorbed only if its energy matches a permitted transition, in accordance with selection rules. In molecules, there are discrete electronic transitions as well as vibrational and rotational substructure; in solids, broad bands or gaps reflect the electronic band structure. The probability of absorption at a given wavelength is described by an absorption cross section, which depends on the material, environment, and temperature.

The spectral dependence of absorption is often summarized by the absorption coefficient or molar extinction coefficient and described experimentally by Beer-Lambert Law: A = εlc, relating absorbance to path length and concentration. Absorption can be influenced by broadening mechanisms, such as thermal motion, collisions, and lifetime effects.

Applications span spectroscopy, photography, solar energy, and detector design. Absorption spectra reveal chemical composition and electronic structure, while controlled absorption underpins devices from solar cells to optical sensors.