photophysics

Photophysics is the branch of science that studies the interaction of light with matter at the level of physical processes that accompany absorption, emission, and energy transfer. It explores how electronic, vibrational, and spin states of molecules or condensed-phase systems are excited by photons and how they return to lower energy states. Central concepts include the Jablonski diagram, which summarizes electronic states and transition pathways; the Franck-Condon principle, which explains the relative probabilities of transitions during electronic excitation; and the distinction between radiative and nonradiative decays.

When a system absorbs a photon, it is promoted to an excited electronic state, often followed by rapid vibrational relaxation to the lowest vibrational level of that state. Relaxation can occur radiatively as fluorescence, emitting a photon, or nonradiatively via internal conversion. If the excited state is a triplet, intersystem crossing may produce phosphorescence, which is typically longer-lived due to spin-forbidden transitions. Energy can also be transferred to other chromophores via mechanisms such as Förster resonance energy transfer (FRET) or Dexter energy transfer, or converted to electrical energy in photoinduced charge transfer processes. Quantities of interest include the quantum yield and lifetimes of excited states, as well as absorption and emission spectra and rate constants such as Einstein A and B coefficients.

Photophysics underpins many applications, including light-emitting devices, solar cells, and fluorescence-based imaging and sensing. Techniques such as absorption spectroscopy, steady-state and time-resolved fluorescence spectroscopy, and transient absorption measurements probe the excited-state dynamics. The field informs the design of materials with specific optical properties by controlling electronic structure, vibronic coupling, and nonradiative loss pathways.