Singletransition

Singletransition is a term used in physics to denote a process in which a system undergoes a single energy-level transition between two states, typically accompanied by emission or absorption of a photon with a well-defined energy difference. In atomic and molecular spectroscopy, a single transition produces a discrete spectral line whose position corresponds to the energy difference between the initial and final states, and whose intensity is governed by the transition dipole moment and the population of the initial state. The line is often broadened by lifetime effects and instrumental resolution.

In theoretical treatments, the singletransition or two-level approximation models the system as effectively having only two relevant states and ignores coupling to other nearby states. This simplification underpins the use of Einstein A and B coefficients to describe spontaneous and stimulated emission and absorption. The singletransition picture is particularly useful for isolated atoms or molecules with large energy gaps to other states, or when external perturbations are small.

Singletransition models contrast with multi-step or multi-photon processes, where the system undergoes a cascade of transitions or simultaneous transitions involving more than one photon. In such cases, spectral features can involve multiple lines, continuum processes, or non-linear effects, and a single-line interpretation may be insufficient.

Applications of singletransition analysis include the calibration of spectroscopic instruments, the determination of energy level schemes, and the interpretation of astrophysical spectra where isolated lines provide clean probes of physical conditions. The term is not universally standardized, and authors may refer to a "single-transition approximation" or a "two-level model" depending on context.