linesemission

Line emission is the process by which atoms, ions, or molecules emit photons at specific wavelengths as they transition from higher to lower energy levels. This produces discrete spectral features known as emission lines, in contrast to the continuous spectrum created by broad thermal emission. Emission lines can arise from spontaneous radiative decay, recombination of ions with electrons, or collisional excitation followed by radiative decay.

In many astrophysical plasmas, especially at low densities, emission lines originate from transitions that are forbidden or semi-forbidden, meaning they occur with low probability in dense environments but can be observed in nebulae due to longer available lifetimes. Line emission serves to identify chemical elements and ions, to diagnose physical conditions such as temperature and density, and to reveal motions through Doppler shifts. The intensity of a line depends on the population of the upper level, the transition probability (Einstein A coefficient), and the local excitation conditions.

Line profiles are shaped by broadening mechanisms including natural broadening, thermal (Doppler) broadening, and pressure broadening. In expanding or moving sources, lines may be broadened, shifted, or split. Ratios of spectral lines from the same ion or from different ions provide diagnostic tools for electron temperature, density, and ionization state.

Common examples include hydrogen Balmer lines, such as H-alpha at 656.3 nm and Lyman-alpha at 121.6 nm, and forbidden lines like [O III] 5007 Å or [N II] 6584 Å. Molecular lines, such as CO rotational transitions, are prominent in the radio and infrared. Line emission is routinely measured with spectrographs in astronomy to study composition, physical conditions, and kinematics of astronomical objects.