Quasielastic

Quasielastic is a term used in physics to describe processes in which the energy transferred between a probe and a target is small compared with the probe’s energy, so the interaction is nearly elastic but not strictly so. The designation is applied across different areas where internal degrees of freedom or slow dynamics of the target allow only a tiny amount of energy to be exchanged.

In nuclear and particle physics, quasielastic scattering refers to reactions in which a high-energy probe, such as an electron or neutrino, scatters off a bound nucleon inside a nucleus with the nucleus remaining intact or in a low-lying excited state. The nucleus is often treated as a collection of quasi-free nucleons, and the cross section is approximated by the sum of scattering from individual nucleons. Final-state interactions between the ejected nucleon and the residual nucleus can modify the observed spectrum, introducing broadening or shifts that must be modeled in order to extract information about nucleon structure and nuclear dynamics.

In condensed matter and materials science, quasielastic scattering is observed as a broad, near-zero-energy-transfer feature in spectra measured by neutron or light scattering. Quasielastic neutron scattering (QENS) is commonly used to study diffusive motion, rotational dynamics, and relaxation processes in liquids, polymers, and biological systems. The width and shape of the quasielastic peak provide information about diffusion coefficients, jump lengths, residence times, and molecular reorientation rates on picosecond to nanosecond timescales.

Quasielastic lies between elastic (zero energy transfer) and inelastic (finite energy transfer to excitations) regimes, offering a window into dynamic processes without large energy losses. The interpretation depends on the specific system, probe, and temperature, and often requires detailed theoretical modelling.