 Collisional excitation

A collision can transfer kinetic energy to an atom or ion and excite a bound Electron. Subsequent radiative de-excitation removes that energy from an optically thin gas. The thermal excitation rate is strongly reduced when the transition energy exceeds $k_{B}T$, giving the threshold behavior of atomic line cooling.

Collisional excitation is a process in which an atom or molecule absorbs energy during a collision with a particle, such as another atom, molecule, or electron. This energy transfer can promote an electron within the atom or molecule to a higher energy state, or excited state. Here's how it works: 1. **Encounter**: During a collision, kinetic energy from the colliding particle (which can be a gas particle or an electron) is transferred to the target atom or molecule.