Collisional excitation
= Collisional excitation
{title2=$A+e^-\to A^*+e^-$}
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 $k_BT$, giving the threshold behavior of <atomic line cooling>.