Lyman-alpha emission
= Lyman-alpha emission
{c}
{title2=$2p\to1s+\gamma$}
Radiative decay from the <hydrogen> $2p$ level to $1s$ produces a Lyman-alpha photon. In an optically thin collisionally excited gas, escape of this photon removes the excitation energy deposited by an <Electron> collision. The excitation threshold is responsible for the rapid temperature sensitivity of this cooling channel.