In the simplest case, plasma is formed by applying a potential difference (of a few 100 V to a few kV) between two electrodes that are inserted in a cell (or that form the walls of the cell). The cell is filled with a gas (an inert gas or a reactive gas) at a pressure ranging from a few mTorr to atmospheric pressure. Due to the potential difference, electrons that are emitted from the cathode, are accelerated away from the cathode, and give rise to collisions with the gas atoms or molecules. If e* is an excited electron in plasma, the following types of reactions can occur in plasma: