Laser apparatus includes hydrogen or deuterium buffer gas and dispenser reservoirs which contain a hydride or deuteride as appropriate. As gas molecules from the buffer gas are lost within the discharge tube due to trapping and sputtering processes, hydrogen or deuterium is given off from the surfaces of the reservoirs at a greater rate to maintain the vapor pressure within the discharge tube at a substantially constant level. Thus the laser apparatus may operate in a "sealed-off" mode, that is without requiring that buffer gas be flowed through it.
