Excimers are formed in a gas (30,130) by applying a pulsed potential between a first electrode (14,114) and a counter electrode (26, 126) so that corona discharge occurs, substantially without arcing, when the potential is on. The pulses or on-times of the potential desirably are about 100 microseconds or less. Use of a pulsed potential provides greater efficiency than a constant potential. Where the excimer-forming gas is a pure inert gas, the gas desirably contains less than 10 ppm water vapor.