The disclosed system functions to generate ozone with improved efficiency. Oxygen gas is passed through an electric field, and while in the field is irradiated with brief bursts of high energy electrons. This produces a large number of secondary electrons in the gas and these electrons taken energy from the electric field. The secondary electrons efficiently dissociate oxygen molecules from the gas, thereby producing oxygen atoms which immediately combine with oxygen to form ozone. The electric field is pulsed on during the electron beam irradiation, and thereafter pulsed off in order to maximize energy deposition by electrons and minimize energy deposition by ions. The disclosed system may also be used to generate other substances in gas phase ionic or free-radical reactions in which a suitable gas or mixture of gases is passed through the discharge and made to react in it. For example, the system can be used to produce hydrogen peroxide or hydrazine from suitable gas mixtures of H2O + O2 and NH3 + N2, respectively.