In this paper, we study the chemical reactions of medium, and obtain the relationship of the sidelight spectrum and the pumping rate of the excimer excited states such as KrF*, ArF*, Kr2F* on time, medium pressure and medium composition. The results show that the optimum value of medium pressure is 0.8atm, the optimum ratio of the partial pressure Ar/Kr/F2 equals to 91.375%/8.4%/0.225%, and the optimum dwell time of electron beam should be more than 50ns.