A surprisingly long delay time (of the order of 10 &mgr;sec) is found necessary for maximum enhancement of the output power and energy from the main pulse of a 0.1‐liter‐volume Blumlein‐driven XeF excimer laser when another fast‐discharge pulse of about 0.1‐&mgr;sec duration and 10&percent; of the main discharge energy is used to preionized or otherwise ’’precondition’’ the gas mixture. At such optimal timing, a factor‐of‐5 enhancement of the UV laser output (3510<&lgr;<3537 A˚) has been observed in a He : Xe : NF3mixture at an overall generation efficiency of 1.3&percent;.