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A self-filtering unstable resonator with magnification |M| = 5 has been applied to a pulsed UV-preionized XeCl excimer laser. In agreement with numerical results it is experimentally shown that less than two cavity round trips are needed to establish a steady state lowest-order mode. A diffraction-limited laser beam with a brightness of 1.5 x 10(13) W cm(-2) sr(-1) has been obtained.

The single-pass/double-pass amplified spontaneous emission technique has been used to measure the net gain at 222.4 nm in a discharge-pumped UV-preionized KrCl laser. The net gain has been measured as a function of pump rate and total pressure in He- and Ne-based gas mixtures. A maximum net gain of 0.15 cm(-1) has been obtained in a 0.09% HC1/10.1% Kr/1.5% He/88.3% Ne mixture at a total pressure of 355 kPa and at a specific power loading of approximately 21 MW/cc. The data presented are compared with previous net gain measurements performed using the passive absorption cell method.