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Irradiance values have been measured for the onset of laser-induced plasma formation in Xe, Ar, N(2), and O(2) at pressures from 760 to 25 Torr at wavelengths of 1.064, 0.532, 0.355, and 0.266 microm. These values have been compared with the results of other workers who used similar experimental setups. There is agreement within a factor of 4 when irradiance values are compared and within a factor of 2 when ratios of irradiance values for different gasses are compared. Comparisons among workers who used widely different pulse lengths indicate that the onset of plasma formation is measured better by energy fluence than by irradiance.

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We outline what is to our knowledge the first experimental demonstration of an excited-state Faraday filter. The filter consists of potassium vapor between crossed polarizers in a dc magnetic field and operates on the 4P((1/2)) ? 8S((1/2)) transition in potassium. The 4P((1/2)) state is populated by a linearly polarized, 10-ns light pulse from a dye laser operating at 769.9 nm. Another linearly polarized, 10-nsec pulse at 532.33 nm traverses the pumped volume of the K cell and is absorbed from the 4P((1/2)) state to the 8S((1/2)) state. The transmission of the filter is approximately 3.5% at 532.33 nm with a bandwidth of less than 10 GHz.

The operation of a narrow-linewidth optical f ilter based on the 4P((1/2)) ? 8S((1/2)) excited-state transition in potassium vapor is reported. The 4P((1/2)) state is excited by a circularly polarized, 769.9-nm, 10-ns pulse from a dye laser. A linearly polarized, time-sequenced, and spatially overlapped probe pulse at 532.33 nm completes the transition to the 8S((1/2)) state. The peak filter transmission is ~40% with a bandwidth of less than 4 GHz. Corroborative experimental results suggest that the rotation of probe-pulse polarization by an induced circular birefringence is the dominant mechanism behind the filter operation.