Abstract

When an aluminum or indium target in a molecular hydrogen buffer gas is exposed to a 500-mJ, 20-nsec pulse from a line-focused ArF 1930-Å laser, light from the neutral resonance lines appears promptly. It is emitted in a beam directed along the axis of the focal volume with a divergence less than 18 mrad. On axis, the 3s4s2S1/2–3s3p2P3/2 transition of Al is more than 100 times the intensity of the transition from the same excited state to the 3s3p2P1/2 ground state, but the ratio of intensities decreases rapidly with increasing angle from the axis. The intensity anomaly is evidence of amplification of spontaneous emission from a population inversion in the 3s4s2S1/2 state of Al. The resulting stimulated emission depends critically on the presence of the H2 buffer gas at a density of the order of 1019 cm−3. It lasts only as long as the excitation provided by the ArF excimer laser. However, it is absent for excitation by a similar KrF laser at 2480 Å, a wavelength below the threshold for single-photon ionization of the target. The relative intensities of the resonance doublet, and the variation of intensity with line focus length and buffer-gas density, indicate a gain–length product of approximately 8 cm−1 under optimum conditions. Photoionization followed by rapid recombination in the presence of H2 is suspected as the primary mechanism responsible for the inversion and the gain.

© 1991 Optical Society of America

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