LASING IN THE ALUMINUM AND INDIUM RESONANCE LINES FOLLOWING PHOTOIONIZATION AND RECOMBINATION IN THE PRESENCE OF H2

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-angstrom 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 3s4s S-2 1/2-3s3p P-2 3/2 transition of Al is more than 100 times the intensity of the transition from the same excited state to the 3s3p P-2 1/2 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 3s4s 2S1/2 state of Al. The resulting stimulated emission depends critically on the presence of the H2 buffer gas at a density of the order of 10(19) 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 angstrom, 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.