A high-gain harmonic-generation free-electron laser is demonstrated. Our approach uses a laser-seeded free-electron laser to produce amplified, longitudinally coherent, Fourier transform-limited output at a harmonic of the seed laser. A seed carbon dioxide laser at a wavelength of 10.6 micrometers produced saturated, amplified free-electron laser output at the second-harmonic wavelength, 5.3 micrometers. The experiment verifies the theoretical foundation for the technique and prepares the way for the application of this technique in the vacuum ultraviolet region of the spectrum, with the ultimate goal of extending the approach to provide an intense, highly coherent source of hard x-rays.

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Continuously growing cultures of E. coli B/r were irradiated with a fluence of broad-band near-ultraviolet radiation (315-405 nm) sufficient to cause extensive growth delay and complete cessation of net RNA synthesis. Chloramphenicol treatment was found to stimulate resumption of RNA synthesis, similar to that observed with chloramphenicol treatment after amino-acid starvation. E. coli strains in which amino-acid starvation does not result in cessation of RNA synthesis ("relaxed" or ret strains) show no cessation of growth and only a slight effect on the rate of growth or of RNA synthesis. These findings show that such near-UV fluences do not inactivate the RNA synthetic machinery but affect the regulation of RNA synthesis, in a manner similar to that produced by amino-acid starvation. Such regulation is believed to be mediated through alterations in concentration of guanosine tetraphosphate (ppGpp), and our estimations of ppGpp after near-UV irradiation are consistent with such an interpretation. These data, combined with earlier published data, strongly suggest that the mechanism of near-UV-induced growth delay in E. coil involves partial inactivation of certain tRNA species, which is interpreted by the cell in a manner similar to that of amino-acid starvation, causing a rise in ppGpp levels, a shut-off of net RNA synthesis, and the induction of a growth delay.Near-ultraviolet radiation (near-UV; 300-380 nm) is present in sunlight, but is absorbed only marginally, if at all, by proteins and nucleic acids. Yet the well-known deleterious effects of far-ultraviolet radiation (far-UV; below 300 nm), such as killing, induction of mutation, and inhibition of growth, are found to occur in bacteria throughout the near-UV region (1, 2).Of these effects, the one induced by the lowest fluences of near UV is the inhibition of growth*. Bacterial studies have shown that growth delay has a narrow action spectrum, peaking at 340 nm. Earlier suggestions that quinones might be the chromophores and oxidative phosphorylation the cellular target (1) have not been supported by recent work showing that coenzyme Q is resistant to near-UV radiation in vivo (3) and that ATP synthesis shows a threshold response (B. Lakchaura, T. Fossum, and J. Jagger, J. Bacteriol., 125, in press). In addition, it is unlikely that DNA is the target for near-UV-induced growth inhibition (4).Inhibition of growth clearly must involve inhibition of macromolecular synthesis. Swenson et al. (5) He has also shown that the action spectrum for induction of this inhibition of RNA synthesis is similar to the action spectrum for induction of growth delay in E. coli, and that the growth delay action spectrum fits very closely to the absorption spectrum of tRNAs that contain the unusual nucleoside 4-thiouridine (4Srd). It is known that near-UV irradiation in vitro of such tRNAs can induce an adduct between the 4Srd in the 8-position and a cytidine in the 13-position of the tRNA, and that this lowers the rate at which the tRNA accepts amino acids (7). The f...

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We report on an experimental investigation characterizing the output of a high-gain harmonic-generation (HGHG) free-electron laser (FEL) at saturation. A seed CO2 laser at a wavelength of 10.6 microm was used to generate amplified FEL output at 5.3 microm. Measurement of the frequency spectrum, pulse duration, and correlation length of the 5.3 microm output verified that the light is longitudinally coherent. Investigation of the electron energy distribution and output harmonic energies provides evidence for saturated HGHG FEL operation.