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The propagation of high-power short-pulse laser beams over considerable distances in air is studied both experimentally and via numerical simulations. Filaments are formed after 5–10 m and their propagation over distances in excess of 200 m is reported for the first time. The lateral dimensions of the filaments are found to range from about 100 μm to a few millimeters in diameter. The early values of plasma electron density have been inferred to be a few times 1016 cm−3 using longitudinal spectral interferometry. For 500 fs pulses and a wavelength of 1053 nm, the energy in the filament can be quite high initially (∼8 mJ) and is found to stabilize at about 1.5–2 mJ, after about 35 m. A simple model based on the nonlinear Schrödinger equation coupled to a multiphoton ionization law appears to describe several experimental results quite well.

Ultrafast x-ray techniques can, in principle, allow us to more directly watch the time evolution of matter, with atomic spatial resolution and with time resolution on the scale of atomic motions such as the making and breaking of chemical bonds, in order to more directly observe the fundamental molecular dynamics underlying the concept of ‘‘mechanism’’ in inorganic, organic, and biochemical reactions. As a step toward this goal, we have observed a chemical reaction process, photoinduced dissociation of gas phase SF6 molecules, detected by ultrafast near-edge x-ray absorption spectroscopy with time resolutions of 1.5–3 ps, near the sulfur K edge at a photon energy of 2.48 keV (4.98 A).

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Psoralea corylifolia L. fruit extracts exhibited osteoblastic proliferation stimulating activity in UMR106 cell line cultured in vitro. The flavonoids of corylin and bavachin were isolated and identified as active principles by activity-guided fractionation. The results suggested that Psoralea corylifolia L. fruit extracts and corylin and bavachin might stimulate bone formation or have potential activity against osteoporosis.

Hard x-ray spectra (10–100 keV) created in high contrast, 400 fs, laser pulse interaction with solid targets, have been studied for laser intensities in the 1017–1019 W/cm2 range. The target atomic numbers (Z) extended from Z=13 to Z=73. The measured conversion efficiency at Ag Kα emission line was 10−3% at 5×1018 W/cm2. It has been confirmed that the hot electron temperature increased as (Iλ2)1/3 and the fraction of laser energy in hot electrons follows scaling law of (Iλ2)3/4.

Analysis is presented of K-shell spectra obtained from solid density plasmas produced by a high contrast (1O":l) subpicosecond laser pulse (0.5 pm) at 10'8-10'9 W/cm'. Stark broadening measurements of He-like and Li-like lines are used to infer the mean electron density at which emission takes place. The measurements indicate that there is an optimum condition to produce x-ray emission at solid density for a given isoelectronic sequence, and that the window of optimum conditions to obtain simultaneously the shortest and the brightest x-ray pulse at a given wavelength is relatively narrow. Lower intensity produces a short x-ray pulse but low brightness. The x-ray yield (and also the energy fraction in hot electrons) increases with the laser intensity, but above some laser intensity (IO's W/cm* for Al) the plasma is overdriven: during the expansion, the plasma is still hot enough to emit, so that emission occurs at lower density and lasts much longer. Energy transport measurements indicate that approximately 6% of the laser energy is coupled to the target at lOI* W/cm* (1% in thermal electrons with T,=O.6 keV and 5% in suprathermal electrons with The25 keV). At Zh*= 10's W pm2/cm2 (no prepulse) around 10" photons are emitted per laser shot, in 2n srd in cold K, radiation (2-9 A, depending on the target material) and up to 2X lo*' photons are obtained in 271. srd with the unresolved transition array (UTA) emission from the Ta target. 0 1995 American Institute of Physics.