[en] Laser ablation is used in laser plasma thrusters (LPT), in which the created plasma provides the thrust that is used to stabilize the trajectory of satellites in space. To allow the use of IR laser diodes, an IR absorber has to be added to the polymer. As a measure of the energy released during ablation of such polymers, the shockwave velocity in air is measured with shadowgraphy. The measured shockwave velocities of a cross-linked glyzidyl azide polymer (GAP) incorporating carbon particles (as broad range absorber), reveal that the shockwave velocity decreases with the increasing irradiation wavelength (193-1064 nm) for a given fluence. In addition, the shadowgraph images reveal that for irradiation with shorter wavelengths, the amount of solid fragments decreases and more gaseous products are released. Comparing the shockwave propagation of GAP and a triazene polymer reveals that both polymers exhibit similar shockwave velocities at UV irradiation wavelength, whereas with 1064 nm irradiation, the shockwaves generated using GAP propagate faster. These results are probably due to the change of the absorption site, the mechanism of ablation, and the different decomposition enthalpies

[en] The dynamic processes during ablation are studied by nanosecond-interferometry and shadowgraphy. Most commercial polymers exhibit poor laser ablation properties, therefore special triazene polymers, with superior ablation properties were developed. The photochemical active triazene group absorbs around 330 nm whereas the absorption around 200 nm is due to the photostable aromatic groups. The ns-interferometry shows that the etching of the triazene polymer starts and ends with the laser beam after irradiation at 193 and 308 nm. Shadowgraphy of the triazene polymer and polyimide reveal that the speed of the aerial shockwave increases with fluence and is higher for irradiation at 193 nm than for 308 nm. Shockwaves with equal or higher velocities are observed for the triazene polymer compared to polyimide

[en] Molten carbonate fuel cells (MCFCS) are being developed for large scale power generation. One source of fuel for MCFCs is coal gasifiers. This in turn presents the problem of coal gas contaminants and their effect on the MCFC. Of these contaminants (Pigeaud, June 1991), HCl is the contaminant that can have the greatest adverse effect. Those results indicate that HCI has the effect of first being adsorbed by the carbonate, second it converts to alkali chloride and particularly KCl, and third it volatilizes as KCl gas and is swept from the fuel cell. This has the effect of lowering the K+ content of the fuel cell, plugging the gas flow tubes, and effectively causing a shutdown of the fuel cell because of the changing eutectic mixture and a raising of the melting point. Because of this, our effort has been directed toward finding a suitable diagnostic for HCl. With this diagnostic we propose to investigate the physical parameters associated with HCl absorption and conversion to KCl. We also plan on taking this diagnostic to industry to test on a working MCFC to determine if the calculations for the HCl removal coupled with our measurements are verified in the test results

[en] We report the use of a ferroelectric liquid-crystal (FLC) mask as an optical encoder for development of a solid-state Fourier-transform spectrometer. For this demonstration a 1x64 element array was striped and used as a 1x4 element device. The device intersected dispersed radiation and encoded each spectral component with a carrier signal by applying half-wave potential to each of the four striped (1x16) FLC elements, which varied the transmitted amplitude of the light from 0.03% to 28% of full scale. The light was spectrally recombined and imaged onto a photomultiplier and the resulting carriers (and their amplitudes) detected by Fourier transformation of the time-varying signal. Spectra of colored-glass filters were taken to demonstrate the possibilities of the instrument

[en] The fabrication of iron (Fe) films epitaxially grown on single crystal magnesium oxide (MgO) substrates is presented. Our goal is the creation of ideal crystalline iron samples for shock physics experiments using ultrafast x-ray diffraction. There are several reasons for the choice of the Fe/MgO system. Iron is known to undergo shock-induced structural changes, and MgO is selected because it is a) transparent to the laser light used to generate the shock in the Fe film and b) there is only a 4% lattice mismatch between Fe and MgO. Issues such as generating a specific crystallographic orientation in the iron film and technical challenges in the synthetic process are discussed. The ultimate goal for producing a single crystal Fe film on MgO is to allow the merger of theory and experiment. A new ultrafast x-ray diffraction experiment will permit the detection of laser-driven shock-induced phase changes in the crystalline Fe samples. These experiments will complement an elegant MD investigation of a shocked Fe crystal.p

[en] A method is described for generating shock waves with 10-20 ps risetime followed by >200 ps constant pressure, using spectrally modified (clipped) chirped laser pulses. The degree of spectral clipping alters the chirped pulse temporal intensity profile and thereby the time-dependent pressure (tunable via pulse energy) generated in bare and nitrocellulose-coated Al thin films. The method is implementable in common chirped amplified lasers, and allows synchronous probing with a <200 fs pulse

[en] We report a direct measurement of temperature in a shocked metal using Doppler broadening of neutron resonances. The 21.1-eV resonance in ¹⁸²W was used to measure the temperature in molybdenum shocked to ∼63 GPa. An explosively launched aluminum flyer produced a planar shock in a molybdenum target that contained a 1-mm thick layer doped with 1.7 at. %¹⁸²W. A single neutron pulse, containing resonant neutrons of less than 1 μs duration, probed the shocked material. Fits to the neutron time-of-flight data were used to determine the temperature of the shocked molybdenum

[en] We report the measurement of the two-photon detachment spectrum of the ¹D^e resonance of H^- just below the n=2 threshold. The excess photon detachment resonance fits a Fano profile with energy 10.872(2) eV (relative to H^- ground state), a width of 0.0105(10) eV, and a shape parameter of -8(2). To our knowledge, this describes the first time direct multiphoton excitation of a resonance has been observed in any negative atomic ion. copyright 1995 The American Physical Society

[en] Laser induced fluorescence (LIF) excitation spectra recorded for the vibrational bands in the Mg(3s3p ¹P₁)·Xe(¹Π₁)left-arrow Mg(3s3s ¹S₀)·Xe (X ¹Σ⁺) system have been analyzed, yielding absolute vibrational assignments and values of ω_exe=1.585±0.02 and ω_e=97.5±1.0 cm^-1 for the ¹Π₁ state of ²⁴Mg¹³²Xe. From a Birge--Sponer extrapolation, the well depth of this state is estimated to be 1500 cm^-1. Simulations of rotationally structured spectra of three of the most intense vibrational bands are consistent with R^double-prime_e=4.56±0.12 A for the X ¹Σ⁺ state. From Morse function extrapolation of the excited state rotational constants from the simulations, and Franck--Condon intensity simulations of the ¹Π₁ left-arrow X ¹Σ⁺ vibrational progressions, R^'_e for the ¹Π₁ state is estimated to be 3.07±0.10 A. The ¹Π₁ state of MgXe fluoresces strongly. The corresponding ¹Π₁ states of ZnXe and CdXe do not fluoresce, but ''action'' spectra from the production (via predissociation) of metal atom ³P_J states are observed. Possible reasons for these differences are discussed in terms of spin--orbit induced predissociation. It is concluded that predissociation of the MgXe(¹Π₁) state is not observed because the crossing between the repulsive ³ summation ⁺₁ and the attractive ¹Π₁ potential curves does not occur until energies higher than those accessible experimentally

[en] We report the first observation of nonresonant excess photon absorption (for ionization as well as detachment) in competition with the single photon process. A 35keV negative hydrogen ion beam is irradiated with focused Nd:YAG laser pulses; the 1.165eV photon energy exceeds the electron binding energy by 0.41eV. The time of flight spectrum of detached electrons exhibits the absorption of one and two photons. The detached electrons exit with a P wave angular distribution for the one-photon detachment and primarily a D wave for the two photon excess photon detachment. copyright 1997 The American Physical Society