[en] We demonstrate single-shot x-ray laser induced time-of-flight photoelectron spectroscopy on semiconductor and metal surfaces with picosecond time resolution. The LLNL COMET compact tabletop x-ray laser source provides the necessary high photon flux (>10¹²/pulse), monochromaticity, picosecond pulse duration, and coherence for probing ultrafast changes in the city, chemical and electronic structure of these materials. Static valence band and shallow core-level photoemission spectra are presented for ambient temperature Ge(100) and polycrystalline Cu foils. Surface contamination was removed by UV ozone cleaning prior to analysis. In addition, the ultrafast nature of this technique lends itself to true single-state measurements of shocked and heated materials. Time-resolved electron time-of-flight photoemission results for ultra-thin Cu will be presented

[en] An 84.5 eV Ni-like Pd ion 4d - 4p x-ray laser source generated by the LLNL Compact Multipulse Terawatt (COMET) tabletop system has been used to probe the electronic structure of various metals and semiconductors. In addition to the ∼4 - 5 ps time resolution, the probe provides the necessary high photon flux (>10¹²/pulse), narrow line width (ΔE/E∼2 x 10^-5) and coherence for studying valence band and shallow core electronic structure levels in a single shot. We show some preliminary results of room temperature and heated thin foil samples consisting of 50 nm Cu coated on a 20 nm C substrate. A 527 nm wavelength 400 fs laser pulse containing 0.1 - 2.5 mJ laser energy is focused in a large 500 x 700 (micro)m² (FWHM) spot to create heated conditions of 0.07 - 1.8 x 1012 W cm^-2 intensity

[en] Detailed knowledge of how materials respond to strong shocks or other extreme conditions on rapid timescales (such as laser heating) are required to support LLNL missions of national security and stockpile stewardship. This project started in FY01 to develop and demonstrate a new pump-probe characterization capability for investigating ultrafast changes in the chemical and electronic structure of materials under extreme conditions with picosecond time resolution. The LLNL COMET (Compact Multipulse Terawatt) [1] is a compact 15 TW laser facility operating at 1054 nm wavelength, and utilizes the technique of chirped pulse amplification to produce two high power beams at a rate of 1 shot every 4 minutes. A short pulse length varied from 500 fs to 25 ps and a long 600 ps (FWHM) pulse is focused in a high intensity line focus with a traveling wave geometry to generate an intense Ni-like Pd ion 4d-4p x-ray laser (XRL) line at 14.7 nm (84.5 eV). Total energy in the two beams is of order 3-7 J to produce lasing where the peak-to-peak delay between the laser pulses is found to be optimal at 700 ps with the short pulse arriving after the long pulse. Typical COMET x-ray laser characteristics are summarized in Table 1. High photon flux/shot, high monochromaticity, and picosecond pulse duration when combined with small source area and beam divergence properties of the 14.7 nm line [2] give ultra-high peak brightness ∼ 10²⁴-10²⁵ ph. mm^-2 mrad^-2 s^-1 (0.1% BW)^-1. Overall, the 14.7 nm peak brightness is 5-6 orders of magnitude higher than 3rd generation synchrotron undulator sources. However, third generation synchrotron undulator sources still have higher average brightness of 0.5-6 x 10¹⁸ ph.mm^-2 mrad^-2 s^-1 (0.1% BW)^-1 at 50-10 nm, respectively. The technique of electron time-of-flight (e-ToF) spectroscopy requires a monochromatic, ps pulsed source for efficient x-ray laser induced photoelectron spectroscopy (PES)

[en] Time-resolved soft x-ray photoelectron spectroscopy is used to probe the non-steady-state evolution of the valence band electronic structure of laser heated ultra-thin (50 nm) metal foils and bulk semiconductors. Single-shot soft x-ray laser induced time-of-flight photoelectron spectroscopy with picosecond time resolution was used in combination with optical measurements of the disassembly dynamics that have shown the existence of a metastable liquid phase in fs-laser heated metal foils persisting 4-5 ps. This metastable phase is studied using a 527 nm wavelength 400 fs laser pulse containing 0.3-2.5 mJ laser energy focused in a large 500 x 700 (micro)m² spot to create heated conditions of 0.2-1.8 x 10¹² W cm^-2 intensity. The unique LLNL COMET compact tabletop soft x-ray laser source provided the necessary high photon flux, highly monoenergetic, picosecond pulse duration, and coherence for observing the evolution of changes in the valence band electronic structure of laser heated metals and semiconductors with picosecond time resolution. This work demonstrates the continuing development of a powerful new technique for probing reaction dynamics and changes of local order on surfaces on their fundamental timescales including phenomena such as non-thermal melting, chemical bond formation, intermediate reaction steps, and the existence of transient reaction products

[en] Preventing the corrosion and oxidation of uranium is important to the continued development of advanced nuclear fuel technologies. Knowledge of the surface reactions of uranium metal with various environmental and atmospheric agents, and the subsequent degradation processes, are vitally important in 21st century nuclear technology. A review of the oxidation of actinide elements and their use in catalysis summarizes the present understanding of the kinetics and mechanisms of the reaction in dry and humid air. Researchers have recently used N₂⁺ and C⁺ ion implantation to modify the near surface region chemistry and structure of U to affect the nucleation and growth kinetics of corrosion and to passivate the surface. These researchers used Auger electron spectroscopy (AES) in conjunction with sputter depth profiling to show that the implanted surfaces had compositional gradients containing nitrides and carbides. In addition to chemical modification, ion implantation can create special reactive surface species that include defect structures that affect the initial absorption and dissociation of molecules on the surface, thus providing mechanical stability and protection against further air corrosion

[en] Synchrotron radiation is used to study the electronic structure of the energetic material 1,3,5-triamino-2,4,6-trinitrobenzene (TATB). Element and site-specific density of unoccupied electronic states in TATB is probed by x-ray-absorption spectroscopy at the C, N, and O K edges. In addition to illuminating the electronic structure of TATB, the absorption data enable the understanding of microscopic changes occurring in such compounds due to various damage mechanisms. The absorption data are further supplemented by data from the core-level and valence-band photoelectron spectroscopy of thin films of TATB

[en] Carboxyl terminated Self-Assembled Monolayers (SAMs) are commonly used in a variety of applications, with the assumption that the molecules form well ordered monolayers. In this work, NEXAFS verifies well ordered monolayers can be formed using acetic acid in the solvent. Disordered monolayers with unbound molecules present in the result using only ethanol. A stark reorientation occurs upon deprotonation of the endgroup by rinsing in a KOH solution. This reorientation of the endgroup is reversible with tilted over, hydrogen bound carboxyl groups while carboxylate-ion endgroups are upright. C1s photoemission shows that SAMs formed and rinsed with acetic acid in ethanol, the endgroups are protonated, while without, a large fraction of the molecules on the surface are carboxylate terminated

[en] Chemical vapour deposition (CVD) is used for the production of fused silica optics in high-power laser applications. However, relatively little is known about the ultraviolet laser damage threshold of CVD films and how they relate to intrinsic defects produced during deposition. We present here a study relating structural and electronic defects in CVD films to 355 nm pulsed-laser damage threshold as a function of post-deposition annealing temperature (T_HT). Plasma-enhanced CVD based on SiH₄/N₂O under oxygen-rich conditions was used to deposit 1.5, 3.1 and 6.4 µm thick films on etched SiO₂ substrates. Rapid annealing was performed using a scanned CO₂ laser beam up to T_HT ∼ 2100 K. The films were then characterized using x-ray photoemission spectroscopy, Fourier transform infrared spectroscopy (FTIR) and photoluminescence spectroscopy. A gradual transition in the damage threshold of annealed films was observed for T_HT values up to 1600 K, correlating with a decrease in non-bridging silanol and oxygen deficient centres. An additional sharp transition in damage threshold also occurs at ∼1850 K indicating substrate annealing. Based on our results, a mechanism for damage-related defect annealing is proposed, and the potential of using high-T_HT CVD SiO₂ to mitigate optical damage is also discussed. (paper)

[en] Recent experiments were carried out on the Prague Asterix Laser System (PALS) towards the demonstration of a soft x-ray laser Thomson scattering diagnostic for a laser-produced exploding foil. The Thomson probe utilized the Ne-like zinc x-ray laser which was double-passed to deliver ∼1 mJ of focused energy at 21.2 nm wavelength and lasting ∼100 ps. The plasma under study was heated single-sided using a Gaussian 300-ps pulse of 438-nm light (3ω of the PALS iodine laser) at laser irradiances of 10¹³-10¹⁴ W cm^-2. Electron densities of 10²⁰-10²² cm^-3 and electron temperatures from 200 to 500 eV were probed at 0.5 or 1 ns after the peak of the heating pulse during the foil plasma expansion. A flat-field 1200 line mm^-1 variable-spaced grating spectrometer with a cooled charge-coupled device readout viewed the plasma in the forward direction at 30^o with respect to the x-ray laser probe. We show results from plasmas generated from ∼1 (micro)m thick targets of Al and polypropylene (C₃H₆). Numerical simulations of the Thomson scattering cross-sections will be presented. These simulations show electron peaks in addition to a narrow ion feature due to collective (incoherent) Thomson scattering. The electron features are shifted from the frequency of the scattered radiation approximately by the electron plasma frequency ±ω_pe and scale as n_e^1/2

[en] Multilayers are artificially layered structures that can be used to create optics and optical elements for a broad range of x-ray wavelengths, or can be optimized for other applications. The development of next generation x-ray sources (synchrotrons and x-ray free electron lasers) requires advances in x-ray optics. Newly developed multilayer-based mirrors and optical elements enabled efficient band-pass filtering, focusing and time resolved measurements in recent FLASH (Free Electron LASer in Hamburg) experiments. These experiments are providing invaluable feedback on the response of the multilayer structures to high intensity, short pulsed x-ray sources. This information is crucial to design optics for future x-ray free electron lasers and to benchmark computer codes that simulate damage processes