David T. Attwood

0.1–10‐keV x‐ray‐induced electron emissions from solids—Models and secondary electron measurements

Analytical models are presented describing the x‐ray‐excited emission of ’’no‐loss’’ photoelectrons and Auger electrons and the energy distribution of emitted secondary electrons. The secondary electron energy distribution is given in terms of the electron kinetic energy EK, work function W, photon energy Eo, and mass photoionization coefficient μ (Eo), as proportional to Eoμ (Eo) EK(EK+W)−4. Techniques of electron spectral measurements utilizing uniform field preacceleration and limited acceptance angle spectrometers are discussed. Secondary electron energy distributions are measured at about 10−8 Torr from thick evaporated films of gold and aluminum at photon energies 277, 1487, and 8050 eV. The shapes of these distributions do not depend significantly upon photon energy. The full width at half‐maximum (FWHM) of these distributions are 3.9, 6.7, and 4.4 eV for Au and ion‐cleaned Au and Al photocathodes, respectively. The data agree well with the model predictions.

Zone‐plate coded imaging of laser‐produced plasmas

The first successful demonstration of high‐resolution tomographic imaging of a laboratory plasma using a coded imaging technique is reported. Zone‐plate coded imaging has been used to image the x‐ray emission from laser‐compressed D‐T‐filled microballoon targets. The zone‐plate camera exhibited a planar resolution ∼8 μm and had a tomographic resolution of ∼74 μm. This allowed three‐dimensional viewing of the target emission from a single shot in planar slices.

Holographic microinterferometry of laser-produced plasmas with frequency- tripled probe pulses

Holographic microinterferometry is used to study plasmas produced by high‐intensity subnanosecond pulses from a Nd laser. The probing pulse is frequency tripled to 3547 A in order to reduce refractive errors due to steep density gradients and to reduce spatial resolution requirements. Measured electron densities as high as one‐fourth critical for 1.06 μm are presented. Factors concerning the extension of these measurement to 1021 electrons/cm3 are discussed.

Microscopic interferometry of laser‐produced plasmas

Holographic microinterferometry is applied to the study of laser‐produced plasmas. A 150‐psec 1.06‐μm light pulse from a neodymium laser is used for both production and diagnostics of the plasma. An interferogram, obtained 0.5 nsec after irradiation by a 1/2‐J pulse, is presented. It is pointed out that spatial resolution is limited by plasma motion during passage of the 100‐psec frequency‐doubled diagnostic pulse.

Secondary electron energy distributions for gold as excited by C Kα (277 eV) and Al Kα (1487 eV) x rays

The secondary electron energy distributions for a gold photocathode as excited by C Kα (277 eV) and Al Kα (1487 eV) x rays have been measured. The shapes of the energy distributions are essentially the same for these two x‐ray photon excitation energies. For thick evaporated gold samples on glass substrates (at 150 °C and 3×10−8 Torr), the secondary electron energy distributions peak at about 1 eV and have a FWHM of about 4 eV. As measured immediately after ion cleaning, the distributions peak at about 2 eV and have a FWHM of about 6.6 eV. Approximately 5 h after ion cleaning, the measured distributions appear as those obtained before ion cleaning. The work function of the evaporated gold photocathode temporarily increases by 1 eV upon ion cleaning.

Composite x-ray pinholes for time-resolved microphotography of laser compressed targets

Composite x-ray pinholes having dichroic properties are presented. These pinholes permit both x-ray imaging and visible alignment with micron accuracy by presenting different apparent apertures in these widely disparate regions of the spectrum. Their use is mandatory in certain applications in which the x-ray detection consists of a limited number of resolvable elements whose use one wishes to maximize. Mating the pinhole camera with an x-ray streaking camera is described, along with experiments which spatially and temporally resolve the implosion of laser irradiated targets.

Application of x rays to the analysis of DNA packaging in mammalian sperm

X-ray microscopy has been performed on unlabeled and gold labeled rat sperm nuclei using a tantalum x-ray laser (44.83 angstrom) and an x-ray zone plate lense. Transmission images of nuclei labeled with gold using an antibody to protamine 1 show large clusters of gold as well as individual 400 angstrom gold particles coating the surface of the chromatin. Images of the same nuclei obtained with a scanning x-ray microscope demonstrate that the initial exposure of the sperm nucleus to a photon intensity of 3.0 X 1014 W/cm2 for a duration of 500 ps did not destroy or grossly distort their structure. Other nuclei labeled with an antibody to protamine 2 were partially decondensed during the labeling process and contained large vacuole-like regions. Images were also obtained of unlabeled sperm nuclei. Data obtained from one unlabeled nucleus imaged with both the atomic force and x-ray laser microscopes was used to determine the thickness of the chromatin and estimate the amount of water that must be associated with the DNA-protamine complex in air-dried nuclei. These results suggest that even air-dried sperm nuclei are extensively hydrated and that tightly bound water may comprise as much as one-third of the volume of the dried sperm nucleus.

Laser frequency doubling in the presence of small-scale beam breakup

Frequency multiplication of intense laser pulses in narrow acceptance angle crystals is commonly accomplished with large conversion efficiency. However, the conversion efficiency can be seriously affected by degradation of the laser pulse, such as occurs when small-scale intensity fluctuations exponentiate due to nonlinear refractive effects within the laser. We present evidence that beam breakup such as this can cause a significant fraction of the laser energy to lie outside the acceptance angle of the crystal, thus diminishing conversion efficiency.

At‐Wavelength Interferometry of High‐NA Diffraction‐Limited EUV Optics

Recent advances in all‐reflective diffraction‐limited optical systems designed for extreme ultraviolet (EUV) lithography have pushed numerical aperture (NA) values from 0.1 to 0.3, providing Rayleigh resolutions of 27‐nm. Worldwide, several high‐NA EUV optics are being deployed to serve in the development of advanced lithographic techniques required for EUV lithography, including the creation and testing of new, high‐resolution photoresists. One such system is installed on an undulator beamline at Lawrence Berkeley National Laboratory’s Advanced Light Source. Sub‐A‐accuracy optical testing and alignment techniques, developed for use with the previous generations of EUV lithographic optical systems, are being extended for use at high NA. Considerations for interferometer design and use are discussed.

Movies At The Nanoscale Using Extreme Ultraviolet Laser Light

We report on the first demonstration of stop-motion imaging with ~50 nm spatial resolution using an extreme ultraviolet laser. Images of an AFM tip resonating at ~270 kHz were acquired with 1 ns temporal resolution.