H. van Brug

Laser‐generated plasma as soft x‐ray source

We present some properties of laser‐generated plasma emission in the soft x‐ray region. Detailed spectral distributions are reported for different target materials (C, Mg, Al, Si, Ti, Fe, Ni, Cu, Mo, Ag, Sn, Ta, Au, Pb, and Bi) in the energy range from 100 to 800 eV. Furthermore, we include measurements of the angular distribution, laser power dependence, and source size of the soft x rays. The efficiency for soft x‐ray generation in our energy range peaks at a laser power density of 1012 W/cm2, which can be obtained already with standard laser systems. A comparison is made with other x‐ray sources. To demonstrate the unique properties of laser‐generated plasmas as soft x‐ray source, we report the first single‐shot EXAFS measurement in the soft x‐ray region.

Time‐resolved x‐ray monitoring of laser ablation of and plasma formation from Si

We present x‐ray absorption measurements on silicon clusters and silicon plasmas produced by pulsed laser irradiation of bulk silicon. The results are compared with earlier time‐resolved x‐ray absorption measurements on amorphous silicon foils under pulsed laser irradiation. Clusters are formed at an irradiance as low as 3.0 J/cm2. At an irradiance of 14 J/cm2 ionized species up to Si4+ are formed. We find a removed Si layer thickness of 80 A at an irradiance of ≊6 J/cm2, at 15 ns pulse duration.

A time‐resolved x‐ray absorption study of amorphous Si during pulsed laser irradiation

We report the first time‐resolved x‐ray absorption measurements on pulsed laser irradiated Si in the photon energy range from 90 to 300 eV, at irradiation energy densities up to 4 J/cm2, and at several delay times between irradiation pulse and x‐ray probe pulse. The absorption spectra recorded at a delay time of 12 ns can be classified in three categories. Below ≊0.17 J/cm2 only thermal damping of extended x‐ray absorption fine‐structure (EXAFS) oscillations is observed. Between ≊0.17 J/cm2 and ≊1 J/cm2 annealing takes place. Several changes are observed, among which a ≊7‐eV LII,III edge shifts towards higher energy, while the LI edge remains approximately at the same position. This edge shift behavior is explained by a largely reduced 3s density of states of the conduction band. This supports the idea that pulsed laser molten Si has a metalliclike structure. Thermal damping in this region is so strong that EXAFS oscillations are no longer visible. This is consistent with a lattice temperature above or eq...

A polychromator for (S)EXAFS using a laser generated plasma as soft X-ray source

Abstract A grating polychromator of novel design is presented. The design consists of a toroidal mirror and a concave grating and is intended for use in a SEXAFS experiment with use of a laser generated plasma soft X-ray source. The spectral range from 200 eV to 800 eV is dispersed on a 30 mm long, 5 mm high target. The resolution is measured to have a value of approximately 6 eV over the whole spectral range, as was predicted theoretically. The acceptance angle of the system is 2 × 10 −5 sr.

Soft X-Ray Emission of Laser-Produced Plasmas: Comparison for 30-ps and 20-ns Laser Pulses.

Abstract Soft x-ray emission spectra (250–875 eV) are presented for plasmas, produced by picosecond and nanosecond frequency-doubled Nd:YAG-glass laser pulses incident on 14 different target materials. The emitted spectra have been corrected for various apparatus functions which enables a direct comparison between plasmas produced by pico- and nanosecond laser pulses. The relative integrated emission intensity as a function of Z number, obtained from the corrected spectra, shows an oscillatory behavior, with distinct maxima for those elements exhibiting a dominant line emission in our photon energy window. We found for our two pulse lengths an approximately equal conversion efficiency from laser light into x-ray photons. General suggestions are given as to what target material should be used for different applications using the laser plasma as x-ray source in the energy range studied.

A multichannel X-ray detector

A multichannel X-ray detector with a spatial resolution of 0.13 mm and a maximum of 200 effective channels is described. The detector consists of a conversion electrode to convert X-rays into electrons, a microchannelplate phosphor screen assembly to amplify the electron signal, a self-scanning array containing 1024 photodiodes and optics to form an image from the phosphor screen on the self-scanning array. A magnetic field is used to form an image of the electrons from the conversion electrode on the channelplate. The resolution of the detector is determined by the magnetic imaging system and the channelplate/phosphor-screen assembly.

Reflection Extended X-Ray Absorption Fine-Structure Measurements on Ni/C and Nixsiy/C Multilayered Reflection Coatings

A new method is described to measure bond lengths in the interface between thin layers, by using multilayered reflection coatings both as a dispersive element in an x‐ray beam, and as an object of measurement. Extended x‐ray absorption fine‐structure (EXAFS) oscillations can be observed in the reflected spectrum. From multilayered Ni/C and NixSiy/C reflection coatings mounted in a double‐crystal monochromator we determined, from these oscillations above the Ni LII,III edge, bond lengths equal to 1.8, 2.1, and 2.3 A, which can be assigned to Ni–C, Ni–Si, and Ni–Ni, respectively. NixSiy/C coatings deposited in a 10−6 Torr H2 atmosphere show better resolution than those without. The difference is ascribed to H2 preventing the formation of silicides and the diffusion of the Ni into the carbon; this results in more amorphous layers. A new and simple method is described for obtaining, in the EXAFS analysis, a high‐resolution radial distribution function (RDF) from a small k‐vector space chi function. This metho...

Ni‐C multilayer reflectivity and photoelectron yield in the NiL‐edge region

We analyze measurements of the x‐ray reflectivity and photoelectron yield of a Ni‐C multilayer (103 layers; d=31 A) in the NiL‐edge region (700–950 eV). The measured reflectivity is shown to be consistent with the one calculated using x‐ray scattering factors  f1 and  f2 for Ni as obtained from the photoelectron yield (∼f2). The analysis yields a new set of ‘‘effective’’ values for  f1 in the NiL‐edge region.

Figured Multilayer Structures Applied to a Laser Produced Plasma

A prime application of figured multilayer structures as X-ray optics is the combination with a laser produced plasma. As a source of soft X-rays, a laser-plasma has proven its attractive properties: a high instantaneous brightness, good shot-to-shot stability and nearly pointlike dimensions. The combination of laser-plasma and X-ray optics would give improved performance in e.g. plasma diagnostics, rapid detection of low Z elements in solid samples, pulsed laser irradiated materials and lithographic imaging of submicron patterns. In the latter application, a parabolic multilayer structure has the potential to convert the emission of the point source into a large parallel and homogeneous beam to prevent several distortions in the lithographic imaging.

Use Of A Laser Produced Plasma In Time-Resolved Transmission And Reflection Spectroscopy

An apparatus for timeresolved X -ray absorption measurements is discussed. The apparatus consists of a 15 ns, 20 J Nd:YAG /glass laser system, a polychromator for soft X -rays and a multichannel detector system. X -ray emission spectra of a laser produced plasma (LPP) for 15 different target materials are shown. Time resolved X -ray absorption measurements (90 -190 eV) on clouds of Si particles, produced by laser irradiation of bulk Si, are presented. From these measurements the following results were obtained: Clusters are formed at an irradiance as low as 3.0 J /cm2; at an irradiance of 14 J /cm2 ionized species up to Si4± are formed; at an irradiance of =6 J /cm2 at 15 ns pulse duration, a Si layer thickness of 80 A is removed.An apparatus for time-resolved X-ray absorption measurements is discussed. The apparatus consists of a 15 ns, 20 J Nd:YAG/glass laser system, a polychromator for soft X-rays and a multichannel detector system. X-ray emission spectra of a laser produced plasma (LPP) for 15 different target materials are shown. Time resolved X-ray absorption measurements (90-190 eV) on clouds of Si particles, produced by laser irradiation of bulk Si, are presented. From these measurements the following results were obtained: Clusters are formed at an irradiance as low as 3.0 J/cm2; at an irradiance of 14 J/cm2 ionized species up to Si 4+ are formed; at an irradiance of J/cm2 at 15 ns pulse duration, a Si layer thickness of 80 A is removed.