In this article we describe a new slit grating spectrograph which is based on an e-beam written 10 000 linepairs/mm freestanding transmission diffraction grating. In combination with a thinned, back-illuminated charge coupled device (CCD), the spectrograph allows for real-time spectroscopy of laser-produced plasma x-ray sources within the wavelength region λ=1–20 nm. Calibration of grating and CCD allow for the possibility to measure absolute photon fluxes, currently within the wavelength region λ=1–6 nm. The compact spectrograph is easy to align and flexible in its use. Absolutely calibrated spectra were obtained from a liquid-jet laser-plasma source in the water window, with a spectral resolution λ/Δλ⩾330 at λ=3.37 nm. A simple change in experimental geometry allowed single-shot spectra to be recorded with λ/Δλ⩾60 at the same wavelength. In addition, spectra from this laser-plasma source were measured within the range λ=9–20 nm.

A slit grating spectrograph for quantitative soft x-ray spectroscopy

We report a method for achieving advanced photon diagnostics of x-ray free-electron lasers (XFELs) under a quasi-noninvasive condition by using a beam-splitting scheme. Here, we used a transmission grating to generate multiple branches of x-ray beams. One of the two primary diffracted branches (+1st-order) is utilized for spectral measurement in a dispersive scheme, while the other (−1st-order) is dedicated for arrival timing diagnostics between the XFEL and the optical laser pulses. The transmitted x-ray beam (0th-order) is guided to an experimental station. To confirm the validity of this timing-monitoring scheme, we measured the correlation between the arrival timings of the −1st and 0th branches. The observed error was as small as 7.0 fs in root-mean-square. Our result showed the applicability of the beam branching scheme to advanced photon diagnostics, which will further enhance experimental capabilities of XFEL.

A beam branching method for timing and spectral characterization of hard X-ray free-electron lasers.

Absolute measurements of the soft x-ray emission (50 eV < hv < 1000 eV) from laser plasmas were made with transmission gratings, including pinhole-grating combinations for spatially resolved spectroscopy. Kodak 101–01 film was absolutely calibrated as a function of wavelength with the help of a bolometer, the laser plasma being used as a source. An example of a quantitatively analyzed, space-resolved pinhole grating spectrum recorded on film will be shown.

Absolute soft x-ray measurements with a transmission grating spectrometer

In order to design extreme ultraviolet (EUV) sources for nanolithography, xenon EUV emission has been experimentally studied in a plasma generated by the interaction of a high-power laser with a droplet jet. A theoretical model assuming that the resulting plasma is optically thick allows one to find the distribution of the relevant ions and transitions involved in the emission process. Atomic physics computations are performed using the HULLAC code to give a detailed account of the transitions involved. The importance of 4p–4d, 4d–4f, and 4d–5p transitions is stressed, as well as the need for configuration-interaction treatment of the Δn=0 transitions. Comparisons of a modeled local thermodynamical equilibrium spectrum with experiment provides qualitative agreement and permits an estimate of the plasma temperature, density, and dimensions.

Emissive properties of xenon ions from a laser-produced plasma in the 100–140 Å spectral range: Atomic-physics analysis of the experimental data

Diffraction efficiencies of 44.8–5.4-A x rays by a free-standing gold transmission grating of 1.0-μm period have been measured, using characteristic line radiation. Partial transparency of the grating affects the efficiency at the shorter wavelengths and causes an increase in the efficiencies for orders m ≥ 1 over the long-wavelength values. Model calculations for a trapezoidal wire cross section are in good agreement with the measured efficiencies and allow the wire cross section to be determined. Gratings of this type have a high efficiency for wavelengths as short as 5 A and are, therefore, well suited for applications both in x-ray and extreme uv astronomy.

Transmission grating efficiencies for wavelengths between 5.4 A and 44.8 A.

Efficiencies for diffraction of 45–275-eV x rays into orders by interferometrically formed, electrodeposited, gold transmission gratings have been measured on the 4° beam line at the Stanford Synchrotron Radiation Project (SSRP). Anomalous dispersion affects the observed efficiency since the gold is partially transmitting to x rays. Model calculations which include anomalous dispersion are in good agreement with observations. With a suitable choice of material and thickness, a grating can be optimized for a given wavelength range by reducing the zero order transmission and enhancing the higher orders. Even orders are suppressed for a grating with equal slit and wire sizes.

Diffraction grating transmission efficiencies for XUV and soft x rays.

This paper describes an experimental arrangement utilized at SSRL for obtaining high‐quality Fraunhofer diffraction spectra from a group of specially‐fabricated gold transmission gratings prepared at IBM. The data taken will ultimately be utilized to estimate the optical constants of gold in the 120 eV‐640 eV range, but in the present paper our focus is on the new advances achieved in the attained spectral quality of our measurements in this range and on the improvements that are plausible in future utilizations of the same experimental scheme.

R. Z. Bachrach

Papers

Diffraction grating transmission efficiencies for XUV and soft x rays.

High Quality Fraunhofer Diffraction Spectra Taken at SSRL in the Soft X‐Ray Range