A Fourier Transform Soft X-ray spectrometer (FT-SX) has been designed and is under construction for the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory as a branch of beamline 9.3.2. The spectrometer is a novel soft x-ray interferometer designed for ultra-high resolution (theoretical resolving power E/{delta}E{approx}10{sup 6}) spectroscopy in the photon energy region of 60-120 eV. This instrument is expected to provide experimental results which sensitively test models of correlated electron processes in atomic molecular physics. The design criteria and consequent technical challenges posed by the short wavelengths of x-rays and desired resolving power are discussed. The fundamental and practical aspects of soft x-ray interferometry are also explored

Duarte, R. M.

Howells, M. R.

Hussain, Z.

Moler, E. J.

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L j v r -  - LBL-377 1 1 UC-410 1 Lawrence Berkeley Laboratory e UNIVERSITY OF CALIFORNIA Accelerator & Fusion Research Division Presented at the 1 lth International Conference on Vacuum Ultraviolet Radiation Physics, Tokyo, Japan, August 27- September 1,1995, and to be published in the Proceedings Design and Performance of a Soft X-Ray Interferometer for Ultra-High Resolution Fourier Transform Spectroscopy E.J. Moler, 2. Hussain, R.M. Duarte, and M.R. Howells August 1995 r : " *  Y - . '& '* Prepared for the U.S. Department of Energy under Contract Number DEAC03-76SF00098 I DISCLAIMER This document was prepared as an account of work sponsored by the United States Government While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor The Regents of the University of California. nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark. manufacturer, or otherwise. does not necessarily constitute or imply its endorsement, recommendation. or favoring by the United States Government or any agency thereof, or The Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Govemment or any agency thereof, or The Regents of the University of California. Lawrence Berkeley National Laboratory is an equal opportunity employer. I i LSBL-279 UC-410 DESIGN AND PERFORMANCE OF A SOFT X-RAY INTERFEROMETER FOR ULTRA- HIGH RESOLUTION FOURIER TRANSFORM SPECTROSCOPY" E.J. Mole@, Z. Hussaina, R.M. Duartea, M.R. Howellsa aAdvanced Light Source Lawrence Berkeley National Laboratory University of California Berkeley, CA 94720 bThe University of California Department of Chemistry Berkeley, CA 94720 Paper submitted at the 11th International Conference on Vacuum Ultraviolet Radiation Physics, Tokyo, Japan, August 27 - September 1,1995 W s  work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy, under Contract No. DE-ACO3-76SFOOO98. DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. tight Scurce Note: i" Design and Performance of a Soft X-ray Interferometer For Ultra-high ResoIution Fourier Transform Spectroscopy E J. Moler.ab * a RM. Duartep M. R. Howelk? ahwren= Berkeley National Laboratory, Berkeley, CA 94720 &JMOLER@LBL.GOV, The University of California, Dept of Chemistry, Berkeley, CA 94720 A Fourier Transform Soft X-ray spectrometer (FT-SX) has been designed and is under construction for the Advanced Light Source (ALS) at Lawrence Berkelcy National Laboratory as a branch of beamline 9.3.2. The spectrometer is a novel soft x- ray interferometer designed for ultra-high resolution (theoretical resolving power E/AJ2-106) spec&oscopy in the photon energy region of 60-120 eV. This instrument is expected to provide experimental results which sensitively test models of correlated electron processes in atomic and molecular physics. The design criteria and consequent technical challenges posed by the short wavelengths of x-rays and desired resolving power are discussed. The fundamental and practical aspects of soft x-ray interferometry are also explored. . 1. INTRODUCTION AND SCIENTIFIC MOTIVATION Correlated electron motion is at the center of any chemical process and thus constitutes a very important arena of basic scientific research. The many-body nature of the problem precludes analytical solutions leaving only approximations to the quantum problem potentially tractable. The central problem in theoretical electron correlation studies is to find the appropriate approximations that describe all of the main characteristics of the experimentally observed features of such systems. It is natural to start with excitations of the helium atom since this is the simplest correlated electron system and, with the availability of synchrotron radiation sources, increasingly detailed experimental observations have become available. The FT-SX spectrometer has been designed to probe this prototypical system with unprecedented resolving power. Of particular interest are the regions of overlapping double excitation series where classically chaotic behavior is mixed with quantization effects [l]. . 2. SYNCHROTRON RADIATION AND RESOLUTION LIMITS OF STANDARD SOFT X-RAY MONOCHROMATORS Synchrotron radiation has proved to be an extremely useful tools in basic atomic and molecular physics studies. The high photon flux and tunable photon energy enables detailed studies of excitation processes which are considerably challenging to the theoretical physics community. Third generation light sources, such as the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, coupled with high resolution beam lines are providing extremely detailed observations of scientifrcaUy important systems such as the helium double excitation series, a correlated electron system, in the soft x-ray range. However, there is more to be gained with increased resolving power in x-ray absorption experiments. The highest resolving power soft x-ray beam l ies available consist of a Iow emittance synchrotron source and a highly optimised, grating based monochromator, usually a Spherical Grating Monochromator.(SGM). The low emittance source alIows for relatively high photon flux while using . the small aperatures necessary to operate the SGM at maximum resolving power. While the claim of experimentally achieved resolving power can vary depending on the interpretation of the data and caution of the investigators, it is apparent that the theoretically maximum resolving power of a spherical grating monochromator [23, (E/AE=50,000) has been closely approached by Kaindle, et al. in the region of the helium double excitations (- 65 eV) [3]. 3. SOFT X-RAY INTERFEROMETRY AND ESSENTIALS OF FOURIER TRANSFORM SPECTROMETRY To achieve a higher resolving power, a different approach musk be used other than the "traditional" grating based monochromator. It has been known in optics for a long time that interferometers can * 1 DRf VE SHAf T TO MOVfNG (76) FLEXURE MNG€ FIXED FRAME 25.4 (4) MfRROR PRISMJ (ON MOVJ'NG STAGE) 36.83 Cf77-1 Figure 1. Schematic of Soft X-ray Interferometer. always, in principle, outperform gratings in terms of maximum resolving power or phase space acceptance for equivalent resolving power[4]. In practice, interferometry with x-rays is very difficult due to the short wavelengths. It is only recently that the mechanical, optical, and metrological technology have advanced enough to practically build a soft x-ray interferometer for Fourier transform spectrometry. The basic operation of a Fourier transform spectrometer is to split an incident beam into two parts and recombine them coherently after introducing a phase delay. The Fourier transform of the resulting interference pattern, as a function of phase delay or path-length difference, gives the energy spectrum of the incident beam. The Michelson interferometer, which is widely used in infra-red spectroscopy, is the most common example of such an instrument. The path-length- difference is introduced by moving one or more mirrors to vary the geometric distance between the beam splitter(s) and the detector. The key I r (2) BEAM SPLl TTER components of any interferometer are the beam splitters, which usually determine the optical aperature and operating range of an interferometer, and the mirror translation stage, which determine the maximum resolution of the instrument. The resolution of any optical spectrometer depends on the maximum path length difference introduced between the various parts of the beam. More precisely, 1 6V=- 2ALd where is the maximum path-lengthdifference introduced and 6V is the maximum resolution in wavenumber. If one can maintain the alignment of the mirrors to ensure proper overlap of the beams at the detector, the resolution of an interferometer can be arbitrarily small by simply increasing AL. A grating, on the other hand, has a limit to the 2 1 Figure 2. Silicon Beam Splitter . maximum path-length-difference which it can accomodate. 4. OVERVIEW OF I?"-SX BRANCHLINE The FT-SX spectrometer is a modified Mach- Zehnder type interferometer designed to operate in the 60-120 eV photon energy range (wavelength 10- 20 nm) with a theoretical resolution of -65 peV.It is a permanent end-station branch of beamline 9.3.2 at the ALS. The synchrotron radiation is first pre- filtered by the SGM monochromator, operated in a moderate resolution mode with a 100 Vmm grating, to improve the SignaVnoise ratio of the absorption ' spectrum measured by the interferometer. The x- rays are then deflected by a simple plane mirror downstream of the monochromator exit slit then pass through the FT-SX spectrometer and a conventional gas ionization cell and impinge on a solid state detector. The entire interferometer is mounted in a high-vacuum chamber separated from the UHV beamline by a thin foil window. The vacuum vessel is mounted on an optical table to damp vibration. The moving mirror stage is driven by a double action, hydraulic piston which is external to the vacuum vessel. The mirror position in the interferometer is measured with a commercially available HEWLI?T-PACK4RD heterodyne laser interferometer with a resolution of 3 A and an actual rms noise of 4.5 A. Both the piston drive and the laser interferometer are mounted on the optical table. The heart of the spectrometer is a flexture-hinged linear motion stage upon which is mounted four mirrors permanently aligned by optical contacting to a custom, high precision prism. 5. SOME TECHNICAL CHALLENGES AND SOLUTIONS A detailed discussion of the design and specification considerations for a soft x-ray interferometer is beyond the scope of this paper. Here we present just a few of the key features of the m-SX spectrometer. BEAM SPLT?TERS Beam splitters for soft x-rays are not readily available. We have chosen to use a wavefront dividing beamsplitter which is essentially an x-ray quality mirror with slots. The difficulty in fabricating such a device is to maintain a tolerable slope error on the reflecting arts (flatness e1 p a d  and surface roughness -3 1 rms) while creating. 3 slots which are small enough to allow coherent splitting of the incident x-ray beam. Additionally, the thin "bars" of mirror left between the slots must have an optical coating to achieve reasonable reflectivity. Based on the coherence properties of the incident x-ray beam we determined a slot width of 50 pn and a period of 100 pn was sufficient to . split the x-ray beam coherently. The scheme developed to fabricate the beam splitters is as follows: a) a single crystal Si wafer is polished to an x-ray quality finish with a specific orientation relative to the (110) crystal plane. b) the back side of the wafer is ultrasonically machined to thin the center of the wafer where the slots will be and to cutout chamfers to allow the grazing incident x-rays to pass through the slots. c) the wafer is masked and the slots are chemically etched with an etchant that preferentially cuts along the [110] direction. d) the optical coating is applied in a manner to maintain a stress free thin film. For the ALS spectrometer we have selected molybdenum. The beam splitters have been successfully fabricated by Rockwell Power Systems Division of Rockwell International. technical challenges posed by the short wavelength of the x-rays and the desired, ultra-high resolving power have been met using state-of-the-art mechanical, optical, and metrological technology. We believe this general scheme may be adapted to other uses such as high throughput and high resolving power x-ray flourescence spectrometers. ACKNOWLEDGEMENTS We wish to thank the ALS designers, engineers, and technicians. This work was supported by U.S. DOE contract number DE-AC03-76SF00098. REFERENCES 1. Excitation States in Atomic Helium. Physical Reveiw Letters, 1991.66: p. 3441-3443. Domke, M., et al., Extensive Double- 2. News, 1995. (June). Kaindl, G.K., et aL, Synchrotron Radiation 3. Kaindl, G.K., Personal Communication, 1995. 4. Interfrometric Spectroscopy. 1979, Chichestex John WiIey and Sons. ' Chamberlain, J., The Principles of THE MECHANTCAL MIRROR STAGE The mirror stage must allow enough motion to introduce the desired path-length-difference between the two halves of the x-ray beams and must maintain an angular alignment which ensures coherent recombination of the two beams at the detector. For the FT-SX spectrometer the total range of motion is - 1 cm and the angular tolerance is 50.1 arc-seconds across the entire range. We have achieved this by using "cartwheel" type flexural hinges (see fig. 1) which were wire-EDM cut from a monolithic maraging steel slab. 6. CONCLUSION An FT-SX spectrometer is currently under construction at the Advanced Light Source at Lawrence Berkeley National Laboratory. The 4 

Design and performance of a soft X-ray interferometer for ultra-high resolution Fourier transform spectroscopy

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Design and performance of a soft X-ray interferometer for ultra-high resolution Fourier transform spectroscopy

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