Instrumentation

The endstation consists of 5 main components:

(1) A chamber with an Ion Time-Of-Flight (I-TOF) analyzer in use either with a channeltron or a small (50 mm radius) hemispherical analyzer for electron-ion coincidence measurements.

(2) A rotatable chamber (about the beam axis) with 5 angle-resolving, electrostatic Electron Time-Of-Flight (E-TOF) analyzers.

(3) A Photoabsorption Cell.

(4) A rotatable chamber (about the beam axis) with a high-resolution, rotatable (about the axis perpendicular to the beam axis), angle-resolving, electrostatic Hemispherical Electron Analyzer (HEA).

(5) A rotatable chamber (about the beam axis) with a rotatable Polarized X-Ray-Emission Spectrometer.

(6) A chamber with a Magnetic Mass Spectrometer, formerly used by J.A.R. Samson and now in operation through Wayne Stolte.

(7) A Microwave Discharge System to create atomic fragments of gases such as O₂, N₂, CL₂ etc. This system can be mounted to any of the other instruments for experiments on atomic species.

Both TOF experiments are in parallel operation at different ALS beamlines during scarce 2-bunch operations. Each of the 5 systems has a post-doc and/or graduate student at UNLV with primary responsibility for its development. Present status is summarized below.

The Ion-TOF system also is fully operational. This analyzer, which relies on space focussing to achieve good mass and charge-state resolution, was designed and constructed by UNLV graduate student D. Hansen for use at the ALS. As with the electron-TOF system, care was taken to preserve ultimate timing resolution, both in the detector design and in electronics. Also, larger apertures than those used in the ion-TOF developed by Levin and Sellin² were adopted to increase signal. In a change from initial plans, a small hemispherical electron analyzer, designed and built by Samson, was included in this system to provide a low-energy-electron trigger for ion-TOF spectra. The "hemi-TOF" combination awaits full operation of BL 9.3.1 (see Beamline 9.3.1) for testing. Meanwhile, a channeltron is being used as an electron trigger for ion-TOF measurements when the ALS is not running in 2-bunch mode.

The Electron-TOF system, developed primarily by UNLV post-doc 0. Hemmers (Ph.D. Advisor: U. Becker), is fully operational as a stand-alone system with 4 analyzers built at the U. Tennessee shop (3 built with endstation funding, 1 built independently by UT post-doc R. Wehlitz (I.A. Sellin, J.C. Levin)).

Undergradute students J. Daniels and G. Fisher helped to put together the electron-TOF experiment at the ALS.

This figure shows the back view of the electron-TOF system. Three analyzers can clearly be seen. Two are sticking out from the back flange and one to the lefthand side. The fourth analyzer is at the righthand side (4 o'clock position) behind the handle for the chamber rotation. The manifold for the gasinlet system is visible at the lower part of the chamber. Click on the figure to see it in full size.

These analyzers are based on a new design that includes cylindrical focussing to preserve accurate timing while dramatically improving the collection efficiency for highly retarded electrons. For example, in measurements with this apparatus, electrons with 1 keV initial kinetic energy were retarded to 50 eV with no loss in throughput. Because electron-TOF energy resolution is proportional to the final kinetic energies of the electrons, this new design allows efficient electron spectroscopy with energy resolution comparable to or better than most conventional electrostatic analyzers. To maintain ultimate timing resolution for this new generation of TOF analyzers, significant care was taken in the design of the microchannel-plate detectors and impedance-matched conical anodes. Likewise, the best commercially available electronic modules were purchased, allowing simultaneous operation of up to 4 TOF analyzers with 8192 data points per analyzer and a fixed downtime of only 0.8 µs per analyzer for each event.

The data aquisition software was developed by WMU post-doc B. Langer (now MBI Berlin) and the coincidence electronic including software by K. Wieliczek (FHI Berlin). The coincidence electronic has 6 input channels and can handle a total count rate of 200,000 cts/sec. As a result, this system provides timing and data-collection capabilities that meet or exceed those of any other electron-TOF system in use with SR. Finally, the analyzers are mounted in a chamber which rotates about the x-ray beam. In consultation with team theorists, the chamber was designed with two additional analyzer mounting ports 54.7^o out of the plane perpendicular to the x-ray-beam direction, a geometry which permits direct and sensitive measurement of non-dipolar angular-distribution parameters.

Coincidence measurements require additional electronics. At present, the endstation is capable of doing (up to sixfold) electron-electron, electron-ion, and ion-ion coincidence measurements with the recently obtained electronics. Electron-electron coincidence measurements have been initiated using electronics borrowed from U. Becker's group in Germany. Experts in electron-electron coincidence experiments from Germany (J. Viefhaus, FHI of MPG) and ion-ion coincidence experiments from France (M. Simon, LURE) visited the team in Berkeley for to provide advice in this area.

A Photoabsorption Cell was built by R.C.C. Perera and is in use at BL 9.3.1.¹ A shorter cell, intended as the sample region for the x-ray-emission spectrometer, is under development.

For the high-resolution angle-resolved Hemispherical Electron Analyzer, R. Carr arranged donation of a large hemisphere (radius = 146 mm) complete with a high-energy retarding lens, a 2-D position-sensitive detector, and associated electronics. This device is presently being tested by UNLV post-doc H. Wang (Ph.D. Advisor: B. Crasemann). The larger radius significantly enhances throughput at a given energy resolution, but makes multi-angle measurements more difficult. Nevertheless, the hemisphere will be affixed on a vacuum chamber that permits partial rotation about both the photon propagation and polarization axes, providing coverage of approximately 30% of 4 srad.

Finally, the X-Ray-Emission Spectrometer was under development by former UNLV post-doc Glans. Ray tracing of prospective designs is complete. Two key components require mention. First, the crystal bender, necessary to focus the x-rays emitted from the sample for good energy resolution, is based on a design used for SR x-ray mirrors. This design uses a flexure device, machined to achieve precise and variable cylindrical curvature in vacuo. Perera is managing its development. Second, the x-ray detector needs high spatial resolution to obtain good x-ray-energy resolution. Actually, there will be 2 detectors: a microchannel-plate device with a 2-D position-sensitive anode purchased from Quantar Technology, and a home-built 2-D gas proportional counter offered for the team's use by S. Cramer (UC Davis), a collaborator on BL 9.3.1. The detectors are complementary in that the Quantar device is more efficient for lower energies, where the Be window of the proportional counter absorbs most strongly, whereas at higher energies the proportional counter is more efficient.

References

1. R.C.C. Perera, W. Ng, G. Jones, D. Hansen, J. Daniels, 0. Hemmers, P. Glans, S. Whitfield, H. Wang, and D.W. Lindle, Rev. Sci. Instrum. (in press); R.C.C. Perera, W. Ng, Y. Uehara, M. Simon, P. Neill, D.L. Hansen, S.B. Whitfield, and D.W. Lindle, abstract submitted to The American Physical Society's Division of Atomic, Molecular, and Optical Physics (DAMOP) Annual Meeting (1996).

2. J.C. Levin, D.W. Lindle, N. Keller, R.D. Miller, Y. Azuma, N. Berrah Mansour, H.G. Berry, and I.A. Sellin, Phys. Rev. Lett. 67, 968 (1991).

Send comments to Oliver Hemmers.

Last updated on June-7-1999.