New Results in Atomic Physics at the Als*

LBL-36919 LSBL-251 UC-410

NEW RESULTS IN ATOMIC PHYSICS AT THE ALS*

Alfred S. Schlachter Advanced Light Source Accelerator and Fusion Research Division Lawrence Berkeley Laboratory University of California Berkeley, CA 94720, USA

December 1994

*This 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-AC03-76SF00098.

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Portions of this document may be illegible in electronic image products. Images are produced from the best available original document. NEW RESULTS IN ATOMIC PHYSICS AT THE ADVANCED LIGHT SOURCE

Fred Schlachter

Advanced Light Source, Accelerator and Fusion Research Division, Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720, USA

ABSTRACT

New results in atomic physics at the Advanced Light Source demonstrate the opportunities available in atomic and molecular physics at this synchrotron light source. The unprecedented brightness allows experiments with high flux, high spectral resolution, and nearly 100% linear polarization.

The Advanced Light Source is. the world's first low-energy third- generation synchrotron radiation source. It has been running reliably and exceeding design specifications since it began operation in October 1993. It is available to a wide community of researchers in many scientific fields, including atomic and molecular science and chemistry. A schematic diagram of the beamlines available or soon to be available is shown in Fig. 1.

Surface and Materials Science Magnetic Metrology and Microscopy Standardsx „ , Spectromicroscopy X-Ray Surface and Microscopy' Materials Science Chemical Dynamics

Protein Atomic and- Crystallography Molecular Science

Atomic and Materials Science Chemical and Surface Science Fluorescence X-Ray Microprobe X-Ray Optics Development Materials Science Diagnostic and Biology Beamline EUV Projection Lithography

1 Beamline Source Research Energy Range Avail. 3.1 Bend magnet Diagnostic beamline 200-280 eV 1994 5.0 W16 wiggler Protein crystallography 4-13 keV 1996 6.1 Bend magnet High-resolution ^one-plate microscopy 250-600 eV Now i 6.3 Bend magnet Metrology and standards 50-4000 eV Now 7.0.1 U5 undulator Surfaces and materials, 70-1200 eV Now "spectromicroscopy 7.0.2 U5 undulator Coherent optics experiments 70-650 eV 1995 7.3.1 Bend magnet Magnetic microscopy 600-900 eV 1995 7.3.2 Bend magnet Spectromicroscopy 100-1500 eV 1996 8.0 U5 undulator Surfaces and materials 70-1200 eV Now 8.5 Bend magnet Infrared spectromicroscopy 0.1-2 eV 1995 9.0.1 U8 undulator -+ U10 Atomic physics and chemisby 20-300 eV Now 9.0.2 U8 undulator -»U10 Chemical dynamics 5-30 eV 1995 9.3.1 Bend magnet Atomic and materials science 700 eV-6 keV 1995 9.3.2 Bend magnet Chemical and materials science 50-1500 eV Now 10.31 Bend magnet Materials science and advanced 3-12 keV Now microprobe instrumentation 10.3.2 Bend magnet LIGA, total reflection x-ray fluorescence 3-12 keV 1994 11.0 EW20 elliptical wiggler Materials science and biology, magnetic 50 eV-10 keV 1996 materials 12.0 US undulator EUV projection lithography, optics 60-320 eV 1996 development Fig. 1. Layout of the ALS beatnlines for 1994-1996 with table describing their energy ranges, availability, and research applications.

Undulators at the ALS provide beams of exceptional brightness. The narrow beam of radiation from an undulator allows high flux to pass through the narrow entrance slit of the monochromator, thus allowing high flux with high spectral resolution. The spectral resolution of an ALS spherical grating monochromator is approximately 10,000.

40 jirad

W@130A,1%BW

16mW@ 1SOA,xl%BW-

^X-ray 'light bulb* X c .A^io2 to104 e JIC AX S Q. JoC Ql 500 ©V Photon energy Photon energy

Fig. 2. Diagram of the angular divergence of bend magnet and undulator radiation.

2 The brightness of radiation from undulators at third-generation sources exceeds that from bend magnets by many orders of magnitude, and exceeds that from undulators at second-generation sources typically by an order of magnitude or more. This high brightness allows new experiments to be done in many fields. High brightness, for example, allows the beam to be focused to a very small spot, ranging from 50 (im to less than 1000 A, depending upon the optical configuration; this small focused spot of radiation is valuable for many sorts of microscopy.

Wavelength (A) 104 103 10* IO1 10°

Fig. 3. With the use of more than one undulator harmonic, it is possible to cover a wide spectral range. The spectral brightness envelopes are shown for the three ALS undulators and wiggler, the ALS bend magnets, undulators at the Advanced Photon Source (APS), and representative insertion-device and bend-magnet sources at the National Synchrotron Light Source (NSLS) and the Stanford Synchrotron Radiation Laboratory (SSRL). The discontinuities in the undulator curves represent a shift from the envelope of the fundamental to that of the third harmonic, etc. The spectral brightness of the undulators greatly exceeds that of other sources shown in the drawing.

The prime tool for research in atomic and molecular physics is beamline 9.0.1, a spherical-grating monochromator beamline which utilizes an 8-cm-period undulator that is 4.5 m long. Typical performance of this undulator is shown in Fig. 4.

3 0.5-

Wavelength (A)

Fig. 4. Spectrum of the 8-cm-period undulator taken by a transmission grating spectrometer, showing harmonics from the 3rd to the 82nd (and beyond) at a value ofK = 5.24 (25 mm gap) [1].

The layout of. Beamline 9.0.1 is shown in Fig. 5; performance is shown in Fig. 6 [2].

Horizontal beam defining aperture Photon Dtfltadion beam H. Vertical sratings position fc condensing ** 1 Sfidmg exttsGt

•Experimental

Horizontal focusing mrmx Vertical refocusing mirror

Horizontal beam-d^ining aperture

Fig. 5. Layout of beamline 9.0.1.

4 14 10 1—r

Gi G2 ^JIDK linae/mm\ (900 lines/mm)

w c o (2100 fines/mm) o 1013 £L /

1012 20 40i 100 200 400 Photon energy (eV) Fig. 6. Calculated resolved flux vs. photon energy after the exit slit for each diffraction grating on beamline 9.0.1 [3]. The resolved flux is computed as the width of the entrance slit is varied to fix the slit-width-limited resolving power at 10,000. In practice, grating aberrations and slope errors prevent this value from ever being achieved. The calculations are based on the predicted flux from the undulator, neglecting field errors and using the first or third harmonic. The calculations include mirror absorption, aberration losses at the entrance slit, and a diffraction efficiency for square-wave gratings, in first order, with shadozving [4].

The need for high spectral resolution in atomic physics is clear from studies by the Berlin group [5] of doubly excited autoionizing states of helium measured by photoabsorption. An example is shown in Fig. 7, showing small spectral features to be resolved.

60 61 63 63 64 65 S4.ll 64.13 64.15

Fig. 7. Doubly excited autoionizing state of helium measured by photoabsorption at BESSY [1].

5 Several groups have recently worked on this beamline for research in atomic and molecular physics. They have all utilized the high spectral resolution, the high flux, and the essentially 100% linear polarization of the radiation, typically to study photoionization of atoms and molecules, with measurement of electron emission as a function of the angle relative to the polarization of the radiation. An example is shown in Fig. 8, showing measurement of electrons emitted from doubly excited autoionizing states of helium, which Domke et al. [1] have recently shown to have a particularly rich spectrum in experiments at BESSY. These measurements at the ALS by the Caldwell/Krause [6] group have demonstrated the high spectral resolution of the beamline, which allows measurements in electron emission which were previously only feasible in photoabsorption.

Photon Energy (eV)

Fig. 8. He(sp, 2n+, n = 5-15) autionizing resonances measured in photoemission on undulator beamline 9.0.1 at the ALS. This spectrum recorded at 0° consists of402 points and required ten minutes to record. Note the appearance of the weak He(sp, In-, n = 5-7) resonances. (Courtesy of Caldwell et al.)

Another example is shown in Fig. 9, showing measurement of electrons emitted from argon, where electron-energy spectra were obtained using a time- of-flight technique by the group of Nora Berrah [7]. Again, these measurements demonstrate the high spectral resolution obtainable with the ALS and the high flux which allows measurements to be made in angle- and energy-resolved electron emission.

6 Ar 3s23p6('S) -*3s3p

Photon Energy (eV)

Fig. 9. Argon 3s autoionizing resonance spectrum measured using photoelectron spectroscopy by Nora Berrah (Western Michigan University) and her research team on undulator beamline 9.0.1 at the ALS.

Helium and other rare gases will continue to be studied by several spectroscopies. One group (University of Nevada at Reno) has begun measurements of fluorescence emission from doubly excited autoionizing states of helium. Another (Bozek, et al.) is bringing a high-resolution hemispherical electron-energy analyzer to the beamline. The group of Jim Samson has brought a magnetic-sector ion analyzer and detector to the beamline. Adam Hitchcock has begun studies of photodissociation of small molecules. All these instruments will eventually provide comprehensive tools for the study of photoexcitation and photoionization of atoms and molecules. Experiments of additional complexity are also planned. Duane Jaecks is planning experiments on alignment in photoionization. Francois Wuilleumier and his collaborators are planning experiments on laser-excited atoms at the ALS. Lew Cocke and collaborators plan to study ionization of helium by recoil- ion spectroscopy along with the group of Samson. Eventually experiments on the photoionization of ions will be performed. Thus, a wide variety of research in atomic and molecular physics is underway at the ALS at the present time, with access to beamtime providing the primary limitation to research opportunities. Other tools are or will shortly be available at the ALS. Beamline 9.0.2 is a facility, also using the 8-cm-period undulator (soon to be replaced by one with a period of 10 cm), which will begin operation early in 1995. Two beamlines will be available: one with 1016 photons/sec with a 2% energy width, the second a high-

7 resolution normal-incidence monochromator with a resolution of the order of 100,000 [5]. Arthur Suits at the Chemistry Department, University of California Berkeley, is responsible for this beamline.

Molecular beam

U8Undutator

Zx*5*10' 3 = 2 x 1011 photons/sec

Excimer Experimental Laser Station E AX 3 = 101< photons/**

Fig. 10. Layout of the chemical dynamics beamline (9.0.2) at the ALS, showing molecular beams, lasers, and undulator radiation [8].

A bend-magnet beamline for soft x rays, beamline 9.3.1, will begin operation in 1995. The double-crystal monochromator will produce a flux of 1011 photons/sec. in a 0.5 eV bandpass over the energy range 700 eV to 6 keV. Dennis Lindle at the University of Nevada at Las Vegas is responsible for this beamline. An additional capability will become available within two years: elliptically polarized radiation. This will be useful for atomic and molecular physics as well as for studies in materials science and biology. In conclusion, the Advanced Light Source provides new opportunities in atomic and molecular physics and chemistry, especially for high-resolution studies, where the high brightness of the ALS can be effectively utilized. New results demonstrate these capabilities.

8 References

1. D. A. Mossessian, P. A. Heimann, E. Gullikson, R. K. Kaza, J. Chin, and J. Akre, Nuclear Instruments & Methods in Physics Research, 347, 244-248 (1994).

2. P. Heimann, D. Mossessian, and J. Bozek, Advanced Light Source project document LSBL-239, Lawrence Berkeley Laboratory, Berkeley, California (1995).

3. P. Heimann, Advanced Light Source project document LSBL-110, Lawrence Berkeley Laboratory, Berkeley, California (1991).

4. J. M. Bennet, Ph.D. Thesis, University of London (1971).

5. M. Donke, C. Xue, A. Puschmann, T. Mandel, E. Hudson, D. A. Shirley, G. Kaindl, C. H. Greene, H. R. Sadeghpour, and H. Petersen, Phys. Rev. Lett. 66,1306 (1991); M. Domke, G. Remmers, and G. Kaindl, Phys. Rev. Lett. 69,1171 (1992).

6. C. D. Caldwell, A. Mertzel, S. P. Frigo, S. B. Whitfield and M. O. Krause, Synchrotron Radiation News 8,23 (1995).

7. Results kindly provided by Nora Berrah and co-workers; published in a report from the organizers of IWP94 in Synchrotron Radiation News 8,13 (1995).

8. M. Koike, P. A. Heimann, A. H. Kung, T. Namioka, R. DiGennaro, B. Gee, N. Yu, Nuclear Instruments & Methods in Physics Research, 347,282-286 (1994).