A Real-Time Imaging System for Stereo Atomic Microscopy at SPring-8’s BL25SU

Tomohiro Matsushita1, Fang Zhun Guo1, Takayuki Muro1, Fumihiko Matsui2, and Hiroshi Daimon2

1JASRI/SPring-8, Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo, 679-5198, Japan †Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST),8916-5 Takayama,

Ikoma, Nara 630-0192, Japan

Abstract. We have developed a real-time photoelectron angular distribution (PEAD) and Auger-electron angular distribution (AEAD) imaging system at SPring-8 BL25SU, Japan. In addition, a real-time imaging system for circular dichroism (CD) studies of PEAD/AEAD has been newly developed. Two PEAD images recorded with left- and right-circularly polarized light can be regarded as a stereo image of the atomic arrangement. A two-dimensional display type mirror analyzer (DIANA) has been installed at the beamline, making it possible to record PEAD/AEAD patterns with an acceptance angle of ±60° in real-time. The twin-helical undulators at BL25SU enable helicity switching of the circularly polarized light at 10Hz, 1Hz or 0.1Hz. In order to realize real-time measurements of the CD of the PEAD/AEAD, the CCD camera must be synchronized to the switching frequency. The VME computer that controls the ID is connected to the measurement computer with two BNC cables, and the helicity information is sent using TTL signals. For maximum flexibility, rather than using a hardware shutter synchronizing with the TTL signal we have developed software to synchronize the CCD shutter with the TTL signal. We have succeeded in synchronizing the CCD camera in both the 1Hz and 0.1Hz modes.

Keywords: Photoelectron diffraction, Electron holography, photoemission, other methods of structure determination. PACS: 61.14.Qp, 61.14.Nm , 79.60.-i, 61.18.-j

INTRODUCTION

The photoelectron diffraction technique is an atomic structural analysis method using photoelectrons. The stereo atomic microscope [1] and photoelectron holography [2-4] are based on the photoelectron diffraction technique. The stereo atomic microscope utilizes two photoelectron diffraction patterns excited by left- and right-circularly polarized light. The two patterns can be regarded as stereo image of the atomic arrangement, and no computer processing is required to observe the atomic structure. On the other hand, photoelectron holography utilizes one or more photoelectron diffraction patterns. Recently, new reconstruction algorithms, which can reconstruct a three-dimensional atomic arrangement with about 0.02 nm resolution from a single-energy photoelectron hologram or an Auger-electron hologram, have been proposed [5-7]. The features of these techniques are,

1. An initial model for atomic structure is not required for the atomic structural analysis. 2. Local three-dimensional structure around the target atomic site is observable. 3. Not the electron cloud, but the nuclei position is observable. 4. Perfect long-range order is not required. 5. High surface-sensitivity. 6. Electronic structure (spin etc.) is observable. These methods are now being applied to the atomic structural analysis of the bulk, the surface, and the local

structures around dopants[5-10]. These methods require photoelectron angular distributions (PEAD) of the core-level photoelectron or Auger-electron angular distributions (AEAD) over nearly 2π-steradian (half sphere) with a resolution of about 1 degree. A conventional photoelectron diffraction measurement system is usually composed of a light source (synchrotron radiation or an X-ray tube), a conventional electron analyzer and a sample manipulator

1164

Downloaded 25 Apr 2007 to 163.221.235.155. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

with two rotation axes. A PEAD/AEAD pattern is recorded by sweeping two rotation axes to give 2π-steradian coverage, and long measurement times (several hours) are required. Real-time measurement is impossible.

We have developed a real-time measurement system of PEAD/AEAD. In addition, a real-time imaging system for circular dichroism (CD) of the PEAD/AEAD has been newly developed. This system proves extremely useful for stereo atomic microscope and photoelectron holography studies.

INSTRUMENT

Figure 1 shows a schematic view of the constructed real-time measurement system for PEAD/AEAD installed at the soft x-ray beamline BL25SU of SPring-8, Japan. The major components are twin helical undulators, a grating monochromator, and a two-dimensional display-type analyzer (DIANA) [11]. The twin helical undulators [12,13] are composed of two helical undulators ID1 and ID2, and a set of kicker magnets. ID1 and ID2 generate left- and right-helicity radiation, respectively. The kicker magnets bump the electron orbit at each undulator, deflecting one radiation component off-axis. By changing the excitation of the kicker magnets periodically, the helicity of the circularly polarized radiation passing to the beamline optics can be periodically switched. Currently a switching frequency of 0.1, 1 or 10 Hz is available. The beamline monochromator is a constant deviation type with varied line-spacing plane gratings (VLSPG) covering an energy region of 0.22 - 2 keV [14,15]. The resolving power of the monochromator is more than 10,000 over the whole energy region. The monochromatic light is incident on a sample, with the resulting emission of photoelectrons. Emitted electrons with kinetic energies corresponding to the pass energy of the DIANA are focused to an aperture, and the electrons that pass through the aperture are projected onto a screen. The two-dimensional PEAD/AEAD pattern directly appears on the screen. The PEAD/AEAD pattern is detected by a CCD camera located outside the vacuum chamber. The acceptance angle covered by the screen is ±60°. Therefore, DIANA can be used to record PEAD/AEAD patterns in real time.

FIGURE 1. A schematic view of the real-time measurement system for the PEAD/AEAD.

In addition, in order to study the CD of the PEAD/AEAD pattern in real time, it was necessary to synchronize the

helicity switching and the CCD camera shutter. The sequences for the 0.1Hz and 1Hz modes are shown in Fig.2. In the 1Hz mode, the ID1 light is turned on for 0.3 sec, then both components are turned off for 0.2 sec, the ID2 light is turned on for 0.3sec, and both components are again turned off for 0.2sec. At SPring-8, the VME computers that control the ID and monochromator are controlled over a local network. Synchronization by directly querying the current light helicity over the network, however, is not feasible due to the inherent delays. Therefore, we make a direct connection between the VME computer that controls kicker magnets and the measurement computer with two BNC cables. The two BNC cables correspond to the ID1 light signal and the ID2 light signal. A TTL “high” signal corresponds to the particular ID being “on”.

1165

Downloaded 25 Apr 2007 to 163.221.235.155. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

FIGURE 2. A diagram of ID1 and ID2 light of 1Hz mode and 0.1Hz mode.

FIGURE 3. A diagram of the computer sequence, in order to synchronize the CCD camera shutter and the helicity switching of ID25.

In order to maximize flexibility for the measurements, we did not develop a hardware shutter synchronizing with

the TTL signal, but adopted a software synchronization system. A diagram of the software sequence is shown in Fig. 3. In the case of the 1Hz mode, the time of “Image translation, calculation and display” is quite important, since it must be complete while both lights are turned off. If the calculation time is longer than 0.2sec, the exposure time is reduced, and if it becomes longer than 0.5sec, the sequence collapses. In particular, the translation time of the image of CCD camera is the key to realize this sequence. We have selected for the camera the “Sensicam QE” of “PCO imaging Co.”. The camera spec is high-resolution (1376 x 1040 pixels), has a 12-bit dynamic range, and a fast frame rate of 10fbps. In addition, it is necessary to make the calculation and display time as short as possible. Therefore, we did not adopt Labview or other interpreted languages but constructed new original software using the native compiler of “Borland C++ Builder”.

PERFORMANCE

The first test was the real-time measurement of AEAD. An LVV AEAD pattern from a Cu(001) sample was successfully recorded with a 0.1sec exposure time at 5~8 frame/sec. Synchronization with the helicity switching was then tested by changing the frequency of the helicity. In the 0.1 Hz mode, an exposure time of about 3.8sec is available. The 1 Hz mode was also tested, and it was possible to synchronize the CCD camera with a 0.16sec exposure time. The loss time is about 0.14sec, mainly due to the two DI readings before and after exposure.

The two PEAD patterns excited with left- and right-circularly polarized light can be regarded as a stereo image of the atomic arrangement. An example of such a stereo atomic microscope image pair is shown in Fig. 4 [8]. These images are sets of Cu (001) PEAD patterns from a Cu (001) surface recorded with a photoelectron energy of 600eV.

1166

Downloaded 25 Apr 2007 to 163.221.235.155. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp

The helicity of the photons used for the patterns shown in the left and right images were cw and ccw respectively. These two patterns form a stereo photograph of the atomic arrangements for the left eye and the right eye.

FIGURE 4. Two-dimensional PEAD patterns recorded for Cu(001) 3p excited by cw (left) and ccw (right) helicity light. The kinetic energy is 600eV [8]. A three-dimensional image can be seen by looking at the left image with the left eye, and the right image with the right eye.

CONCLUSIONS

We have constructed a real-time measurement system for PEAD/AEAD at BL25SU of SPring-8. We have confirmed that it is possible to observe AEAD patterns with 0.1sec exposure time at 5~8 frame/sec. In addition we have also constructed a system for synchronizing the periodic photon helicity switching with the CCD camera shutter by using newly developed software. We succeeded in synchronizing at both 1Hz and 0.1Hz modes. We have achieved 0.16sec exposure time at 1Hz mode, and confirmed that it is possible to measure the CD of the AEAD pattern at 1Hz. This system enables real-time atomic stereo microscopy and circular-dichroism photoelectron holography.

REFERENCES

1. H. Daimon, Phys. Rev. Lett., 86, 2034 (2001). 2. A. Szöke, in Short Wavelength Coherent Radiation: Generation and Applications, AIP Conf. Proc. No. 147, 361 (1986). 3. J. J. Barton, Phys. Rev. Lett., 61, 1356 (1988). 4. J. J. Barton, Phys. Rev. Lett., 67, 3106 (1991). 5. T. Matsushita, A. Agui and A. Yoshigoe, Europhys. Lett., 65, 207 (2004). 6. T. Matsushita, A. Yoshigoe and A. Agui, Europhys. Lett., 71, 597 (2005). 7. T. Matsushita, A. Agui and A. Yoshigoe, J. Electron. Spectrosc. Relat. Phenom., 144-147, 1175 (2005). 8. F. Z. Guo, F. Matsui, T. Matsushita and H. Daimon, J. Electron. Spectrosc. Relat. Phenom., 144-147, 1067 (2005). 9. F. Z. Guo, T. Matsushita, K. Kobayashi, F. Matsui, Y. Kato, H. Daimon, M. Koyano, Y. Yamamura, T. Tsuji and Y. Saitoh, J.

Appl. Phys., 99, 024907 (2006). 10. F. Matsui, H. Daimon, F. Z. Guo and T. Matsushita, Appl. Phys. Lett., 85, 3737 (2004). 11. M. Kotsugi, T. Miyatake, K. Enomoto, K. Fukumoto, A. Kobayashi, T. Nakatani, Y. Saitoh, T. Matsushita, S. Imada, T.

Furuhata, S. Suga, K. Soda, M. Jinno, T. Hirano, K. Hattori and H. Daimon, Nucl. Inst. and Meth. in Physics Res. A, 467-468, 1493 (2001).

12. T. Hara, T. Tanaka, T. Tanabe, X. -M. Marećhel, K. Kumagai and H. Kitamura, J. Synchrotron Rad., 5, 426 (1998). 13. T. Hara, K. Shirasawa, M. Takeuchi, T. Seike, Y. Saitoh, T. Muro and H. Kitamura, Nucl. Inst. and Meth. in Physics Res. A,

498, 496 (2003). 14. Y. Saitoh, H. Kimura, Y. Suzuki, T. Nakatani, T. Matsushita, T. Muro, T. Miyahara, K. Soda, S. Ueda, H. Harada, M.

Kotsugi, A. Sekiyama and S. Suga, Rev. Sci. Instrum., 71, 3254 (2000). 15. T. Muro, T. Nakamura, T. Matsushita, H. Kimura, T. Nakatani, T. Hirono, T. Kudo, K. Kobayashi, Y. Saitoh, M. Takeuchi,

T. Hara, K. Shirasawa and H. Kitamura, J. Electron. Spectrosc. Relat. Phenom., 144-147, 1101 (2005).

1167

Downloaded 25 Apr 2007 to 163.221.235.155. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jsp