- Email: [email protected]
PLS photoemission electron microscopy beamline
Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
PLS photoemission electron microscopy beamline Tai-Hee Kanga,b,*, Ki-jeong Kima, C.C. Hwanga, S. Raha, C.Y. Parkb, Bongsoo Kima a
Beamline Research Division, Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, South Korea b Department of Physics and BK21 Sung Kyun Kwan University, Suwon 440-746, South Korea
Abstract The performance of a recently commissioned beamline at the Pohang Light Source (PLS) is described. The beamline, which is located at 4B1 at PLS, is a Varied Line Spacing (VLS) Plane Grating Monochromator (PGM) beamline. VLS PGM has become very popular because of the simple scanning mechanism and the fixed exit slit. The beamline which takes 3 mrad horizontal beam fan from bending magnet, covers the energy range 200–1000 eV for Photoemission Electron Microscopy (PEEM), X-ray Photoelectron Spectroscopy (XPS) and Magnetic Circular Dichroism (MCD) experiments. Simplicity of the optics and high flux with medium resolution were the design goals for these applications. The beamline consists of a horizontal focusing mirror, a vertical focusing mirror, VLS plane grating and exit slit. The source of PLS could be used as a virtual entrance slit because of its small size and stability. The flux and the resolution of the beamline at the experimental station have been measured using an ion chamber and a calibrated photodiode. Test images of PEEM from a standard sample were taken to illustrate the further performance of the beamline and PEEM station. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Qe; 42.15.Eq; 61.18.j; 79.20.La Keywords: Synchrotron; Beamline; Varied line spacing grating; PEEM; Soft X-rays
1. Introduction Since Tonner and Harp [1] demonstrated the first element specific imaging of surfaces with photoemission electron microscopy (PEEM) using tunable X-ray source from synchrotron radiation in 1988 the synchrotron radiation source has
*Corresponding author. Beamline Research Division, Pohang Accelerator Laboratory, POSTECH, San31, Hyoja-dong, Pohang, 790-784, South Korea. Tel.: +82-54-279-1535; fax: +82-54-279-1599. E-mail address: [email protected] (T.-H. Kang).
improved the performance of PEEM. Recently, PEEM has not only developed the high spatial resolution, 420 nm [2,3], but also been used as mesoscopic imaging techniques for observations of magnetic domain [4,5], superconductors [6], geological samples [7], biological samples [8] and chemical gas reaction on metal [9,10]. Therefore, PLS had planned and constructed the 4B1 beamline for utilizing the versatile applications of PEEM. The design concept of this beamline is similar to the calibration beamline 6.3.2 at Advanced Light Source (ALS) designed by Underwood and Gullikson [11] which was based on a
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 4 1 7 - X
582
T.-H. Kang et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
varied line spacing plane grating monochromator (VLS-PGM) [12]. Here, we briefly report on parameters of the beamline optics, the performance of the beamline and test images of the Ni mesh on SiOx with the Hg lamp and the synchrotron radiation tuned to Ni core levels.
2. Beamline and end-station The need for a high-resolution, high-throughput monochromator that is simple and compact, with a stable geometry that can be accurately determined, is met by a VLS-PGM monochromator of the type originally described by Hettrick and
Underwood [13]. Underwood and Gullikson designed and constructed the calibration beamline 6.3.2 at ALS for the purpose of a highresolving power (E=DE 7000), high-throughput (109–1012 photons s1 per 0.1% BW) and simple design (no physical entrance slit) with three VLSplane gratings. For more simplicity we had designed 4B1 PEEM beamline which has a medium resolving power (E=DE 1000) in the 200–1000 eV energy range with only one grating instead of three gratings for a high resolution. The grating, which is a varied line spacing holographic recorded ion-beam etched laminar plane grating, covers the whole energy range and corrects the spherical aberration of vertical focusing mirror
Fig. 1. The optical design layout of 4B1 PEEM beamline at PAL. There are two mirrors and one grating. Entrance slitless operation is allowed. The simple fixed exit slit is located at the focusing position.
Table 1 Optical parameters of mirrors, grating, out-of-plane aperture and focusing position Element
Dist. from source (m)
M1 Out-of-plane aperture M2 Grating Focus positionb & Exit slit
11.5
a b
Radius (m),
l w (mm),
y (deg)
Coating
229.88
750 85
3.0
Au/Cr
214.96 1
320 75 220 40
3.0 172a
Au/Cr Au
14.0 15.0 15.5
Included angle of monochromator The fixed exit is located at this point
24.0
T.-H. Kang et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
(VFM), so that the monochromator is essentially aberration-free over its tuning range [11]. VLSplane grating could have a very simple and stable mechanism design. The energy is tuned by a simple rotation of the grating about its center with no rotating or translating mirrors or no moving exit slit. The grating has central groove densities of 1000 mm1 and maximum efficiency at 750 eV. Fig. 1 shows an optical layout of the beamline. The spherical horizontal and the spherical vertical focusing mirrors image the beam from the bending magnet on the fixed exit slit, which are almost unit and 15 : 9 demagnification, respectively. The VFM and grating, which are only 50 cm apart, were installed in the same holder and housing chamber in order to reduce the number of factors for the alignment. The small size and high stability of the PLS source [14] allows one to remove the physical entrance slit. The idea of entrance slitless operation gives some advantages such as maximum photon flux, shortening the length of beamline, simplifying the mechanical design and making the alignment easier. The exit slit has three fixed sizes of 300, 200, 100 mm, respectively, in order to simplify the mechanical design. The exit slit, which is shown in Fig. 1, is located at the focus point. Table 1 shows the optical parameters of grating, the mirrors, the out-of-plane aperture and focus position. The end-station consists of PEEM station, X-ray photoelectron spectroscopy (XPS) station and magnetic circular dichroism (MCD) station along the beam path downstream of the focusing point. The XPS station will serve the experimental research on the chemical adsorption and/or desorption on surface such as the selective molecular cleavage studied by Park et al. [15]. The MCD station under construction will serve the dichroism phenomena studies of magnetic materials. The mechanical out-of-plane aperture for MCD, which is located just before the VFM, can select the circular polarization of the photon beam. The left or right hand circularly polarized beam can be obtained by means of a movable outof-plane aperture that selects light from above or below the horizontal plane. The sample integration Omicron IS-PEEM is located 50 cm downstream of the exit slit. Because of sample integration stage
583
Fig. 2. The photon flux of beamline is shown. The flux is maximized at 750 eV. The broken curve is the predicted flux from a PLS bending magnet source assuming 100% efficient optics.
Fig. 3. The medium resolving power was observed with Ar and N2 gases. (a) the photoionization spectrum of gas-phase Ar closed to Ar L2;3 threshold, (b) the vibration resonant spectrum of N2 1s ! p*. The fitted peaks, which have 400 meV Gaussian broadening, are shown.
584
T.-H. Kang et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
Fig. 4. PEEM images of a Ni mesh on silicon oxide. 853 eV energy was used to take the chemical element contrast of Ni and silicon. Insets are the Ni mesh image using Hg lamp (4.9 eV) and X-ray absorption spectroscopy of Ni. Recording frames with 1 eV increment step of photon energy obtained the Ni X-ray absorption spectra from 835 eV to 885 eV.
the PEEM instrument is stable against vibration which makes worse the spatial resolution of the PEEM image. We have obtained the PEEM images with standard samples for commissioning beamline and illustrating the further performance of the beamline and PEEM.
3. Performance The beamline is being commissioned, and the focused beam size, photon flux, resolution have been characterized. The focused visible light at exit
slit was measured 0.4 mm (V) 0.8 mm (H) using the Hamamatsu MOS linear sensor S3904-1024, and the beam size of the soft X-ray is assumed to be smaller. Three end-stations have a different beam size}PEEM (1 1.5 mm), XPS (2.5 3 mm), MCD (4 5 mm)}because the end-station is installed downstream of the exit slit and there are no re-focusing mirrors between them. The photon flux from the beamline has been carefully measured using photodiode IRD AXUV100 and compared to the data of ionization gas cell [16]. Fig. 2 presents the photon flux that has a maximum value more than 2 1011 photons s1
T.-H. Kang et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
per 0.1% BW at 750 eV. This flux is enough to make images of PEEM that has 200 nm spatial resolution, especially chemical contrast image of the transition metal. A refocusing mirror will be installed to make a micro-spot size beam on sample at PEEM for the less than 70 nm high spatial resolution. For measuring the monochromator resolution, the gas cell, which was located just behind the XPS station, was used with Ar and N2 gases. A 100 mm fixed slit was used to measure the resolution. The medium resolving power, (E=DE¼ 100021200) was observed in the whole range, the results are presented in Fig. 3. Fig. 3(a) and (b) show the photoionization spectrum of gas-phase Ar close to Ar L2;3 thresholds and N2 1s ! p* in gas-phase N2, respectively. For investigating the performance of PEEM station and the beamline, the preliminary PEEM images which have a big field of view were obtained with Ni mesh on SiOx sample using the Hg lamp (4.9 eV) and the synchrotron tuned to Ni 2p core levels (2p3/2=853 eV, 2p1/2=870 eV). Fig. 4 shows that the Ni mesh PEEM images were taken with a Hg lamp and synchrotron radiation with 200 mm field of view. The inset in Fig. 4 presents the X-ray absorption spectra on Ni 2p levels. The spatial resolution was less interesting in this case because the sample was made by Ni mesh wire (30 mm diameter) put on a SiOx wafer. The long diameter of the mesh disturbed the wellfocused image in the vertical direction. From this test we illustrate the further performance of the beamline and PEEM station.
Acknowledgements This work at PLS is supported by Ministry of Science and Technology and POSCO. This experi-
585
ment is also supported by the Korean Science and Engineering Foundation through the AtomicScale Surface Science Research Center (ASSRC).
References [1] B.P. Tonner, G.R. Harp, Rev. Sci. Instrum. 59 (1988) 853. [2] W. Swiech, G.H. Fecher, Ch. Ziethen, O. Schmidt, G. Scho¨nhense, K. Grzelakowski, C.M. Schneider, R. Fomter, H.P. Open, J. Kirschner, J. Electron Spectrosc. Relat. Phenom. 84 (1997) 171. [3] S. Anderson, H.A. Padmore, R.M. Duarte, T. Renner, T. Stammler, A. Scholl, M.R. Scheinfein, J. St hr, L. Seve, B. Sinkovic, Rev. Sci. Insrum. 70 (1999) 3973. [4] A. Scholl, J. Sto¨hr, J. Lu¨ning, J.W. Seo, J. Formpeyrine, H. Siegwart, J.-P. Locquet, F. Nolting, S. Anderson, E.E. Fullerton, M.R. Scheinfein, H.A. Padmore, Science 287 (2000) 1014. [5] G. Scho¨nhense, J. Phys. : Condens. Matter 11 (1999) 9517. [6] Y. Hwu, J. Electron. Spectrosc. Relat. Phenom. 8 (1997) 149. [7] T. Droubay, G. Mursky, B.P. Tonner, J. Electron. Spectrosc. Relat. Phenom. 84 (1997) 159. [8] G. De Stasio, G. Magaritondo, J. Electron. Spectrosc. Relat. Phenom. 84 (1997) 137. [9] Y. Yamaguchi, S. Takakusagi, Y. Sakai, M. Kato, K. Asakua, T. Iwasawa, J. Moclec. Catal. A; Chemic. 141 (1999) 129. [10] H.H. Rotermund, J. Electron. Spectrosc. Relat. Phenom. 98-99 (1999) 41. [11] J.H. Underwood, E.M. Gullikson, J. Electron. Spectrosc. Relat. Phenom. 92 (1998) 265. [12] W.R. Mckinney, Rev. Sci. Instrum. 63 (1) (1992) 1410. [13] M. Hettrick, J.H. Underwood, AIP Conf. Proc. 147 (1986) 237. [14] I.S. Ko, J.Y. Huang, D.K. Seon, C.B. Kim, T.-Y. Lee, J. Korean Phys. Soc. 35 (1999) 411. [15] J.H. Moon, Y.-H. La, J.Y. Shim, B.J. Hong, K.J. Kim, T.-H. Kang, B. Kim, H. Kang, J.W. Park, Langmuir 16 (2000) 2981. [16] J.A.R. Samson, Techniques of VACUUM ULTRAVIOLET SPECTROSCOPY, Pied Publications, Lincoln, Nebraska, 1980, (Chapter 8).
PLS photoemission electron microscopy beamline Tai-Hee Kanga,b,*, Ki-jeong Kima, C.C. Hwanga, S. Raha, C.Y. Parkb, Bongsoo Kima a
Beamline Research Division, Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, South Korea b Department of Physics and BK21 Sung Kyun Kwan University, Suwon 440-746, South Korea
Abstract The performance of a recently commissioned beamline at the Pohang Light Source (PLS) is described. The beamline, which is located at 4B1 at PLS, is a Varied Line Spacing (VLS) Plane Grating Monochromator (PGM) beamline. VLS PGM has become very popular because of the simple scanning mechanism and the fixed exit slit. The beamline which takes 3 mrad horizontal beam fan from bending magnet, covers the energy range 200–1000 eV for Photoemission Electron Microscopy (PEEM), X-ray Photoelectron Spectroscopy (XPS) and Magnetic Circular Dichroism (MCD) experiments. Simplicity of the optics and high flux with medium resolution were the design goals for these applications. The beamline consists of a horizontal focusing mirror, a vertical focusing mirror, VLS plane grating and exit slit. The source of PLS could be used as a virtual entrance slit because of its small size and stability. The flux and the resolution of the beamline at the experimental station have been measured using an ion chamber and a calibrated photodiode. Test images of PEEM from a standard sample were taken to illustrate the further performance of the beamline and PEEM station. # 2001 Elsevier Science B.V. All rights reserved. PACS: 07.85.Qe; 42.15.Eq; 61.18.j; 79.20.La Keywords: Synchrotron; Beamline; Varied line spacing grating; PEEM; Soft X-rays
1. Introduction Since Tonner and Harp [1] demonstrated the first element specific imaging of surfaces with photoemission electron microscopy (PEEM) using tunable X-ray source from synchrotron radiation in 1988 the synchrotron radiation source has
*Corresponding author. Beamline Research Division, Pohang Accelerator Laboratory, POSTECH, San31, Hyoja-dong, Pohang, 790-784, South Korea. Tel.: +82-54-279-1535; fax: +82-54-279-1599. E-mail address: [email protected] (T.-H. Kang).
improved the performance of PEEM. Recently, PEEM has not only developed the high spatial resolution, 420 nm [2,3], but also been used as mesoscopic imaging techniques for observations of magnetic domain [4,5], superconductors [6], geological samples [7], biological samples [8] and chemical gas reaction on metal [9,10]. Therefore, PLS had planned and constructed the 4B1 beamline for utilizing the versatile applications of PEEM. The design concept of this beamline is similar to the calibration beamline 6.3.2 at Advanced Light Source (ALS) designed by Underwood and Gullikson [11] which was based on a
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 4 1 7 - X
582
T.-H. Kang et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
varied line spacing plane grating monochromator (VLS-PGM) [12]. Here, we briefly report on parameters of the beamline optics, the performance of the beamline and test images of the Ni mesh on SiOx with the Hg lamp and the synchrotron radiation tuned to Ni core levels.
2. Beamline and end-station The need for a high-resolution, high-throughput monochromator that is simple and compact, with a stable geometry that can be accurately determined, is met by a VLS-PGM monochromator of the type originally described by Hettrick and
Underwood [13]. Underwood and Gullikson designed and constructed the calibration beamline 6.3.2 at ALS for the purpose of a highresolving power (E=DE 7000), high-throughput (109–1012 photons s1 per 0.1% BW) and simple design (no physical entrance slit) with three VLSplane gratings. For more simplicity we had designed 4B1 PEEM beamline which has a medium resolving power (E=DE 1000) in the 200–1000 eV energy range with only one grating instead of three gratings for a high resolution. The grating, which is a varied line spacing holographic recorded ion-beam etched laminar plane grating, covers the whole energy range and corrects the spherical aberration of vertical focusing mirror
Fig. 1. The optical design layout of 4B1 PEEM beamline at PAL. There are two mirrors and one grating. Entrance slitless operation is allowed. The simple fixed exit slit is located at the focusing position.
Table 1 Optical parameters of mirrors, grating, out-of-plane aperture and focusing position Element
Dist. from source (m)
M1 Out-of-plane aperture M2 Grating Focus positionb & Exit slit
11.5
a b
Radius (m),
l w (mm),
y (deg)
Coating
229.88
750 85
3.0
Au/Cr
214.96 1
320 75 220 40
3.0 172a
Au/Cr Au
14.0 15.0 15.5
Included angle of monochromator The fixed exit is located at this point
24.0
T.-H. Kang et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
(VFM), so that the monochromator is essentially aberration-free over its tuning range [11]. VLSplane grating could have a very simple and stable mechanism design. The energy is tuned by a simple rotation of the grating about its center with no rotating or translating mirrors or no moving exit slit. The grating has central groove densities of 1000 mm1 and maximum efficiency at 750 eV. Fig. 1 shows an optical layout of the beamline. The spherical horizontal and the spherical vertical focusing mirrors image the beam from the bending magnet on the fixed exit slit, which are almost unit and 15 : 9 demagnification, respectively. The VFM and grating, which are only 50 cm apart, were installed in the same holder and housing chamber in order to reduce the number of factors for the alignment. The small size and high stability of the PLS source [14] allows one to remove the physical entrance slit. The idea of entrance slitless operation gives some advantages such as maximum photon flux, shortening the length of beamline, simplifying the mechanical design and making the alignment easier. The exit slit has three fixed sizes of 300, 200, 100 mm, respectively, in order to simplify the mechanical design. The exit slit, which is shown in Fig. 1, is located at the focus point. Table 1 shows the optical parameters of grating, the mirrors, the out-of-plane aperture and focus position. The end-station consists of PEEM station, X-ray photoelectron spectroscopy (XPS) station and magnetic circular dichroism (MCD) station along the beam path downstream of the focusing point. The XPS station will serve the experimental research on the chemical adsorption and/or desorption on surface such as the selective molecular cleavage studied by Park et al. [15]. The MCD station under construction will serve the dichroism phenomena studies of magnetic materials. The mechanical out-of-plane aperture for MCD, which is located just before the VFM, can select the circular polarization of the photon beam. The left or right hand circularly polarized beam can be obtained by means of a movable outof-plane aperture that selects light from above or below the horizontal plane. The sample integration Omicron IS-PEEM is located 50 cm downstream of the exit slit. Because of sample integration stage
583
Fig. 2. The photon flux of beamline is shown. The flux is maximized at 750 eV. The broken curve is the predicted flux from a PLS bending magnet source assuming 100% efficient optics.
Fig. 3. The medium resolving power was observed with Ar and N2 gases. (a) the photoionization spectrum of gas-phase Ar closed to Ar L2;3 threshold, (b) the vibration resonant spectrum of N2 1s ! p*. The fitted peaks, which have 400 meV Gaussian broadening, are shown.
584
T.-H. Kang et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
Fig. 4. PEEM images of a Ni mesh on silicon oxide. 853 eV energy was used to take the chemical element contrast of Ni and silicon. Insets are the Ni mesh image using Hg lamp (4.9 eV) and X-ray absorption spectroscopy of Ni. Recording frames with 1 eV increment step of photon energy obtained the Ni X-ray absorption spectra from 835 eV to 885 eV.
the PEEM instrument is stable against vibration which makes worse the spatial resolution of the PEEM image. We have obtained the PEEM images with standard samples for commissioning beamline and illustrating the further performance of the beamline and PEEM.
3. Performance The beamline is being commissioned, and the focused beam size, photon flux, resolution have been characterized. The focused visible light at exit
slit was measured 0.4 mm (V) 0.8 mm (H) using the Hamamatsu MOS linear sensor S3904-1024, and the beam size of the soft X-ray is assumed to be smaller. Three end-stations have a different beam size}PEEM (1 1.5 mm), XPS (2.5 3 mm), MCD (4 5 mm)}because the end-station is installed downstream of the exit slit and there are no re-focusing mirrors between them. The photon flux from the beamline has been carefully measured using photodiode IRD AXUV100 and compared to the data of ionization gas cell [16]. Fig. 2 presents the photon flux that has a maximum value more than 2 1011 photons s1
T.-H. Kang et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 581–585
per 0.1% BW at 750 eV. This flux is enough to make images of PEEM that has 200 nm spatial resolution, especially chemical contrast image of the transition metal. A refocusing mirror will be installed to make a micro-spot size beam on sample at PEEM for the less than 70 nm high spatial resolution. For measuring the monochromator resolution, the gas cell, which was located just behind the XPS station, was used with Ar and N2 gases. A 100 mm fixed slit was used to measure the resolution. The medium resolving power, (E=DE¼ 100021200) was observed in the whole range, the results are presented in Fig. 3. Fig. 3(a) and (b) show the photoionization spectrum of gas-phase Ar close to Ar L2;3 thresholds and N2 1s ! p* in gas-phase N2, respectively. For investigating the performance of PEEM station and the beamline, the preliminary PEEM images which have a big field of view were obtained with Ni mesh on SiOx sample using the Hg lamp (4.9 eV) and the synchrotron tuned to Ni 2p core levels (2p3/2=853 eV, 2p1/2=870 eV). Fig. 4 shows that the Ni mesh PEEM images were taken with a Hg lamp and synchrotron radiation with 200 mm field of view. The inset in Fig. 4 presents the X-ray absorption spectra on Ni 2p levels. The spatial resolution was less interesting in this case because the sample was made by Ni mesh wire (30 mm diameter) put on a SiOx wafer. The long diameter of the mesh disturbed the wellfocused image in the vertical direction. From this test we illustrate the further performance of the beamline and PEEM station.
Acknowledgements This work at PLS is supported by Ministry of Science and Technology and POSCO. This experi-
585
ment is also supported by the Korean Science and Engineering Foundation through the AtomicScale Surface Science Research Center (ASSRC).
References [1] B.P. Tonner, G.R. Harp, Rev. Sci. Instrum. 59 (1988) 853. [2] W. Swiech, G.H. Fecher, Ch. Ziethen, O. Schmidt, G. Scho¨nhense, K. Grzelakowski, C.M. Schneider, R. Fomter, H.P. Open, J. Kirschner, J. Electron Spectrosc. Relat. Phenom. 84 (1997) 171. [3] S. Anderson, H.A. Padmore, R.M. Duarte, T. Renner, T. Stammler, A. Scholl, M.R. Scheinfein, J. St hr, L. Seve, B. Sinkovic, Rev. Sci. Insrum. 70 (1999) 3973. [4] A. Scholl, J. Sto¨hr, J. Lu¨ning, J.W. Seo, J. Formpeyrine, H. Siegwart, J.-P. Locquet, F. Nolting, S. Anderson, E.E. Fullerton, M.R. Scheinfein, H.A. Padmore, Science 287 (2000) 1014. [5] G. Scho¨nhense, J. Phys. : Condens. Matter 11 (1999) 9517. [6] Y. Hwu, J. Electron. Spectrosc. Relat. Phenom. 8 (1997) 149. [7] T. Droubay, G. Mursky, B.P. Tonner, J. Electron. Spectrosc. Relat. Phenom. 84 (1997) 159. [8] G. De Stasio, G. Magaritondo, J. Electron. Spectrosc. Relat. Phenom. 84 (1997) 137. [9] Y. Yamaguchi, S. Takakusagi, Y. Sakai, M. Kato, K. Asakua, T. Iwasawa, J. Moclec. Catal. A; Chemic. 141 (1999) 129. [10] H.H. Rotermund, J. Electron. Spectrosc. Relat. Phenom. 98-99 (1999) 41. [11] J.H. Underwood, E.M. Gullikson, J. Electron. Spectrosc. Relat. Phenom. 92 (1998) 265. [12] W.R. Mckinney, Rev. Sci. Instrum. 63 (1) (1992) 1410. [13] M. Hettrick, J.H. Underwood, AIP Conf. Proc. 147 (1986) 237. [14] I.S. Ko, J.Y. Huang, D.K. Seon, C.B. Kim, T.-Y. Lee, J. Korean Phys. Soc. 35 (1999) 411. [15] J.H. Moon, Y.-H. La, J.Y. Shim, B.J. Hong, K.J. Kim, T.-H. Kang, B. Kim, H. Kang, J.W. Park, Langmuir 16 (2000) 2981. [16] J.A.R. Samson, Techniques of VACUUM ULTRAVIOLET SPECTROSCOPY, Pied Publications, Lincoln, Nebraska, 1980, (Chapter 8).