F i g . 1 . T h e m a g n e t i c s t r u c t u r e a n d m o v e m e n t o f t h e d o u b l e - a r r a y undulator. λ u and D indicate the magnetic period length and the phase shift distance, respectively. L i g h t p o l a r i z a t i o n h a s b e c o m e i n c r e a s i n g l y i m p o r t a n t f o r s u c h s o f t X - r a y s p e c t r o s c o p i c t e c h n i q u e s a s m a g n e t i c c i r c u l a r d i c h r o i s m ( M C D ) an d ci rc ul ar di ch ro is m ( CD ) me as ur em en ts . Su ch e x p e r i m e n t a l m e t h o d s r e q u i r e g o o d p o l a r i z a t i o n quality and the fast switching of photon helicity. To promote high-sensitivity studies and to improve the signal-to-noise ratio for circular dichroism, we have d e v e l o p e d a n e w M C D m e a s u r e m e n t s y s t e m coupled with the right and left circular polarizations ( RCP and LCP ) switching [1] . B L 2 3 S U i s a s o f t X - r a y b e a m l i n e u s e d f o r s p e c t r o s c o p i c s t u d i e s f o r a w i d e v a r i e t y o f a p p l i c a t i o n s , s u c h a s s u r f a c e c h e m i s t r y , b i o l o g y , and condensed matter physics [2] . The light source ( ID23 ) is an APPLE-2 type undulator [3] . Figure 1 s h o w s t h e s t r u c t u r e a n d m o t i o n o f p e r m a n e n t magnet rows of the undulator. This planar double- a r r a y u n d u l a t o r g e n e r a t e s a n y k i n d o f e l l i p t i c a l polarization of light by adjusting the relative position PERIODIC POLARIZATION SWITCHING FOR MCD MEASUREMENTS of pairs of magnet rows (phase shift) and changes t h e p h o t o n e n e r g y i n t h e s o f t X - r a y r a n g e b y v a r y i n g t h e g a p d i s t a n c e b e t w e e n t h e u p p e r a n d lower jaws. F i g u r e 2 p r o v i d e s a n o u t l i n e o f t h e M C D - measurement-control system at beamline BL23SU , w h i c h i s b a s e d o n t h e S P r i n g - 8 p u b l i c b e a m l i n e model [4] . Experiments using MCD measurements can be performed with a personal computer ( MCD- P C ) b y u s i n g s o p h i s t i c a t e d a p p l i c a t i o n s s o f t w a r e . The beamline workstation ( BL-WS ) is connected to t h e M C D - P C a n d c o n t r o l s t h e b e a m l i n e i n s e r t i o n d e v i c e ( I D 2 3 - g a p / p h a s e ) a n d t h e m o n o c h r o m a t o r [5 ] , wh ic h es ta bl is he s th e se le ct ed mo no ch ro ma ti ze d p h o to n e n e rg y, ma i n ta i n i n g th e ro b u st n e ss o f th e system. T h e M C D s a m p l e c u r r e n t ( I 1 ) a n d m o n i t o r cur ren t ( I 0 ) are col lec ted by the MCD -PC . Dur ing the MCD measurements, the undulator control, the monochromator control, and the data accumulation Gap open Gap open Phase shift h = 25 mm w = 50 mm row 3 e - D row 4 Phase shift D row 2 row 1 d= 3mm λ u =120 mm z x y Fig. 2. Schematic illustration of the MCD measurement control system at b e a m l i n e B L 2 3 S U w i t h I D 2 3 . E x p e r i m e n t e r s c a n c a r r y o u t t h e M C D me as ur em en ts vi a se ve ra l SP ri ng -8 ne tw or ks , in cl ud in g BL -S R- LA N an d BL-USER-LAN. VMEs and BL-X-terminals are omitted. a r e o p e r a t e d s i m u l t a n e o u s l y . F o r i n s t a n c e , a measurement cycle proceeds as follows: (i) photon energy tuning; ( ii ) ID23 phase-shift to provide RCP X - r a y s ; ( i i i ) s i m u l t a n e o u s s a m p l i n g o f I 0 a n d I 1 ; ( iv ) ID 23 ph as e- sh if t to pr ov id e LC P X- ra ys ; an d ( v ) s a m p l i n g a g a i n o f I 0 a n d I 1 . A t t h i s p o i n t t h e cycle returns back to (i) to be continued. Figure 3 s h o w s t h e f i r s t r e s u l t s o f M C D m e a s u r e m e n t s , mad e for the of Fe2 p abs orp tio n of Fe met al in a m a g n e t i c f i e l d o f 0 . 4 T e x c i t e d b y R C P a n d L C P soft X-rays with a polarization switching frequency of 0.1 Hz. We believe that this circular dichroism mea sur eme nt sys tem wit h per iod ic pho ton -he lic ity sw it ch in g by a va ri ab ly -p ol ar iz in g un du la to r pr ov id es g r e a t o p p o r t u n i t i e s f o r t h e s t u d y o f a v e r y w e a k circular dichroism signals in the soft X-ray region. It should be noted that the closed orbit distortion o f t h e s t o r a g e r i n g c a u s e d b y t h e I D 2 3 d r i v e i s mi ni mi ze d us in g th e st ee ri ng ma gn et s in st al le d in the upstream and downstream of the undulator. ID23 Monochromator MCD station e-beam Polarized SR Au monitor SPring-8 Storage-Ring COD Beamline Workstation BL-USER-LAN STM current ID Gap Phase Grating rotation Monitor I 0 current Sample I 1 current MCD Measurement PC Firewall XBPM Other beamline STM3 STM4 STM1 STM2 BL-SR-LAN Fig. 3. Fe L 2,3 absorption spectra of Fe metal in a magnetic field 0.4 T excited by right and left polarized X-rays with a polarization switching frequency of 0.1 Hz. A k a n e A g u i a , T a k e s h i N a k a t a n i a , b a n d T o m o h i r o Matsushita b (a) SPring-8 / JAERI (b) SPring-8 / JASRI E-mail: agui @ spring8.or.jp 730 720 710 700 Photon Energy ( eV ) Intensity ( arb. units) Left circular polarization Right circular polarization Fe 2 p → 3 d absorption Magnetic field 0.4 T References [1 ] A. Ag ui , A. Yo sh ig oe , A. Yo ko ya , Y. Sa it oh , Y. Hiramatsu, T. Shimada, T. Nakatani, T. Matsushita, Y. Miyahara, H. Tanaka, M. Takeuchi, T. Bizen, S. Sasaki, M. Takao, H. Aoyagi, T.P. Kudo, K. Satoh, S. Wu , an d H. Oh ku ma , to be pu bl is he d in Re v. Sci. Instrum. [2] A. Yokoya et al. , J. Synchrotron Rad. 5 (1998) 10. [3 ] S. Sa sa ki , Nu cl . In st ru m. Me th . A 34 7 (1 99 4) 83. [4] T. Nakatani et al. , J. Synchrotron Rad. 5 (1998) 536. [5] Y. Saitoh, Y. Teraoka, A. Agui, A. Yoshigoe, A. Y o k o y a , t o b e p u b l i s h e d N u c l . I n s t r u m . M e t h . (2000).
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