772 Nuclear Instruments and Methods in Physics Research A246 (1986) 772-775 North-Holland, Amsterdam

U S I N G T H E C I R C U L A R L Y P O L A R I Z E D C O M P O N E N T S O F S Y N C H R O T R O N R A D I A T I O N F O R

C I R C U L A R D I C H R O I S M M E A S U R E M E N T S B E T W E E N 400 A N D 3500

J 6 r g S C H I L L E R a n d Jose f H O R M E S

Physikalisches Institut der Universiti~t Bonn, Nussallee 12 D 5300 Bonn 1, FRG

The out of plane components of synchrotron radiation are circularly polarized to a high degree. This property offers the possibility to overcome the energy limitation of about 9 eV for circular dichroism (CD) and magnetic circular dichroism (MCD) measurements which is set by the optical components normally used to produce circularly polarized light. We report the first CD spectra which were taken using the circularly polarized components of synchrotron radiation. The light source was the Bonn 2.5 GeV synchrotron and a special monochromator constructed to monochromatize the off plane components of the radiation. Our experiments reveal several problems which make CD measurements very difficult. Some of these problems which we discuss in detail arise from the intrinsic properties of a synchrotron as compared to a storage ring but most are more general and have to be considered for further work on monochromators for the circularly polarized components of synchrotron radiation.

1. Introduction

For optically active molecules the absorbance A _ for left circularly polarized (lcp) light is different from the absorbance A + for right circularly polarized (rcp) light at the same wavelength. In circular dichroism (CD) experiments this difference is measured in an absorp- tion type experiment. In favourable cases CD spectra provide the information needed to determine the ab- solute stereochemical configuration of the investigated chiral molecule [1].

CD measurements require circularly polarized light. Polarization is achieved by using optical components such as linear polarizers followed by a quarter wave plate or a photoelastic modulator [2]. Because of the absorption of these components CO measurements were restricted until now to energies below - 9 eV [3].

According to theory [4], synchrotron radiation con- sists of two linearly polarized components. In the plane of the electron orbit the light is 100% plane polarized with the electric vector parallel to the electron orbit. For observations out-of-plane the intensity of the light which is polarized perpendicular to the electron orbit in- creases. Therefore it is possible to get circularly polarized light by fixing a suitable angle of observation [5]. In this way it should be possible to overcome the energy limita- tion for CD measurements.

2. Experimental

At the 2.5 GeV synchrotron of the Physikalisches Institut in Bonn a monochromator is available which was specially designed to monochromatize the circularly

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polarized off plane components of the synchrotron radi- ation. This monochromator has been used successfully for measurements on spin-polarized photoelectrons. Re- suits of these experiments and the performance of the monochromator are described in the literature [5,6].

In fig. 1 a schematic view is given of the experimen- tal setup as used for the CD measurements. A spherical mirror projects the tangent point of the electron beam ( = entrance slit) onto the exit slit of the monochroma- tor. A reflecting holographic plane grating (4960 1/mm) faces the tangent point in a Littrow mounting giving a resolution of 0.5 ,~ with an exit slit width of 1.3 mm. A flag in front of the grating is used to shut off either the upper (lcp) or the lower (rcp) half of the beam.

As the pertinent signals are very small CD measure- ment should be made using modulat ion techniques. Thus we had to modify the original setup of the mono- chromator replacing the beam flag by a chopper. Be- cause of the 50 Hz pulse structure of the radiation from the synchrotron a 25 Hz synchronous motor drive was

" id G a s F t o w L~qu Cell Aperture

Fitter Exit ~

Concave ~ ' ~ - ' ~ ' ~ ~ ; LI].] ~ Detector Mirror ~_....-- I0 nn ~"-~'-~-. .~ ~

Chopper , ~ ~ ( I TODifferentiot ."]-'-~A~")~ ~ " ~ /7 I I Pumping Stage

........ Y - " f . l I I Beam

Fig. 1. Schematic diagram of the apparatus.

J. Schiller, J. Hormes / Circular dichroism measurements between 400 and 3500.4 773

used. For chopping the beam a steel plate Of 0.2 x 40 x 110 mm 3 was soldered onto the shaft of a magnetic clutch vacuum feedthrough. The system could be verti- cally adjusted relative to the electron orbit by a stepper motor and was placed at a distance of 2.6 m from the tangent point thus defining a vertical acceptance inter- val ~ with 1.15 mrad < I~ [ < 3.8 mrad. With this setup a circular polarization of about 70% was measured between 1650 and 3000 ,~. This figure is lower than the value of 85% reported earlier [5] the decrease probably being caused by aging effects of the optical components. The phase of the chopper can be set between 0 and 180 ° thus allowing the shutter to cross the synchrotron plane during the dark interval of the synchrotron cycle lasting about 4 ms. The phase is controlled by an optical reflecting sensor.

The original exit slit of the monochromator was replaced by stainless steel jaws which were gold coated on the illuminated side, thus reflecting radiation not passing through the exit slit onto a monitor photomulti . plier (EMI 9789 QB). In this way beam splitting ele- ments within the exit slit aperture could be avoided. An effective slit width of 1.5 mm was used and the slit was located near the circle of least confusion.

A windowless absorption cell was developed which in combination with an effective differential pumping stage allowed a pressure times absorption length of 0.1 Torr × 160 cm without reducing the acceptance of _+ 3.8 mrad. An EMI 9635 B photomultiplier with a photo- cathode of 44 mm diameter was used as a detector located behind a sodium salycilated window which closed the absorption cell.

The reported spectra were taken by counting single photons. The counting rates for lcp ( Z ) and rcp light ( Z + ) were normalized to a constant integrated monitor flux and corrected for the dark phase counting rates. The CD signal AA was then obtained as l o g ( Z + / Z ) . In this way difficulties arising from the operation of the synchrotron such as changes in the vertical position of the electron beam and intensity fluctuations could be compensated for.

A C A M A C system controlled by a Data General N O V A on-line computer was used for data collection and storage. The computer also controlled the stepping motors of the grating and the vertical chopper move- ment, the gas inlet system and the beam shutter system.

3. Spurious CD signals

Space constraints imposed an asymmetric setup of the monochromator with deflection angles et v = 5 ° and ct H = 16 ° as shown in fig. 1. This asymmetry caused severe problems for measuring CD spectra which could be traced out only by using a scaled model of the monochromator with a mercury lamp as a light source

-11- - - S y n c h r o t r o n Prone - -

Fig. 2. Light distribution at the photocathode of the detector -- 2a v sin a H (cf fig. ] and section 3).

in the visible. Fig. 2 shows the light distribution at the photocathode of the detector multiplier. Turning the exit slit clockwise around the direction of light propa- gation for just a few degrees can fill the quadrants CD, AC or AB with green light whereas AB, BD and CD respectively turn yellow. During this procedure the image E of the bar representing the chopper shaft still remains fixed. Thus the spectrum A A will always show the derivatioe structure of the absorption unless the exit slit is properly aligned at an angle of - c~ against the vertical position (cf fig. 2) and no light is intercepted after having passed the exit slit. At the 10 m monochromator it turned out that a correction as described could not be achieved for the whole wavelength region and longer periods of time. The reason for this is that the position of the electron beam (the "entrance slit" for our mono- chromator) was not stable thus changing the optical properties of the system. So wavelength differences of about 0.1 A between lcp and rcp light occurred giving rise to spurious A A / A signals of about 10 -2.

Further difficulties were caused by the high stray light level of the monochromator of about 5%. Though we were able to reduce this level to 0.5% it was still responsible for further CD artefacts because the optical paths for lcp and rcp light are separated in the mono- chromator. Thus the ratio Z + / Z of the first order light is in general different from the stray light ratio.

Fig. 3 shows the CD artefacts at the spectrum of the X ~ A transition of gaseous CO which gives no genuine C D signal (curves 2 and 3). Curve 1 gives the distribu- tion of stray light with wavelengths longer than 3000 ,~. The spectrum of the evacuated cell closed with a LiF window is given in curve 4. Increasing the optical den- sity forces AA towards its stray light limit (cf traces 2 and 3 in fig. 3). Though these signals may exceed "derivat ive artefacts" by an order of magnitude it is in

IV(d). MISCELLANEOUS

774 J. Schiller, J. Hormes / Circular dichroism measurements between 400 and 3500

A+-A_ ~W%,~

0.25

- 2 t ,

0.00

ca) I I

1500 1400 ~l~l

0.00 ~

0.12 4 b)

Fig. 3. Spectrum of the X ~ A transition of gaseous CO: (a) CD spectra showing the spurious signals, (b) absorption spec- tra. (1) stray light spectrum for X > 3000 ft., (2) CO at 10 Torr, (3) CO at 5 Tort, (4) spectrum of the empty cell with a LiF window.

I

0.00 --- A+-A_

- 0.04

a) I 3500 3000 2500 MAI

0.50 I I

log +

0.00

I

4

-0 .50

b) 2

Fig. 4. Spectrum of camphorsulfonic acid (CSA) at a concentration of 1 mol. in H20: (a) CD spectra, (b) absorption spectra. (1) (+)-CSA, (2) HzO solute, (3) racemic CSA, (4) ( - )-CSA.

some cases possible to compensate these effects by including stray light spectra in the data reduction pro- cess.

At wavelengths between 400 and 800 A, we have used an A1 filter of 1000 ~. thickness to get rid of the stray light. It turned out to be possible to decrease the spurious signals by an order of magnitude but they were still present mainly because of different Z + / Z ratios for first and second order radiation.

4. Results

We have measured the spectra of several optically active molecules (e.g. limonene, camphor, butanol, menthone) in the gas phase over the wavelength region between 400 and 800 A without finding A A / A signals higher than 10 -2 which was our sensitivity limit as demonstrated in section 3. Therefore we decided to measure the spectrum of camphorsulfonic acid (CSA) which has a A A / A of about 0.07 and a rather broad absorption structure so that derivative effects should not to be too serious [7].

Fig. 4 shows the original spectra obtained for the various CSA modifications. The difference of traces 2 ( H 2 0 solute) and 3 (racemic CSA) gives the stray light artefacts, the difference of curves 1 ( (+) -CSA) and 3 and 4(( - )-CSA) respectively gives the desired CD spec- tra of ( + ) and ( - ) CSA. The A A / A value derived from these spectra is 0.03. The deviation from the value reported in the literature (0.068) is probably caused by the low degree of polarization used for these experi- ments.

5. Conclusion

The spectra shown in fig. 4 are the first CD spectra which were taken by using the circularly polarized com- ponents of synchrotron radiation and to our knowledge the first spectra taken by a combined photon count ing/modula t ion technique. Our experiments dem- onstrate that it is feasible to use the circular polariza- tion of SR for C D / M C D experiments. However, our results show very clearly that future monochromators have to be designed very carefully for their special task

J. Schiller, J. Hormes / Circular dichroism measurements between 400 and 3500 A 775

of obtaining CD spectra of a quality comparable to those taken by conventional techniques and synchro- tron radiation in the VUV [3].

We thank Professor U. Heinzmann for fruitful ad- vice and Professor F. von Busch for his assistance in preparing the manuscript. This work was supported by the Bundesministerium for Forschung und Technologie (BMFT) from funds for synchrotron radiation research which is gratefully acknowledged.

References

[1] S.F. Mason, Molecular Optical Activity and the Chiral Discriminations (Cambridge University Press, 1982).

[2] O. Schnepp, S. Allen and E.F. Pearson, Rev. Sci. Instr. 41 (1970) 1136.

[3] P.A. Snyder and E.M. Rowe, Nucl. Instr. and Meth. 172 (1980) 345.

[4] J. Schwinger, Phys. Rev. 75 (1949) 1912. [5] U. Heinzmann, B. Osterheld and F. Sch~fers, Nucl. Instr.

and Meth. 195 (1982) 395. [6] U. Heinzmann and F. Sch~fers, J. Phys. B13 (1980) L415. [7] G.C. Chen and J.T. Yang, Anal. Letters 10 (1977) 1195.

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