status of natural and magnetic circular dichroism instrumentation using synchrotron radiation

364 Nuclear Instruments and Methods in Physics Research 222 (1984) 364-371 North-Holland, Amsterdam

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

Patricia A n n S N Y D E R

Department of Chemistry, Florida Atlantic University, Boca Raton, Florida 33431, USA

Since synchrotron radiation is intense, highly linearly polarized, a spectral continuum and collimated, it is ideal (necessary) for magnetic and natural circular dichroism measurements in certain regions of the spectrum. What these regions are, the importance of the measurements and considerations in instrumental design which preserve the ideal properties of synchrotron radiation are discussed. Then the design, present status and future plans are reviewed for each functioning instrument. Finally the instrumental design challenges of the future are surveyed.

1. Introduction

A system which can rotate the plane of polarization of linearly polarized radiation is called optically active. Since the discovery of optical activity, its study has lead to major scientific discoveries in chemistry, physics and biology. For instance, Pasteur discovered enantiomeric pairs of optically active molecules [1] and Faraday showed the intimate connection of light and electromag- netism from his discovery of magnetic optical activity [2]. In recent times, the study of optical activity has been extended to new regions of the spectrum [3].

Optical rotation arises because of different responses to left and right circularly polarized radiation. Subs- tances that are optically active in the absence of exter- nal influences are said to be naturally optically active and have a natural circular dichroism spectrum. All substances in a magnetic field are optically active and therefore have a magnetic circular dichroism spectrum.

Circular dichroism studies have been difficult or impossible in certain regions of the spectrum. The de- velopment of a magnesium fluoride polarizer [4] and a calcium fluoride stress plate quarter wave modulator [5-7], as well as modified Hinteregger lamps [8,9] were responsible for the initial circular dichroism measurements in the vacuum ultraviolet region [9-20]. The calcium fluoride quarter wave retarder also helped open the infrared region to circular dichroism measurements [21-23]. (The use of zinc selenide opened the infrared region even further [24].) Synchrotron radiation is highly linearly polarized, a spectral continuum, collimated and intense. These properties are ideal (necessary) for natural and magnetic circular dichroism measurements in certain regions of the spectrum. Use of synchrotron radiation has already extended natural and magnetic circular dichroism measurements to higher energies and resulted in better resolution in the vacuum ultraviolet

0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

region [25-28]. In the future it is anticipated that the use of synchrotron radiation will allow natural and magnetic circular dichroism measurements in new regions of the infrared, as well as to energies higher than 125 nm.

Before considering the present status of natural and magnetic circular dichroism instrumentation using synchrotron radiation, I will define natural and magnetic circular dichroism and discuss the types of information they provide. This discussion is presented in sect. 2. In sect. 3 the present status of the instrumentation which uses synchrotron radiation will be discussed and in sect. 4, future instrumental developments will be discussed.

2. The definition and importance of natural and magnetic circular dichroism

Both magnetic and natural circular dichroism are types of absorption spectroscopy. Normal absorption spectroscopy measures the absorption (c) of isotropic light as a function of energy, while magnetic and natural circular dichroism measure the difference in absorption of left and right circular polarized light ((L- ~R) as a function of energy.

Molecules with no plane or center of symmetry, which means virtually all biological molecules, exhibit natural circular dichroism. Natural circular dichroism spectra are sensitive to molecular conformation. Since conformation is so intimately related to function for biological molecules, natural circular dichroism is used in their study and it is a reasonable goal to extract structural information from natural circular dichroism spectra. Natural circular dichroism studies of biological molecules have clearly demonstrated the added information and greater sensitivity [13,29-30] obtained by studies in the vacuum ultraviolet region. The studies of

P.A. Snyder / Natural and magnetic circular dichroism instrumentation 365

biological molecules have been carried out mainly with conventional sources. Since the bands are broad, wide slit widths and long time constants are not detrimental. However, use of synchrotron radiation has already con- tributed in this area with the results on Z - D N A [31].

Natural circular dichroism has been used to obtain the configuration (the arrangement of the groups in a molecule relative to the asymetric carbon atom) of molecules. The most successful case has been the octant rule for ketones [32]. However, extension of measurements into the vacuum ultraviolet on ketones has shown that while the results have been correct (i.e. the right configuration is obtained), the true basis of the rule was not understood [32].

However, if the configuration and conformation of a molecule is known, the natural circular dichroism spectrum can be used to obtain information on the excited state symmetries. It is the more theoretically tractable molecules which absorb in the vacuum ultraviolet. Therefore, natural circular dichroism data in the vacuum ultraviolet is leading to a better understanding of both the theory and the excited states present for these molecules [32-36].

Magnetic circular dichroism is a powerful technique for obtaining information on the electronic structure, number of transitions and assignment of transitions. It is particularly powerful when applied to molecules which belong to symmetry groups which have some degenerate states. For M C D [37]

a~= . 4 ~ +(B+c/~r)s,~,

where s is the absorption band shape, v the frequency,

H the magnetic field and T the temperature. The three terms, A, B and C, are hnearly dependent on the field strength, but they have different dependences on the temperature and absorption band shape. A derivative band shape, with respect to the absorption band shape, is diagnostic of a degenerate electronic state and is called an A term. If the magnetic circular dichroism peak has the same band shape as the absorption, it is called a B term. B terms are present for any transition. C terms have the same band shape as the absorption, but they are temperature dependent. C terms arise when the ground state is paramagnetic. For interpretation, it is necessary to sort out the various types of terms and evaluate their magnitude. Therefore, sufficient resolution is necessary for proper interpretation. This means synchrotron radiation is particularly useful (necessary) for magnetic circular dichroism measurements in the vacuum ultraviolet region. The use of synchrotron radiation for these measurements has resolved some long standing controversies on transition assignments in benzene and ethylene [27,28].

3. Present instrumentation

While the cause of magnetic and natural circular dichroism is different, the instrument is the same, ex- cept for the addition of a magnet for magnetic circular dichroism measurements. For magnetic circular dichroism, the measured circular dichroism is proportional to the magnetic field (the light path is parallel to the applied magnetic field), and the sign of the dichroism for a given wavelength depends upon the relative direc-

Table 1 Present status of natural and magnetic circular dichroism instrumentation using synchrotron radiation.

Natural circular dichroism Magnetic circular dichroism Location

Functioning from 125 to 310 nm functioning from 125 to 310 nm with a 7 T field (room temperature

• bore); matrix isolation planned in the future

planned in the future (2.3 T field from electromagnet)

Functioning from 180 nm to infrared (1.5 #), future plans to modify equipment for measurements to 160 nm

functioning from 160 to 330 nm for matrix isolation with a 6 T field; future plans for gas phase studies when a room temperature bore magnet is obtained

functioning from 160 to 250 nm with a 4.7 T field (Faraday rotation "type" measurements). It is anticipated that these measurements can be carried out to higher energies (future magnet with 6 T field)

Synchrotron Radiation Center, University of Wisconsin~Madison

National Synchrotron Light Source, Brookhaven National Laboratory (originally at Surf II, National Bureau of Standards)

Bonn 2.5 GeV synchrotron

Bonn 0.5 GeV synchroton

VI. SPECIAL TECHNIQUES

366 P.A. Snyder / Natural and magnetic circular dichroism instrumentation

tions of the light and the magnetic field. (As a conse- quence of this, if light is reflected back through the sample, the magnetic circular dichroism signal would be twice as big.) Therefore, either the field or the light may be modulated. Having mentioned these two differences, it should be kept in mind that the following discussion is true for both magnetic and natural circular dichroism.

Noise on a circular dichroism spectrum is inversely proportional to the square root of the number of pho- tons [38]. Therefore, circular dichroism instruments need bright sources and efficient optics. In the vacuum ultraviolet region, this has been a problem. While natural and magnetic circular dichroism measurements have been carried out in the vacuum ultraviolet without synchrotron radiation, these measurements have been difficult, time consuming and limited in both their resolution and energy capabilities [9-20]. However, these measurements clearly showed the usefulness of data in the vacuum ultraviolet region. Since synchrotron radiation is more intense that conventional vacuum ultraviolet sources, it is ideal for these measurements. In addition, since synchrotron radiation is linearly polarized in the plane of the electron orbit, no polarizer is necessary if the beam line and monochromator are designed to preserve the polarization. The collimation of synchrotron radiation allows the light to pass through the bore of a magnet without additional optics.

The first natural and magnetic circular dichroism measurements using synchrotron radiation were carried out on Tantulus I at the Synchrotron Radiation Center, University of Wisconsin-Madison [25-27]. I will discuss the design of the equipment for these measurements and then I will discuss the instruments which have been developed since then. Table 1 summarizes the present capabilities of various instruments.

Fig. 1 shows the schematic of the instrument constructed using synchrotron radiation for natural circular dichroism measurements. For magnetic circular dichroism measurements, a superconducting magnet with a room temperature bore (7 T field) is placed around the

sample cell. All reflections are s reflections, so the linear polarization of the light is maintained (actually en- hanced). The lm Seya-Namioka monochromator has a 8.3 ,~/mm dispersion and the capability of obtaining a 0.4 ,~ spectra bandwidth. A degree of linear polarization of 99% is expected at the exit of the monochromator.

The exit and entrance mirrors are both gold coated cylindrical mirrors. After exiting the monochromator, the light passes through a lithium fluoride window, which separates the sample chamber from the ring, beam line and monochromator. The linearly polarized light then passes through a calcium fluoride stressed plate quarter wave modulator (Hinds). The quarter wave modulator has its fast axis at 45 ° to the plane of the linearly polarized light. It oscillates at 50 kHz, making a quarter wave plate with its fast axis alternately at + 45 ° and - 4 5 ° to the plane of the polarized light and therefore producing left and right circularly polarized light. The circularly polarized light proceeds through the sample cell and is detected by a photomultiplier tube. With no sample in the light path, the photomultiplier produces a d c signal which is proportional to the light intensity at that wavelength. With an optically active sample in the light path the signal is modulated at 50 kHz, since the absorption for right and left circularly polarized light is different. Therefore, the dc signal has an ac signal superimposed on it. The magnitude of the ac signal is a measure of the circular dichroism at that wavelength. It can be shown that to a good approxima- tion; [38]

AL(~- ) - -AR(~. ) = CIAc/IDc,

where A is the absorbance. The constant C is de- termined by calibration with a standard. The Idc is kept constant with a dc controller. The dc controller keeps Idc constant by changing the voltage applied to the photomultiplier tube. Fig. 2 shows the schematic of the dc controller which was constructed for this purpose [26]. The ac signal is detected and rectified by a lock-in amplifier and then outputted to one pen of a two pen

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recorder. The other pen follows the change in the voltage supplied to the photomuitiplier and is therefore related to the absorption of light by the sample.

These measurements clearly showed that the use of synchrotron radiation can allow measurements to higher energies and with better resolution than with the use of conventional vacuum ultraviolet sources. Magnetic and natural circular dichroism measurements were made to 125 nm. (The calcium fluoride mlodulator is responsible for the 125 nm limit.) The natural circular dichroism measurements were made with a 1.7 A spectral band-

width from 200 to 148 nm and a 4.1 ,~ spectral bandwidth from 148 to 125 nm. Fig. 3 shows the natural circular dichroism spectrum of (+)-3-methylcyclopentanone from 170 to 125 nm [26]. These measurements on (+)-3-methylcyclopentanone go to higher energies than previous measurements, which were limited by the use of a magnesium fluoride polarizer to 135 nm [9]. In addition, these measurements have better resolution. The best spectral bandwidth previously reported in this region is 8 A. The magnetic circular dichroism measurements were made with a 0.4 ,k, spectral bandwidth (limited by the monochromator and not by the intensity of the source). The magnetic circular dichroism measurements on benzene not only solved a long standing controversy on the assignment of the R and R' Rydberg Series (to 1A2u and 1Elu states respectively), but clearly showed the need for a monochromator which is capable of better resolution [27]. For this purpose, a 4 m monochromator will be installed on Aladdin, see sect. 4.

Fig. 4 shows raw magnetic circular dichroism data for deuterated ethylene (C2D4) in the 133 nm region. These data show the capabilities of the instrument in this region, as well as showing that there are additional transitions in this region (as well as in regions not shown), which have not been recognized by studies of the absorption spectrum. (The spectral bandwidth is 0.4

and the time constant is 12.5 s.) The next natural circular dichroism instrument to be

Fig. 4. Magnetic circular dichroism spectrum of deuterated ethylene in the vapor phase.


developed using synchrotron radiation was at Surf II (National Bureau of standards). The idea was to de- velop the instrument on Surf II for use at the National Synchrotron Light Source, Brookhaven National Laboratory, where it is now located. The following is a description of that instrument [31,39,40].

The light from the mirror box passes through a calcium fluoride window and is focused on the entrance slit of a Czerny-Turner 0.5 m focal length monochromator (Minuteman 305 MV). The light exits the monochromator, passes through a second calcium fluoride window, a magnesium fluoride polarizer and is reflected and focused by a 30 ° angle-of-incidence toroidal mirror. The light then passes through a calcium fluoride stress plate quarter wave modulator, a beam splitter, a sample cell and impinges on a photomultiplier tube. (Only one of the beams from the polarizer is allowed to reach the sample cell). The layout of the electronic system is similar to that in fig. 1. However, the absorption is obtained by knowing the relationship between the gain and the voltage of the photomultiplier tube [40]. (A beam splitter allows changes in light intensity to be monitored by a second photomultiplier tube and is used for the absorption measurements.) The second calcium fluoride window, which isolates the sample chamber from the monochromator, allows samples (liquids in cells) to be changed in about 10 rain. Instead of a recorder, the signal is collected and reduced by a Textronic "desk top" computer system [39]. Mea- surements have been made to 180 nm in the vacuum ultraviolet [31] and plans are being made to extend the capabilities of the instrument to 160 nm.

The above instrument has recently been developed to allow measurements in the infrared region. While natural circular dichroism measurements have been carried out with conventional light sources in the infrared, it is hoped that synchrotron radiation will decrease the baseline problems which have plagued infrared measurements. Also, in the future, use of synchrotron radiation may allow measurements on parts of the infrared not previously possible. At wavelengths below about 100 #, synchrotron radiation is more intense than conventional sources.

The next two instruments were reported at about the same time. I will discuss the one at the Bonn 2.5 GeV synchrotron first. These measurements are the first matrix isolated vacuum ultraviolet magnetic circular dichroism measurements with synchrotron radiation [41].

The reflections in this system are all s reflections. The degree of polarization has been measured and is 97% throughout the region of interest. The synchrotron radiation is collected by a first mirror (reflection angle 7.5 ° ) and then reflected upward by an entrance slit mirror (gold coated with a reflection angle of 3.75 °) and focused vertically on the horizontal entrance slit. After being monochromatized, the light is focused on the

sample holder by a cylindrical mirror after the exit slit. The light passes through a CaF 2 stress plate modulator before arriving at the sample. The monochromator is a 1 m normal incidence monochromator (McPherson type from Acton Research Corporation) mounted in the vertical direction. The grating has 1200 lines/mm. The slits are adjustable between 5 #m and 1 mm. The sample is mounted in a split coil superconducting magnet capable of 6 T fields. The system has been designed such that three samples can be run before breaking the vacuum. The system does not keep /de constant, but instead actually measures both Iac and/de. A photomultiplier before the superconducting magnet is used to monitor the light for use in determining Ioc. (Idc = I d / l m where 14 is the dc signal detected and I m is the monitor signal.) This method allows the absorption and magnetic circular dichroism of a sample to be obtained simultaneously. The output signals are given to a four channel recorder and simultaneously collected by a CAMAC system and given to an on-line Nova-Data General computer. To date, measurements have been carried out to 160 nm with this instrument and it is anticipated that measurements to shorter wavelengths will be possible. In the near future, a 6 T field magnet (1 m long) with a room temperature bore will be used for gas phase work.

The final instrument which I will describe has been developed at the Bonn 0.500 GeV synchrotron [42-45]. This instrument is unique in both the experimental setup and the goal of the measurements. It is a Faraday rotation "type" of measurement and therefore the plane polarized radiation from the synchrotron is utilized directly. The plane polarized light passes through a gaseous sample in a magnetic field (4.7 T possible field), is focused on the entrance slit of a 3 m monochromator by a mirror, passes out the exit slit, is detected and the intensity is recorded.

The holographic grating in the monochromator has strong polarizing properties and therefore acts as an analyzer. As a result, scanning an absorption line gives a Faraday rotation pattern which extends from the wings through the core of the absorption line. The method to obtain relative oscillator strengths from this data has been published, as well as oscillator strengths obtained in this way [42-44]. At the present time, measurements have been made to 160 nm and it is believed that it will be possible to go to shorter wavelengths with this system. Future measurements will be made with a new magnet (6 T field, warm bore 10 cm diameter and 1 m long).

4. Future instrumentation

The future instrumental design challenges are in the areas of better resolution in the vacuum ultraviolet



region, extending circular dichroism measurements to higher energies and extending measurements into certain regions of the infrared. Only the first two of these will be discussed below.

Better resolution in the vacuum ultraviolet region will be realized shortly. A 4 m monochromator has been designed and built (with all s reflections) and will be installed on Aladdin, at the Synchrotron Radiation Center, University of Wisconsin-Madison [46]. This monochromator has a possible spectral bandwidth at least ten times smaller than the present 1 m Seya-Namioka monochromator on Tantulus 1. In addition, the intensity of the 4 m monochromator will be four times greater than the 1 m monochromator at the same spectral bandwidth and beam current.

Measurements could be extended to 105 nm from 125 nm by use of a lithium fluoride quarter wave modulator. However, to extend measurements beyond 105 nm, new methods to produce circularly polarized light are necessary. (Optical rotation measurements could be made using the linear polarization of synchrotron radiation.) Also, the question of sample contain- ment is more difficult in this region. These problems must be surmounted, while still maintaining enough light intensity for reasonable signal to noise at the necessary spectral bandwidth.

Table 2 lists proposed methods for producing circularly polarized light. The three reflection device [47], helical wiggler and undulator [49-52], light emitted above and below the plane of the ring [48] and the silicon crystal [53] do not lend themselves to easy modu- lation. Therefore, these methods would be difficult to use for natural circular dichroism measurements. How- ever, they could be used for magnetic circular dichroism measurements by modulating the magnetic field. If measurements are made using the present methods for obtaining data, the recent design of two linear undulators (one vertical and one horizontal) which produces very intense light with modulated left and right circular polarization is very promising [54].

The use of synchrotron radiation as a tool for magnetic and natural circular dichroism measurements is in its infancy. Experimentally and theoretically, the future of these measurements promises to be both challenging and exciting.

Table 2 Methods to produce circularly polarized light at higher energies (h < 105 nm).

1. Three reflection circular polarizer [47] 5-30 eV 2. Synchrotron radiation above and below the plane [48] 3. Helical wigglers or undulators [49-52] 4. Silicon crystal [53] (10-20 keV) 5. Two linear undulators (one vertical and one horizontal) [54]

This work was supported by the National Science Foundat ion under grants CHE 8108534, D M R 77-21888 and CHE 77-08311, by the Research Corporation and by the Divsion of Sponsored Research at Florida Atlantic University. The assistance of the staff of the Synchrotron Radiation Center, University of Wiscon- sin-Madison, and the National Synchrotron Light Source, Brookhaven National Laboratory is gratefully acknowledged. In particular, I gratefully acknowledge Ednor M. Rowe, Director, Synchrotron Radiation Center, University of Wisconsin-Madison.

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status of natural and magnetic circular dichroism instrumentation using synchrotron radiation

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