- Email: [email protected]
The first use of synchrotron radiation for vacuum ultraviolet circular dichroism measurements
Nuclear I n s t r u m e n t s and Methods 172 (1980) 3 4 5 - 3 4 9 © N o r t h - H o l l a n d Publishing C o m p a n y
THE FIRST USE OF SYNCHROTRON RADIATION FOR VACUUM ULTRAVIOLET CIRCULAR DICHROISM MEASUREMENTS *
Patricia Ann SNYDER Department of Chemistry, Florida Atlantic University Boca Raton, Florida 33431 USA and
Ednor M. ROWE The Synchrotron Radiation Center, University of Wisconsin-Madison Stoughton, Wisconsin 53589 USA
S y n c h r o t r o n radiation (SR) from m o d e r n electron storage rings is highly linearly polarized and m o r e intense than conventional v a c u u m ultraviolet c o n t i n u u m sources. These unique properties make SR ideal for construction of a vacuum ultraviolet circular dichroism (CD) i n s t r u m e n t . We report the first use of SR for CD m e a s u r e m e n t s and describe the instrumental setup. These measurements were carried o u t in the wavelength range 1 3 2 5 - 2 0 5 0 A on ( + ) - 3 - m e t h y l c y c ! o p e n t a n o n e at the S y n c h r o t r o n Radiation Center of t h e University of Wisconsin-Madison. The signal to noise ratio, and therefore the resolution, was the best ever obtained for CD m e a s u r e m e n t s in this wavelength range. This resulted in the observation of new structure and dramatic peak height changes in the CD spectrum. In addition, these m e a s u r e m e n t s showed that t h e extension of CD m e a s u r e m e n t s to higher energies is possible through the use of s y n c h r o t r o n radiation.
1. What is circular dichroism?
then different amounts of right and left circularly polarized light are absorbed (eL 4= eR). Fig. 2 shows the absorption of isotropic, right circularly polarized, and left circularly polarized light for a molecule with no plane or center of symmetry and the CD for that molecule. Since eL may be larger or smaller than eR, the CD will be positive or negative depending on which circular polarization is absorbed more strongly. For this reason, CD often shows a number of bands in a region where the absorption.spectrum shows only one. Therefore, CD gives additional information on
Circular dichroism (CD) spectroscopy is a type of absorption spectroscopy. Normal absorption spectroscopy investigates the excitation of a molecule from the ground state to an excited electronic state by measuring the absorption (e) of isotropic light as a function of frequency. CD is the difference in absorption for left and right circularly polarized light (eL -ep,). Circularly polarized light is electromagnetic radiation in which the projection of the electric field onto a fixed plane perpendicular to the direction of propagation describes a circle with time. If the electric vector rotates to the right with time as the observer looks at the beam, the light is called right circularly polarized. Fig. 1 illustrates left and right circularly polarized light.
t =variable z :const
circularly,
2. Information contained in circular dichroism For molecules metry, right and absorbed equally If a molecule has
with a pIane or center of symleft circularly polarized light are at a given wavelength (eL = eR). no plane or center of symmetry,
righthanded
lefthanded
projection on plane for right-handed t = const z = variable
* See acknowledgement at the end of the paper.
Fig. 1. Circularly polarized light. 345
VII. DETECTORS AND O T H E R SUBJECTS
P.A. Snyder, E.M. R o w e / S y n c h r o t r o n radiation in UVCD measurements
346
6
ABSORPTION ~right-honded ./f--~,~ isotropic //L/'/-- - ~ left-handed X CIRCULARDICHROISM
sary to avoid too strong absorption of light. In a well designed CD system, the statistical (shot) noise is the limiting factor in the signal to noise ratio (s/n) [1]. It can be shown that the s/n is proportional to the sensitivity of the photocathode (o), the luminous flux (O) and inversely proportional to the bandwidth (df), i.e., s/n -
~(a~ 1O-erect) 1/2 A e c l (df)l/2
,
+
cL + cR cm =
Fig. 2. The abso rp tion of light for an a s y m m e t r i c molecule and the circular dichroism of t h a t molecule.
the number of transitions in a given region of the spectrum. Most optically active molecules can be mentally divided into a number of optically inactive groups. The group primarily involved in the transition is called the chromophoric group, and the other groups are called vicinal (neighboring) groups. Perturbation of the electronic states of the chromophore by the vicinal groups leads to altered electronic states and optically active transitions. Since the vicinal groups cause the perturbation which results in CD, the CD of a molecule contains conformation information. It is this sensitivity to conformation which has made CD particularly interesting to biochemists. CD is often used to observe conformational changes, but if the configuration and unperturbed electronic properties of all the groups are known, then the conformation o~" the molecule can, in theory, be extracted from the CD spectrum. On the other hand, if the conformation and configuration of a molecule are known, then the CD spectrum can be used to obtain information on excited state symmetries (assign transitions).
3. Difficulties in the measurement of circular dichroism Since CD exists only in a region of absorption, measurements must be carried out in spectral regions where the light is already strongly diminished by the absorption itself. CD constitutes only a small fraction of the absorption (~1/10000). Therefore, only very weak dichroisms maybe measured because it is neces-
2
where ~ is a constant, e is the extinction coefficient, c is the concentration and l is the pathlength. Assuming that we are working with a photomultiplier tube with the highest photocathode sensitivity possible, and at the optimum concentration (OD = 0.86) the s/n can be increased by a larger light flux. Generally this has been accomplished by opening the monochromator slits, and slit widths giving resolutions of 16 to 32 A have been used to get enough light in the vacuum ultraviolet region [ 2 - 9 ] . Since a larger time constant results in a smaller bandwith, time constants of 10 to 30 s are commonly used. This necessitates a slow scan rate for the CD spectrum.
4. Advantages of synchrotron radiation for circular dichroism SR from modern electron storage tings is highly linearly polarized and more intense than conventional vacuum ultraviolet continuum sources. These properties make SR ideal for CD measurements because of the better s/n resulting from the higher light flux. The other advantages of SR arise from the way in which circularly polarized light is produced. Experimentally, circularly polarized light has been obtained by the use of a linear polarizer followed by a 1/4 X retarder. The 1/4 X retarder has its fast axis 45 ° to the plane of polarization. Depending on whether the fast axis is at plus or minus 45 ° to the polarization of the light, one obtains right or left circularly polarized light. Since SR is linearly polarized, a polarizer is not necessary. This has two advantages. First, use of a linear polarizer diminishes the light intensity. Therefore, the s/n with SR is better because no polarizer is necessary. In addition, it is the magnesium fluoride polarizer which has limited the range of circular dichroism measurements to 1350A. Without the
P.A. Snyder, E.M. Rowe / Synchrotron radiation in UVCD measurements
polarizer it should be possible to make CD measurements to the limit of the calcium fluoride t/4 X retarder (~1250 A). Use of a lithium fluoride 1/4 X retarder would allow measurements to 1150 )I,.
347
Signal Detected by PM Tube (for a sample with a negative CD) ~ -r'
I/4 X plate gives left-handed light
~ .
V4 X plate ~ /
/
~ !sotropic /
t.c L ,-X- y - 5. Experimental setup
h
""@/4 X plate gives right-handed light
I
CD is a modulation spectroscopy. The 1/4 X retarder is modulated by squeezing and stretching such that the fast axis is alternatively at +45 ° and - 4 5 ° to the linearly polarized light at a frequency of 50 kHz. Thus the sample sees an alternation of left and right circularly polarized light. At a constant wavelength with an optically active sample in the beam, the photomultiplier sees an ac signal superimposed on a dc signal. See fig. 3. The sample in fig. 3 absorbs less right circularly polarized light than left. The dc is kept constant by a feedback system and the sign and magnitude of the ac signal is detected by a lock-in amplifier. Since CD = K IAC/IDc, where K is a constant, and IDC is kept constant, measurement of IAc and calibration yield the CD. Figure 4 shows a schematic of the instrument. After the plane polarized light leaves the monochromator, it passes through the modulated 1/4 ~k retarder and is circularly polarized. The light passes through the sample cell and hits a fluorescent screen of sodium salicylate. The visible light is then split and measured by two photomultiplier tubes. One photo-
//
/
\
] oc
-l-
m
X =const TAc CD ~ - Toc Fig. 3. The signal as seen by the photomuliplier tube.
multiplier measures the light intensity to determine the absorption. The other photomultiplier is connected to the feedback system and the lock-in amplifier for CD determination. The output of both photomultiplier tubes is recorded simultaneously on a two channel recorder. The design is similar to previous instruments [ 2 9]. The main differences are that no polarizer is necessary and the presence of the beam splitter which allows simultaneous measurement of CD and absorption. Also, in addition to the recorder, there is a data acquisition system.
",x
..
.......
"A%t.2Z D&-r A
Ad.@Ol'%l T I O N [
1 ~0. ,"01.,.4 ~ 0 "f" N 12. J
Fig. 4. A schematic of the instrument. VII. DETECTORS AND OTHER SUBJECTS
348
P . A . S n y d e r , E . M , R o w e / S y n c h r o t r o n radiation in U V C D m e a s u r e m e n t s
6. Results Figure 5 shows the spectrum of (+)-3-methylcyc!opentanone in the vapor phase with and without SR between 2050 A and about 1600 A. The better resolution with SR radiation results in observation of more peaks and also dramatic peak height changes. The first CD band doubles in height. The spectral bandwidth with SR is 2 A. The time constant was 4 s for the measurements with SR. Fig. 6 shows the spectrum of (+)-3-methylcyclopentanone from 1600 to 1325 A, which is an extension of the CD spectrum to a higher energy than previous measurements. Also, since the resolution is better (6 A), more peaks are visible. Figure 7 shows a portion of the spectrum of ( - ) ~-pinene with and without SR. With SR more vibrational structure is clearly evident.
'
I
'
I
'
I
I
'
T.ouT AA SYNCHROTRON IIIIRAD/ATION~ ~ ~
8 4 A~ 0
-8
2000
0 --4 I
1700
i
I
I
1600
I
1500
I
J
:
1400
XA
Fig. 6. Circular dichroism spectrum of (+)-3-methylcyclopentanone in the vapor phase (1600-1325 A).
7. Future developments The same properties which make SR ideal for vacuum ultraviolet CD measurements make SR ideal for vacuum ultraviolet magnetic circular dichroism (MCD) measurements. In addition, the collimation of SR means there is plenty of room for a magnet. Since all molecules have MCD, the better resolution will be particularly useful for MCD measurements of small molecules which usually have considerable structure in their absorption spectra. CD measurements could be extended to higher energies by using: (a) a lithium fluoride quarter-wave retarder would allow measurements to ] 150 A. (b)
T
-4
Ae 4
p900 r800 x (A)
I WITH
i700
p600
~]
Wavelength A 1800 1600 I i
'
Without I0 ~- Synchrotron Radiation 86(-)-a-pinene 4-2-0 I0
-16L
8!-20 -
6-
-24~
4-
With Synchrotron Radiation
I
-28-
,
2000
i
1900
O-1800 x (A)
i 1700
1600
Fig. 5. Circular dichroism spectrum of (+)-3-methylcyclopentanone in the vapor phase (2050-1600A) with and without [4] SR.
60 56 52 Frequency (cm-~x I0-3)
Fig. 7. Circular dichroism spectra of (-)-c~-pinene with and without [10] SR.
P.A. Snyder, E.M. Rowe / Synchrotron radiation in UVCD measurements
An appropriate reflection in place of the quarter-wave retarder. (c) Off axis light could be used. (SR is polarized. The light with the electric vector parallel to the electron orbit is labelled I//. If the observation angle relative to the plane of the electron orbit is zero, the light is 100% plane polarized (I//). As the observation angle becomes greater, the light contains more perpendicularly polarized light (/1). Therefore, the light goes from plane polarized to eliptically polarized, to circularly polarized as the observation angle increases from zero.) (d) If and when they become available, a helical wiggler could be used. (A wiggler is a magnet that makes electrons wiggle transverse to their normal path and if it is appropriately designed, circularly polarized light results.) Acknowledgement is made to the National Science Foundation (Grant No. DMR 77-21888), to the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant No. PRF-9238-66), to the Division of Sponsored Research, Florida Atlantic University, and to the Camille and Henry Dreyfus Foundation Program for
.349
Innovation in Chemistry Education Grant to the University of Florida and Florida Atlantic University Cooperative PhD program for support o f this research.
References [1 ] L. Velluz, M. Legrand and M. Grosjean, Optical circular dichroism, (Weinheim Verlag Chemie, (1965). [2] S. Feinleib and F.A. Bovey, Chem. Commun. 1968 (1968) 978. [ 3 ] O. Schnepp, S. Allen and E.F. Pearson, Rev. Sci. lnstrum. 41 (1970) 1136. [4] W.C. Johnson, Jr., Rev. Sci. Instr. 42 (1971) 1283. [5] K.P. Gross and O. Schnepp, Rev. Sci. Instr. 48 (1977) 362. [6] E.S. Pysh, Ann. Rev. Biophys. Bioeng. 5 (1976) 63. [7] S. Brahms, J. Brahms, G. Spach and A. Brack, Proc. Natl. Acad. Sci. 74 (1977) 3208. [8] A. Gedanken and M. Levy, Rev. Sci. Instr. 48 (1977) 1661. [9] A.F. Drake and S.F. Mason, Tetrahedron 33 (1977) 937. [10] P.A. Snyder, 5th Int. Conf. on Vacuum ultraviolet radiation physics, Vol. Ill (CNRS Meudon, France, 1977) p. 97.
VII. DETECTORS AND OTHER SUBJECTS
THE FIRST USE OF SYNCHROTRON RADIATION FOR VACUUM ULTRAVIOLET CIRCULAR DICHROISM MEASUREMENTS *
Patricia Ann SNYDER Department of Chemistry, Florida Atlantic University Boca Raton, Florida 33431 USA and
Ednor M. ROWE The Synchrotron Radiation Center, University of Wisconsin-Madison Stoughton, Wisconsin 53589 USA
S y n c h r o t r o n radiation (SR) from m o d e r n electron storage rings is highly linearly polarized and m o r e intense than conventional v a c u u m ultraviolet c o n t i n u u m sources. These unique properties make SR ideal for construction of a vacuum ultraviolet circular dichroism (CD) i n s t r u m e n t . We report the first use of SR for CD m e a s u r e m e n t s and describe the instrumental setup. These measurements were carried o u t in the wavelength range 1 3 2 5 - 2 0 5 0 A on ( + ) - 3 - m e t h y l c y c ! o p e n t a n o n e at the S y n c h r o t r o n Radiation Center of t h e University of Wisconsin-Madison. The signal to noise ratio, and therefore the resolution, was the best ever obtained for CD m e a s u r e m e n t s in this wavelength range. This resulted in the observation of new structure and dramatic peak height changes in the CD spectrum. In addition, these m e a s u r e m e n t s showed that t h e extension of CD m e a s u r e m e n t s to higher energies is possible through the use of s y n c h r o t r o n radiation.
1. What is circular dichroism?
then different amounts of right and left circularly polarized light are absorbed (eL 4= eR). Fig. 2 shows the absorption of isotropic, right circularly polarized, and left circularly polarized light for a molecule with no plane or center of symmetry and the CD for that molecule. Since eL may be larger or smaller than eR, the CD will be positive or negative depending on which circular polarization is absorbed more strongly. For this reason, CD often shows a number of bands in a region where the absorption.spectrum shows only one. Therefore, CD gives additional information on
Circular dichroism (CD) spectroscopy is a type of absorption spectroscopy. Normal absorption spectroscopy investigates the excitation of a molecule from the ground state to an excited electronic state by measuring the absorption (e) of isotropic light as a function of frequency. CD is the difference in absorption for left and right circularly polarized light (eL -ep,). Circularly polarized light is electromagnetic radiation in which the projection of the electric field onto a fixed plane perpendicular to the direction of propagation describes a circle with time. If the electric vector rotates to the right with time as the observer looks at the beam, the light is called right circularly polarized. Fig. 1 illustrates left and right circularly polarized light.
t =variable z :const
circularly,
2. Information contained in circular dichroism For molecules metry, right and absorbed equally If a molecule has
with a pIane or center of symleft circularly polarized light are at a given wavelength (eL = eR). no plane or center of symmetry,
righthanded
lefthanded
projection on plane for right-handed t = const z = variable
* See acknowledgement at the end of the paper.
Fig. 1. Circularly polarized light. 345
VII. DETECTORS AND O T H E R SUBJECTS
P.A. Snyder, E.M. R o w e / S y n c h r o t r o n radiation in UVCD measurements
346
6
ABSORPTION ~right-honded ./f--~,~ isotropic //L/'/-- - ~ left-handed X CIRCULARDICHROISM
sary to avoid too strong absorption of light. In a well designed CD system, the statistical (shot) noise is the limiting factor in the signal to noise ratio (s/n) [1]. It can be shown that the s/n is proportional to the sensitivity of the photocathode (o), the luminous flux (O) and inversely proportional to the bandwidth (df), i.e., s/n -
~(a~ 1O-erect) 1/2 A e c l (df)l/2
,
+
cL + cR cm =
Fig. 2. The abso rp tion of light for an a s y m m e t r i c molecule and the circular dichroism of t h a t molecule.
the number of transitions in a given region of the spectrum. Most optically active molecules can be mentally divided into a number of optically inactive groups. The group primarily involved in the transition is called the chromophoric group, and the other groups are called vicinal (neighboring) groups. Perturbation of the electronic states of the chromophore by the vicinal groups leads to altered electronic states and optically active transitions. Since the vicinal groups cause the perturbation which results in CD, the CD of a molecule contains conformation information. It is this sensitivity to conformation which has made CD particularly interesting to biochemists. CD is often used to observe conformational changes, but if the configuration and unperturbed electronic properties of all the groups are known, then the conformation o~" the molecule can, in theory, be extracted from the CD spectrum. On the other hand, if the conformation and configuration of a molecule are known, then the CD spectrum can be used to obtain information on excited state symmetries (assign transitions).
3. Difficulties in the measurement of circular dichroism Since CD exists only in a region of absorption, measurements must be carried out in spectral regions where the light is already strongly diminished by the absorption itself. CD constitutes only a small fraction of the absorption (~1/10000). Therefore, only very weak dichroisms maybe measured because it is neces-
2
where ~ is a constant, e is the extinction coefficient, c is the concentration and l is the pathlength. Assuming that we are working with a photomultiplier tube with the highest photocathode sensitivity possible, and at the optimum concentration (OD = 0.86) the s/n can be increased by a larger light flux. Generally this has been accomplished by opening the monochromator slits, and slit widths giving resolutions of 16 to 32 A have been used to get enough light in the vacuum ultraviolet region [ 2 - 9 ] . Since a larger time constant results in a smaller bandwith, time constants of 10 to 30 s are commonly used. This necessitates a slow scan rate for the CD spectrum.
4. Advantages of synchrotron radiation for circular dichroism SR from modern electron storage tings is highly linearly polarized and more intense than conventional vacuum ultraviolet continuum sources. These properties make SR ideal for CD measurements because of the better s/n resulting from the higher light flux. The other advantages of SR arise from the way in which circularly polarized light is produced. Experimentally, circularly polarized light has been obtained by the use of a linear polarizer followed by a 1/4 X retarder. The 1/4 X retarder has its fast axis 45 ° to the plane of polarization. Depending on whether the fast axis is at plus or minus 45 ° to the polarization of the light, one obtains right or left circularly polarized light. Since SR is linearly polarized, a polarizer is not necessary. This has two advantages. First, use of a linear polarizer diminishes the light intensity. Therefore, the s/n with SR is better because no polarizer is necessary. In addition, it is the magnesium fluoride polarizer which has limited the range of circular dichroism measurements to 1350A. Without the
P.A. Snyder, E.M. Rowe / Synchrotron radiation in UVCD measurements
polarizer it should be possible to make CD measurements to the limit of the calcium fluoride t/4 X retarder (~1250 A). Use of a lithium fluoride 1/4 X retarder would allow measurements to 1150 )I,.
347
Signal Detected by PM Tube (for a sample with a negative CD) ~ -r'
I/4 X plate gives left-handed light
~ .
V4 X plate ~ /
/
~ !sotropic /
t.c L ,-X- y - 5. Experimental setup
h
""@/4 X plate gives right-handed light
I
CD is a modulation spectroscopy. The 1/4 X retarder is modulated by squeezing and stretching such that the fast axis is alternatively at +45 ° and - 4 5 ° to the linearly polarized light at a frequency of 50 kHz. Thus the sample sees an alternation of left and right circularly polarized light. At a constant wavelength with an optically active sample in the beam, the photomultiplier sees an ac signal superimposed on a dc signal. See fig. 3. The sample in fig. 3 absorbs less right circularly polarized light than left. The dc is kept constant by a feedback system and the sign and magnitude of the ac signal is detected by a lock-in amplifier. Since CD = K IAC/IDc, where K is a constant, and IDC is kept constant, measurement of IAc and calibration yield the CD. Figure 4 shows a schematic of the instrument. After the plane polarized light leaves the monochromator, it passes through the modulated 1/4 ~k retarder and is circularly polarized. The light passes through the sample cell and hits a fluorescent screen of sodium salicylate. The visible light is then split and measured by two photomultiplier tubes. One photo-
//
/
\
] oc
-l-
m
X =const TAc CD ~ - Toc Fig. 3. The signal as seen by the photomuliplier tube.
multiplier measures the light intensity to determine the absorption. The other photomultiplier is connected to the feedback system and the lock-in amplifier for CD determination. The output of both photomultiplier tubes is recorded simultaneously on a two channel recorder. The design is similar to previous instruments [ 2 9]. The main differences are that no polarizer is necessary and the presence of the beam splitter which allows simultaneous measurement of CD and absorption. Also, in addition to the recorder, there is a data acquisition system.
",x
..
.......
"A%t.2Z D&-r A
Ad.@Ol'%l T I O N [
1 ~0. ,"01.,.4 ~ 0 "f" N 12. J
Fig. 4. A schematic of the instrument. VII. DETECTORS AND OTHER SUBJECTS
348
P . A . S n y d e r , E . M , R o w e / S y n c h r o t r o n radiation in U V C D m e a s u r e m e n t s
6. Results Figure 5 shows the spectrum of (+)-3-methylcyc!opentanone in the vapor phase with and without SR between 2050 A and about 1600 A. The better resolution with SR radiation results in observation of more peaks and also dramatic peak height changes. The first CD band doubles in height. The spectral bandwidth with SR is 2 A. The time constant was 4 s for the measurements with SR. Fig. 6 shows the spectrum of (+)-3-methylcyclopentanone from 1600 to 1325 A, which is an extension of the CD spectrum to a higher energy than previous measurements. Also, since the resolution is better (6 A), more peaks are visible. Figure 7 shows a portion of the spectrum of ( - ) ~-pinene with and without SR. With SR more vibrational structure is clearly evident.
'
I
'
I
'
I
I
'
T.ouT AA SYNCHROTRON IIIIRAD/ATION~ ~ ~
8 4 A~ 0
-8
2000
0 --4 I
1700
i
I
I
1600
I
1500
I
J
:
1400
XA
Fig. 6. Circular dichroism spectrum of (+)-3-methylcyclopentanone in the vapor phase (1600-1325 A).
7. Future developments The same properties which make SR ideal for vacuum ultraviolet CD measurements make SR ideal for vacuum ultraviolet magnetic circular dichroism (MCD) measurements. In addition, the collimation of SR means there is plenty of room for a magnet. Since all molecules have MCD, the better resolution will be particularly useful for MCD measurements of small molecules which usually have considerable structure in their absorption spectra. CD measurements could be extended to higher energies by using: (a) a lithium fluoride quarter-wave retarder would allow measurements to ] 150 A. (b)
T
-4
Ae 4
p900 r800 x (A)
I WITH
i700
p600
~]
Wavelength A 1800 1600 I i
'
Without I0 ~- Synchrotron Radiation 86(-)-a-pinene 4-2-0 I0
-16L
8!-20 -
6-
-24~
4-
With Synchrotron Radiation
I
-28-
,
2000
i
1900
O-1800 x (A)
i 1700
1600
Fig. 5. Circular dichroism spectrum of (+)-3-methylcyclopentanone in the vapor phase (2050-1600A) with and without [4] SR.
60 56 52 Frequency (cm-~x I0-3)
Fig. 7. Circular dichroism spectra of (-)-c~-pinene with and without [10] SR.
P.A. Snyder, E.M. Rowe / Synchrotron radiation in UVCD measurements
An appropriate reflection in place of the quarter-wave retarder. (c) Off axis light could be used. (SR is polarized. The light with the electric vector parallel to the electron orbit is labelled I//. If the observation angle relative to the plane of the electron orbit is zero, the light is 100% plane polarized (I//). As the observation angle becomes greater, the light contains more perpendicularly polarized light (/1). Therefore, the light goes from plane polarized to eliptically polarized, to circularly polarized as the observation angle increases from zero.) (d) If and when they become available, a helical wiggler could be used. (A wiggler is a magnet that makes electrons wiggle transverse to their normal path and if it is appropriately designed, circularly polarized light results.) Acknowledgement is made to the National Science Foundation (Grant No. DMR 77-21888), to the donors of the Petroleum Research Fund, administered by the American Chemical Society (Grant No. PRF-9238-66), to the Division of Sponsored Research, Florida Atlantic University, and to the Camille and Henry Dreyfus Foundation Program for
.349
Innovation in Chemistry Education Grant to the University of Florida and Florida Atlantic University Cooperative PhD program for support o f this research.
References [1 ] L. Velluz, M. Legrand and M. Grosjean, Optical circular dichroism, (Weinheim Verlag Chemie, (1965). [2] S. Feinleib and F.A. Bovey, Chem. Commun. 1968 (1968) 978. [ 3 ] O. Schnepp, S. Allen and E.F. Pearson, Rev. Sci. lnstrum. 41 (1970) 1136. [4] W.C. Johnson, Jr., Rev. Sci. Instr. 42 (1971) 1283. [5] K.P. Gross and O. Schnepp, Rev. Sci. Instr. 48 (1977) 362. [6] E.S. Pysh, Ann. Rev. Biophys. Bioeng. 5 (1976) 63. [7] S. Brahms, J. Brahms, G. Spach and A. Brack, Proc. Natl. Acad. Sci. 74 (1977) 3208. [8] A. Gedanken and M. Levy, Rev. Sci. Instr. 48 (1977) 1661. [9] A.F. Drake and S.F. Mason, Tetrahedron 33 (1977) 937. [10] P.A. Snyder, 5th Int. Conf. on Vacuum ultraviolet radiation physics, Vol. Ill (CNRS Meudon, France, 1977) p. 97.
VII. DETECTORS AND OTHER SUBJECTS