Kinetic Energy Dependence of Spin Filtering of Electrons Transmitted

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Kinetic Energy Dependence of Spin Filtering of Electrons Transmitted Through Organized Layers of DNA Richard A. Rosenberg, Joshua M Symonds, Vijayalakshmi Kalyanaraman, Tal Markus, Thomas Michael Orlando, Ron Naaman, Ernesto A Medina, Floralba A López, and Vladimiro Mujica J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp402387y • Publication Date (Web): 03 Jul 2013 Downloaded from http://pubs.acs.org on July 9, 2013

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The Journal of Physical Chemistry

Submitted to Journal of Physical Chemistry C

Kinetic energy dependence of spin filtering of electrons transmitted through organized layers of DNA Richard A. Rosenberg1*, Joshua M. Symonds2, Vijayalakshmi Kalyanaraman1, Tal Markus3, Thomas M. Orlando2, Ron Naaman3, Ernesto A. Medina4 Floralba A. López5, and Vladimiro Mujica6,7,8 1

Advanced Photon Source, Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439

2

School of Chemistry and Biochemistry and School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332 3

Department of Chemical Physics, Weizmann Institute, Rehovot 76100, Israel

4

Laboratorio de Física Estadística de Sistemas Desordenados, Centro de Fisica, IVIC, Apartado 21827, Caracas 1020A, Venezuela. 5

Quimicofísica de Fluidos y Fenómenos Interfaciales (QUIFFIS), Departamento de Quımica, Universidad de los Andes, Mérida 5101, Venezuela.

6

Department of Chemistry, Northwestern University, 2148 Sheridan Rd. Evanston, IL 60208, USA

7

Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85287 USA. 8

Center for Nanoscale Materials, Argonne National Laboratory, Argonne IL 60439, U.S.A. *Corresponding author: email [email protected], phone: 630-252-6112

ACS Paragon Plus Environment


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We present an experimental and theoretical study of the kinetic energy dependence of spin filtering of electrons by organized layers of DNA adsorbed on a gold substrate. When Au 4f7/2,5/2 levels are ionized by circularly polarized X-rays, the emitted electrons will be spin polarized. The spin distribution depends on the particular sub level and is opposite for right versus left circularly polarized light. If the DNA overlayer preferentially attenuates one spin over another, then there should be a circular dichroism (CD) in the X-ray photoelectron spectroscopy (XPS) spectra observed with the different polarizations. Using synchrotron radiation excitation, XPS CD measurements were made of electrons with kinetic energies in the range 30 to 760 eV. In all cases there was no evidence of any significant dichroism. These results are explained by a model in which the longitudinal polarization is strongly dependent on the k-vector, and hence the energy or the de Broglie wavelength which are simply connected to the magnitude of this vector of the incoming electrons. For a helix with a fixed number of turns, this dependence is due to a coherent process associated with multiple scattering. This model predicts that there is a window of energies where changes in the polarization should be expected. Two competing effects determine this window: The energy has to be small enough to allow for at least double scattering, but large enough so that the de Broglie wavelength probes the chiral structure. Also at very low energies the spin-orbit interaction weakens and no polarization results. Keywords: spin polarized electrons, chiral molecule, x-ray photoelectron spectroscopy, circular dichroism, helical, scattering.


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1. Introduction It is widely believed that external delivery of pre-biotic material formed in interstellar molecular clouds may have been an important factor in the genesis of life. If an interstellar chemical process is viable it must be demonstrated that the requisite molecules can be formed and, more importantly, chirality can be achieved, which is essential for the origin of life as we know it. It has been conjectured that chiral selective chemistry induced by spin polarized electrons interacting with a racemic mixture of chiral molecules could result in an enatiomeric excess of one through selective destruction of the other. Research in this area has been reviewed recently.1 Early research was primarily directed at studying the interaction of high energy, spin-polarized electrons produced by nuclear β decay or accelerators. Demonstration of chiral selective reactions was generally inconclusive. Recently it has been shown that low energy (0-10 eV) spin-polarized, secondary electrons, produced by irradiation of a magnetic substrate, can show significant chiral selectivity.2 This was achieved by determining the reaction rate for a model chiral molecule, R- and S-2-Butanol, adsorbed on a magnetic substrate using X-ray photoelectron spectroscopy (XPS). Cross sections for dissociation of the chiral carbon were determined using time-dependent XPS. It was found that the dissociation cross section for a given chirality was dependent on the substrate magnetization direction, which, in turn, determines the sign of the spin polarization. Naaman and coworkers have shown that there is a strong asymmetry in the transmission of low energy (0.5-2 eV) spin-polarized electrons through organized chiral overlayers.3,4 In particular they found that spin filtering effects as high as 60% could be



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observed in organized DNA films. The effect could be unambiguously attributed to electron transmission through the chiral monolayers and not to the initial polarization of the photo-emitted electrons, since it was not dependent on the helicity of the photon source. The selectivity increased as the length of the DNA increased and was dependent on the purity of the film. A few theoretical models have been proposed to explain this behavior5-9 by computing transport through helices with discrete scattering units, spin-orbit coupling and a scalar potential. Although all these models predict large polarizations (comparable to experimental measurements) on the basis of the meV scale spin-orbit interaction, none of them have addressed the higher kinetic energy electrons studied in the present work. References 6 and 7 display a spin asymmetry and oscillatory behavior in a very low energy regime (tens of meV) and do not explore energies beyond a few eV. Reference 8 predicts significant asymmetries in the 0.1 to 2 eV region. On the other hand in ref. 5, Cahen, Naaman and Vager predict the existence of extremely high electric fields (~106 V/cm) due to charge transfer in adsorbed polypeptides on the surface. If one were to invoke such fields to induce spin polarization through spin-orbit interaction, the expectation is for stronger polarization effects for energies an order of magnitude larger than those currently explored in experiments, due to the kinetic energy dependence of the spin-orbit coupling. Thus, experimental verification of the kinetic energy dependence of spin-dependent scattering can be used as a benchmark for the various models.


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Our theoretical model8,9 was the first to explicitly include the spin-orbit interaction as an essential ingredient to explain the spin polarization behavior observed in the experiments. We have argued that an enhanced spin-orbit interaction, whose effective magnitude can be as much as six orders of magnitude larger that the bare spin-orbit coupling in the Dirac equation, arises from the high density of the monolayers which in turn induces substantial orbital penetration into the nuclear region. This enhanced effective coupling between the momentum of the scattered electron and its spin, together with the chiral nature of the molecules in the monolayer modifies both the longitudinal and the transverse components of the spin polarization. In our model the longitudinal polarization is strongly dependent on the k-vector, and hence the energy or the de Broglie wavelength are simply connected to the magnitude of this vector of the incoming electrons. For a helix with a fixed number of turns, this dependence is due to a coherent process associated with multiple scattering. In fact, our model predicts that there would be a window of energies where changes in the polarization should be expected. Two competing effects determine this window: the de Broglie wavelength must be small enough to be comparable to the spatial extension of the helix, and it must be large enough so that the residence time of the electron in the scattering region allows for at least double scattering to take place. These constraints provide a framework to understand the absence of spin polarization for high energies. In fact, we will show below that the longitudinal component of the polarization, the most important component from the point of view of the experiments, vanishes for large values of the magnitude of the k-vector.



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In the present paper we present an investigation of the interaction of higher energy (>30 eV) spin polarized electrons with chiral overlayers, an energy regime that has hitherto been unexplored, either experimentally or theoretically. Our approach is to examine circular dichroism in the X-ray photoelectron spectra of spin-orbit split core levels of the substrate. High degrees of spin polarization have been previously reported for W 4f and Au 4f levels.10,11 The handedness of the polarization depends on the helicity of the X-rays. If there is an asymmetry in the scattering of the electrons by the chiral overlayer, then this should be reflected in the XPS circular dichroism (XPS CD) obtained by the difference of spectra acquired using right and left circularly polarized light, in a analogous fashion to the results of Naaman and coworkers using low energy (6.4 eV) laser light.4 Measurements were performed with higher energy X-rays at the Advanced Photon Source (APS) and lower energy X-rays at the Synchrotron Radiation Center (SRC). As shown in Fig. 1 this allowed us to study electron kinetic energy regimes in the 410 to 760 eV range (APS), with short de Broglie wavelength and relatively long inelastic mean free paths and in the 30 to 50 eV range (SRC), with an intermediate de Broglie wavelength and a minimum inelastic mean free path. Note that if incoherent, inelastic scattering were important the XPS CD effect should be strong in the SRC measurements. Furthermore the spin polarization of the 4f electrons is largest for kinetic energies greater than ~80 eV,10 which should lead to enhanced XPS CD for higher energy electrons. Finally, note that significant spin filtering effects resulting from spin-dependent scattering in ferromagnetic thin film overlayers have been observed for electrons with kinetic energies as high as 720 eV.12-14


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The system we investigate is a self-assembled monolayer of double-stranded DNA adsorbed on a Au substrate. By changing the X-ray energy (and hence the electron kinetic energy) we can determine the effect of the electron momentum on the scattering process.

Figure 1. Calculated inelastic mean free path15 (dashed line) and de Broglie wavelength (solid line) for electrons with kinetic energies in the range 0.5 to 800 eV. The vertical bars show the range of kinetic energies used in the APS and SRC. experiments as well as previously reported laser studies.3,4 2. Experimental The samples were prepared at the Weizmann Institute according to a method described previously.3,4 They consisted of 40 base pairs of 3’ thiolated DNA on a clean 200 nm thick polycrystalline gold film on a Si substrate. They were shipped in sealed vials under a nitrogen atmosphere to laboratories in the United States and not opened until shortly before they were loaded into a load lock chamber, which was immediately evacuated.



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The measurements were performed at two different synchrotron radiation sources. Experiments utilizing X-rays in the range 500 to 850 eV were performed on beamline 4-ID-C at the APS, which utilizes an electromagnetic undulator that produces circularly polarized (~98%) X-rays. The polarization can be switched at speeds up to 1 Hz. XPS data were measured using a CLAM 2 electron energy analyzer. The angle (Φ) between the incoming X-ray beam and the analyzer axis was 35 degrees. XPS CD spectra were acquired by either switching the polarization at each point or by taking two separate scans with opposite polarization. Measurements performed with lower energy X-rays (120 to 140 eV) were carried out on the U9 APPLE PGM beamline (circular polarization ~80%) at the SRC using a Scienta SES200U electron energy analyzer with Φ set at 50 degrees. Data were acquired by taking two separate scans with opposite polarization. To minimize possible effects of radiation damage, each 2 to 5 min. scan was acquired at a fresh spot on the sample. Measurements performed at APS indicate that the time constant for radiation damage is ~30 minutes for power densities of ~80 mW/cm2 (950 eV). The APS data presented here were all obtained at lower photon energies where the power density was at least a factor of two lower. At SRC the power density was ~70 mW/cm2, so that during the typical 4 minutes it took to acquire a spectrum, ~10 % of the irradiated molecules may have undergone photolysis. All measurements were performed at room temperature. The experimental geometry is shown in Fig. 2.


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Figure 2. Schematic diagram of the experimental setup showing the geometry employed.

Characterization of the samples was performed at both APS and SRC. At APS measurements were made of the C 1s, N 1s, O 1s XPS spectra as well as O K X-ray absorption spectroscopy (XAS) on beamline 4-ID-C. XAS data were obtained by measuring the total-electron yield from the sample as a function of photon energy. At SRC the characterization was carried out on the HERMON beamline, which allowed access to higher energies than the U9 APPLE PGM beamline. Polarization dependent N K-edge XAS obtained using linearly polarized light are shown in Fig. 3. The two sharp peaks at 399 and 401 eV are due to N 1s excitation to π* orbitals which are localized on the base pairs, while the broad peak at 407 eV is due to excitation to σ* orbitals. As the angle between the X-ray beam and the sample normal increases, the E vector becomes more aligned with the π* orbitals for the roughly vertically protruding DNA. This enhances the N 1s π* intensity as has been observed in previous



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studies.16-18 The C 1s XPS spectrum, shown in the insert, is also similar to previously reported results.19,20

Figure 3. Polarization dependent X-ray absorption measurements at the N K edge on double stranded DNA absorbed on gold. As the angle decreases the Poynting vector of the X-rays becomes more aligned with the π* orbitals of the nitrogen atoms in the base pairs, leading to increased intensity of the two sharp, lower energy peaks. The inset shows a C 1s XPS spectrum taken at a photon energy of 394 eV.

3. Theoretical Approach We follow closely our previous work8,9 on molecular spin polarization and filtering. We consider that the Hamiltonian of the system is given by H=T+V

(1)

Where T is the kinetic energy operator and V is the scattering potential.


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The elastic scattering of spin-1/2 particles by a potential requires the consideration of the spin density matrix, which can be written in a compact way in terms of two spin functions

( + , − ), corresponding to the two spin projections along a given

space axis.  ρ+ +  ρ− +

ρ=

ρ+ − ρ+ +

  

(2)

The matrix elements are functions of all the variables characterizing the scattering problem, including the wave vector corresponding to the incoming and outgoing states, k a and kb and the scattering potential V in equation (1). The scattering amplitude is defined by

fν ' ν = −

µ k aν ' t k bν 2π h2

(3)

where µ is the reduced mass of the electron and t is the scattering operator given by the Lippmann-Schwinger equation t = V + VGV

(4)

and G is the Green function corresponding to the Hamiltonian in (1) G(z) = (z − H )−1

(5)

{

The spin polarization P is defined in terms of the Pauli matrices σ = σ x , σ y , σ z as


}


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P = Tr (ρσ )

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(6)

The density matrices corresponding to the incoming and outgoing electron beams, ρ and ρ ' are related by

ρ ' = fρ f †

(7)

It is useful to rewrite the full scattering amplitude in terms of the Pauli matrices

f(k a ,k b ) = −

µ  h 0 (k a ,k b )1+h1 (k a ,k b )σ x + h 2 (k a ,k b )σ y +h 3 (k a ,k b )σ z  2π 2 

(8)

where the functions hi (k a ,k b ) , which contain all the information about the scattering event, are to be determined. The scattered polarization component

when the incident state is a completely

unpolarized state ρ = (1 / 2 ) 1 can be computed from the output density matrix through the relation

P' =

Tr (σρ ') Tr ρ

(9)

where we normalize to the input intensity. In terms of the expansion in Eq. 8, the polarization can then be written as

( ( ) (

Pl = 2 ℜ hl ho† + ℜ ihm hn†

))

(10)

where the indices (l, m, n) are taken in cyclic order and ℜ denotes the real part of the argument. As described previously8,9 we consider a potential that includes both a purely spatial component and a spin-orbit term


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V ( r, σ ) = V ( r ) 1 + ασ ⋅  ∇V ( r ) ∧ p 

(11)

In the case of a molecule, the spatial component in (11) is essentially the molecular electrostatic potential that could be calculated directly from the quantum electronic density; p is the electron’s momentum, and α = −h / ( 2me ) is the bare Dirac-Pauli spin2

orbit coupling. If the spin-orbit component is absent, the only term present in the scattering amplitude expression is the one proportional to the unit matrix in equation (8); hence there is no relevant change in the polarization as a result of the scattering process. We immediately note in equation (10) a central feature of polarization effects in electron scattering: it is an interference effect between the function related to spin- orbit interaction

and the spin-independent term

. One needs the presence of both

to observe changes in the polarization. A short discussion on the norm of the polarization is in order. The regular definition of polarization normalizes to the output intensity, and determines the fraction of spin alignment referred to that intensity, so the total spin polarized current will not be assessed. In order to obtain the absolute spin polarized current we must refer to the incident intensity. This is what is closest to the quantity measured in the experiments. 4. Results Some typical Au 4f7/2,5/2 XPS CD results obtained at the APS are shown in Fig. 4. Data obtained at 500 eV and emission angles of 0 and 45 degrees are shown in (a) and (b), respectively, and results obtained at 650 and 850 eV are shown in (c) and (d). These data span the range of ~410 to 760 eV kinetic energy. Data were analyzed by two methods. In the first method, the individual LCP and RCP spectra were normalized



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to the incident intensity as monitored by the photocurrent from a gold grid inserted into the X-ray beam, then the normalized spectra were subtracted from one another.

Figure 4. Four Au 4f XPS CD spectra taken at the APS at different X-ray energies and/or electron emission angles, θ (Fig. 2).

However, due to variations in the baseline, this method sometimes resulted in artifacts where the difference spectrum showed both 4f7/2,5/2 peaks to be slightly positive or negative, but this was not reproducible. In the second approach, spectra obtained with LCP and RCP X-rays were scaled so that their intensities ranged between 0 and 1, then their difference was taken. This method of analysis essentially scales to the height of the Au 4f7/2 peak, so that dichroism effects will only be observed in the 4f5/2 peak. Although this approach is somewhat artificial, it avoids errors that result from improper normalization and baseline correction. For all these results, as well as numerous others, no significant dichroism could be observed. The results were the same whether the data was collected by taking LCP and RCP spectra consecutively or by changing


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the polarization at each data point. XPS CD measurements were also made of electrons emitted from Au 4d5/2,3/2 levels. Results are shown in Fig. 5. Again, no dichroism could be observed.

Figure 5. Two Au 4d XPS CD spectra taken at the APS at different X-ray energies and electron emission angles, θ (Fig. 1).

At the SRC, numerous CD measurements were made at photon energies of 120, 125, 130, 135, and 140 eV. Some typical results are shown in Fig. 6. The 135 eV spectra in (a) are the average of 8 separate CD measurements, while the 120 eV data in (b) are the average of 3 CD measurements. Again, no meaningful dichroism could be observed.

Figure 6. Two Au 4f XPS CD spectra taken at the SRC at different X-ray energies.



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In order to get a grasp of the limits of dichroism detection, we have simulated the expected results for various degrees of attenuation of polarized Au 4f7/2,5/2 electrons using previously published, spin-polarized XPS data.11 A typical experimental spectrum was manipulated so as to construct the reported distribution of up and down spinpolarized electrons. The results are shown in Fig. 7(a).

Figure 7. (a) Simulated spin-resolved Au 4f XPS spectrum for LCP X-rays using experimental data manipulated to yield previously published results.11 For RCP X-rays the spectra would have the opposite spin distribution. (b-f) Simulated Au 4f CD spectra using the results from (a) and attenuating the spin-down electrons by the amount indicated.


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Next, the individual spin polarized components were attenuated by varying degrees, which depended on the X-ray helicity, and the expected Au 4f spectra and their dichroism, as observed with a non spin-resolved detector, was constructed. Results of this procedure are shown in Figs. 7(b-f). Based on these model results, the limits of detectable attenuation should be 1-2 % for data with a high signal to noise ratio (SNR) (Figs. 4(c,d), Fig. 6(a)), 3-4 % for moderate SNR (Figs. 4(a,b)) and ~5 % for lower SNR data (Figs. 5(a,b), Fig. 6(b)). 5. Discussion Naaman and coworkers have found significant spin filtering effects for low energy (~1 eV) electrons using both spin integrated4 and spin-resolved3 detection schemes. Spin selectivity ranging from 30% to 60% was reported. If such large attenuations were effective at the higher kinetic energies studies here, they would definitely be observable (Fig. 7(b)). Since the samples used in the present work and those used in the laser studies were produced in the same laboratory, we are led to conclude that the energy or wavelength of the electron is an important factor. We are unaware of any previous reports of spin filtering effects of DNA overlayers at high kinetic energies. There has been one report of asymmetric photoelectron transmission through chirally-sculpted, polycrystalline gold.21 Using 690 eV LCP and RCP X-rays, MacLaren and coworkers observed strong dichroism in the orbital angular momentum leading to attenuation of both the Au 4f7/2 and Au 4f5/2 peaks. As mentioned previously, we occasionally observed very weak effects of this nature, but



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these were not reproducible and believed to arise from problems stemming from normalization, baseline correction, and/or radiation damage of the DNA (for longer scans). Incoherent inelastic scattering of the electrons could play a role in the dichroism. The measurements at APS were performed with energies in the range 410 to 760 eV whose corresponding inelastic mean free path15 (IMFP) is 1.1 to 1.7 nm (Fig. 1). At SRC the electron kinetic energies were near the minimum IMFP, 0.6 to 0.9 nm (Fig. 1), while the laser studies were done with very low kinetic energies, which should have a very long IMFP. Thus, if incoherent scattering played a role, the effects should be strongest for data acquired at the SRC, which is not the case. Also note that the spindependent inelastic mean free path for scattering by a magnetic film is at a minimum (0.2 to 0.5 nm) near 50 eV kinetic energy, and around 1.5 nm for 720 eV electrons.12-14 Therefore, if purely magnetic interactions dominated we might expect to observe XPS CD in these measurements. The present results indicate that spin-dependent scattering in organized DNA is inefficient at high kinetic energies, but strong at lower kinetic energies. For the range of kinetic energies encountered in the APS experiments the de Broglie wavelength is less than 0.1 nm and for the SRC measurements it is 0.1 – 0.3 nm (Fig. 1). The laser experiments yielded electrons with wavelengths greater than 1 nm, so evidently the wave vector of the electron is an important consideration as discussed in the following section. 6. Comparison to theoretical model


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Fig. 8 presents the polarization perpendicular to the sample

, as a function of the

incident wave vector k integrated over the forward scattering directions (θ = (0, π/2) and φ = (0,2π) ). One can readily see that the relevant energy regime for chirality effects, in the double scattering approximation, is between zero and 30 eV (in a.u. in the figure), decaying very strongly beyond. Note that the energies distribution shown in the inset in Fig. 8 is higher than that calculated previously using a similar methodology.8

Figure 8. Predicted electron polarizations following scattering of electrons of a chiral overlayer using the model discussed in the text. The rectangles designate the range of kinetic energies studied at APS, SRC in this work and the previous laser-based experiments.3,4

The energy dependence arises from the relation between wavelength and molecular dimensions. In the current work the molecule has more physical dimensions (those of DNA), which also adds another peak at k = 1.1 (inset of Fig. 8) in the energy dependence. Since longitudinal polarization requires at least double scattering, the electrons must be deflected at large angles for intermediate dispersion events.



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Nevertheless, large deflections can only occur with the denominator handicap ∝ |ka −kb|3 the momentum transfer, derived from the Fourier transform of the Hartree-Fock atomic potential seen by the scattering electron.7 If k is increased by a factor of two, the spin-orbit interaction effects can be reduced by an order of magnitude, concomitantly reducing longitudinal polarization. There is a corresponding effect at low k vectors due to large wavelengths not resolving the chiral structure and the reduction of spin-orbit effects. So there will be a window in energies where longitudinal polarization is observed. The theory ideally works well when the wavelength of the electrons is smaller than the smallest distance between scattering events; this is well achieved for energies above 10 eV, for chiral molecules with interatomic spacing’s of the order of 3Å. For lower energies, the correct interferences are not well accounted for and multiple scattering events are increasingly important. Thus the double scattering approximation, presented here, only gives the right trend but, quantitatively, the polarizations will be lower than those observed experimentally.3 It is important to mention that despite the fact that our theoretical model is not valid for de Broglie wavelengths

larger than the interatomic distance, it

reproduces qualitatively the behavior in this range, even though it results in polarizations smaller than the experimental ones. However, in assessing the importance of this limit one should bear in mind that the effect is observed for spatially extended chirality, as opposed to a single chiral center. On the other hand, our theory predicts remarkably well the strong quenching of the polarization effect for large k

observed experimentally even though we are limiting ourselves to the consideration of second order scattering events. Note that higher order scattering


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involves high momentum transfer from collision to collision; this is only possible for elastic scattering at low energies, where the interaction is strongest. Each single scattering order brings in an extra inverse power of the momentum transfer for the scattering amplitude, so at higher energies, these momentum transfer denominators rapidly reduce the contribution from higher orders. On the other hand, double scattering events mix in transverse polarization as well (not measured to our knowledge and unrelated to the chiral structure) so full longitudinal polarization is never achieved.

7. Conclusions Using XPS CD measurements we have searched for spin filtering effects of organized DNA overlayers adsorbed on a gold substrate for electrons with kinetic energies in the range 30 to 760 eV. In all cases no significant spin polarization was observed. However, previous studies3,4 have observed significant spin filtering of low energy electrons by DNA. These results are consistent with a spin-dependent scattering theory in which changes in the longitudinal polarization require both a spatially extended chiral object and multiple scattering. This conditions determine the existence of an energy window, strongly dependent on the magnitude the k-vector, that explains the observed experimental results. Two competing effects determine this window: the de Broglie wavelength must be small enough to be comparable to the spatial extension of the helix and it must be large enough so that the residence time of the electron in the scattering region allows for at least double scattering to take place. According to this model, the strongest effects should be observed for electrons with k~0.5 (3-4 eV) (Fig. 8). There should be virtually no observable effects for the high



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energy electrons studied at APS and SRC. There may be some possibility of observing such effects in the lowest energies studied at SRC (Fig. 6(b)), but these measurements were complicated due to the presence of low energy secondary electrons, which resulted in a sloping background. The elastic and inelastic scattering of electrons with energy between 5 - 25 eV has been examined theoretically with a multiple scattering method and experimentally.22 The theory did not incorporate any polarization interactions. Both the theoretical and experimental results show that constructive interference could contribute to fragmentation of DNA via excitation of low-energy Feshbach resonances. These are spatially localized on the bases and water present in the major grooves. There are also shape-resonaces at energies below a few eV which could be excited by electrons, The present results show that spin dependent scattering is also most efficient at low energies where fragmentation via resonant excitation occurs.23,24 The polarity of the scattered electrons will depend on the helicity of the scattering molecules. Thus, the present results reinforce the role of low-energy, spin-polarized electrons in chiralspecific chemistry.1,2

Acknowledgements The work performed at the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-06CH11357. This work is based in part upon research conducted at the Synchrotron Radiation Center, University of Wisconsin-Madison, which


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is supported by the National Science Foundation under Award No. DMR-0537588. V.M. acknowledges the support of NSF through Award No. NSF CHE-1124895. TMO and JMS wish to acknowledge support from the U.S. Department of Energy, Office of Science, Contract No. DE-FG02-02ER15337. We would like to thank Nir Eliyahu for experimental support.



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TOC IMAGE


Kinetic Energy Dependence of Spin Filtering of Electrons Transmitted

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