Circular Dichroism in VUV and Soft XRay Molecular Photoionization

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Circular Dichroism in VUV and Soft X-Ray
Molecular Photoionization

• Ivan Powis
• University of Nottingham

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Outline

• Chirality (Handedness)
• biological
• molecular
• Photoionization of Chiral Molecules
• Experimental Circular Dichroism Angular
  Distribution of randomly oriented chiral
  molecules
• Valence Shell Ionization Camphor
• C 1s core Ionization - Camphor
• Theoretical Understanding

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Leonardo da Vinci

• Leonardo had many talents artist, scientist,
  engineer, anatomist, architect..
• He made important studies of proportion and
  symmetry

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Left and Right Handedness
John the Baptist By Leonardo da Vinci
Right hand and left hand are non-superimposable
Right hand is a mirror image of left hand
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Chirality in Micro-Organisms
N. pachy - The most abundant planktonic
foraminifer (microscopic marine organism) at high
latitudes

• Neogloboquadrina pachyderma
• Coils left when sea temperature lt 8 C
• Coils right when sea temperature gt 8 C

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Chiral Molecules
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The Homo-Chirality of Life

• Amino Acids are the chiral molecular building
  blocks of life.
• Both L- and D- forms can exist, but only the
  L-form occurs naturally in living organisms

Alanine The smallest chiral Amino Acid
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• Discriminate chiral objects by interaction with
  chiral environment or other chiral objects
• Since life is chiral it frequently discriminates
  between chiral enantiomers
• Pharmaceuticals
• Odours

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Chiral Odour Molecules
See- http//www.leffingwell.com/chirality/chirali
ty.html
S-() carvone - caraway
R-(-) carvone - spearmint
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Circular Dichroism

• Difference in response to left-right circularly
  polarised radiation
• CD in total absorption
• Electric-magnetic dipole interaction
• Weak (lt0.01)
• CD in differential cross-section is more promising

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Photoelectron Angular Distributions
(also for achiral molecules)
p0 linear polarization p1 left/right
circularly polarized
1 I. Powis, J. Chem. Phys., 112, 301 (2000) I.
Powis, J. Phys. Chem. A104, 878 (2000).
D-Glyceraldehyde1
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Circular Dichroism in Angular Distribution of
photoelectrons
I. Powis, J. Chem. Phys., 112, 301 (2000).

• Electric dipole approximation
• P1 term arises from interference between l, l1
  partial waves

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Camphor

• Rigid bicyclic carbonyl
• HOMO is nO lone pair
• Solid at room temperature
• Heat to approx. 120 to obtain vapour pressure
• Problem of condensation !

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Ion Image

• Camphor
• (135 C)
• 2 bar Ar
• 50 ?m nozzle

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SU5 at Super-ACO
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SU5- Circularly Polarized VUV

• 6-40 eV
• Fully variable polarization
• Polarimeter
• Gas Filter

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Undulators for circularly polarized light

• A pair of horizontal and vertical magnetic arrays
  with a phase shift between the poles creates
  helical electron beam hence helical (circularly
  polarised radiation) is emitted

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The SU5 dedicated polarimeter
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Polarization on SU5
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Electron-Ion Coincidence Imaging (Valence
Ionization)
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Position Sensitive Delay Line Anode
NI Data Acq Board

• Spatial resolution better than 80 ?m as inferred
  from the mask

xxt1-xt2

• High linearity across the surface

yyt1-yt2
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Camphor Valence PES
E. E. Rennie, I. Powis, U. Hergenhahn, O.
Kugeler, G. Garcia, T. Lischke, and S. Marburger,
J. Elec. Spectrosc. Rel. Phen. 125, 197 (2002).
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(a)
(b)
(c)
y (mm)
x (mm)
x (mm)
x (mm)
Coincident ion corrected electron image
Ion image
Raw electron image
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Dichroism Images Camphor 9.2 eV
G. A. Garcia, L. Nahon, M. Lebech, J.-C. Houver,
D. Dowek, and I. Powis, J. Chem. Phys. 119 xyz
(2003)
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Camphor CDAD 9.2 eV
G. A. Garcia, L. Nahon, M. Lebech, J.-C. Houver,
D. Dowek, and I. Powis, J. Chem. Phys. 119 xyz
(2003)
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The new AR-imaging PEPICO DELICIOUS
Dichroism and ELectron Ion Coincidences
in IOnization Using Synchrotron
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Electron/Ion VMI Spectrometer
Repeller
Electrode de masse
PSD
Lentille de Einzel
Plaque déflectrice
Extracteur
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He ions _at_ 25 eV
Ar e- _at_ 16 eV
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hv 9.6 eV
hv 10.3 eV
R- Camphor
S-Camphor
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Camphor CDAD Images _at_10.3 eV
Polar Basex Inversion 3D
R-()
S-(-)
HOMO
HOMO-1
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Camphor Valence CDAD Experimental Results
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Camphor CO C1s PES
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CDAD in Core Ionisation of Camphor
Results obtained in the variable polarisation
UE56-PMG2 at BESSY II (Berlin)
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BESSY Electron Spectrometer
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Camphor Core CDAD
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Further Work

• This effect is not just a feature of the camphor
  molecule,
• it appears to be more general

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Conclusions

• (Anti-)symmetry of effect (change with helicity
  of light, molecule) as expected from pure
  electric dipole approx.
• This also precludes significant non-dipole
  contributions (different symmetry for E1-E2,
  E1-M1)
• Magnitude up to 12 asymmetry seen (cf
  absorption CD of lt 0.01)
• Final state scattering effect (since C 1s
  spherical orbital)
• Expect to be general phenomenon (eg carvone) and
  not associated with CO group
• Rapid polarization switching will improve S/N

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Acknowledgements

• L Nahon
• D Dowek
• J-C Houver
• M Lebech

• Gustavo Garcia
• C Harding

• U Hergenhahn
• E Rennie
• O Kugeler
• T Lischke

• EPSRC
• EU
• University of Nottingham
• The Royal Society
• INTAS