1. Charge-transfer effects in Raman Scattering of Individual Molecules
Gilad Haran
Chemical Physics Department
Weizmann Institute of Science
FRISNO, EIN-BOKEK, February 2004

2. Surface-Enhanced Raman Scattering
Electromagnetic Enhancement
on a nanosphere
metal dielectric function
medium dielectric function

3. The ‘Chemical’ (Charge Transfer) Mechanism
A new charge transfer band is formed when a molecule is adsorbed on a metal surface
Avouris and Demuth., 1981
Molecular levels
Vacuum
level EF
HOMO
LUMO
Metal levels 

4. Substrates supporting
Single-molecule SERS
100
200 nm
Colloids
Silver islands

5. Electromagnetic Enhancement
The local field can be huge!
From Xu et al., PRE 2000
-local, incident field
G=1012
G=1011

6. Exploring smSERS in dimers
Oligo-thiophene
10 – 50 nm
POSTER BY TALI DADOSH, Tuesday

7. SERS of Rhodamine 6G Hildebrandt and Stockburger, 1984
Very large cross-section
Involvement of halide ions
Frequency (cm-1)
Weiss & Haran, JPC B (2001) 105, 12348

8. Single-molecule Raman spectrometer
532 nm laser
Spectrograph+CCD camera
microscope
scanning stage

9. SERS spectrum of a single molecule
Frequency (cm-1)

10. Fluctuations in total intensity of
a series of molecules
Intensity scale
Time (seconds)

11. Fluctuations in total intensity

12. SERS spectrum of a single molecule
Frequency (cm-1)

13. Spectral fluctuations in one molecule
Time (seconds)
Intensity scale
Raman shift (cm-1)

14. Similar behavior seen in crystal violet molecules

15. The EM selection rule
E>>E
E E
How many equilibrium orientations? ~1-2
But in R6G- semi-continuous fluctuations!
Also – no correlation between different parts of spectrum

16. Resonance Raman-Charge Transfer
Resonance Raman transition within this band is responsible for surface enhancement (RR-CT). s0 s1
Pyridine on electrodes, Arenas et al., 1996

17. 774 cm-1 614 cm-1 1650 cm-1 C-C stretches (A term Raman scattering?)
Frequency (cm-1)
C-C stretches (A term Raman scattering?)
Bend vibrations

18. x Polarized Raman measurements parallel perpendicular
Raman scattered light
parallel
polarizing prism x
perpendicular
POSTER BY TIMUR SHEGAI, Monday

19. Probing the Raman Scattering Tensor
In resonance-enhanced scattering involving a
non-degenerate electronic excitation – a single-element tensor

21. Angular dependence of 

22. Distribution of 0 The low-frequency bands have a different tensor
than that of high-frequency bands

23. Hildebrandt & Stockburger, 1984
A CT band in R6G?
773 cm-1
Hildebrandt & Stockburger, 1984

24. On resonance:  Metal levels Molecular levels LUMO Vacuum level EF
HOMO

25. Smoluchowski’s smoothing effect
Wandelt, 1987
The local work function can vary along the surface.
Methods to measure:
Photoemission of adsorbed xenon (PAX)
STM

26. Possible causes for local work function changes at an adsorbed molecule
Motion of silver adatoms / surface features
Diffusion of the adsorbed molecule

27. Slowing down of fluctuations in glycerol-
a viscosity effect
Haran, Israel J. Chem. 2004

28. Laser power effect on whole-spectrum correlation functions

29. Dependence of correlation times on laser power

30. Are we heating the system (colloid + molecule)?
water
Q - amount of heat/unit time
- density of silver
c – specific heat of silver
 - heat diffusivity in water
Assuming: illumination intensity 100W/cm2
absorption cross section cm2

31. Possible effect of EM field on the adatom diffusion constant?
Ds~10 Å2/sec
Depends exponentially on electrode potential
A linear dependence expected for oscillating fields
From Hirai et al., Appl. Surf. Sci. 1998

32. Possible role for surface roughness relaxation?
The relaxation time depends on surface tension and surface diffusion
Can  or DS can depend on the electromagnetic field?
PROBABLY NOT!
- surface tension
DS- diffusion coefficient
Lukatsky, Haran & Safran, PRE (2003) 67,

33. Photodissociation can lead to sampling of different surface areas
CT!

34. Quantifying fluctuations by using
ratios between Raman band intensities
I614cm-1/ I1650cm-1

35. Distribution of ratio values
R=I614cm-1/I1650cm-1

36. Probability function for local work function fluctuations

37. Distribution of ratio values
R=I614cm-1/I1650cm-1
Assuming
Haran, Israel J. Chem. 2004

38. Conclusions
SERS fluctuations are due to modulation of charge transfer.
This modulation is due to lateral motion of molecules and sampling of different local work functions.
Lateral diffusion is facilitated by light.
Analysis of spectral fluctuations leads to better understanding of molecule-surface interactions involved in SERS.

39. Thanks to:
Amir Weiss
Timur Shegai
Yamit Sharaabi
Dima Lukatsky
Sam Safran
Tali Dadosh
Paulina Płochocka
Israel Bar-Joseph

40. Timur
Yamit