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

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

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

4. Substrates supporting Single-molecule SERS 0100200 100 200 0 nm ColloidsSilver islands

5. Electromagnetic Enhancement The local field can be huge! From Xu et al., PRE 2000 -local, incident field G=10 12 G=10 11

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

7. Frequency (cm -1 ) SERS of Rhodamine 6G Hildebrandt and Stockburger, 1984 Very large cross-section Involvement of halide ions 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. Time (seconds) Intensity scale Fluctuations in total intensity of a series of molecules

11. Fluctuations in total intensity

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

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

14. Similar behavior seen in crystal violet molecules

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

16. Resonance Raman-Charge Transfer Pyridine on electrodes, Arenas et al., 1996 Resonance Raman transition within this band is responsible for surface enhancement (RR- CT). s0s0 s1s1

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

18. Polarized Raman measurements polarizing prism Raman scattered light parallel perpendicular x 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 773 cm -1 A CT band in R6G?

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

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

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)? R water Q - amount of heat/unit time  - density of silver c – specific heat of silver  - heat diffusivity in water Assuming: illumination intensity 100W/cm 2 absorption cross section 10 -10 cm 2

31. From Hirai et al., Appl. Surf. Sci. 1998 D s ~10 Å 2 /sec Depends exponentially on electrode potential A linear dependence expected for oscillating fields Possible effect of EM field on the adatom diffusion constant?

32. Possible role for surface roughness relaxation? The relaxation time depends on surface tension and surface diffusion Can  or D S can depend on the electromagnetic field? PROBABLY NOT!  - surface tension D S - diffusion coefficient Lukatsky, Haran & Safran, PRE (2003) 67, 062402

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

34. Quantifying fluctuations by using ratios between Raman band intensities I 614cm -1 / I 1650cm -1

35. Distribution of ratio values R=I 614cm -1 /I 1650cm -1

36. Probability function for local work function fluctuations

37. Distribution of ratio values R=I 614cm -1 /I 1650cm -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. Amir Weiss Timur Shegai Yamit Sharaabi Thanks to: Dima Lukatsky Sam Safran Tali Dadosh Paulina Płochocka Israel Bar-Joseph

40. Timur Yamit