Gilad Haran

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Charge-transfer effects in Raman Scattering of
Individual Molecules
Gilad Haran Chemical Physics Department Weizmann
Institute of Science
FRISNO, EIN-BOKEK, February 2004
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Surface-Enhanced Raman Scattering
Electromagnetic Enhancement
on a nanosphere
metal dielectric function
medium dielectric function
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The Chemical (Charge Transfer) Mechanism
A new charge transfer band is formed when a
molecule is adsorbed on a metal surface
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Substrates supporting Single-molecule SERS
Colloids
Silver islands
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Electromagnetic Enhancement
The local field can be huge!
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Exploring smSERS in dimers
Oligo-thiophene
POSTER BY TALI DADOSH, Tuesday
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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
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Single-molecule Raman spectrometer
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SERS spectrum of a single molecule
Frequency (cm-1)
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Fluctuations in total intensity of a series of
molecules
Intensity scale
Time (seconds)
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Fluctuations in total intensity
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SERS spectrum of a single molecule
Frequency (cm-1)
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Spectral fluctuations in one molecule
Time (seconds)
Intensity scale
Raman shift (cm-1)
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Similar behavior seen in crystal violet molecules
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The EM selection rule
E??gtgtE?
How many equilibrium orientations? 1-2
But in R6G- semi-continuous fluctuations! Also
no correlation between different parts of spectrum
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Resonance Raman-Charge Transfer
Resonance Raman transition within this band is
responsible for surface enhancement (RR-CT).
Pyridine on electrodes, Arenas et al., 1996
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C-C stretches (A term Raman scattering?)
Bend vibrations
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Polarized Raman measurements
Raman scattered light
parallel
polarizing prism
perpendicular
POSTER BY TIMUR SHEGAI, Monday
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Probing the Raman Scattering Tensor
In resonance-enhanced scattering involving a
non-degenerate electronic excitation a
single-element tensor
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Angular dependence of ?
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Distribution of ??0
The low-frequency bands have a different tensor
than that of high-frequency bands
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A CT band in R6G?
773 cm-1
Hildebrandt Stockburger, 1984
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Molecular levels
Metal levels
LUMO
Vacuum level
?
EF
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Smoluchowskis smoothing effect
The local work function can vary along the
surface.

• Methods to measure
• Photoemission of adsorbed xenon (PAX)
• STM

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Possible causes for local work function changes
at an adsorbed molecule

• Motion of silver adatoms / surface features

• Diffusion of the adsorbed molecule

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Slowing down of fluctuations in glycerol- a
viscosity effect
Haran, Israel J. Chem. 2004
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Laser power effect on whole-spectrum correlation
functions
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Dependence of correlation times on laser power
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Are we heating the system (colloid molecule)?
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Possible effect of EM field on the adatom
diffusion constant?

• Ds10 Å2/sec
• Depends exponentially on electrode potential
• A linear dependence expected for oscillating
  fields

From Hirai et al., Appl. Surf. Sci. 1998
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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, 062402
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Photodissociation can lead to sampling of
different surface areas
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Quantifying fluctuations by using ratios between
Raman band intensities
I614cm-1/ I1650cm-1
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Distribution of ratio values RI614cm-1/I1650cm-1
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Probability function for local work function
fluctuations
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Distribution of ratio values RI614cm-1/I1650cm-1
Haran, Israel J. Chem. 2004
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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.

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Thanks to
Amir Weiss Timur Shegai Yamit Sharaabi
Dima Lukatsky Sam Safran
Tali Dadosh Paulina Plochocka Israel Bar-Joseph
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