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

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

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 

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

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

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

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

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

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

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

Fluctuations in total intensity

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

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

Similar behavior seen in crystal violet molecules

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

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

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

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

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

Angular dependence of 

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

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

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

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

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

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

Laser power effect on whole-spectrum correlation functions

Dependence of correlation times on laser power

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 10-10 cm2

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

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

Photodissociation can lead to sampling of different surface areas CT!

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

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

Probability function for local work function fluctuations

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

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.

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

Timur Yamit