Charge-transfer effects in Raman Scattering of Individual Molecules

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 Metal levels Vacuum level Molecular levels LUMO EF HOMO Avouris and Demuth. , 1981

Substrates supporting Single-molecule SERS 200 100 200 0 nm Colloids Silver islands

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

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

SERS of Rhodamine 6 G 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)

Intensity scale Time (seconds) Spectral fluctuations in one molecule 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 R 6 G- 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 (RRCT). s 1 s 0 Pyridine on electrodes, 1996 Arenas et al. ,

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

Polarized Raman measurements Raman scattered light parallel polarizing prism perpendicular x 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

A CT band in R 6 G? 773 cm-1 Hildebrandt & Stockburger, 1984

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

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

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 - surface tension DS- diffusion coefficient Can or DS can depend on the electromagnetic field? PROBABLY NOT! 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 I 614 cm-1/ I 1650 cm-1

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

Probability function for local work function fluctuations

Distribution of ratio values R=I 614 cm-1/I 1650 cm-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 Dima Lukatsky Sam Safran Yamit Sharaabi Tali Dadosh Paulina Płochocka Israel Bar-Joseph

Timur Yamit