Raman spectroscopy: Basic principles and applications

1 Christian HessRaman spectroscopy : Basic principles and applications Basic principles - Resonance Raman scattering - Surface Enhanced Raman Scattering (SERS) Instrumentation -Spectrometer- Excitation sources Raman in catalysis- In situ cells- In situ Raman (of working catalysts) Raman (1928) Basic principles - Resonance Raman scattering - Surface Enhanced Raman Scattering (SERS) Instrumentation -Spectrometer- Excitation sources Raman in catalysis- In situ cells- In situ Raman (of working catalysts)C.

2 Hess, 2006 Why Raman spectroscopy ? Information on rotational and vibrationallevels Raman effect small but accessible by use of lasers Complementaryinformationto IRspectroscopyphomonuclear diatomic molecules, low frequency range In situ analysisof organic and inorganic compounds Analysis of aqueous solutions and solids(powders) Using resonance and surface enhancement effects ~1010pTrace gas/single molecule analysis- molecular structureIntroductionC. Hess, 2006 Spatial charge separation under influence of electric fieldEpinduced dipole moment : = E(1) : polarizabilitypExample: polarizability changes during CO2vibrationsClassical descriptionC.

3 Hess, 2006 Example: Polarizability changes CO2 Vibrational modes of CO2C. Hess, 2006 Example: Polarizability changes CO2 Vibrational modes of CO2C. Hess, 2006 Spatial charge separation under influence of electric fieldEpinduced dipole moment : = E(1) : polarizability Electric fieldEdue to electromagn. wave with frequency 0E= E0cos 2 0t(2) p = E0cos 2 0t (3)pemission of light at same frequency 0pI= (2/3c3) * *== (16 4 2E02/3c3) 04(4)Classical description222 dtd C. Hess, 2006 Internal vibrational motion with EigenfrequencyvMq= q0cos 2 Mt(5) Polarizability pdevelop in series = q=0+ ( / q)q=0q+ higher order terms(6)p = E(7)= ( q=0+ ( / q)q=0q0cos 2 Mt) E0cos 2 0t= q=0E0cos 2 0t+ ( / q)q=0q0E0[cos 2 ( 0 - M)t+ cos 2 ( 0+ M)tRayleigh Stokes/Anti-StokesClassical descriptionC.]

4 Hess, descriptionStokesAnti-StokesRayleighinel astic impactinelastic impactelastic impact EM= 0 EM< 0 EM> 0 = 0 vib< 0 = 0 vib> 0 = 0 = 0 = 1 = 0 = 1 = 0 = 1 = 0 = 1h 0h( 0 - vib)h 0h( 0 + vib)h 0h 0h 0h 0~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~pharmon ic oscillator: v= 1C. Hess, 2006 Raman intensity?pIs= Ni R(i f) IL(8) Ni: initial state population R(i f): Raman cross section for transitionEi EfIL: Laser intensitypthermal equilibrium: Boltzmann distribution for stateNiat TNi= N0exp(-ih vib/kT)(9)plower energy state: higher initial state populationI(Stokes) > I(Anti-Stokes) descriptionpExample: Stokes/Anti-Stokes intensities of CHCl3C.

5 Hess, descriptionStokesAnti-Stokes-800-4000400 800020000400006000080000 IntensityRaman shift (cm-1)514nm excitation neat CHCl3C. Hess, 2006 Basic principles - Resonance Raman scattering- Surface Enhanced Raman Scattering (SERS) Instrumentation -Spectrometer- Excitation sources Raman in catalysis- In situ cells- In situ Raman (of working catalysts)C. Hess, 2006 Introduction to Resonance Raman scatteringpPolarizability tensor (single e state):: , Electric dipole moment operator with incident/scattered light + =viwwgevevgev0g0ev.

6 0 11 h {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample non-resonant Ramang1g0 Stokesresonance virtual levelVisresonant Ramang1g0 Stokeselectronic resonance virtual levelVisevC. Hess, 2006 {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample non-resonant Ramanv=1v=0 Stokesresonance virtual levelVisresonant Ramanv=1v=0 Stokeselectronic resonance virtual levelVisExample: -Carotin514nm632nmWavelength (nm)AbsorbanceC.}}}}}}}}}}}}}}}}}}}}}}}} }}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}} }}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}

7 Hess, 2006 Example: -CarotinC=C stretch040080012001600200001000020000300 00 IntensityRaman shift (cm-1) -carotin, 9mW, 20s exp 514nm excitation 632nm excitationenhancementfactor = 10C. Hess, 2006 Basic principles - Resonant Raman spectroscopy - Surface Enhanced Raman Scattering (SERS) Instrumentation -Spectrometer- Excitation sources Raman in catalysis- In situ cells- In situ Raman (of working catalysts)C. Hess, 2006 Introduction to SERS Normal Raman scatteringI( L)N molecules with R,freeI( S)INRS( S) = N I( L) R,freeC.

8 Hess, 2006 SERS mechanism - enhancement factorsSurface-enhanced Raman scatteringI( L) A( L)2N with R,ads, N NI( S) A( S)2 ISERS( S) = N I( L) A( L)2A( S)2 R,ads Rough metal nanostructure(1) field enhancement(2) Chemical enhancementmax. 10-100max. ~ 106(isolated Ag,Au)rdMoleculemetal 0EM( )A( )=E0( )max. ~ 108(coupled) C. Hess, 2006 {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample SERS substrateSEM images of the silver nanowire ML on Si waferDDeposited ML shows domains of aligned silver nanowiresA.}}}}}}}}}}}}}}}}}}}}}}}}}}}}} }}}}}}}}}}}}}}}}}}}

9 Tao, F. Kim, C. Hess, J. Goldberger, R. He, Y. Sun, Y. Xia, P. Yang, Nano Lett. 3 (2003) 1229 {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample SERS substrateUV-VIS absorption spectra of silver nanowire MLDB roadened extinction 500-600 nm due to wire-wire couplingtransverselongitudinalRaman {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple}}}}}}}}}}}}}}}}}}}}}}}}}}}} }}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}} }}}}

10 WellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample SERS example: 1-hexadecanthiol/Ag-LB film (C-S)t (C-C)CH3rockCH2wagCH2twist/scissor Enhancement factor EF:EF = [ISERS]/[IRaman] [Mb]/[Mads] thiol head group ( )2, closed-packed layerDmolecule conc. on 1014/cm2D532nm, band at 1295 cm-1 DEF = 2 105C. Hess, 2006 {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample {{{{{{{{{{{{{{{{{{{{{{{{Gas inlet Gas outlet Thermocouple wellQuartz chips O-ringHigh-temperaturecementQuartz woolHeating wireQuartz windowSample SERRS example.}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}} }}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}}