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On the Differences between SERS and Infrared Reflection Absorption Spectra of CO 2 on Cold-deposited Copper

On the Differences between SERS and Infrared Reflection Absorption Spectra of CO 2 on Cold-deposited Copper . M.Lust, A.Pucci, Universität Heidelberg A.Otto, W. Akemann, Universität Düsseldorf, EU, Germany, .

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On the Differences between SERS and Infrared Reflection Absorption Spectra of CO 2 on Cold-deposited Copper

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  1. On the Differences between SERS and Infrared Reflection Absorption Spectra of CO2 on Cold-deposited Copper M.Lust, A.Pucci,Universität Heidelberg A.Otto,W. Akemann, Universität Düsseldorf, EU, Germany, • SERS of CO2 /Cu, an electronic effect. (We will learn from electron energy loss spectroscopy-EELS) • IRRAS and SERS of CO2

  2. Conformations and vibrations of stable neutral CO2 and metastable anionic CO2- H.-J. Freund, M.W. Roberts Surface Science Reports 25 (1996) 225 273 Vibrations of linear CO2: IR-bending mode 667 cm-1, IR-antisymmetric CO-stretch 2349 cm-1 But what about Raman active symmetric CO-stretch?

  3. symmetric CO-stretch is replaced by Fermi resonance of CO2 one period of bending time two periods of symm. stretch FR I 1286 cm-1 FR II 1388 cm-1 symm.stretch 2  bending

  4. Only higher component of the Fermi dyad FII appears in SERS FR II 1658 - - FR I infrared (not Raman) - active bending mode of linear, neutral CO2 observed in SERS FRI + FRII in thick CO2 films condensed on sapphire W. Akemann, A. Otto, The effect of atomic scale surface disorder on bonding and activation of adsorbates: Vibrational properties of CO and CO2 on copper, Surf. Sci. 287/288 (1993) 104-109

  5. Learning from gas phase energy loss spectroscopy kinetic electron energy electron energy above EF enery levels of adsorbed CO2 above EFermi electron in gas CO2, gas phase * * hot electron in metal Laser photon energy Coulomb relaxation CO2, adsorbed EFermi surface of metal

  6. M. Allan, J.Phys.B: At. Mol. Opt. Phys. 35 (2002) L387–L395 electron energy loss spectroscopy in gaseous CO2 Eprimary=Eresidual + Energy Loss 2Πu shape resonance at 3.0 - 3.6 eV primary kinetic electron energy (eV)

  7. Coulomb relaxation of an adsorbate, here CO/Cu(111) fig 8 from H.Ehrhardt et al., Phys.Rev.173 (1968)222 negative ion resonance 2π* at about 2.0eV energy above EF Coulomb relaxation ca 3.5 eV CO 2π* in vacuum +7.0 eV Vacuum-level +5eV CO 2π* at Cu(111) +3.6eV EF

  8. M. Allan J. Phys. B: At. Mol. Opt. Phys. 35 (2002) L387–L395 EF Laser photon energy 1.916eV 2Πu shape resonance at 3.0 - 3.6 eV assumption of Coulomb relaxation of about 3.5eV, like CO/Cu(111) Excitation of FR II, bending and (weakly) antisymmetric stretch, but not of FR I. This is also observed in SERS

  9. What we have learned from EELS „activated“ anionic CO2- only FRII observed FR II Infrared bending mode observed FR I

  10. Special „catalytic active sites“for „CO2-activation“into anionic CO2(bands of anionic CO2 at 1185 cm-1 and 769cm-1 W. Akemann, A. Otto, The effect of atomic scale surface disorder on bonding and activation of adsorbates: Vibrational properties of CO and CO2 on copper, Surf. Sci. 287/288 (1993) 104-109

  11. A more general aspect: All signals are from SERS active sites „activated“ anionic CO2- only FRII observed FR II There are plenty of molecules at non SERS active sites, they should show FRI and FRII. Why are they not observed? The EM enhancement is not strong enough! Infrared bending mode observed FR I

  12. Summary of SERS of CO2 / CO2- on Cu Ground state vibrations of linear CO2 (bending, FR II, asymmetric stretch) are observed by electron transfer metal to CO2 „activated“ CO2- is only formed at „chemical active sites“, a subgroup of SERS active sites. Groundstate vibrations of bent CO2- (bending, symmetric stretch (no Fermi Resonance for CO2- )) are observed by electron transfer from CO2- to the metal EM enhancement is not sufficient for observing CO2 at terrasses.

  13. IRRAS of CO2/ Cu : either tilted orientation or mixed parallel and perpendicular orientation Why are bands of CO2- (also IR active) not observed in IRRAS?

  14. IRRAS and SERS of C2H4

  15. The free C2H4 molecule – four vibration modes of interest + = towards viewer - = away from viewer

  16. IRRAS spectra of C2H4 on Cu(111) at T=90K, effect of post-deposition of Cu given in monolayers (ML)), M.Binder, O.Skibbe, Heidelberg , unpublished SERS bands appear 1.35ML 0.96ML 0.72ML 0.48ML 0.29ML 0.17ML 0.03ML 0ML 1.35ML 0.96ML 0.72ML 0.48ML 0.29ML 0.17ML 0.03ML 0 ML Band of IR active vibration disappears Exposure of C2H4 at least until saturation, values between 1.8L and 5.6L ML=monolayer; 1L=Langmuir=1s*1.333*10-6mbar

  17. IRRAS Studies of Adsorbed Ethene (C2H4) on Clean and Oxygen-Covered Cu(110) Surfaces, J.Kubota et al. J. Phys. Chem. 1994,98, 7653-7656 Raman CH2 scissor Raman CC stretch defects are oxidized, Raman lines disappear, IR active wagging line (expected on ideal Cu(110)) reappears IR CH2 wagging defect induced Raman lines, no intensity of the expected IR wagging mode

  18. Summary 1) The SERS spectra of CO2 and C02- are caused by an electronic effect IRRAS of CO2 on cold deposited Cu looks „normal“ Sublte atomic scale roughness effects on Cu (111) make the IR vibrations of C2H4 dissappear. Not all the differences between SERS and IRRAS of the same samples are understood. read A.Otto, M.Lust, A.Pucci,G.Meyer, “SERS active sites“, facts and open questions Canadian Journal of Analytical Sciences and Spectroscopy, 52(3) 150-171 (2007)

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