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, 1)SERS of CO 2 /Cu, an electronic effect. (We will learn from electron energy loss spectroscopy-EELS) 2)IRRAS and SERS of CO 2

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

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

FR II FR I Only higher component of the Fermi dyad FII appears in SERS infrared (not Raman) - active bending mode of linear, neutral CO 2 observed in SERS W. Akemann, A. Otto, The effect of atomic scale surface disorder on bonding and activation of adsorbates: Vibrational properties of CO and CO 2 on copper, Surf. Sci. 287/288 (1993) FRI + FRII in thick CO 2 films condensed on sapphire

surface of metal E Fermi electron energy above E F CO 2, adsorbed ** enery levels of adsorbed CO 2 above E Fermi CO 2, gas phase kinetic electron energy ** hot electron in metal electron in gas Laser photon energy Coulomb relaxation Learning from gas phase energy loss spectroscopy

M. Allan, J.Phys.B: At. Mol. Opt. Phys. 35 (2002) L387– L395 2 Π u shape resonance at eV E primary =E residual + Energy Loss electron energy loss spectroscopy in gaseous CO 2 primary kinetic electron energy (eV)

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

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

FR II FR I only FRII observed „activated“ anionic CO 2 - Infrared bending mode observed What we have learned from EELS

Special „catalytic active sites“ for „CO 2 -activation“ into anionic CO 2 (bands of anionic CO 2 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 CO 2 on copper, Surf. Sci. 287/288 (1993)

FR II FR I only FRII observed „activated“ anionic CO 2 - Infrared bending mode observed A more general aspect: All signals are from SERS active sites 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!

Summary of SERS of CO 2 / CO 2 - on Cu Ground state vibrations of linear CO 2 (bending, FR II, asymmetric stretch) are observed by electron transfer metal to CO 2 „activated“ CO 2 - is only formed at „chemical active sites“, a subgroup of SERS active sites. Groundstate vibrations of bent CO 2 - (bending, symmetric stretch (no Fermi Resonance for CO 2 - )) are observed by electron transfer from CO 2 - to the metal EM enhancement is not sufficient for observing CO 2 at terrasses.

IRRAS of CO 2 / Cu : either tilted orientation or mixed parallel and perpendicular orientation Why are bands of CO 2 - (also IR active) not observed in IRRAS?

IRRAS and SERS of C 2 H 4

The free C 2 H 4 molecule – four vibration modes of interest Mode CH 2 wagging CH 2 scissorC-C stretch gas frequ. 950 cm cm cm cm -1 Activity IRRaman + = towards viewer- = away from viewer

IRRAS spectra of C 2 H 4 on Cu(111) at T=90K, effect of post-deposition of Cu given in monolayers (ML)), M.Binder, O.Skibbe, Heidelberg, unpublished 1.35ML 0.96ML 0.72ML 0.48ML 0.29ML 0.17ML 0.03ML 0 ML 1.35ML 0.96ML 0.72ML 0.48ML 0.29ML 0.17ML 0.03ML 0ML ML=monolayer; 1L=Langmuir=1s*1.333*10 -6 mbar Exposure of C 2 H 4 at least until saturation, values between 1.8L and 5.6L Band of IR active vibration disappears SERS bands appear

IRRAS Studies of Adsorbed Ethene (C 2 H 4 ) on Clean and Oxygen-Covered Cu(110) Surfaces, J.Kubota et al. J. Phys. Chem. 1994,98, defect induced Raman lines, no intensity of the expected IR wagging mode defects are oxidized, Raman lines disappear, IR active wagging line (expected on ideal Cu(110)) reappears Raman CC stretch Raman CH 2 scissor IR CH 2 wagging

Summary 1) The SERS spectra of CO 2 and C0 2 - are caused by an electronic effect IRRAS of CO 2 on cold deposited Cu looks „normal“ Sublte atomic scale roughness effects on Cu (111) make the IR vibrations of C 2 H 4 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) (2007)