Vibrationalspectroscopy振动光谱

Vibrational spectroscopy Chemical composition: finger print Bonding orientation: adsorption structure on surfaces Infrared Spectroscopy (IR)High Resolution Electron Energy Loss Spectroscopy (HREELS)Surface Enhanced Raman Spectroscopy (SERS)Second Harmonic Generation (SHG)Photo-acoustic Spectroscopy (PAS)Inelastic electron tunneling Spectroscopy (IETS)Inelastic Neutron Scattering (INS) Surface Infrared spectroscopyRefs: Y.J. Chabal, Surf. Sci. Rep. 8, 211 (1988) F.M. Hoffman, Surf. Sci. Rep. 3, 107 (1983)Transmission IR Spectroscopy-supported metal cataysts- IR transparent samples (Si) Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) -refocus the diffusively scattered IR beam-high surface area catalytic samples-low surface area single crystalsReflection-Absorption IR Spectroscopy ( RAIRS ) -specular reflected IR beam -single crystal samplesMultiple Internal Reflection Spectroscopy ( MIR ) orAttenuated Total Reflection (ATR) -total internal reflection -SAM , polymer films BackgroundTransmission and absorption modeTransmittance T = I/I0 = exp(kcl)Absorbance A = eclk: absorption coefficient; e : absorptivityc : concentration; l : cell thickness Imaginary part of refractive index n = k n = n + ik for absorbing medium n = n for dielectric non-absorbing medium-needs to take reference and sample spectra-not popular for surface studies due to the large bulk contribution-I0 I- + ReflectionThe reflection anglesSnells lawn1/n2 = sinqi/sinqtCrtical angle: qc= sin-1(n2/n1)Intenstiy of the reflected light - Depend on polarizationsFresnels equationsn = n + iks-polarized light : | the plane of incidenceR s = (n-secq)2+k2/ (n+secq)2+k2p-polarized light : the plane of incidenceRp = (n-cosq)2+k2/ (n+cosq)2+k2- qi must be large: grazing incidence for thin films on reflective surface Ep x Es qi qrqt xthe plane of incidence Phase shift , electric field, intensity of p-polarized light as a function of incidence angle from a metal surface 0 incidence angle 90 Phase shift on refelctions 1800 p-pol :q|s-pol: q 0 incidence angle 90 Surface intensity function (E/Eo) 2secq n =3, k=30204060 Surface electric field E/E0s-polarized light at the surface - uniform phase shift - vanishing E field at the surfacep-polarized light at the surface - dependent on incidence angle - strong E field at large incidence angle, ie, grazing incidence Absorbance is proportional toE2 and area of surface as 1/cos qI E2/cos q Adsorbate covered surfaceDielectric constant e = (n+ik)2Vcauum e1n1Adsorbate e2(n2, k2)Metal e3(n3, k3)Ro R RRoAbsorption function A = (R- R 0) /R0 = DR/Re3 e21, d e2 and cosq e3-1 DRp/Rp = (8pdsin2q/lcosq)Im(-1/e2)a large reflectivity change at high incidence angle d Surface selection rule-The electric field of light and the molecule interact with surface electrons-The incident light must be p-polarized-Only vibrations with a dipole moment perpendicular to the surface-The incident light should be reflected at grazing incidence+ - + +-+m M mMmimage mimage IR inactive IR activemfi = 0, dm/dr 0- for lying down molecules, molecular and image dipoles are cancelled out- for upright molecules, molecular and image dipoles are enhanced Surface IR spectra of adsorbed moleculesIdentification of adsorbate: high resolution : 2-4 cm-1Orientation of adsorbed molecule by surface dipole selection ruleHow to confirm the metal-adsorbate bond ? - frequency shift of internal modes compared to gas-phase spectra - additional metal-molecule vibration: 800 cm-1Frequency shift of internal and external modes for adsorbed layers - weakening of metal-molecule bond: n decreases as coordination of surface atoms increases - formation of adsorbate islands - compression structuresDR/R: 0.110 -3 often small: sufficient for submonolayer sensitivity for molecule with strong dynamic dipole momentDR/R roughly linear with coverage, but not a good indicator of population Peak width and intensityhomogeneous broadening - coupling to phonon - electron-hole creationinhomogeneous broadening - inhomogeneous distribution of harmonic oscillator - intermolecular interactionenergy transport between molecule and surfacedipole-surface interaction: dynamic dipole interaction Instrumentation: RAIRS J.E. Reutt-Robey et al, JCP 93, 9113 (1990) Instrumentation: MIR IR IR finger print Modes of vibration IR spectra of CO on Pd(100)Lower frequency shift compared to that of gas phase ? - Interaction with the vibrating dipole with the image dipole - Chemical effect due to backdonation, which change the CO bond strengthHigher frequency shift as coverage - vibrational coupling : dipole-dipole, dipole-metal electrons - chemical effect: reduced backdonation into antibonding orbitals - electrostatic effect due to charge transfer between the metal and moelcule - intermolecular repulsion-threefold:site : 18001900 cm-1-bridge site: 19002000 cm-1-on top site: 20002100 cm-1 IRRAS spectraCO on Pd(111) Diffuse reflectance IR spectra High Resolution Electron Energy Loss Spectroscopy- Inelastic scattering of low energy electron beam- Energy loss due to the vibrational excitation - observe vib. modes parallel and perpendicular to the surface- Lower resolution 3meV (=24 cm-1 )(compare with IR 2 cm-1) - Submonolayer sensitivity- can observe surface-atom vib. freq. 50 eV- off specular angle- lower scattering cross section the the dipole scatteringNegative ion resonance scattering- short range interaction- electron trapped in empty Rydberg state of adsorbate - temporary negative ion- enhancement of vib. Intensity over relatively narrow range of Ei- very small cross- section off resonance- molecular orientation on surface Peak positions for different adsorption states Instrumentation Examples: CO on W(100)565 cm-1 ; W-C stretching630 cm-1 : W-O stretching363 cm-1 W-CO (on top)2081 cm -1 CO stretchingCO(g): 2140 cm-1 Interaction ions with solidEvacE F EiAuger neutralizationResonance ionizationResonance neutralizationQuasi-resonance neutralization- Charge transfer: neutralization of ion and electronic excitation- Kinetic energy transfer: sputtering, scatteringe Atomic and nuclear collisionImpact parameter (b) scattering process energy transfer (Tc)1 A inelastic excitation 10eV of valence electrons10-1 A inelastic excitation 100eV of L-shell electrons10 -2 A inelastic excitation 1 keV of K-shell electrons 10-4 A elastic scattering 100keV from nuclei Ion scattering spectroscopyLow energy ion scattering (LEIS): 0.5 3 keVMedium energy ion scattering (MEIS): 10500 keVHigh energy ion scattering (HEIS) orRutherford backscattering spectroscopy (RBS): 0.5 5 MeV Binary elastic collisionKinematic factor K= E1/Eo E1/Eo = (M22 M12)sinq)1/2+M1cosq) /(M2+M1)2M1,M2 : mass of incident atom and targetq = scattering angle Ion trajectroy Blocking, shadowing, and channeling effect- scattering cross section at a certain angle depend on atomic potentials of incident and substrate atoms-scattering depend on incident angle and impact parameter-lower ion energy, larger shadow cone Scattering cross section2pbdb = s(q) 2psinqdqs(q) = b(db/d q)/sin q = # of scattered paricles into dW/total # of incident particlesRutherford formulads /dW = Z1 Z2e2/4Ecsinqc/22 Ec = M2/(M1+M2)Eoqdbb dq Quantitative analysisTotal # of particles of impurity mas M3, atomic number Z3, surfacedensity N3(atoms/cm2)The measured yield Y3Y3 = N3 (ds /dW) DW QQ: measured # of incident particlesDW : solid angle accepted by detector- N3 can be determined typically with an accuracy better than 10% Stopping power and depth resolutionElectronic stoppingduring going inElastic scattering Electronic stopping during going outFinal Energy of a particle at normal incidenceE1 = Eo DEin Es - DEout-the rate of energy loss dE/dx depends on mass of projectiles, traget, and incident energy-for 0.52.0 MeV, dE/dx is independent of energy-Depth resolution: 30100 Energy spectrum Channeling and blocking Surface peaks Energy distribution of sputtered species Sputtering yield: ion energy dependence Sputtering yield: dependence on element Sputtering yield: angle dependence-varies 1/cosq-Drop at grazing incidence angle Secondary Ion Mass Spectrometry (SIMS)Ion beam S+-Sensitive to top most layers-Chemical composition-Structural informations-Very high sensitivity-Imaging: 1001000nm-Depth profiling: 5nm-Ion yield depends on surface concentration and sputtering yield-Organic anlaysis: m/z = 500040,000-Matrix effect: secondary ionization mechanism -Destructive: implantation, mixing, sputtering, ion beam induced surface chemistry, radiation induced atomic redistribution Massdetect sputtered species (neutrals, ions)from the sample S SIMS modes-Static SIMS - low sputter rate 1nA/cm2 10 /hr - nondestructive- Submonolayer analysis -Dynamic SIMS - high sputter rate 10 mA/cm2 100 mm/hr - destructive- Depth profiling1nA/cm 2=10-9A/cm2/1.6x10-19 C= 6.3x109 ions/sec-cm2= 6.3x109 ions/sec-cm2 1015atoms/cm2= 1.6x10-5 ML InstrumentationIonization methods: -electron impact - microwave-field ionization-laser ablationIon sourceAr+ ionO2+: for electropositive elementsCs +: for electronegative elementsLiquid metal: Ga+, In+- small beam size Mass spectrometerQuadrupole-inexpensive, compactDouble focusing electrostatic/magnetic sector-high transmission-High mass resolutionTime of flight-high molecular weight From Jeol Example Imaging SIMS-scan ion beam or ion detector-Beam size 10nm-Resolution 100mm Thermal desorption spectroscopyTemperature programmed desorpion-measure desorbing molecules by heating the surface using mass spectrometerQuadrupole mass spectrometerheater Adsorbed molecules-Heat of adsorption if Eads =Edes-Surface coverage: peak area -Adsorption sites: peak position-Intermolecular interaction-Kinetics of desorption : peak shape Analysis of TPDRedhead, Vacuum 12, 203 (1963)The rate of desorptionrd = -dq/dt = koqn exp(-Ed/kT) n: order of reaction ko : prexponential factor q : coverage Ed: activation barrier for desorptionThe sample temperature varies linearly T(t) = T0 + bt b = dT/dt : heating rateK/s r d = -dq/dT = (1/b)koqn exp(-Ed/kT) coverage kd=k0eEa/kTTPD spectraTemperatureIntensity Ea = 24kcal/molb= 10 K/secn=1ko=1013 1/secEd,ko q, b : Desorption temperatureko q,n: peak shapeq: Peak area: Zero-order desorption kinetics-independen of coverage-exponential increase with T-common leading edge-Rapid drop-T max move to higher T with coverage-Pseudo zerp-order for strong intermolecular interactions between adsorbates Intensity T/Krd = -dq/dt = ko exp(-Ed/kT) First-order desorption kinetics n =1Intensity T/Krd = -dq/dt = koqexp(-Ed/kT) q exp(-Ed/kT)-rate proportional to coverage-balance between q and exp(-Ed/kT)-Tpeak independent of q-Asymmetric line shape-T peak as Ed -Molecular desorption Second order desorption kinetics n=2 T/Krd = -dq/dt = koq2 exp(-Ed/kT)-rate proportional to coverage-balance between q2 and exp(-Ed/kT)-Tpeak varies with q-symmetric line shape-Common trail of peaks-Recomnative desorption-Pseudo-2 nd order for strong intermolecular interactions Intensity Fractional order desorption kinetics- indicaIndicate cluster formation on the surfaceDesorption from edge of clustersEffect of activation barrier Ed=50400kJ/molIntensity E d 10 20 30 40 50Ed Tpeak peak width At saturation coverageEd/RTp= 30kJ/mol Effect of pre-exponential factor k0 =1011 1015 1/secIntensity k0 =1015 k0 =1011T/K-oscillation frequency for adsorbate particlesEffect of heating rateInte nsity T/Kb= dT/dt =18.5b= 18.5 b= 17.5 CO/Ni(110) Determination of activation barrier EdThe maximum rate in the desorption ratedrd/dt =0, konqn-1 exp(-Ed/kT) =b Ed/kTp2-Ed/kT = ln (b/ kTp2 )+ln(Ed/ konqn-1 )Plot of ln vs 1/T at constant initial coverage: EdTp E dko/b=1014/K ko/b=1010/KOther methods:Chan, Aris, Weinberg, Appl. Surf. Sci. 1, 360 (1978)Habenschaden, Kuppers, Surf. Sci., 138 L148 (1984)D.A. King, Surf. Sci. 47, 384 (1975)