Introduction to Near Edge X- ray Absorption Fine Structure of Molecules Maddalena Pedio TASC-INFM Trieste

Transcription

1 Introduction to Near Edge X- ray Absorption Fine Structure of Molecules Maddalena Pedio TASC-INFM Trieste NEXAFS Basic Principles Low Z element XAS Examples: Anisotropy in Solids 6H SiC Di-atomic Molecules Complex molecules

2 X-Ray Absorption Spectroscopy (XAS) Absorption Coefficient and Cross Section The central physical quantity is the absorption coefficient α = ε α 4πk λ = n k ( ω) = if W if c W n n~ = n ε 2 = 2nk hω α = σn ik ~ ε iε r = ε1 + 2 The cross section σ is related to the transition probability of an interaction process. It is typically calculated by perturbation theory. σ is a measurable quantity and has the dimensions of an AREA. Its units are the BARN: 1 barn= cm 2

3 h p 1 λ = k = 2m( hν E ) 2 0 h (N)EXAFS

4 Low-Z element XAS The study of C, N and O is crucial for many topics of fundamental and technologic interest, such as molecular solids, catalysis and chemisorption processes, oxides =>SOFT X-RAYS Control of the detection depth hundreds mev=>high photon energy Resolution Once a core hole has been created it will decay. The dominant decay mechanisms are fluorescence emission or Auger electron emission. Because of the Heisemberg uncertainty relationship a short core-hole lifetime (rapid x-ray emissionand/or Auger decay) is associated with a large uncertainty in energy. The lifetime broadening leads to Lotentzian lineshapes 0.5 FWHM=Γ L =h/ τ = /τ ev Light Z elements (O, C, N 1s) XAS required very high photon flux. For K edge cross section, the absorption coefficient depends on Z as Z 4.

5 The main properties of NEXAFS technique are: Involves core level so NEXAFS is element specific because the x-ray absorption edges of different elements have different energies. NEXAFS is also very sensitive to the bonding environment of the absorbing atom. The NEXAFS spectrum exhibits considerable fine structure above each elemental absorption edge. Another great asset of NEXAFS spectroscopy is its polarization dependence. Linearly polarized x-rays are best suited for covalent systems like low-z molecules, macromolecules and polymers, which possess directional bonds.

6 Carbon configurations Carbon covalent C-C C bonds within 'molecule' variable sp hybridisation sp 2 sp 2 pure sp 2 pure sp 3

7 C K-edge

8 Optic absorption What is absorbed is not transmitted The transmission of photons induced by the optics of a beamline working in soft X ray energy range typically shows absorption features due to the materials of the optics (SiC, Cr ) themselves and to the presence of contaminants (residual gas -> C, N, O etc) In the hard X the transmission results a smoother function of the photon energy

9 Optics absorption examples

10 BEAR Optics ELETTRA GATE 2 GATE 1 GATE 3 Pol Sel GATE 6 GATE 5 GATE 4 G1200 G1800 M1 GNIM M2 P1 BPM GATE 7 RIF EX SLITS P2 MONO C K edge O K edge Photon flux measured at the end station of BEAR beamline

11 Monitors of the lights Gas cells Mesh with high degree of transmission (>80%) Photodiodes

12 The transmission experimental scheme providing the absorption coefficient Direct Method I(d,R, µ) = I 0 (1-R) 2 e -αd (1-R) 2 loss of intensity due to the 2 interfaces, exponential derives from the integration of the linear relation di=-αi(x)dx R given by the Fresnel relations, i.e. a combination of incidence angle, complex dielectric constants of the two media at the interfaces and light (s or p) incidence conditions. The absorption coefficient is related to the cross section of the process: α ( ω) = nσ ( ω) = 2 r λ f 0 2 Where n is the density of scattering centers, r 0 the classical radius of electron, λ the wavelength and f 2 the imaginary part of the atomic scattering factor

13 Detection Modes Transmission experiment is not always possible: solid thick samples adsorbtion of adlayers on surfaces Incoming Photons Reflected Photons Sample

14 Detection Modes: Indirect Methods and use of the different recombination channels Fluorescence Photon emission Indirect Methods By-products of the core hole deexcitation process are collected. The thus-measured signal is expected to mimic the behavior of α(ω) Electron Yield (Total vs. Partial) Ion Desorption Auger emission Photons Ions Sample

15 Detection mode is related to the detection depth of XAS measurements

16 X-Ray Absorption Spectroscopy (XAS) How Partial Spectral Components May Contribute to the Measurement

17 X-Ray Absorption Spectroscopy (XAS) Electron Yield Detection Modes

18 X-Ray Absorption Spectroscopy (XAS) Electron Yield Detection Mode

19 From raw data to the absorption coefficient of an adsorbate: a naive model-1 adsorbate Substrate Λ electron free collision path θ g = π/2 -θ i θ i d d θ g V d d d = = sinθ cosθ g Λ i The excited volume contributing to the measured signal can be written as V = S Λ cosθ Section of the light beam d is the transverse section of the spot at the surface for light impinging with grazing angle θ g on the surface In a linear approximation di = Iα dx the drain current induced by the light impinging on the sample Y = S I α cosθ Λ Dr 0 C M Function converting the absorbed photon into the electrons contributing to the drain current Optical absorption coefficient

20 X-Ray Absorption Spectroscopy (XAS) Electron Inelastic Mean Free Path vs.photon MFP Surface vs. Bulk Sensitivity

21 From raw data to the absorption coefficient of an adsorbate: a naive model-2 adsorbate Substrate Λ Experiment whose goal is the measure of the optical absorption coefficient α a of an overlayer, with electron mean free collision path Λ a, deposited onto a semi-infinite substrate of optical absorption coefficient α s, with electron mean free collision path Λ s Y S ω, t) = I0( ω, t) ΛSαS ( ω) CS ; I0( ω, t) = I ( ω) f ( t) cosθ DO( 0 Y DO ( ω, t) = Y W DO DO ( ω, t) ( ω, t) I0( ω) is the photon distribution due to source plus transport optics and f(t) is the function taking into account the time dependence of such distribution (e.g. due to time decay, fluctuations etc.) First step: measurement of substrate drain current, Y DO, before depositing the overlayer, mesh W DO Second step: measurement of drain current, Y DA, in presence of the overlayer of thickness d the incoming flux is simultaneously monitored for normalization/comparison purposes by measuring the drain current, W DA

22 From raw data to the absorption coefficient of an adsorbate: a naive model-3 Two cases must be considered a) overlayer thickness Λ < d s s s a a D C d S t I C d S t I t Y α θ ω α θ ω ω ) ( cos ), ( cos ), ( ), ( ' 0 ' 0 ' Λ + = mesh drain current W D (ω,t ) S S S S a S S S W s s s W a a W DO D d d C C t f I t f I C d C t f I t f I C d C t f I t f I t Y t Y Λ Λ + Λ = Λ Λ + = ) ( ) ( ) ( ) ( ) ( ) ( ) ( ') ( ) ( ) ( ) ( ) ( ) ( ) ( ) ( ), ( ), ( ' 0 ' 0 ' 0 ' ' ω α α ω α ω ω α ω ω α ω ω ω ω

23 From raw data to the absorption coefficient of an adsorbate: a naive model-4 b) overlayer thickness d > Λ At these coverages the contribution of the substrate can be considered negligible, the constant term Λ S d vanishes and Λ S the drain current ratio is proportional to the overlayer absorption coefficient

24 AY and TY C K-edge K NEXAFS of a 3C SiC ordered film (>500 Å) obtained by C 60 decomposition on Si(100) grown ex situ and exposed to air before measurements. The two spectra have been taken simultaneously. The thickness of detection in the Auger Yield mode is about 10 Å,, while for Total Yield is higher 1 order of magnitude. The difference in the two detection mode is due mainly to oxygen contamination and trace of nitrogen

25 Anisotropy in solids Silicon Carbide structures in different polytypes 6H-SiC structure

26 NEXAFS anisotropies mainly within ev above the edge Absorption Intensity (arb. int.) A B NEXAFS C K-edge 3C SiC Normal Incidence 6H SiC 75 C K-edge NEXAFS of 3C SiC 8000 Å ordered film taken at normal incidence in total yield detection mode and 6H SiC monocrystal taken at nearly normal (75 ) and grazing incidence (15 ) in fluorescence yield mode. 6H SiC 15 Grazing Inc Photon Energy (ev)

27 C K-edge A B 3C SiC extended NEXAFS 71 atoms 47 atoms E-E MT

28 Surface NEXAFS: Atomic adsorbates O/Ni(110) strong anisotropy O 2p empty states are superimposed in the [001] azimuth => increasing of cross section (label 3 atoms)

29 Surface NEXAFS: Atomic adsorbates Multiple Scattering Missing Row Model Saw-tooth Model Unreconstructed Surface Among the MS paths that lead to the enhancement of the intensity in the NEXAFS, are those scattering events that involve the central atom the nearest neighbor atoms and also neighboring adsorbate atoms (Oxygens labeled 3 in the [001] azimuth). Not only the local symmetry but also the adsorbate configuration are responsible for the pronounced azimuthal dependence.

30 NEXAFS of di-atomic molecules

31 Surface NEXAFS: Exploiting the X-Ray Polarization π*-resonance σ*-resonance

32 NEXAFS intensity for a diatomic molecule adsorbed on a substrate. Variation of light incidence angle is used to determine alignment of adsorbate bonds: cos 2 α law for the π and σ excited empty molecular orbital.

33 NO single molecule NEXAFS N K-edge for an angle between the E vector and the molecular axis of 0, 60 and 90 degrees. Simulations of the NEXAFS spectra by the MXAN code (M. Benfatto and S. della Longa) The new version of MXAN calculates correctly the polarization dependance (cos 2 θ law) for the π and σ excited empty molecular orbital of the NO molecule and their energy separation as a function of the NO distance: R=1.2 Å E(π σ)=14.5 ev, R=1.3 Å E(π σ)=10.2 ev Our simulations correctly reproduce the NO molecole in gas phase spectra. Effect in the N K-edge induced by the variation of N-O distance

34 Empirically (the so called rule of thumb) the kind of bond results related to the energy position of the σ* resonance, for example in molecules containing C- C bonds,

35 Complex molecules Polarization dependent NEXAFS spectra of benzene chemisorbed on Ag(110), illustrating the capability of the technique to determine molecular orientations following the maximum of the p orbital intensity as a function of the incidence angle of light.

36 Surface NEXAFS: Exploiting the X-Ray Polarization

37 Surface NEXAFS: Exploiting the X-Ray Polarization

38 Surface NEXAFS: Exploiting the X-Ray Polarization

39 Examples for carbon atoms in multi-atom molecules with different bonding configurations, often found in polymers. The spectra exhibit chemical shifts within each group similar to XPS spectra but more importantly considerably different fine structure for carbon in different molecular groups. This clearly illustrates the power of NEXAFS to distinguish chemical bonds and local bonding. In many ways it is superior to XPS, which does not provide local structural information.

40 ANNEALING TEMPERATURE DEPENDENCE OF C 60 ON SILICON SURFACES: BOND EVOLUTION AND FRAGMENTATION AS DETECTED BY NEXAFS C 60 Interaction with ordered surfaces leads to different bond as detected by e- spectroscopies C60 on Silicon surfaces NEXAFS results on Si(100) and Si(111) Fragmentation

41 C60 Buckyball diameter 7.1 Å Single bond length 1.45 Å Double bond length 1.40 Å All C sites are equivalent Truncated icoesahedron Magic Cluster Large molecule acts as e- acceptor + super alkene

42 C 60 interacting with Ordered Surfaces Interaction with ordered surfaces Different bond as detected by e- spectroscopies A. J. Maxwell, P. A. Brühwiler, D. Arvanitis, and J. Hasselstroe m M. K.-J. Johansson, N. Mårtensson, Phys. Rev. B 7312 (1998) Examples Ionic bond C60/Au(110) Covalent bond C60/Pt(111) C60/Si(111), Si(100), Ge(100) C60 Fragmentation SiC production

43 C 60 /Si(111) 1185 K 1070 K 1185 K 1070 K 950 K 950 K 900 K 900 K 650 K 650 K Multilayer Multilayer 284 s incidence E in plane Photon Energy (ev) p incidence E normal Photon Energy (ev) Dependence on the E direction polarization ALOISA Beamline Molecular states detectable in GI 292

44 Dependence on the polarization of E vector C 60 /Si(111) E near normal ~7 degree GI p incidence R1=90 STM model: Si atoms moves toward C atoms Pascual et al. Surf. Sci 1.0 E in plane NI s incidence R1= Photon Energy (ev) hν e e ALOISA

45 C 60 /Al XAS A. J. Maxwell, P. A. Brühwiler, D. Arvanitis, J. Hasselström, and N. Mårtensson, PRL 79, (1997)

46 Two detection mode C K-edge Auger Yield C 60 /Si(100) 1060 K C K-edge Drain Current C 60 /Si(100) 1060 K 1040 K 1020 K 1000 K 900 K 1 ML 650 K 1040 K 1020 K 1000 K 900 K 1 ML 650 K Multilayer Multilayer Photon Energy (ev) Photon Energy (ev) 294 BEAR beamline E in plane

47 Difference curve C 60 fragmentation Auger Yield C K edge C /Si(100) 60 1 ML C 60 /Si(100) T ann =1000 K C 1s Photoemission Spectra Difference curve(1040 K-1060 K) C - C Internsity (arb. units) 1000K 1040K 1060K C - Si Photon Energy (ev) Molecular states visible at 1040 K in Auger mode: Surface sensitive Binding Energy (ev)

48 STM performed on both Si(100) and Si(111) during C 60 fragmentation revealed that all the cages break

49 C 60 fragmentation starts at a temperature of about 1020 K. By changing the probing depth of NEXAFS, the residual molecular states of the fragmented C 60 cage are localized on the topmost surface layer, Molecular states at the more external site with respect to the C-Si interface. The feature intensities of the electric field direction for the C 60 /Si(111) system suggest that the molecular fragment have π character and are directed almost orthogonal to the surface.

50 From Raw Data to the absorption coefficient- example pentacene 1

51 From Raw Data to the absorption coefficientexample pentacene 2

52 In the common interpretation frameworks the process is described in terms of transition probability between eigenstates of the unperturbed Hamiltonian and the total x-ray absorption cross section (that is proportional to the SPECTRUM) The calculation of the transition probability is treated with the following approximations: - Dipole approximation, used to treat the radiation field in its interaction with matter - One-electron approximation 2 2 r r σ ( ω) = 4π α ψ p Dψ δ ( hω f, i f i E f E i ) These assumptions correspond to the calculation of the cross section for the radiation-matter interaction at the first order of perturbation i.e. in the context of the linear response theory, valid for the spectroscopies

53 X-Ray Absorption Spectroscopy (XAS) Single-Particle Energy Level Scheme Site selectivity Orbital Selectivity via Dipole selection rules l --> (l ± 1) The photon energy is swept while crossing a core level threshold The XAS profile is expected to mimic the unoccupied DOS projected onto the excited atomic site via the dipole selection rules

54 The fine structure of XAS edges can be interpreted as the Dipole transition from the localized core level of the element of interest into unoccupied electronic states The one-electron cross section may be further decomposed into the product of an energy dependent matrix element overlap factor P(E) and a projected density of states N(E) with appropriate symmetry σ(e) ~ P(E) N(E)

55 NEXAFS interpretation frameworks: two different approach have been developed: Electron Transitions (molecules LCAO) Solids: Band structure calculations -A periodic potential is assumed -Final states are Bloch electronic States -Information of the different contributions to the LDOS relative to that initial state core level of that element The initial state is the core level and the final are the unoccupied Bloch states n, k. 2 2 r r σ ( ω) 4π α ψ e ψ δ ( hω E k, n f i k, n E core )

56 NEXAFS interpretation frameworks: two different approach have been developed: (cfr. M. Benfatto contribution) Multiple Scattering calculations -Final states are scattering states -The potential is calculated for a cluster of atoms (up to about 10 Å from the central atom) in real space with Green s Function Formalism Information on the local geometrical arrangement of the excited atom, even though further details can be obtained by using self consistent calculations r r σ ω + r r ( ) ψ f p D ImG ( E f ) p Dψ i = σ atom + σ scatter

57 The interpretation in real space is in terms of scattering events of the photoemitted electron that interacts with the surrounding atoms, the number of scattering processes depending on the energy above the edge while in the electronic states band space we speak of transitions from a filled core electronic state to an empty electronic state (or molecular states) following the dipole selection rules In any case the dipole transitions dominate the process of photoabsorption, an electron from a core level having angular momentum l is excited into the l+1 finale states. If the spectrum is taken at a K edge the dipole selection rule leads to a outgoing p wave (or in terms of electronic states the local density of states of p character)

58 A unifying scheme of NEXAFS interpretation based on the multiple scattering theory is possible. Natoli and Benfatto demonstrated it analytically by using the Green function formalism and the KKR band calculation method. Roughly speaking the multiple scattering in the real space cluster model can be interpreted as the exact formulation of the X-ray absorption fine structure problem with infinitely many scattering events included. Both the calculation frameworks are valid in the context of the approximation that the single electron processes dominate following the dipole selection rules.