Synchrotron X-ray Absorption Spectroscopy

Transcription

1 Synchrotron X-ray Absorption Spectroscopy Near-ege Spectra (I) Graham N. George Ingri J. Pickering Toay Near-ege spectra Nomenclature Selection rules an spectra What are near-ege spectra sensitive to? Pseuo Voigt peak fitting analysis X-ray Absorption Spectroscopy EXAFS oscillations (k 3 -weighte) Near-ege spectrum

2 Nomenclature There are a large number of names an acronyms in use they all refer to the same thing or are closely relate Ege Spectra Near-Ege Spectra Near-Ege X-ray Absorption Fine Structure (NEXAFS) X-ray Absorption Near-Ege Structure (XANES) Nomenclature Sometimes, but not always, XANES is use to refer to the region just above the ege, which is more reaily calculable using multiple scattering theory. XANES } } near-ege EXAFS Nomenclature What is a White Line? The term white line refers to an intense absorption in the near-ege. The nomenclature ates from the ays when spectra were recore on strips of photographic film, an such intense absorption peaks showe up as a heavily expose line on the evelope film. White Line photographic film White Line spectrum

3 X-ray absorption near-ege spectra Intense features arise ue excitation of transitions from the core level to vacant levels, close to the highest occupie molecular orbital. vacant orbital hν p, l=1 nucleus electron electron-hole 1s, l=0 s, l=0 What is a near-ege spectrum The photoelectron is excite to a variety of boun states lying below the threshol energy. Transitions to boun states observe spectrum Core level Threshol, E 0 Near-ege spectra X-ray absorption is given by Fermi s Golen Rule: µ ( E) = ψ i H ψ f ψ i - the initial state wavefunction ψ f - the final state wavefunction H -the interaction If we wish to quantify spectra, we have two alternatives evaluate the integral as completely as possible (molecular orbital approach) or use multiple scattering theory. Molecular orbital approach. A chemistry perspective the X-ray excites transitions between the core level an a molecular orbital. Quantification is non-trivial, but this approach is highly successful in unerstaning spectra. Multiple scattering approach. A physics perspective the X-ray excites a lowenergy photo-electron which unergoes extensive multiple scattering by nearby atoms. This success of this approach is limite (to ate). It usually cannot moel features ue to low-lying boun-state transitions.

4 What is a near-ege spectrum? Molecular orbital approach - transitions to bounstate molecular orbitals. σ* LUMO+1 S1s π* S1s σ* S OH O π* LUMO Spectral linewiths Two components contribute to the spectral linewith the core-hole lifetime an the optical resolution. Core-hole lifetime. Heisenberg s uncertainty principal states that: 1 E t h Thus, comparing high an low energy eges, we expect the higher energy ege to have shorter core hole lifetimes ( t) an corresponingly broaer experimental linewith ( E) (assuming that the spectroscopic resolution is not limiting). This as a Lorentzian component to the lineshape. Spectral linewiths Example aqueous solution of molybate [MoO 4 ] - measure at the K-ege (1s excitation) an the L I ege (s excitation). These are very similar groun states, an no significant ifferences in the nature of the near-ege transitions are expecte. The spectra have been offset by ev an ev, respectively. The K ege is has a much shorter core-hole lifetime than the L I ege, an has corresponing broaer linewiths. L I ege K ege

5 Spectral linewiths Spectrometer Resolution In a moern EXAFS beamline this is usually only a function of the monochromator. Each monochromator material has an inherent energy resolution - the Darwin with of the crystal. This as a Gaussian component to the overall experimental lineshape function. The experimental lineshape is expecte to be approximate by a convolution of a Gaussian an a Lorenztian ue to monochromator an lifetime broaening, respectively. This is known as a Voigt lineshape function in practice it can be approximate by the sum of a Gaussian an Lorenztian a pseuo Voigt lineshape function. Near-ege Spectra We can write Fermi s Golen Rule as: µ ψ i e ψ i( r ) ( e p) k f If we use a series expansion of the exponential, an examine just the first term, we get what is calle the ipole-allowe transitions. These are the most intense transitions observe, an can be thought of as being stimulate by an oscillating electric fiel. ψ i - the initial state wavefunction ψ f - the final state wavefunction e - the X-ray electric vector p - the electron momentum vector k - the X-ray forwar propagation vector r - the transition operator (x, y or z) in the molecular axis system Dipole an Quarupole Transitions Dipole transitions are escribe by: µ ψ e p) ψ D i ( f These are the most intense transitions observe, an can be thought of as being stimulate by an oscillating electric fiel, an have l= ±1. Incluing the next term in the series expansion gives quarupole transitions, which have l= ±, an these are escribe by: µ Q ψ i ( e p)( k r) ψ f Quarupole transitions are of low intensity an can be thought of as being stimulate by the electric fiel graient, which is significant ue to the short wavelength of the X-raiation being use.

6 Selection Rules for X-ray absorption near-ege spectra Transition Selection rule Strength K-ege L III -ege Dipole l=±1 Intense 1s np p n Quarupole l=± Weak 1s n p nf Transition K, L I, M I L II, L III, M II, M III M IV, M V Dipole ns n p np n n n f Quarupole ns n np n f n n g Tungsten L-eges Selection Rules XAS L-ege spectra of Na WO 4 W(VI) is 5 0, so we expect strong ipole allowe transitions to the 5 manifol at the L III an L II eges from the p 3/ an p 1/, respectively. No such intense transitions are expecte at the L I near-ege (s excitation). W L III W LII W L I Uranium M-eges Selection Rules XAS M-ege spectra of UO (CH 3 CO ) (H O) U(VI) is 5f 0, so we expect strong ipole-allowe transitions to the 5f manifol at the M V an M IV eges from the 3 3/ an 3 1/, respectively. No such intense transitions are expecte at the M III, M II (3p 3/ an 3p 1/ excitation, respectively) or M I (3s excitation) near-eges. O O OH U O O H O O O M I M IV M III M II M V

7 Dipole an Quarupole transitions Cu K-ege spectrum of [Cu(Imiazole) 4 ](NO 3 ) is 3 9. Spectra arise from 1s excitation, so we expect strong ipole allowe transitions to orbitals with a lot of 4p character, an a single weak quarupole allowe transition to the half-fille 3 level. Quarupole 1s 3 transition Dipole 1s 4p transitions x0 What o we expect about near-ege spectra? Intense features ue to ipole-allowe l=±1 transitions Weak features ue to quarupole-allowe l=± transitions For har X-ray spectra (i.e. E > 1500 ev) the core-hole lies eep within the atom. One consequence of this is that the final state of an absorber with atomic number Z approximates to that of Z+1 i.e. the next element in the perioic table. This can be important when comparing splittings measure from UV-visible electronic spectroscopy with X-ray near-ege spectra e.g splittings of Co + K near-ege spectra correspon optical spittings observe in the iso-structural Fe + compoun. What o we expect about near-ege spectra? For har X-ray spectra (i.e. E > 1500 ev) the ejection of a core electron will cause the outer orbitals to relax to lower energies (e.g. by about 10 ev for Cu K-ege spectra). This causes a corresponing shrinkage of the wave function, an thus reuction in the overlap integrals for molecular orbitals. We therefore expect the spectra to be very atomic in some of their properties.

8 Influence of core hole on electronic structure Ejection of metal core-electron causes outer metal orbitals to relax to lower energies. 4p 3 4p 3p 3p 3 1s metal ligan 1s metal ligan Groun state Final state What are near-ege spectra sensitive to? Oxiation State Pyrococcus furiosus rubreoxin Fe + Fe 3+ What are near-ege spectra sensitive to? Nature of the Ligans Ferric ions with sulfur an oxygen onors [Fe 3+ () 4 ] - [Fe 3+ (OR) 4 ] -

9 What are near-ege spectra sensitive to? Nature of the Ligans P 4 O 10 an P 4 S 10 are isostructural, both with P(V) oxiation state P 4 S 10 P 4 O 10 Covalency of sulfur means that phosphorus appears more reuce than its formal oxiation state What are near-ege spectra sensitive to? Coorination Geometry Oxygen coorinate ferric ions octaheral vs. tetraheral octaheral 1s 3 region tetraheral Transition metal MO 4 anions Similar chemical environments give rise to similar spectra VO 4 - K K K K L I K K CrO 4 KMnO 4 K FeO 4 Na WO 4 Na MoO 4 Ege energy Spectra have been offset to align the lowest energy transition

10 What are near-ege spectra sensitive to? Trigonal vs. Digonal cuprous thiolate compouns Inspection of the chemical literature inicates that Cu(I) prefers two istinct coorination environments linear two-coorinate (igonal) an planar three-coorinate (trigonal) coorination geometries, e.g. with thiolate ligans: Cu - - RS Cu Cuprous thiolate metalloproteins form a very large group of iverse function. Both two an three coorinate examples are known. Ligan Fiel Splitting Isolate atom egenerate p-orbital energies Molecule ligan-fiel splitting, p-orbital egeneracy lifte p z atom energy energy p x p y p z p x p y What are near-ege spectra sensitive to? Trigonal vs. Digonal cuprous thiolate compouns Cu(I) is 3 10, so we expect no quarupole transitions to the 3 manifol, an the lowest energy features in the near-ege shoul be 1s 4p transitions. Let us consier the ligan fiel splitting of the 4p orbitals. igonal trigonal istorte trigonal x z y Cu p x p z p y RS Cu p y p x p z RS Cu p x p z p y

11 Cu K near-ege spectra of igonal an trigonal cuprous thiolates Cu 1s 4p x,y igonal trigonal Cu 1s 4p x Cu K near-ege spectra of igonal an trigonal cuprous thiolates Trigonal vs. Digonal cuprous thiolate compouns The ~8983 ev peak is iagnostic of igonal Cu(I) coorination. It can be use as a fingerprint of this kin of metal coorination. Cu RS Cu 1s 3 transitions of transition metal ions Octaheral Fe 3+ with oxygen coorination a small quarupoleallowe, ipole-forbien 1s 3 peak is observe. The transition has structure is ue to the ligan fiel splitting of the 3 manifol. 1s 3 1s 3

12 Ligan Fiel Splitting Octaherally coorinate metal atom Ligan atom Those -orbitals with lobes irecte towars the ligan atoms will possess higher energies than those with lobes irecte in between the ligans. Energy z x y xy xz yz The energy separation of the orbitals is known as the ligan fiel splitting 1s 3 transitions of transition metal ions The size of the ligan fiel splitting (remember this is an excite state splitting) can tell us about the nature of the metal site. High-Spin vs. Low-Spin Ferrous x y z xy xz yz e g t g Low spin, R ion =0.9 Å x y z xy xz yz e g t g High spin, R ion =0.75 Å Low-spin Fe + occurs with larger, an gives rise to one peak of relatively increase intensity.

13 1s 3 transitons octaheral vs. tetraheral geometry Centrosymmetric (e.g. octaheral symmetry) mixing of metal 4p an 3 orbitals forbien, an the transition is pure quarupole. Non-centrosymmetric (e.g. tetraheral symmetry) mixing of metal 4p an 3 orbitals allowe, an the transition is quarupole, plus ipole-allowe intensity from amixture of metal 4p levels. Analysis by peak econvolution The experimental spectrum is fitte to a calculate spectrum comprise of a sum of pseuo Voigt peaks (I V ) plus a step function for the ege (I 0 ). This is usually one by iteratively minimizing the sum-of-squares of the ifferences between calculate an measure spectra. Each peak shoul comprise a single transition (or group of transitions) to a particular boun state (or states). µ calc( E) = a0i0( E) + aiivi ( E) i I Vi -psuo-voigt peak i I 0 -ege function a 0 - amplitue for ege function a i - amplitue for peak i This metho allows quantitative analysis of quite subtle changes in near-ege spectra. Analysis by peak econvolution I = mi + 1 V G I G = exp I L = ( m) I L ln ( E E ) m [( W + ( E E ) η) ξ ] m [ W + ( E Em ) η] [ W + ( E E ) η] + ( E E ) m m I V - the psuo-voigt function I G - Gaussian peak-shape function I L - Lorentzian peak-shape function m -mixing factor E m -peak position W half-with of peak η - peak skew ξ - ratio of Gaussian to Lorentzian withs

14 Analysis by peak econvolution Sulfur K-ege X-ray absorption near-ege spectra Sulfur K Near-ege spectra of biological moel compouns. SO - 4 RSO - 3 SO - 3 RSO - The spectra are very sensitive to the chemical form of sulfur an can be use to fingerprint forms of sulfur present. x x x x x RS=O R 3 S + RS-Me RS-H RS- S 8 Fe 4 S 4