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Core level spectroscopy of solids. CRC Press, Boca Raton

  • March 2008
DOI:10.1201/9781420008425
Authors:
Frank M F De Groot at Utrecht University
Frank M F De Groot
Akio Kotani
Akio Kotani
Akio Kotani
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Abstract

INTRODUCTION FUNDAMENTAL ASPECTS OF CORE LEVEL SPECTROSCOPIES Core holes Overview of core level spectroscopies Interaction of x-rays with matter Optical transition operators and x-ray absorption spectrum The interaction of electrons with matter X-ray sources Electron sources MANY-BODY CHARGE-TRANSFER EFFECTS IN XPS AND XAS Introduction Many-body charge-transfer effects in XPS General expressions of many-body effects General effects in XPS spectra Typical examples of XPS spectra Many-body charge-transfer effects in XAS Comparison of XPS and XAS CHARGE TRANSFER MULTIPLET THEORY Atomic multiplet theory Ligand field multiplet theory The charge transfer multiplet theory X-RAY PHOTOEMISSION SPECTROSCOPY Introduction Experimental aspects XPS of TM compounds XPS of RE compounds Resonant photoemission spectroscopy Hard XPS Resonant inverse photoemission spectroscopy Nonlocal screening effect in XPS Auger photoemission coincidence spectroscopy Spin polarization and magnetic dichroism in XPS X-RAY ABSORPTION SPECTROSCOPY Basics of XAS Experimental aspects The L2, 3 edges of 3d TM systems Other x-ray absorption spectra of the 3d TM systems X-ray absorption spectra of the 4d and 5d TM systems X-ray absorption spectra of the 4f RE and 5f actinide systems X-RAY MAGNETIC CIRCULAR DICHROISM Introduction XMCD effects in the L2, 3 edges of TM ions and compounds Sum rules XMCD effects in the K edges of transition metals XMCD effects in the M edges of rare earths XMCD effects in the L edges of rare earth systems Applications of XMCD RESONANT X-RAY EMISSION SPECTROSCOPY Introduction Rare earth compounds High Tc Cuprates and related materials Nickel and Cobalt compounds Iron and Manganese compounds Early transition metal compounds Electron spin states detected by RXES and NXES MCD in RXES of ferromagnetic systems APPENDICES Precise derivation of XPS formula Derivation of Eq. (88) in Chapter 3 Fundamental tensor theory Derivation of the orbital moment sum rule Theoretical test of the spin sum rule Calculations of XAS spectra with single electron excitation models REFERENCES INDEX
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Core Level
Spectroscopy
of Solids
Frank de Groot
Akio Kotani
CRC Press
Taylor &Francis Group
Boca Raton London New York
CRC Press is an imprint of the
Taylor & Francis Group, an
Informa
business
Contents
Preface
xv
Acknowledgments
xvii
Authors
xix
Chapter 1
Introduction
1
Chapter
2 Fundamental Aspects of Core Level Spectroscopies
11
2.1 Core Holes
11
2.1.1 Creation of Core Holes
11
2.1.2 Decay of Core Holes
12
2.2 Overview of Core Level Spectroscopies
14
2.2.1 Core Hole Spin—Orbit Splitting
14
2.2.2 Core Hole Excitation Spectroscopies
15
2.2.3 Core Hole Decay Spectroscopies
18
2.2.4 Resonant Photoelectron Processes
19
2.2.5 Resonant X-Ray Emission Channels
22
2.2.6 Overview of the RXES and NXES Transitions
23
2.3 Interaction of X-Rays with Matter
25
2.3.1 Electromagnetic Field
26
2.3.2 Transition to Quantum Mechanics
26
2.3.3 Interaction Hamiltonian
27
2.3.4 Golden Rule
27
2.4 Optical Transition Operators and X-Ray Absorption Spectra
28
2.4.1 Electric Dipole Transitions
29
2.4.2 Electric Quadrupole Transitions
29
2.4.3 Dipole Selection Rules
29
2.4.4 Transition Probabilities, Cross Sections, and Oscillator Strengths
30
2.4.5 Cross Section, Penetration Depth, and Excitation Frequency
31
2.4.6 X-Ray Attenuation Lengths
32
2.5 Interaction of Electrons with Matter
32
2.6 X-Ray Sources
34
2.6.1 Synchrotron Radiation Sources
34
2.6.2 X-Ray Beamlines and Monochromators
35
2.6.3 Other X-Ray Sources
36
2.7 Electron Sources
37
vii
VIII
Chapter 3
Many-Body Charge-Transfer Effects in XPS and XAS
39
3.1 Introduction
39
3.2 Many-Body Charge-Transfer Effects in XPS
40
3.2.1 Basic Description of the XPS Process
40
3.3 General Expressions of Many-Body Effects
42
3.3.1 General Description
42
3.3.2 Generating Function and Dielectric Response
44
3.3.3 XPS Spectrum and Its Limiting Forms
45
3.3.3.1 Slow Modulation Limit
47
3.3.3.2 Rapid Modulation Limit
47
3.4 General Effects in XPS Spectra
47
3.4.1 Screening by Free-Electron-Like Conduction Electrons
47
3.4.2 Screening by Lattice Relaxation Effects
49
3.4.3 Shake-Up Satellites
50
3.4.4 Lifetime Effects
50
3.4.4.1 Auger Transition
50
3.4.4.2 Radiative Transition
51
3.5 Typical Examples of XPS Spectra
52
3.5.1 Simple Metals
52
3.5.2 La Metal
56
3.5.2.1 Final State of Type (A)
59
3.5.2.2 Final State of Type (B)
60
3.5.3 Mixed Valence State in Ce Intermetallic Compounds
62
3.5.4 Insulating Mixed Valence Ce Compounds
67
3.5.5 Transition Metal Compounds
71
3.5.5.1 Model
71
3.5.5.2 Simplified Analysis
72
3.5.5.3 Case A: A
f
> 0(0>
Udc)
74
3.5.5.4 Case B:
0 (0 5_ U
d
c)
74
3.6 Many-Body Charge-Transfer Effects in XAS
76
3.6.1 General Expressions of Many-Body Effects
76
3.6.2 XAS in Simple Metals
76
3.6.3 XAS in La Metal
78
3.6.3.1 Case A:
e
f
<
8F
79
3.6.3.2 Case B:
E
F
80
3.6.4 Ce 3d XAS of Mixed Valence Ce Compounds
81
3.6.5 Ce L
3
XAS
83
3.6.6 XAS in Transition Metal Compounds
87
3.7 Comparison of XPS and XAS
89
Chapter 4
Charge Transfer Multiplet Theory
93
4.1 Introduction
93
4.2 Atomic Multiplet Theory
95
4.2.1 Term Symbols
96
4.2.2 Some Simple Coupling Schemes
98
ix
4.2.3 Term Symbols of d-Electrons
101
4.2.4 Matrix Elements
105
4.2.5 Energy Levels of Two d-Electrons
107
4.2.6 More Than Two Electrons
108
4.2.7 Matrix Elements of the 2p
3
Configuration
109
4.2.8 Hund's Rules
110
4.2.9 Final State Effects of Atomic Multiplets
111
4.3 Ligand Field Multiplet Theory
115
4.3.1 Ligand Field Multiplet Hamiltonian
116
4.3.2 Cubic Crystal Fields
117
4.3.3 Definitions of the Crystal Field Parameters
119
4.3.4 Energies of the 3d
n
Configurations
120
4.3.5 Symmetry Effects in
D
4h
Symmetry
124
4.3.6 Effect of the 3d Spin–Orbit Coupling
125
4.3.7 Consequences of Reduced Symmetry
126
4.3.8 3d° Systems in Octahedral Symmetry
126
4.3.9
Ab Initio
LFM Calculations
132
4.4 Charge Transfer Multiplet Theory
133
4.4.1 Initial State Effects
134
4.4.2 Final State Effects
137
4.4.3 XAS Spectrum with Charge-Transfer Effects
138
4.4.4 Small Charge-Transfer Satellites in 2p XAS
140
4.4.5 Large Charge-Transfer Satellites in 2p XPS
141
4.4.5.1 3d° Compounds
142
4.4.5.2 3d
8
Compounds
143
Chapter
5 X-Ray Photoemission Spectroscopy
145
5.1 Introduction
145
5.2 Experimental Aspects
146
5.3 XPS of TM Compounds
146
5.3.1 2p XPS
146
5.3.2 Zaanen–Sawatzky–Allen Diagram
152
5.3.3 2p XPS in Early TM Systems
154
5.3.4 Effect of Multiplet Coupling an 0 and
U
dd
158
5.3.5 3s XPS
160
5.3.6 3p XPS
164
5.4 XPS of RE Compounds
165
5.4.1 Simplified Analysis for RE Oxides
165
5.4.2 Application of Charge-Transfer Multiplet Theory
169
5.5 Resonant Photoemission Spectroscopy
176
5.5.1 Fundamental Aspects of RPES
177
5.5.2 RPES in Ni Metal and TM Compounds
180
5.5.2.1 3p RPES in Ni Metal
180
5.5.2.2 2p RPES in TM Compounds
182
5.5.2.3 3p RPES in NiO
185
x
5.5.3 3d and 4d RPES of Ce Compounds
185
5.5.4 Resonant XPS
187
5.5.5 Resonant Auger Electron Spectroscopy
188
5.5.6 Reducing the Lifetime Broadening in XAS
191
5.5.7 EQ and ED Excitations in the Pre-Edge of Ti ls XAS of TiO
2
191
5.6 Hard X-Ray Photoemission Spectroscopy
197
5.6.1 2p HAXPS of Cuprates
197
5.6.2 2p HAXPS of V
2
0
3
and La
i
,Sr
x
Mn0
3
198
5.6.3 Ce Compounds: Surface/Bulk Sensitivity
199
5.6.4 Resonant HAXPS of Ce Compounds
202
5.7 Resonant Inverse Photoemission Spectroscopy
205
5.8 Nonlocal Screening Effect in XPS
212
5.9 Auger Photoemission Coincidence Spectroscopy
218
5.10 Spin-Polarization and Magnetic Dichroism in XPS
221
5.10.1 Spin-Polarized Photoemission
221
5.10.2 Spin-Polarized Circular Dichroic Resonant Photoemission
221
Chapter 6
X-Ray Absorption Spectroscopy
225
6.1 Basics of X-Ray Absorption Spectroscopy
225
6.1.1 Metal
L2 ,
3
Edges
228
6.2 Experimental As pects
228
6.2.1 Transmission Detection
229
6.2.2 Energy Dispersive X-Ray Absorption
229
6.2.3 Fluorescence Yield
229
6.2.4 Self-Absorption Effects in Fluorescence Yield Detection
230
6.2.5 Nonlinear Decay Ratios and Distortions in Fluorescence
Yield Spectra
230
6.2.6 Partial Fluorescence Yield
230
6.2.7 Electron Yield
231
6.2.8 Partial Electron Yield
231
6.2.9 Ion Yield
232
6.2.10 Detection of an EELS Spectrum
232
6.2.11 Low-Energy EELS Experiments
233
6.2.12 Space: X-Ray Spectromicroscopy and TEM-EELS
233
6.2.13 Time-Resolved X-Ray Absorption
234
6.2.14 Extreme Conditions
235
6.3
L
2,3
Edges of 3d TM Systems
235
6.3.1 3d° Systems
236
6.3.2 3d' Systems
237
6.3.2.1 V0
2
and LaTiO
3
237
6.3.3 3d
2
Systems
237
6.3.4 3d
3
Systems
238
6.3.5 3d
4
Systems
239
6.3.5.1 LaMnO
3
240
6.3.5.2 Mixed Spin Ground State in LiMnO
2
240
XI
6.3.6 3d
5
Systems
241
6.3.6.1 MnO
241
6.3.6.2 Fe
2
0
3
242
6.3.6.3 Fe
3÷
(tacn)
2
243
6.3.6.4 Fe
3+
(CN)
6
243
6.3.6.5 Intermediate Spin State of SrCo0
3
244
6.3.7 3d
6
Systems
245
6.3.7.1 Effect of 3d Spin—Orbit Coupling in Fe
2
SiO
4
246
6.3.7.2 Co
3+
Oxides
247
6.3.8 3d
7
Systems
248
6.3.8.1 Effects of 3d Spin—Orbit Coupling an the
Ground State of Co
2+
248
6.3.8.2 Mixed Spin Ground State in PrNiO
3
249
6.3.9 3d
8
Systems
251
6.3.9.1 NiO
251
6.3.9.2 High-Spin and Low-Spin Ni
2+
and Cu
3÷
Systems
251
6.3.10 3d
9
Systems
253
6.4 Other X-Ray Absorption Spectra of the 3d TM Systems
254
6.4.1 TM
M
2,3
Edges
254
6.4.2 TM M
1
Edges
255
6.4.3 TM K Edges
255
6.4.4 Ligand K Edges
260
6.4.4.1 Oxygen K Edges of High T
c
Copper Oxides
264
6.4.5 Soft X-Ray K Edges by X-Ray Raman Spectroscopy
264
6.4.5.1 Modifying the Selection Rules
265
6.5 X-Ray Absorption Spectra of the 4d and 5d TM Systems
265
6.5.1
L
2,3
Edges of 4d TM Systems
266
6.5.2 Picosecond Time-Resolved 2p XAS Spectra of [Ru(bpy)
3
]
2+
268
6.5.3 Higher Valent Ruthenium Compounds
269
6.5.4 Pd L Edges and the Number of 4d Holes in Pd Metal
270
6.5.5 X-Ray Absorption Spectra of the 5d Transition Metals
271
6.6 X-Ray Absorption Spectra of the 4f RE and 5f Actinide Systems
272
6.6.1
M
4,5
Edges of Rare Earths
273
6.6.1.1
M
4,5
Edge of Tm
274
6.6.1.2
M
4,5
Edge of La
3+
277
6.6.1.3
M
4,5
Edge of Ce0
2
278
6.6.2
N
4,5
Edges Of Rare Earths
278
6.6.3
L
2,3
Edges of Rare Earths
281
6.6.4
0
4,5
Edges of Actinides
282
6.6.5
M
4
,
:
5
Edges of Actinides
282
Chapter
7
X-Ray Magnetic Circular Dichroism
287
7.1 Introduction
287
7.2 XMCD Effects in the
L
2,3
Edges of TM Ions and Compounds
288
7.2.1 Atomic Single Electron Model
288
7.2.2 XMCD Effects in Ni
2+
293
xii
7.2.3 XMCD of Cr0
2
297
7.2.4 Magnetic X-Ray Linear Dichroism
297
7.2.5 Orientation Dependence of XMCD and XMLD Effects
298
7.2.6 XMLD for Doped LaMnO
3
Systems
299
7.3 Sum Rules
299
7.3.1 Sum Rules for Orbital and Spin Moments
299
7.3.2 Application of the Sum Rules to Fe and Co Metals
302
7.3.3 Application of the Sum Rules to Au/Co-Nanocluster/
Au Systems
304
7.3.4 Limitations of the Sum Rules
308
7.3.5 Theoretical Simulations of the Spin Sum Rule
309
7.4 XMCD Effects in the K Edges of Transition Metals
310
7.4.1 X-Ray Natural Circular Dichroism and X-Ray Optical Activity
311
7.5 XMCD Effects in the M Edges of Rare Earths
312
7.5.1 XMCD and XMLD Effects from Atomic Multiplets
312
7.5.2 Temperature Effects an the XMCD and XMLD
314
7.6 XMCD Effects in the L Edges of Rare Earth Systems
314
7.6.1 Effects of 4f5d Exchange Interaction
315
7.6.2 Contribution of Electric Quadrupole Transition
319
7.6.3 Effect of Hybridization between RE 5d and TM 3d States
319
7.6.4 XMCD at L Edges of R
2
Fe
14
B (R = La–Lu)
320
7.6.5 Mixed Valence Compound CeFe
2
324
7.6.6 Multielectron Excitations
328
7.7 Applications of XMCD
329
7.7.1 Magnetic Oxides
329
7.7.2 Thin Magnetic (Multi)layers, Interface, and
Surface Effects
330
7.7.3 Impurities, Adsorbates, and Metal Chains
332
7.7.4 Magnetic Nanoparticles and Catalyst Materials
333
7.7.5 Molecular Magnets
333
7.7.6 Metal Centers in Proteins
334
Chapter 8
Resonant X-Ray Emission Spectroscopy
335
8.1 Introduction
335
8.1.1 Experimental Aspects of XES (RXES and NXES)
337
8.1.1.1 Detectors for Soft X-Ray XES
338
8.1.1.2 Detectors for Hard X-Ray XES
338
8.1.1.3 X-Ray Raman Allows Soft X-Ray XAS under
Extreme Conditions
338
8.1.2 Basic Description and Some Theoretical Aspects
338
8.2 Rare Earth Compounds
343
8.2.1 Effect of Intra-Atomic Multiplet Coupling
343
8.2.2 Effect of Interatomic Hybridization in Ce0
2
and PrO
2
348
8.2.3 Metallic Ce Compounds with Mixed-Valence Character
351
8.2.4 Kondo Resonance in Yb Compounds
354
XIII
8.2.5 Dy 2p3d RXES Detection of the 2p4f EQ Excitation
357
8.2.6 EQ Excitations in Light Rare Earth Elements
360
8.3 High T
c
Cuprates and Related Materials
363
8.3.1 Cu 2p3d RXES
363
8.3.2 Cu 1s4p RXES
367
8.3.3 Cu ls2p RXES
373
8.3.4 0 ls2p RXES
377
8.4 Nickel and Cobalt Compounds
380
8.4.1 Ni 2p3d RXES in NiO: Charge Transfer Excitations
380
8.4.2 Ni 2p3d RXES in NiO: dd Excitations
384
8.4.3 Ni 2p3d RXES in NiO: Spin-Flip Excitations
386
8.4.4 Ni ls4p RXES of NiO: Pressure Dependence
387
8.4.5 Co 2p3d RXES in CoO and Other Co Compounds
389
8.4.6 Co 1s2p RXES of CoO: Effect of Resolution
389
8.4.7 Co ls2p RXES: Nonlocal Dipole Transitions
391
8.5 Iron and Manganese Compounds
393
8.5.1 Fe ls2p RXES of Iron Oxides: 2D RXES Images
393
8.5.2 HERFD-XAS of Iron Oxides
395
8.5.3 Fe 2p XAS Spectra Measured at the Fe K Edge
397
8.5.4 Valence Selective XAS
397
8.5.5 Mn 2p3d RXES of MnO
399
8.5.6 Mn 2p3d RXES: Interplay of dd and Charge
Transfer Excitations
402
8.5.7 Mn ls4p RXES of LaMn0
3
405
8.5.8 Mn and Ni ls3p XES: Chemical Sensitivity
406
8.5.9 Mn ls3p XES: K Capture Versus X-Ray lonization
408
8.5.9.1 Atomic Multiplet Calculation
409
8.5.9.2 LFM Calculation
410
8.5.9.3 Charge Transfer Multiplet Calculation
410
8.5.9.4 Coherent Calculation of Mn ls3p NXES Spectra
411
8.6 Early Transition Metal Compounds
412
8.6.1 Ca 2p3s RXES in CaF
2
413
8.6.2 Ti 2p3d RXES of Ti0
2
: Polarization Dependence
415
8.6.3 Sc 2p3d RXES of the ScF
3
, ScC1
3
, and ScBr
3
420
8.6.4 TM 2p3d RXES of d"
(n
1, 2, 3) Systems
420
8.6.5 V 2p3d RXES of Vanadium Oxides
423
8.7 Electron Spin States Detected by RXES and NXES
423
8.7.1 Local Spin-Selective Excitation Spectra
423
8.7.2 Spin-Dependent TM ls3p NXES Spectra
425
8.7.3 TM ls3p NXES and Spin-Transitions
426
8.7.4 Local-Spin Selective XAS and XMCD
429
8.8 MCD in RXES of Ferromagnetic Systems
429
8.8.1 Longitudinal and Transverse Geometries in MCD-RXES
429
8.8.2 MCD-RXES in LG of CeFe
2
433
8.8.3 Experiments and Theory of MCD-RXES in TG
435
xiv
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
Appendix F
Precise Derivation of XPS Formula
439
Derivation of Equation 3.88 in Chapter 3
443
Fundamental Tensor Theory
447
Derivation of the Orbital Moment Sum Rule
451
Theoretical Test of the Spin Sum Rule
453
Calculations of XAS Spectra with Single Electron
Excitation Models
457
References
463
Index
483
... The XPS spectra were processed using CasaXPS software (Fairley, 2010). All spectra were fitted using a Shirley background and normalised to bulk signal to allow comparison between spectra (Shirley, 1972). ...
... In addition, simulations were set with an octahedral symmetry and negative 10Dq value to simulate a tetrahedral coordination (Stavitski and de Groot, 2010). Finally, charge transfer parameters were employed to describe the charge transfer effect (De Groot and Kotani, 2008). The charge transfer energy, Δ, gives the energy difference between 3d n and 3d n+1 L states. ...
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Article
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  • Nov 2022
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Implementation of chemical energy storage for a sustainable energy supply requires the rational improvement of electrocatalyst materials, for which their nature under reaction conditions needs to be revealed. For a better understanding of earth‐abundant metal oxides as electrocatalysts for the oxygen evolution reaction (OER), the combination of electrochemical (EC) methods and X‐ray absorption spectroscopy (XAS) is very insightful, yet still holds untapped potential. Herein, we concisely introduce EC and XAS, providing the necessary framework to discuss changes that electrocatalytic materials undergo during preparation and storage, during immersion in an electrolyte, as well as during application of potentials, showing Mn oxides as examples. We conclude with a summary of how EC and XAS are currently combined to elucidate active states, as well as an outlook on opportunities to understand the mechanisms of electrocatalysis using combined operando EC‐XAS experiments.
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Implementation of chemical energy storage for a sustainable energy supply requires the rational improvement of electrocatalyst materials, for which their nature under reaction conditions needs to be revealed. For a better understanding of earth-abundant metal oxides as electrocatalysts for the oxygen evolution reaction (OER), the combination of electrochemical (EC) methods and X-ray absorption spectroscopy (XAS) is very insightful, yet still holds untapped potential. Herein, we concisely introduce EC and XAS, providing the necessary framework to discuss changes that electrocatalytic materials undergo during preparation and storage, during immersion in an electrolyte, as well as during application of potentials, showing Mn oxides as examples. We conclude with a summary of how EC and XAS are currently combined to elucidate active states, as well as an outlook on opportunities to understand the mechanisms of electrocatalysis using combined operando EC-XAS experiments.
XPS photoelectron lines, satellite structures and Wagner plot of Cu(II) β-diketonato complexes explained in terms of its electronic environment
Article
A series of copper(II) β-diketonato complexes [Cu(R¹CHCOCHR²)2], with R¹ = R² = C6H5 (1); R¹ = C6H5 and R² = CH3 (2); R¹ = R² = CH3 (3); R¹ = C6H5 and R² = CF3 (4); R¹ = CH3 and R² = CF3 (5) and R¹ = R² = CF3 (6), was subjected to a systematic XPS study of the Cu 2p core-level photoelectron lines, their satellite structures and the LMM Auger lines. The binding energies of the main Cu 2p core-level photoelectron lines are sensitive towards chemical environment (initial state effects) and are described by linear correlations with the combined Gordy scale group electronegativity of the R-groups on the β-diketonato ligands, ΣχR, and the first electrochemical reduction potential, Epc, of the Cu compounds. The modified Auger parameters were determined from the constructed Wagner plot of the copper(II) β-diketonato complexes, which also showed that the initial state effects are dominant during the photoemission process.
Interpretation of Shakeup Mechanisms in Copper L-Shell Photoelectron Spectra
Article
  • Jul 2022
We report on an original full ab initio quantum molecular approach designed to simulate Cu 2p X-ray photoelectron spectra. The description includes electronic relaxation/correlation and spin–orbit coupling effects and is implemented within nonorthogonal sets of molecular orbitals for the initial and final states. The underlying mechanism structuring the Cu 2p photoelectron spectra is clarified thanks to a correlation diagram applied to the CuO4C6H6 paradigm. This diagram illustrates how the energy drop of the Cu 3d levels following the creation of the Cu 2p core hole switches the nature of the highest singly occupied molecular orbital (H-SOMO) from dominant metal to dominant ligand character. It also reveals how the repositioning of the Cu 3d levels induces the formation of new bonding and antibonding orbitals from which shakeup mechanisms toward the relaxed H-SOMO operate. The specific nature, ligand → ligand and metal → ligand, of these excitations building the satellite lines is exposed. Our approach finally applied to the real Cu(acac)2 system clearly demonstrates how a definite interpretation of the XPS spectra can be obtained when a correct evaluation of binding energies, intensities, and relative widths of the spectral lines is achieved.
An In-depth Theoretical Study of Ligand Field and Charge Transfer Effects on Co 2+ 2p L 2， 3 -edges X-ray Absorption Spectra
Article
Full-text available
Co 2+ 离子的 2p 电子 X 射线 L 2,3 吸收边光谱, 研究了 Co 2+ 离子和周围的配位离子之间的正八面体(O h)晶体场效应和相应的电荷转移效应对于吸收光谱的影 响. 系统讨论了在多重态计算中起作用的所有物理参数对 CoO 和 CoCl 2 的 X 射线吸收光谱特性的特定影响及 其物理机制. 将计算得出的光谱数据和同样具有 O h 对称性结构 Co 2+ 离子的 CoO 和 CoCl 2 实验光谱数据进行了 对比, 在实验光谱数据中发现的特征被确定为来自不同自旋态, 并且光谱强度的变化与晶体场的强度相关, 揭 示了其中包含的电荷转移效应. 本文为低对称性复杂系统的多重态计算提供了一个基础的参考标准, 可以适 用于含有钴元素或其它过渡金属的复杂体系的 X 射线吸收光谱的理论计算. 关键词 多重态计算；X 射线吸收光谱；钴元素；晶体场效应；电荷转移效应 中图分类号 O641 文献标志码 A Abstract The 2p L 2， 3-edges X-ray absorption spectrum of Co 2+ in octahedral (O h)symmetric ligand field is studied theoretically via the multiplet calculation method. The octahedral symmetric crystal field and corre• sponding charge transfer effect between ligands and Co 2+ cation have been investigated. All the parameters used in the multiplet calculation are discussed systematically in regards to their specific effect on X-ray absorp• tion features and related mechanisms. The calculated results are compared with the Ledge X-ray absorption spectra of CoO and CoCl 2 ，both with the same O h symmetrical local structure of the Co 2+ cation. The experimen• tally observed multiplet spectra features are assigned to different spin states and their intensity change is related to the crystal field strength. The underlying charge transfer effect has also been revealed. This study pro• vides a fundamental basis for multiplet calculations of complex systems with lower symmetry associated with cobalt and other transition metals. Keywords Multiplet calculation；X-ray absorption spectrum；Cobalt；Crystal field effect；Charge transfer effect
Pressure-Driven Changes in the Electronic Bonding Environment of GeO 2 Glass above Megabar Pressures
Article
  • May 2022
Noncrystalline oxides under pressure undergo gradual structural modifications, highlighted by the formation of a dense noncrystalline network topology. The nature of the densified networks and their electronic structures at high pressures may account for the mechanical hardening and the anomalous changes in electromagnetic properties. Despite its importance, direct probing of the electronic structures in amorphous oxides under compression above the Mbar pressure (>100 GPa) is currently lacking. Here, we report the observation of pressure-driven changes in electronic configurations and their delocalization around oxygen in glasses using inelastic X-ray scattering spectroscopy (IXS). In particular, the first O K-edge IXS spectra for compressed GeO2 glass up to 148 GPa, the highest pressure ever reached in an experimental study of GeO2 glass, reveal that the glass densification results from a progressive increase of oxygen proximity. While the triply coordinated oxygen [3]O is dominant below ∼50 GPa, the IXS spectra resolve multiple edge features that are unique to topologically disordered [4]O upon densification above 55 GPa. Topological compaction in GeO2 glass above 100 GPa results in pronounced electronic delocalization, revealing the contribution from Ge d-orbitals to oxide densification. Strong correlations between the glass density and the electronic configurations beyond the Mbar conditions highlight the electronic origins of densification of heavy-metal-bearing oxide glasses. Current experimental breakthroughs shed light on the direct probing of the electronic density of states in high-Z oxides above 1 Mbar, offering prospects for studies on the pressure-driven changes in magnetism, superconductivity, and electronic transport properties in heavy-metal-bearing oxides under compression.
Direct and real-time observation of hole transport dynamics in anatase TiO2 using X-ray free-electron laser
Article
Full-text available
  • May 2022
Carrier dynamics affects photocatalytic systems, but direct and real-time observations in an element-specific and energy-level-specific manner are challenging. In this study, we demonstrate that the dynamics of photo-generated holes in metal oxides can be directly probed by using femtosecond X-ray absorption spectroscopy at an X-ray free-electron laser. We identify the energy level and life time of holes with a long life time (230 pico-seconds) in nano-crystal materials. We also observe that trapped holes show an energy distribution in the bandgap region with a formation time of 0.3 pico-seconds and a decay time of 8.0 pico-seconds at room temperature. We corroborate the dynamics of the electrons by using X-ray absorption spectroscopy at the metal L-edges in a consistent explanation with that of the holes. Direct and real-time observation of the excess holes in an element-specific and energylevel-specific manner is a challenging task. Here the authors present a complete hole dynamics of photo-excited anatase TiO2 with 100 femto-second resolution.
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