Soft X ray Spppyectroscopy for Physics (and...

SESAME-JSPS School

Soft XSoft X‐‐ray Spectroscopyray Spectroscopyy p pyy p pyfor Physics (and Chemistry)for Physics (and Chemistry)

Kenta Kenta AmemiyaAmemiya (KEK(KEK--PFPF))yy (( ))

Studies using Soft XStudies using Soft X--rayraySoft X-ray (& VUV) Beamlines

~14/50 at Photon Factory (2.5 & 6.5 GeV)y ( )~5/50 at Spring-8 (8 GeV)

E i t l T h iExperimental TechniquesX-ray Absorption Spectroscopy (XAS)y y ( )Photoemission SpectroscopyX ray ScatteringX-ray Scattering

Applications:Organic Molecules & PolymersMagnetic Materials (Fe Co Ni )Magnetic Materials (Fe, Co, Ni, …)Surface & Thin Film

Absorption Edges in the Soft XAbsorption Edges in the Soft X--ray Regionray Region

M edgeum

ber

L edge

tom

ic N

u L edge

At

K edge

Li.

Absorption-edge Energy (eV)

Soft XSoft X‐‐ray Absorption Spectroscopyray Absorption Spectroscopy

1. Advantages and Disadvantages of Soft X-ray Absorption Spectroscopy (SXAS)

2. SXAS studies on Surface and Thin films

3. Soft X-ray Beamlines

4 N l SXAS T h i D th l d XAS4. Novel SXAS Technique: Depth-resolved XAS

Soft XSoft X--ray Absorption Spectroscopy ray Absorption Spectroscopy (~100(~100--4000 4000 eVeV))

1. Element selectivity Core-hole excitation (1s, 2p…)(C: 290 eV O: 530 eV Fe: 710 eV Ni: 850 eV )

10

12

h

arb.

uni

ts)

C K edge

(C: 290 eV, O: 530 eV, Fe: 710 eV, Ni: 850 eV…)

2. Information on chemical species Characteristic spectral features (π*，σ*…)

4

6

8 = 15 o

= 55 o

= 90 o

on Y

ield

(a

C-S

Characteristic spectral features (π ，σ …)

3. Structural information (bond length, etc.)EXAFS (Extended X-ray Absorption Fine Structure)

0

2

4

rtial

Ele

ctro C S

C-C

S ( te ded ay bso pt o e St uctu e)

4. Information on anisotropy Linear polarization

280 290 300 310

Photon Energy (eV)

Par (molecular orientation, lattice anisotropy)

5. Magnetic information Circular polarization Circular polarizationXMCD (X-ray Magnetic Circular Dichroism)

6. High sensitivity

XAS XAS Measurement in the Soft XMeasurement in the Soft X--ray Regionray Region

0 025

3 ML Fe / Cu(100) Fe L-edge XAS How can we measureX-ray absorption spectrum ?

0 02

0.025

units

)

X-rays

2p3/2→3d(LIII)

~mm ~nm

A0 015

0.02

y (a

rb. u

X rays

Auger electrons

Core hole

0.01

0.015

Inte

nsity

Detector

(core hole relaxation)2p1/2→3d

(LII)

0.005sorp

tion

etecto(Retarding voltage)

Electron yield XAS

0700 710 720 730 740 750

Abs

ect o y e d STotal electron yield (TEY)

Partial electron yield (PEY)700 710 720 730 740 750

Photon Energy (eV)Partial electron yield (PEY)

cf. Fluorescence yield (FY)

Advantages and Disadvantages of SXASAdvantages and Disadvantages of SXASSh t P t ti L thShort Penetration Length

Transmission mode can be available only for a very thin sample on avery thin or without substrate.

Electron yield mode is usually adopted because of high efficiency.Special care is necessary for insulators (powders might be OK).

Fluorescence yield efficiency is very small for light elements.y y y g<1 % for C, N, O Be careful for the self absorption (saturation) effect.

Samples should be usually kept in vacuum (NOT ultra-high vacuum).

Surface Sensitive

Samples should be usually kept in vacuum (NOT ultra high vacuum).Some attempts have been made to realize ambient-pressure or liquid-state measurements.

Surface SensitiveSub-monolayer samples can be investigated.

= several nm for electron yield.)B lk information is hardl obtained especiall in the electron ield mode

Sensitive to Electronic and Magnetic States of light elements

Bulk information is hardly obtained, especially in the electron yield mode.( ~ 0.1 μm for fluorescence yield.)

g gValence electrons can be directly investigated by 1s2p excitation ofC, N, O,… and 2p3d excitation of 3d transition metals (Fe, Co, NI,…).

1. Advantages and Disadvantages of Soft X-ray Absorption Spectroscopy (SXAS)

2. SXAS studies on Surface and Thin films

3. Soft X-ray Beamlines

4 N l SXAS T h i D th l d XAS4. Novel SXAS Technique: Depth-resolved XAS

NearNear--edge Spectroscopyedge SpectroscopyNear edge X ray Absorption Fine Structure (NEXAFS)Near-edge X-ray Absorption Fine Structure (NEXAFS)X-ray Absorption Near-edge Structure (XANES)

Chemical speciesStructural information (orientation)

Determination of Chemical Species

Thiophene (C4H4S) molecule on Au(111)NearNear--edge Spectroscopyedge SpectroscopyDetermination of Chemical Species

Initial oxidation process of SiDipped in thiophene

Molecular O2

Dipped in thiophenesolution

OxidationC6H13SH

Exposed to thiopheneposed to t op e egas in vacuum

Existence of molecular oxygen Different chemical species depending ygin the initial stage of Si oxidation

p p gon preparation processes

Matsui et al., Phys. Rev. Lett. 85, (2000) 630. Sako et al., Chem. Phys. Lett. 413, (2005) 267.

Determination of Atomic StructureDetermination of Atomic StructureE t d d X Ab ti Fi St t (EXAFS)Extended X-ray Absorption Fine Structure (EXAFS)

• Coordination number (N)• Inter-atomic distance (r)• DW factor (σ2)

(k) S 2 NiFi(ki) e2ki2 i

2

e2ri /i (k ) sin(2k r (k ))

EXAFS oscillation

(k) S0 kiri2 e i i e i i

i

sin(2kiri i(ki))

Fe K-edge XAFS spectrum μ(E) of FeOFe K edge XAFS spectrum μ(E) of FeO

Amemiya et al Phys Rev B 59 (1999) 2307Determination of Atomic StructureDetermination of Atomic Structure

Application to surface molecule (CH3O)

CH O/Cu(111)

Amemiya et al., Phys. Rev. B 59, (1999) 2307.

CH3O/Cu(111)

CH3O/Cu(111)

EXAFSNEXAFS

CH3O/Ni(111)

CH3OH (solid)Oscillation period -> O-Cu bond lengthAngle (θ) dependence -> bond angle

adsorption site

Peak B (1s→σ*CO) -> C-O bond length

adsorption site

Peak B (1s→σ CO) C O bond lengthAngle (θ) dependence -> molecular orientation

3 ML F / C (100)

Magnetic structures studied by XMCDMagnetic structures studied by XMCD

0.03s) H li it S i

3 ML Fe / Cu(100)Fe L-edge XMCD X-ray Magnetic Circular Dichroism (XMCD)

0.025

b. u

nits

μ↑↑ (μ+)μ (μ )

Helicity Spin2p3/2→3d

(LIII)Difference in absorption intensities between right- and left-hand circular polarizations

0.015

0.021. Element selectivity

resonant absorptionity (a

rb μ↑↓ (μ－)

2p →3d

0 005

0.01

p2p -> 3d excitation for 3d transition metal3d -> 4f excitation for rare-earth elements

Inte

ns

2p1/2→3d(LII)

0

0.0052. Determination of spin and orbital

magnetic moments Sum rulesor

ptio

n

-0 01

-0.005 Sum rules

3. High sensitivityAbs

o μ↑↓ －μ↑↓

0.01700 710 720 730 740 750

Photon Energy (eV)

XMCD Sum RulesXMCD Sum Rules

Orbital moment

Spin moment

B.T. Thole et al., PRL 68, 1943 (1992).P. Carra et al., PRL 70, 694 (1993).

Utilization of Element Selectivity of XMCDUtilization of Element Selectivity of XMCD

3

4

5

Fe L-edge

b. un

its)

Fe

1

2

3Co L-edge

Inte

nsity

(ar

Co

Fe

680 700 720 740 760 780 800 820 840

0

Photon Energy (eV)

Fe(6 ML)/Co(2 ML)/Cu(001) Fe(5 ML)/Co(2 ML)/Cu(001)

Photon Energy (eV)

Magnetic-field dependence of XMCD at Fe and Co L edges

0.5

1.0

(arb

. uni

ts)

0 0

0.5

1.0

(arb

. uni

ts)

1 0

-0.5

0.0

Inte

nsity

(

Fe Co -1.0

-0.5

0.0

Inte

nsity

(

Fe Co

-30 -20 -10 0 10 20 30

-1.0

H (Oe)-30 -20 -10 0 10 20 30

-1.5

H (Oe)

Angle Dependence of XMCDAngle Dependence of XMCD

0.0

0.5

ity [a

.u.]

0.0

0.5

ity [a

.u.]

0.0

0.5

ity [a

.u.]

-0.5

XM

CD

inte

ns

Grazing Normal

-0.5

XM

CD

inte

ns Grazing Normal

-0.5

XM

CD

inte

ns

Grazing Normal

840 860 880

-1.0

Photon Energy [eV]840 860 880

-1.0

Photon Energy [eV]840 860 880

-1.0

Photon Energy [eV]

Fe (4 ML)Fe (0 7 ML)

Cu(100)Ni (7.5 ML)Fe (4 ML)

Cu(100)Ni (7.5 ML)Fe (0.7 ML)

Cu(100)Ni (7.5 ML)

in plane XMCD reflects magnetic component which is parallel to X-ray beam.

d t i ti f i f ti tiperpendicular

determination of easy axis of magnetization

Information on orbital momentti ti f ti i tin plane estimation of magnetic anisotropy

Abe et al., J. Magn. Magn. Mater. 206 (2006) 86.

1. Advantages and Disadvantages of Soft X-ray Absorption Spectroscopy (SXAS)

2. SXAS studies on Surface and Thin films

3. Soft X-ray Beamlines

4 N l SXAS T h i D th l d XAS4. Novel SXAS Technique: Depth-resolved XAS

3.1. principle - wavelength dispersion -Diff ti ti P i di b t tDiffraction grating： Periodic grooves on a substrate

1 m

10 nm

Principle： Interference between the rays reflected at different grooves

Enhanced when light path difference = msin + sin = nm n: groove density

m: diffraction order

* ≠

depends on ⇒ Wavelength dispersion(conversion of wavelength to angle)

* ≠ (if = , any satisfies the above condition at m = 0)

⇒ zero-th order light

*

3.2. energy resolution - dispersion and focus -

How can we monochromatize by using a diffraction grating?Most basic mode: collimated-light illuminationg

sin + sin = nm

P bl 1 SR i lli d li h

Collimated whitelight

dispersed

Problem 1： SR is not a collimated light！Problem 2: Superposition of diffracted lights

⇒ difficult to be resolved

light

⇒ difficult to be resolvedSolution 1: Collimation of diverging light with a parabolic mirrorSolution 2: Focusing of diffracted lights with another parabolic mirrorg g p

Focused diffracted lights are well resolved in wavelength

Parabolic mirror(collimation)

Parabolic mirror(focusing) Exit slit well resolved in wavelength

at the exit slit !Plane grating

(wavelength selection)

S Dispersion and Focus (dispersion)Source

3.2. energy resolution - dispersion and focus -

The simplest monochromator

Point source

Exit slit(wavelength selection)

Both the “dispersion” and Point source )

“focus” are achieved by

a diffraction grating only. Concave grating

(dispersion & focusing)

Is that really possible?

It is impossible to obtain a perfect focus at all wavelengthp p g

But, ”perfect focus” is not necessary !

Small number of optical elements

3.3. Monochromator operation

Parabolic mirror(collimation)

Parabolic mirror(focusing) Exit slit

How can we select wavelength?

Plane grating

(wavelength selection)

SourceBy moving exit slit?

(dispersion)Source-> Sample should be moved!

i i d itSimple rotation of grating!

sin + sin = nm n: groove densitym: diffraction order

- = const. (included angle)*

( g )“Constant included angle monochromator”

Variable included angle system Wide energy range

Included angle

Variable included angle system Wide energy range

3.4. Beamline components

Overview of a typical soft X-ray beamline

Pre-focusing optics： focuses X rays onto the entrance slit

Pre-focusing optics Monochromator Post-focusing optics

Pre focusing optics： focuses X rays onto the entrance slitMonochromator： from the entrance slit to the exit slitPost-focusing optics： focuses monochromatized X rays onto sample position

22

Higher-order suppression：utilizes energy dependence of reflectivity (or transmittance)

1. Advantages and Disadvantages of Soft X-ray Absorption Spectroscopy (SXAS)

2. SXAS studies on Surface and Thin films

3. Soft X-ray Beamlines

4 N l SXAS T h i D th l d XAS4. Novel SXAS Technique: Depth-resolved XAS

Conventional Technique for Magnetic Depth ProfilingConventional Technique for Magnetic Depth ProfilingSQUID, VSM, MOKE, XMCD…

Gives averaged information over the whole sample.⇒ also averaged in depth

SQUID, VSM, MOKE, XMCD…

3ent

2

netic

Mom

e

0

1

0 1 2 3 4 5 6 7Tota

l Mag

n

Based on an assumptionthat magnetic structure of surface and interface

Film Thickness

Tthat magnetic structure of surface and interface dose not change upon layer growth

Direct technique for depth profiling

XAS XAS Measurement in the Soft XMeasurement in the Soft X--ray Regionray Region

0 025

3 ML Fe / Cu(100) Fe L-edge XAS How can we measure

X-ray absorption spectrum ?

0 02

0.025

units

)

X-rays

y p p2p3/2→3d

(LIII)~mm ~nm

A0 015

0.02

y (a

rb. u

X rays

Auger electrons

Core hole

0.01

0.015

Inte

nsity

Detector

(core hole relaxation)2p1/2→3d

(LII)

0.005sorp

tion

etecto(Retarding voltage)

Electron yield XAS

0700 710 720 730 740 750

Abs

ect o y e d STotal electron yield (TEY)

Partial electron yield (PEY)700 710 720 730 740 750

Photon Energy (eV)Partial electron yield (PEY)

cf. Fluorescence yield (FY)

Principle of DepthPrinciple of Depth--resolved resolved XASXAS

X-raysSurface

X-rays

e-Microchannel plate

Sample

d

e-

p d

e-Auger electrons

XAS data CCD camera

Phosphor screen

e-

XAS data CCD camera

Surface sensitive

Electron yield XAS measurements at different detection angles

A f XAS d i h diff bi d hA set of XAS data with different probing depths

Ni (5.5 ML)OSurface of O/Ni/Cu(100) Surface of O/Ni/Cu(100) ––raw dataraw data--

6

O/Ni(5.5 ML)/Cu(001) Ni L-edge XAS

Cu(100)

(X-ray Absorption Spectrum)

5

6

10

12 (b) e0.6 nm e0.8 nm e1.0 nm

(a)

Surface sensitiveIncrease of L3 peak

4 1.58

rb. u

nits

)

e = 0.6 nm

rb. u

nits

)

2

3

868 870 872

1.0

4

6

ensi

ty (a

r

tens

ity (a

r

Growth of shoulder

1

868 870 872

2In

te

Int

850 860 870 880 890

0

850 860 870 880 890

0 e = 1.0 nm

Ph E ( V)

Decrease of Plateau

Photon Energy (eV)Photon Energy (eV)

K. Amemiya and M. Sakamaki, Appl. Phys. Lett. 98 (2011) 012501.

Extracted XASExtracted XAS

O

Ni (5.5 ML)10

O/Ni(5.5 ML)/Cu(001)Extracted Ni L-edge XAS

) ( )

Cu(100)6

8

1

2

Surface

arb.

uni

ts)

2

4868 870 872

0

nten

sity

(a XAS spectra at surface

shows NiO-like features

4

0

Inner Layers

nits

)In

2

3

ty (a

rb. u

n

NiO thin film

850 860 870 880 890

0

1

Inte

nsit

850 860 870 880 890Photon Energy (eV)

Haverkort et al., Phys. Rev. B 69, 020408(R).

K. Amemiya and M. Sakamaki, Appl Phys Lett 98 (2011) 012501

Extracted XMCD spectraExtracted XMCD spectra

8 Surface

nits

) Ni L-edge XMCD

Appl. Phys. Lett. 98 (2011) 012501.

4

6

ty (a

rb. u

n

O

Ni (5.5 ML)

0

2

Inte

nsit

Cu(100)

Surface layer shows small

negative magnetization.3

4

0

Inner Layers

units

)

g g

1

2

nsity

(arb

.

-1

0Inte

n

840 850 860 870 880 890

Photon Energy (eV)

Depth profile of atomic structureDepth profile of atomic structure

6Co(3 ML)/Ru(0001) Normal Incidence

Layer-resolved EXAFS data

6

1 2

1.3

ts)

4

1.0

1.1

1.2

y (a

rb. u

ni

2 900 1000 1100 1200

Inte

nsity

800 900 1000 1100 1200

0

1st layer (surface) 2nd 3rd

Relaxation of strain

800 900 1000 1100 1200Photon Energy (eV)

Normal incidence: dominated by in-plane distance

Surface shows longer oscillation period: shorter bond length

DepthDepth--resolved EXAFS at grazing incidenceresolved EXAFS at grazing incidence

Grazing Incidence

Longer out-of-plane bond length at surface?

3Grazing Incidence

1st (surface)2nds)

2

2nd 3rd (interface)

rb. u

nits

2

nsity

(ar

Ru

1

Inte

n

800 900 1000 1100Ph t E ( V)Photon Energy (eV)

Preliminary Preliminary analysesanalysesIn plane

0.251 nm

Out of plane

0.260 nm0.251 nm

Ru

0.255 nm 0.252 nm

0.255 nm 0.252 nm

Let’s try soft XLet’s try soft X--ray absorption!!ray absorption!!

You don’t have to be afraid of soft X ray.You don’t have to be afraid of soft X ray.yy