Synchrotron radiation in spectroscopy - KEKSynchrotron radiation in spectroscopy V.V.Mikhailin...

Synchrotron radiation in spectroscopySynchrotron radiation in spectroscopy

V.V.Mikhailin

Optics and Spectroscopy Department, Physics Faculty, M.V.Lomonosov

Moscow State University, Leninskie

Gory,

119991, Moscow, Russia

Synchrotron radiation properties: spectral and angular distribution

6ГэВ

1ГэВ

3

Wavelength,

Radiation power, erg/sec Å, per electronRadiation power, erg/sec Å

mrad, per electron

Angle φ

mrad

Lebedev physical institute, S-60, Moscow

1 2 4 3 25

7

911

1213

86

10

Lebedev physical institute, S-60, Moscow

Siberia-2 and Siberia-1 (RNC “Kurchatov

Institute”)

VUV station (MSU)

X-ray luminescence station (MSU)

VUV station at Siberia-1 storage ring

(1998)

Unique spectral features and time structure of synchrotron radiation allows one to use this kind of excitation in investigation of electronic relaxation processes in insulators with wide band gap. The knowledge of these processes is important for understanding of scintillation efficiency in crystals. Luminescence excitation technique is convenient for study of energy transfer in these systems and for investigation of crystal energy structure. In general, luminescence excitation spectra can be subdivided into several spectral regions:

Direct excitation of lowest defect excited stateIonization of defects by photons with energy below the matrix forbidden gapExcitation of matrix Urbach tailExcitation of excitonsProduction of separated low-energy electron-hole pairsProduction of high-energy electron-hole pairs followed by impact excitation/ionization of defects

Each of these regions is characterized by different role of relaxation channels. Possible channels of energy transfer and relaxation are discussed in the presentation.

Absorption coefficient in wide photon energy range and different processes studied using synchrotron radiation excitation of

luminescence

Excited defect

Ionized defect + electron

Lattice vibrations

Urbach absorption

Exciton

Core exciton

Electron-hole pair

Electron-core hole pair

CONDUCTION BAND

VALENCE BAND

CORE BAND

Multi-ionisation

e eeh

eh

h h

ph

h h

e e

ph

h → Vk + ph

10–16

s 10–14

s 10–12

s 10–10

s 10–8

s time

e–e scattering

threshold

Auger threshold

Eg

2Eg

0

ΔEv

Electron energy

Hole energy

Dynamics of electronic relaxation in wide bandgap

solids

Atomic displacement

Modification of solid:

desorption, localisation, defect creation

Energy transfer

100 as 1 fs 1 ps 1 ns

hνex

Photon emission

hνlum

Energy relaxation processes which can are studied using synchrotron radiation

Excitation range Types of electronic excitations Relaxation processes

Visiblehν ≤ Eg

hν

= 3÷10 eV

Excited and ionized states of defects and impurities. Self-

trapped excitons

Scintillator and phosphor emission. Energy transfer between centers. Defect creation after exciton

annihilation. Intrincic defect quenching.UV

VUVEg ≤

hν ≤ (2÷3)Eg hν

= 5÷20 eV

Electron-hole pairs with energies below the threshold of secondary

excitation creation. Free excitons.

Electron-phonon interaction resulting in thermalization and migration quenching (separation

of electron-holee pair components). Diffusion of excitations. Trapping of excitations. Specific types

of core hole relaxation (core-valence luminescence).

(2÷3)Eg ≤

hν ≤

(5÷10)Eg ,

hν

= 15÷100 eV

Hot excitations with energy higher than the threshold of

secondary excitation creation. Excitation of outermost core

bands.

Electron-electron inelastic scattering and Auger processes resulting in multiplication of electronic

excitations.XUV

Soft XX

hν ≥ (5÷10)Eg , hν

> 50 eV

Core excitations. X-ray fluorescence. Auger processes.

Excited region which contains a hundreds of excitations. Tracks

of ionizing particles.Interaction of large number of electronic excitations.

An example of excitation spectrum in which all of

mentioned above effects are observed

Excitation spectra for two emission bands in BaF2

: self-trapped exciton emission (solid) and core-valence transitions (points)

[A.Belsky

et al.,

LURE (France) + ELETRA (Italy)]

STE

CVL


: self- trapped exciton

emission (solid) and core-valence transitions (points)

Different energies of excitation thresholds

STE

CVL10 eV = Eex

17 eV = Ev-c

STE

CVL




Surface losses

(pecularities

due to radiation penetration depth)

( ) ( )( )

( )ωητωα

ωωη volexp 11

DR

+−

=




Local density effects

STE

CVL

30 eV96 eV160 eV120 eV

Acceleration of decay kinetics and yield quenching in the region of creation of several excitations after

Auger decay

Acceleration of decay kinetics and yield quenching in the region of creation of several excitations after

Auger decay




Yield non-proportionality

STE

CVL

The well-known problem in scintillators: the yield is not proportional to the energy

of the ionizing photon

The well-known problem in scintillators: the yield is not proportional to the energy

of the ionizing photon




Relaxation of core holes

STE

CVL

Different relaxation channels after excitation of

different core levels

Different relaxation channels after excitation of

different core levels

Ionization of defects by photons with energy below the matrix forbidden gapExcitation of matrix Urbach tail

Ionized defect + electron

Urbach absorption Exciton

Yb3+

charge transfer luminescence (CTL) excitation (Guerassimova et al)

CTL spectra and excitation of CTL spectra of sesquioxides

measured with different time windows, temperature 10 K.

Slow/fast emission ratio increases with energy in Urbach

tail region, i.e. slow component increases with delocalization.

Urbach

tail regionExcitonSeparated e-h

pairs

fast

fast

fast

slow

slow

slow

Crystals with Yb3+

CTL (e.g. LuAP:Yb) are used in PET scanners for small

animals

Excitation of matrix Urbach tailExcitation of excitonsProduction of separated low-energy electron-hole pairs

Urbach absorption

Exciton

Electron-hole pair

Urbach

tail effect in PbWO4

excitation spectra

PWO excitation spectra for blue

(top) and green

(bottom) emission bands

3.5 4.0 4.5 5.0 5.5 6.0 6.5

0.5

1.0

1.5

2.03.5 4.0 4.5 5.0 5.5 6.0 6.5

0.0

0.2

0.4

0.6

0.8

1.0

em. 520 nm 30 K 150 K 200 K

Qua

ntum

Yie

ld, a

.u.

hν, eV

em. 400nm 30 K 150 K 200 KQ

uant

um Y

ield

, a.u

.

d-1

)(lg ωα

200K 30 K

150K

ω

Temperature dependence of PWO excitation spectra shows two phenomena:

(1)

dependence of excitation spectrum in Urbach

absorption region due to change of the fraction of absorbed radiation in the sample and

(2) increasing of the slope of quantum yield with T in the region of separated e-h

pairs (see below)

Production of separated low-energy electron-hole pairs

Exciton

Electron-hole pair

Probability of binding

or separation of the components of an electron-hole pair vs

their

energy

0.0

0.2

0.4

0.6

0.8

1.0

Coupled electron and hole

Short-decay emission

Separated electron and holeLong-decay emission

l(E)=

R 0

Lowe

st en

ergy

of

the

ioni

zed

state

Lowe

st en

ergy

of

the

boun

ded

statePr

obab

ility

Energy of an electron-hole pair

R0 increases with decrease of temperature, therefore this probability is temperature dependent:

T

The nature of this curve is the increase of electron-hole separation with pair energy and the decrease of direct recombination with this separation:

The effect of electron kinetic energy on the efficiency of energy transfer to the luminescence center as a function of

temperature (Spassky

et al)

5 10 15 20

0,4

0,6

0,8

1,00

1

b

RT

/LH

eT

ra

tio

Photon energy, eV

a

ZnWO4, a//E

LHeT RT

Re

flectivity, a.u

.

Qua

ntu

m y

ield

, a.

u.

ZnWO4 : (a) –quantum yield spectra at RT and LHeT and reflectivity (thin curve); (b) – the ratio of the above quantum yield spectra.

RT

LHeT Excitation spectrum temperature dependence is described in previous slide

Production of high-energy electron-hole pairs followed by impact excitation/ionization of defects

Electron-hole pairs

MgO:Al

–

threshold of multiplication of electronic excitations

(Ch. Lushchik, Mikhailin

et al)

1

2

1

2

а

б

0

1

2

0

10

20

30

0

1

2

10 20 30 hn, эВ

h, отн

. ед.

R,%

a , 10 см6 -1α, 106 cm-1

hν, eV

η, a

rb.u

n.

α

Luminescence Excitation

Total photoemission (electrons with energy above work function)

Exact position of e-e

scattering threshold located in the region of abrupt decrease of R and α

can be estimated using

measurement of luminescence and total photoemission excitation spectra

R

Two types of recombination channels: excitonic one (upper part) and recombination on a centre

(lower part). Figures on the right display typical energy dependence of

the quantum yield of these channels

Experimentally observed two types of recombination

channels

0

20

40

60

0 5 10 15 20 25

CaWO4

3Eg

2Eg

Eg

Luminescence excitation spectra of intrinsic luminescence of CaWO4

(upper panel) [S. I. Golovkova, A. M. Gurvich, A. I. Kravchenko, V. V. Mikhailin, A. N. Vasil'ev, Phys. Stat. Sol. (a), 77 (1983) 375] and CeF3

[C. Pedrini, A. N. Belsky, A. N. Vasil'ev, D. Bouttet, C. Dujardin, B. Moine, P. Martin, M. J. Weber, Material Research Society Symposium Proceedings, v. 348, pp. 225–234, 1994] (lower panel) and activator luminescence of CaSO4

:Sm

[I. A. Kamenskikh, V. V. Mikhailin, I. N. Shpinkov and A. N. Vasil'ev, Nucl. Instr. and Meth., A282 (1989) 599] (middle panel)

0

20

40

60

0 5 10 15 20 25 30 35 40

CaSO4:Sm

Eg 2Eg3Eg

0

5

10

15

20

25

0 5 10 15 20 25 30 35 40

Photon energy, eV

CeF3

Eg

2Eg 3Eg4f-5d

Eg+E(4f-5d)

More complicated case: an example of crossluminescence quenching at 4d Ba2+ core level

Core exciton

Electron-hole pair

Electron-core hole pair

0 1 2 3103

104

105

Decay curves of BaF2 Auger Free Luminescence

I0(t,0.9 ns)

30 eV

90 eV

110 eV

Cou

nts

Time (ns)

20 40 60 80 100 120 140 1600

1

Lum

ines

cenc

e in

tens

ity (a

.u.)

Photon energy (eV)

0,4

0,6

0,8

Ba 5p

Ba 4d

Que

nchi

ng fu

nctio

n

5p core hole interaction with excitons

and conduction electrons in BaF2 (Belsky et al)

Crossluminescence excitation spectrum CL decay curves

at different excitation energies

Quenching factor at 1.5 ns after excitation pulse

maximum

1.5 ns

Decay is fastest and yield is quenched in the region of 4d Ba2+ absorption region due to interaction of several excitations

Manifestation of core excitons

in in optical

functions in VUV reguion

15 20 25 30 35

5p Ba2+

3p Ca2+

4p Sr2+

5d Pb2+

0(4)

0(3)

0(2)

0(1)

4

3

2

1

Reflectivity, LHeT CaMoO4 SrMoO4 BaMoO4 PbMoO4

Ref

lect

ivity

, a.u

.

Photon energy, eV

Intensity of core exciton peaks correlates with the nature of the bottom of the conduction band

(Kolobanov, Spassky

et al).

Cation

core excitons

are visible only if the lowest states of conduction band are formed from cation

states (Pb

and Ba

molibdates).

Reflectivity shows no structure in core exciton region if the lowest states are formed from complex anion states (Sr

and Ca molibdates).

VB (MoO42-

bond states)

Pb2+

Ba2+

Sr2+

Ca2+

CondB

(MoO42-

antibond

states)

An example of study of energy transfer in wide range in a crystal with complicated electronic structure

Excited defect Ionized defect

+ electron

Urbach absorption

Exciton

Electron-hole pair

5 10 15 20 25 30 35

Eg+Ef-d

Ef-d

Eg

Lum

ines

cenc

e in

tens

ity (a

.u.)

Photon energy (eV)

0 20 40 60 80 100

102

103

104

Time (ns)

16 eV

12 eV

5 eV

Interaction of cerium excitons

in CeF3 (energy transfer)The elementary region of high local density of excitations

(Belsky et al)

At high energy excitation luminescence decay of CeF3

is non exponential and show a strong acceleration

Energy threshold of decay acceleration in CeF3

is at 16 eVAbove 16 eV

inelastic scattering of the primary photoelectron can create two excited ions (Ce3+)*

in close vicinity.The result of their interaction can be written

as(Ce3+)*

+ (Ce3+)*

→ Ce4+

+ e + Ce3+From simulation of interaction the

acceleration of decay is from picosecond

range

An example of study of luminescence excitation spectra in wide region of processes

•Creation of the reversible damage: a) transient defects - close F-H pairs b) Change of electronic state of deep defect levels in the forbidden energy gap•Creation of the irreversible damage: a) stable F-H pairs b) defect conglomerates

The reasons of light yield instability induced by radiation

Benefits of VUV and X-ray SR in radiation damage study

• VUV (especially XUV) and X-ray photons produce the same spectrum of elementary electronic excitations (electron-hole pairs, excitons, core level excitations, initial defect formation stages) as high- energy ionizing particle

• Absorption coefficient in XUV and X-ray region is extremely high (104 to 106 cm-1), therefore accumulated dose in the thin absorption layer becomes huge

• Unique spectral features and time structure, and high intensity of synchrotron radiation allow one to use this kind of excitation in investigation of defects and their creation in insulators with wide band gap.

W10

1

10

10

10

10

10

1010 10 10 1 10 10

6 GeV

5

4

3

2

1

λ, nm

-1

-2

-3

-4

-5

-6-3 -2 -1 2

SR spectral distribution for various electron energy (R=32 m)

How to study radiation effects using luminescence spectroscopy

• Changes of luminescence emission spectra (additional emission bands)

• Changes of decay kinetics (radiation defects can result in sharpening of initial stages of decay and increasing of slow component)

• Changes of energy transfer (radiation defects can change ratio of several relaxation channels)

Usage of SR in X-ray region in the study of PWO radiation hardness

•VEPP-3 (Budker

INP): Flux of 1016

ph/s with energy from 2 to 100 KeV

(“white”

X-rays)

•DCI (Lure, Orsay): Flux of 1012

ph/s of monochromatized

15 KeV

X-ray photons

0 100 200 300 400

0.8

0 .9

1 .0

1 .1

c

b

a

LUM

INES

CEN

CE

INTE

NSI

TY, a

rb.u

n.

T IM E, sec

(a) Green emission (480 nm) – fast degradation at first 10 sec followed by much slower degradation

(b) Blue emission (380 nm) is more stable under irradiation

(c) Few cases of increase of emission in intermediate range (430 nm) under irradiation – the evidence of new emission center production

Dose dependence for different regions of PWO emission spectrum excited by X-ray SR

480 nm

380 nm

430 nm

Time, sec

•

Dose rate is about 1 kGy/sec (in thin absorption layer, d~10-5

cm)•

Degradation / enhancing of emission under irradiation depends on the emission spectral region

•

Fast and slow recovering of radiation defects

•Fast (blue) component –

excitonic

(Pb) emission, (should be linear with excitation intensity)•Slow (green) component –

defect

recombination emission, (should be non-linear (quadratic?) with excitation intensity)

PWO emission spectrum explanation

PbWO4

Modulation of SR spectrum by the grating spectral efficiency with sharp peculiarities allows one to estimate the order of the process

Sharp structure due to Pt covering of the grating dissapiars

in

1st

order fast PWO emission and

2nd

order slow PWO emission

How to measure nonlinear excitation efficiency?

Conclusions

Fundamental mechanisms of electronic relaxation in large bandgap

solids and energy transfer can be

studied by analysis of luminescence excitation spectra and kinetics excited by VUV-X synchrotron radiation photons, especially using Time-Resolved Luminescence VUV Spectroscopy.

High flux of SR enables to simulate and investigate radiation damage effects.

Synchrotron radiation in spectroscopy Synchrotron radiation properties: spectral and angular distributionSlide Number 3Slide Number 4Siberia-2 and Siberia-1 (RNC “Kurchatov Institute”)VUV station at Siberia-1 storage ring (1998)Slide Number 7Absorption coefficient in wide photon energy range and different processes studied using synchrotron radiation excitation of luminescenceSlide Number 9Energy relaxation processes which can are studied using synchrotron radiation An example of excitation spectrum in which all of mentioned above effects are observedExcitation spectra for two emission bands in BaF2: self-trapped exciton emission (solid) and core-valence transitions (points) �[A.Belsky et al., LURE (France) + ELETRA (Italy)] Excitation spectra for two emission bands in BaF2: self-trapped exciton emission (solid) and core-valence transitions (points) �Different energies of excitation thresholds Excitation spectra for two emission bands in BaF2: self-trapped exciton emission (solid) and core-valence transitions (points) �Surface losses (pecularities due to radiation penetration depth) Excitation spectra for two emission bands in BaF2: self-trapped exciton emission (solid) and core-valence transitions (points) �Local density effects Excitation spectra for two emission bands in BaF2: self-trapped exciton emission (solid) and core-valence transitions (points) �Yield non-proportionality Excitation spectra for two emission bands in BaF2: self-trapped exciton emission (solid) and core-valence transitions (points) �Relaxation of core holes Slide Number 18Yb3+ charge transfer luminescence (CTL) excitation (Guerassimova et al)Slide Number 20Urbach tail effect in PbWO4 excitation spectraSlide Number 22Probability of binding or separation �of the components of an electron-hole pair vs their energy The effect of electron kinetic energy on the efficiency of energy transfer to the luminescence center as a function of temperature (Spassky et al)Slide Number 25MgO:Al – threshold of multiplication of electronic excitations (Ch. Lushchik, Mikhailin et al)Two types of recombination channels: �excitonic one (upper part) and recombination on a centre (lower part). �Figures on the right display typical energy dependence of the quantum yield of these channels Experimentally observed two types of recombination channels Slide Number 29Slide Number 30Manifestation of core excitons in in optical functions in VUV reguionSlide Number 32Slide Number 33Slide Number 34Slide Number 35Slide Number 36Slide Number 37Slide Number 38Slide Number 39Slide Number 40Slide Number 41

Synchrotron radiation in spectroscopy - KEKSynchrotron radiation in spectroscopy V.V.Mikhailin...

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