Synchrotron radiation in spectroscopy

Transcription

1 Synchrotron radiation in spectroscopy V.V.Mikhailin Optics and Spectroscopy Department, Physics Faculty, M.V.Lomonosov Moscow State University, Leninskie Gory, , Moscow, Russia

2 Synchrotron radiation properties: spectral and angular distribution Radiation power, erg/sec Å mrad, per electron Radiation power, erg/sec Å, per electron 6ГэВ 3 1ГэВ Angle φ mrad Wavelength,

3 Lebedev physical institute, S-60, Moscow

4 Lebedev physical institute, S-60, Moscow

5 Siberia-2 and Siberia-1 (RNC Kurchatov Institute ) X-ray luminescence station (MSU) VUV station (MSU)

6 VUV station at Siberia-1 storage ring (1998)

7 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 state Ionization of defects by photons with energy below the matrix forbidden gap Excitation of matrix Urbach tail Excitation of excitons Production of separated low-energy electron-hole pairs Production 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.

8 Absorption coefficient in wide photon energy range and different processes studied using synchrotron radiation excitation of luminescence Exciton Urbach absorption Excited defect Core exciton Electron-core hole pair Lattice vibrations Ionized defect + electron Electron-hole pair

9 Dynamics of electronic relaxation in wide bandgap solids 100 as 1 fs 1 ps 1 ns 2E g Multi-ionisation e e e e scattering threshold e h e Atomic displacement Modification of solid: desorption, localisation, e defect creation ph Energy transfer CONDUCTION BAND Photon emission hν lum Electron energy E g hν ex 0 VALENCE BAND ΔE v Auger threshold e h h ph h h V k + ph Hole energy h h CORE BAND s s s s 10 8 s time

10 Energy relaxation processes which can are studied using synchrotron radiation Excitation range Types of electronic excitations Relaxation processes Visible UV hν E g hν = 3 10 ev Excited and ionized states of defects and impurities. Selftrapped excitons Scintillator and phosphor emission. Energy transfer between centers. Defect creation after exciton annihilation. Intrincic defect quenching. VUV E g hν (2 3)E g 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). XUV Soft X X (2 3)E g hν (5 10)E g, hν = ev hν (5 10)E g, hν > 50 ev Hot excitations with energy higher than the threshold of secondary excitation creation. Excitation of outermost core bands. Core excitations. Excited region which contains a hundreds of excitations. Tracks of ionizing particles. Electron-electron inelastic scattering and Auger processes resulting in multiplication of electronic excitations. X-ray fluorescence. Auger processes. Interaction of large number of electronic excitations.

11 An example of excitation spectrum in which all of mentioned above effects are observed

12 Excitation spectra for two emission bands in BaF 2 : self-trapped exciton emission (solid) and core-valence transitions (points) [A.Belsky et al., LURE (France) + ELETRA (Italy)] STE CVL

13 Excitation spectra for two emission bands in BaF 2 : selftrapped exciton emission (solid) and core-valence transitions (points) Different energies of excitation thresholds STE 10 ev = E ex CVL 17 ev = E v-c

14 Excitation spectra for two emission bands in BaF 2 : selftrapped exciton emission (solid) and core-valence transitions (points) Surface losses (pecularities due to radiation penetration depth) η exp ( ω) 1 = 1+ α R( ω) ( ω) η Dτ vol ( ω) STE CVL

15 Excitation spectra for two emission bands in BaF 2 : selftrapped exciton emission (solid) and core-valence transitions (points) Local density effects 30 ev 96 ev 160 ev STE 120 ev Acceleration of of decay decay kinetics and and yield yield quenching in in the the region region of of creation of of several excitations after after Auger Auger decay decay CVL

16 Excitation spectra for two emission bands in BaF 2 : selftrapped exciton emission (solid) and core-valence transitions (points) Yield non-proportionality The The well-known problem in in scintillators: the the yield yield is is not not proportional to to the the energy energy of of the the ionizing photon photon STE CVL

17 Excitation spectra for two emission bands in BaF 2 : selftrapped exciton emission (solid) and core-valence transitions (points) Relaxation of core holes Different relaxation channels after after excitation of of different core core levels levels STE CVL

18 Ionization of defects by photons with energy below the matrix forbidden gap Excitation of matrix Urbach tail Urbach absorption Exciton Ionized defect + electron

19 Yb 3+ charge transfer luminescence (CTL) excitation (Guerassimova et al) Urbach tail region Exciton Separated e-h pairs slow fast fast 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. slow fast slow Crystals with Yb 3+ CTL (e.g. LuAP:Yb) are used in PET scanners for small animals

20 Excitation of matrix Urbach tail Excitation of excitons Production of separated low-energy electron-hole pairs Urbach absorption Exciton Electron-hole pair

21 Urbach tail effect in PbWO 4 excitation spectra Quantum Yield, a.u. Quantum Yield, a.u em. 400nm 30 K 150 K 200 K em. 520 nm 30 K 150 K 200 K lg α( ω) d K 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) hν, ev PWO excitation spectra for blue (top) and green (bottom) emission bands

22 Production of separated low-energy electron-hole pairs Exciton Electron-hole pair

23 Probability of binding or separation of the components of an electron-hole pair vs their energy Coupled electron and hole Short-decay 1.0 emission Separated electron and hole Long-decay emission The nature of this curve is the increase of electron-hole separation with pair energy and the decrease of direct recombination with this separation: 0.8 Probability Lowest energy of the bounded state Lowest energy of the ionized state l(e)=r 0 R 0 increases with decrease of temperature, therefore this probability is temperature dependent: 0.0 Energy of an electron-hole pair T

24 The effect of electron kinetic energy on the efficiency of energy transfer to the luminescence center as a function of temperature (Spassky et al) Quantum yield, a.u. 1 ZnWO 4, a//e LHeT RT a Reflectivity, a.u. ZnWO 4 : (a) quantum yield spectra at RT and LHeT and reflectivity (thin curve); (b) the ratio of the above quantum yield spectra. 0 1,0 RT/LHeT ratio 0,8 0,6 LHeT RT Excitation spectrum temperature dependence is described in previous slide 0,4 b Photon energy, ev

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

26 MgO:Al threshold of multiplication of electronic excitations (Ch. Lushchik, Mikhailin et al) R,% a, 10 см -1 α, 10 6 cm -1 R а α 2 1 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 η, arb.un. h, отн. ед б Luminescence Excitation Total photoemission (electrons with energy above work function) hn, эв hν, ev

27 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

28 CaWO4 2Eg Eg 3Eg CeF3 Eg Eg+E(4f-5d) 2Eg 3Eg 4f-5d Photon energy, ev Eg 2Eg 3Eg CaSO4:Sm Experimentally observed two types of recombination channels Luminescence excitation spectra of intrinsic luminescence of CaWO 4 (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 CeF 3 [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 , 1994] (lower panel) and activator luminescence of CaSO 4 :Sm [I. A. Kamenskikh, V. V. Mikhailin, I. N. Shpinkov and A. N. Vasil'ev, Nucl. Instr. and Meth., A282 (1989) 599] (middle panel)

29 More complicated case: an example of crossluminescence quenching at 4d Ba 2+ core level Core exciton Electron-core hole pair Electron-hole pair

30 5p core hole interaction with excitons and conduction electrons in BaF 2 (Belsky et al) Decay is fastest and yield is quenched in the region of 4d Ba2+ absorption region due to interaction of several excitations Luminescence intensity (a.u.) 1 Ba 5p Ba 4d 0,8 0,6 0,4 Quenching function Counts Decay curves of BaF 2 10 Auger Free Luminescence ns 30 ev I 0 (t,0.9 ns) Crossluminescence excitation spectrum Photon energy (ev) Quenching factor at 1.5 ns after excitation pulse maximum 90 ev 110 ev Time (ns) CL decay curves at different excitation energies

31 Reflectivity, a.u. 5d Pb 2+ 5p Ba 2+ 0(4) 4p Sr 2+ 0(3) Reflectivity, LHeT CaMoO 4 SrMoO 4 BaMoO 4 PbMoO Manifestation of core excitons in in optical functions in VUV reguion 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). CondB (MoO 4 2- antibond states) 0(2) 3p Ca 2+ 1 VB (MoO 4 2- bond states) 0(1) Photon energy, ev Pb 2+ Ba 2+ Sr 2+ Ca 2+

32 An example of study of energy transfer in wide range in a crystal with complicated electronic structure Urbach absorption Exciton Electron-hole pair Excited defect Ionized defect + electron

33 Interaction of cerium excitons in CeF 3 (energy transfer) The elementary region of high local density of excitations (Belsky et al) An example of study of luminescence excitation spectra in wide region of processes Luminescence intensity (a.u.) 5 ev ev ev E E g +E Time (ns) g f-d E f-d Photon energy (ev) At high energy excitation luminescence decay of CeF 3 is non exponential and show a strong acceleration Energy threshold of decay acceleration in CeF 3 is at 16 ev Above 16 ev inelastic scattering of the primary photoelectron can create two excited ions (Ce 3+ ) * in close vicinity. The result of their interaction can be written as (Ce 3+ ) * + (Ce 3+ ) * Ce 4+ + e + Ce 3+ From simulation of interaction the acceleration of decay is from picosecond range

34 The reasons of light yield instability induced by radiation 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

35 Benefits of VUV and X-ray SR in radiation damage study W 10 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 highenergy ionizing particle Absorption coefficient in XUV and X-ray region is extremely high (10 4 to 10 6 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 GeV λ, nm SR spectral distribution for various electron energy (R=32 m) 1

36 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)

37 Usage of SR in X-ray region in the study of PWO radiation hardness VEPP-3 (Budker INP): Flux of ph/s with energy from 2 to 100 KeV ( white X-rays) DCI (Lure, Orsay): Flux of ph/s of monochromatized 15 KeV X-ray photons

38 Dose dependence for different regions of PWO emission spectrum excited by X-ray SR 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 LUMINESCENCE INTENSITY, arb.un nm c b Time, sec a 380 nm 480 nm (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 TIME, sec

39 PWO emission spectrum explanation 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)

40 How to measure nonlinear excitation efficiency? PbWO 4 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 1 st order fast PWO emission and 2 nd order slow PWO emission

41 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.