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Excitation of secondary processes in the vacuum ultraviolet range
Nuclear Instruments and Methods in Physics Research A261 (1987) 107-114 North-Holland, Amsterdam
107
Section IlL VUV and soft X-ray spectroscopy E X C I T A T I O N O F S E C O N D A R Y P R O C E S S E S IN T H E V A C U U M U L T R A V I O L E T R A N G E V.V. M I K H A I L I N
Moscow State University,Moscow, USSR
The paper is a survey of the activity of the laboratory of synchrotron radiation, Moscow University, in the application of SR to the investigation of luminescence excitation of insulators: Mg and Be oxides, Ca, Sr and Ba sulphides, sulphates and halides and of the extrinsic photoeffect of MgO. General tendencies of the luminescence quantum yield behaviour are considered. The contribution of the transitions from core levels to the formation of the quantum yield is discussed.
1. Introduction This paper is a survey of the activity of the laboratory of synchrotron radiation, Physical Faculty of Moscow State University in the application of synchrotron radiation (SR) to the investigation of optical properties of insulators. The staff of the laboratory perform experiments at SR beam lines of several machines: synchrotrons S-60 [1] and Pakhra [2] of the Lebedev Institute, Moscow, Sirius of the Polytechnical Institute, Tomsk [3], storage rings of the Institute of Nuclear Physics, Novosibirsk [4] and the dedicated SR storage ring Siberia-1 of Kurchatov Institute of Atomic Energy, Moscow [5]. Solid-state spectroscopy has benefited much from the application of SR, namely in the VUV and XUV ranges experimental spectra have become more detailed and reliable. Research in this field contributes to our understanding of the electronic structure of solids and fundamental processes in them.
2. Quantum yield of secondary processes In this paper we shall deal mainly with luminescence excitation and shall also touch on the photoemission of wide band gap insulators. The luminescence quantum yield for a fixed energy of exciting photons is related to the following stages of the photon energy transfer: a) absorption of the exciting photon: the absorption coefficient k(h ~), providing information on primary electronic excitations (EE), shows up in the excitation spectrum due to surface losses; b) inelastic scattering of primary EE resulting in the creation of secondary EE [6]; c) transfer of the energy of EE (either electron-hole pairs or excitons) to luminescent centers. Reflection and absorption spectra originate from the first stage of photon absorption, thus, they provide 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
information only on the primary excitations. Besides, the determination of optical constants of powder materials (a number of the substances important for industrial applications are accessible only in powder form) is a task which cannot yet be solved by means of absorption and reflection spectroscopy. Thus, the study of luminescence excitation not only yields information complementary to the data provided by absorption and reflection, but in several cases is a unique method of investigation of EE evolution. We shall consider at first general tendencies of the luminescence quantum yield behaviour [7]. In the energy range of exciting photons limited by the fundamental absorption edge at the low-energy side and the onset of photon multiplication at the high-energy side two main tendencies are observed: (1) a decrease of the luminescence quantum yield with the energy of exciting photons in the samples with predominantly excitonic mechanism of energy transfer to luminescent centers; (2) the opposite tendency, i.e. an increase of the quantum yield in the case of recombination luminescence. The same feature is observed for the photoeffect quantum yield. When the energy of primary EE becomes sufficient to create secondary ones, the quantum yield should show a rise; however, incorporation of such processes as reflection, surface and migration losses, etc. complicates the picture, resulting in a variety of spectra for different phosphors. To study the effect of photon multiplication it is necessary to select it from the contributions of other processes to the measured spectra. Surface losses, e.g., are well described by a simple diffusion theory with a large EE recombination rate at the surface [8,9]. The luminescence quantum yield ~/(hu) is then related to the frequency of the incident light as 1 - R(h~')
7l(hu) Here
X + k(hu)L~lv(hu). ~v(hv) is the luminescence quantum yield III. VUV/SOFT X-RAY SPECTROSCOPY
108
v.v. Mikhailin / Excitation of secondao' processes in the VUV range
originating only from volume processes; k and R are absorption and reflection coefficients, respectively; L is the effective diffusion length. We shall assume that L is independent of the photon energy, this assumption is true for a fast relaxation of EE, when the thermalization length for electrons and holes is less than the diffusion length of thermalized excitations. Surface losses cannot account for the monotonic decrease or increase of the quantum yield which is uncorrelated to the absorption coefficient in the range preceding the onset of photon multiplication. Such a luminescence intensity dependence on the exciting photon energy cannot be explained on the basis of a simple band diagram for a phosphor, since the equations contain only the concentrations of electrons and holes, which are related to the intensity of the photon flux, but not to the energy of the photons creating them. In a series of papers [10-12] devoted to this problem the idea of "genetical" recombination was suggested taking into account the correlation between the electron and the hole created by the same photon. Here we shall consider correlations of this kind for a phosphor described by the following model. An electron e and a hole h created by the absorption of an exciting photon thermalize with a mean thermalization length l which depends on the energy of the incident photon. In the process of scattering they emit predominantly optical phonons (it is assumed that the energies of the electron and the hole are not sufficient yet to create secondary EE in the crystal). One of the following events can then take place: the electron and the hole can bind into an exciton e + h---, ex with further emission of the energy in the emission band of a free or self-trapped exciton or the exciton energy may be transferred to a luminescent center; successive recombination at a positive center e + c + ---, c 0, h + c o ---, c* with further emission from the excited state of the center c* in the respective emission band or radiation-
less relaxation of the center; the electron can be captured by a trap b with further thermal release from the trap: e+b~b
,
b
~b+e.
Fig. 1 shows the quantum yield for the excitonic channel of recombination versus the energy of the exciting photons, the intensity of exciting radiation and temperature, calculated with rather simple assumptions ( E 0 is the photon energy for which the thermalization length of the electron and the hole equals the radius of the recombination sphere). The curves evidently show that low intensities of excitation are characterized by a stronger dependence of ~,, on the energy of exciting photons: the quantum yield steeply decreases when the thermalization lengths of e and h exceed the recombination radius, and the number of e - h pairs with an infinite thermalization length increases. With increasing intensity even the electron which escaped from the recombination sphere has the possibility of recombining with a hole created by the absorption of another photon, so the depth of the depression in ~v decreases. When we take into consideration the temperature dependence of the probability W T for an electron to be released from a trap with activation energy %, the temperature of the sample will also influence the depth of the depression in the spectrum. At low temperatures electrons spend more time in the traps, their diffusion length decreases and the excitation spectrum becomes flatter. In the framework of the discussed model the recombination quantum yield equals 1 - ~v, it increases with the energy of the incident photon. In our model we did not consider the energy range where the multiplication of electronic excitations occurs. For simplicity let us start with crystals with a narrow valence band. At the energy above the threshold of EE multiplication electron-electron collisions are
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Fig. 1. Quantum yield for the excitonic channel of recombination vs the energy of exciting photons h~, the intensity of exciting radiation I, the temperature T and the concentration of c centers. Calculations are presented for k T = 0.08 (a) and C 1 / n 2o = 10- 3 (b).
V. V. Mikhailin / Excitation of seconda~ processes in the VUV range
109
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Fig. 2. Quantum yield vs the energy of exciting photons. (a-c) Theoretical calculations for E0 =10Eg (a), Eo = 0.8Eg (b) and Eo = 0.15Eg (c); (d-f) experimental quantum yield of KI-TI [15] (d), MgO-A1 [16] (e) and BeO [17] (f).
accompanied by the phonon mechanism of EE energy loss mainly due to interaction with optical phonons h f2. Luminescence quantum yields calculated on the basis of a simple model [7,13] for three cases with different migration losses in the range from Eg to 2Eg (curves a-c) are plotted in fig. 2. The calculations were made with the assumption of comparable electron-electron and electron-phonon interactions in the region about 2.5Eg. When the interaction with phonons is weaker spectral features are sharper at energies larger than 2Eg and maxima shift towards lower energies. The depth of the trough at 3Eg is correlated to that at 2 Eg. This can be explained by the fact that immediately after 2Eg electrons and holes with low energy are created and therefore their thermalization length is small. The energy of the exciting quanta increasing up to 3Eg, secondary electrons are distributed in the interval Eg-2Eg, their mean thermalization length l increase leading to the reduction of the quantum yield due to migration losses. If we increase the energy of exciting photons further, secondary EE would also be able to produce new EE, this will result in a linear growth (above (4-5)Eg) of the quantum yield [14]. The theoretical curves (a-c) in fig. 2 can be compared with measured luminescence excitation spectra (d-f) of KI-T1 [15], MgO-A1 [16] and BeO [17]. The spectrum d is characterized by a step-wise growth of the quantum yield without notable depressions due to migration losses. The observed spectral features are mainly due to the structure of the absorption coefficient. Spectral features about 2Eg, which are observed at the background of a steep ~/ rise, are attributed to transitions from core levels. The main tendencies in the spectra a and d are similar: a monotone growth of ~l(hu) showing steps at energies that are multiples of Eg. The curve b can be illustrated by the excitation
spectrum of MgO-A1 (curve e). In this spectrum, besides features attributed to surface losses, a depression of the quantum yield in the range 8-19 eV is observed and another at energies of 26-32 eV. The appearance of the second trough is in good agreement with the proposed scheme of EE multiplication. A still deeper depression attributed to migration losses is observed in the excitation spectrum of BeO (curve f); this spectrum correlates with curve c. We tried to show how the proposed models can be applied to treat the measured luminescence excitation spectra of a series of phosphors. Now we shall proceed to a more detailed analysis of our experimental results.
3. Luminescence excitation and photoemission of MgO Radiation-resistant MgO single crystals have been extensively studied by a number of authors, so the stored information makes this wide band gap insulator a convenient model system for the investigation of EE multiplication. Impurity luminescence of MgO-A1, peaking at 5.3 eV, appears when electrons recombine with holes captured by associations of AI 3+ ions with cation vacancies [18,19]. The reported optical measurements were performed at the S-60 synchrotron of the Lebedev Institute [1]. The experimental setup was based on a normal incidence monochromator using the Wadsworth mounting without entrance silt. Photoemission spectra were measured at the "Sirius" synchrotron of the Polytechnical Institute, Tomsk [3]. All measurements were performed relative to the reference signal from the luminescent sodium salicylate. The results presented were obtained at room temperature. In fig. 3a reflection spectrum of a MgO single crystal III. VUV/SOFT X-RAY SPECTROSCOPY
110
V. V. Mikhailin / Excitation of secondary processes in the VUV range
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in the range 5-25 eV is presented, the angle of incidence was 10 o. In the same figure an absorption spectrum is shown derived from reflectivity data using K r a m e r s - K r o n i g relations. In the fundamental absorption region ( 1 0 s - 2 × 1 0 6 cm 1) anion excitons are created at energies of about 7.58 eV, above Eg separated electrons and holes are produced. In the energy range 7.7-25 eV the holes are created in the valence band (Ell , oxygen levels), at 25-30 eV at the L l levels of oxygen. In the luminescence excitation spectrum steep minima are observed corresponding to the maxima of the reflection and absorption spectra. As was mentioned above, these minima should be attributed to radiationless recombination of electrons and holes at the surface. Besides spectral features due to this effect and that due to selective reflection we observe additional features: the quantum yield decreases with the energy of exciting photons in the ranges 7.6-19 and 26-31 eV and increases at energies from 19.5 to 26 eV and above 32 eV. It is natural to attribute the steep rise of the excitation efficiency to the effect of EE multiplication; depressions of 7/(h v), as was discussed earlier, can be interpreted in terms of a thermalization length increase with the energy of exciting photons. In the presence of any radiationless recombination channels
this effect results in the reduction of the excitation efficiency (see fig. 2). Fig. 3 also shows the excitation spectrum of the extrinsic photoeffect of MgO [3]. A series of maxima in this spectrum coincides with the maxima but not with the minima of the absorption. " M o d u l a t i o n s " by the absorption coefficient are superimposed on the rise of the electron emission efficiency in the energy ranges 10-19 and 22-27 eV and a notable decrease of the number of electrons escaping from the crystal at energies of 19.5-21.5 eV. The energy distribution of the emitted electrons, measured in ref. [3], shows that at h~,= 18.8 eV and 20 eV we deal mainly with fast photoelectrons; increasing the energy h J, up to 24.5 eV results in a decrease of the number of fast photoelectrons, while slow photoelectrons appear with energies of 1 2 eV. The shift of the extrinsic photoeffect excitation spectrum in comparison with the recombination luminescence excitation of MgO characterizes the electron affinity and equals about 2 eV. Consideration of the photoelectric measurements indicates that the first threshold of EE multiplication in MgO is at 19.5 eV, as was mentioned in ref. [16].
4. L u m i n e s c e n c e
excitation
of sulphides of alkali-earth
metals
We present luminescence excitation spectra measured in the energy range 3-35 eV at 300 K for sulphides doped with Ce (4 × 10 -4 g/g). The quantum yield was determined relative to sodium salicylate (fig. 4) [20]. A brief literature survey should be made on the
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Fig. 5. Correlation of spectral features in luminescence excitation (solid line) and reflection (dashed line) [21] spectra of BaS. s8 objects of research. There are many papers dealing with alkali-earth sulphides. They have a rock salt structure, but unlike alkali halide crystals, their chemical bonding has a substantial contribution of covalency, which increases with the atomic number of the cation. Reflection spectra of single crystals have been measured in ref. [21]. The same authors investigated core states of Ca, Sr and Ba cations. The energy-band structure of CaS and SrS has been calculated in refs. [22] and [23], respectively. Stationary luminescence excitation spectra of these compounds in a wide spectral range have been measured for the first time in refs. [24,25]. As in the case of MgO, the accessible data on sulphides of alkaliearth metals makes them a convenient object for the study of EE dynamics in partially covalent compounds. The main tendency of */(hu) in the measured spectra is its growth with exciting photon energies. This growth, however, is not monotonous. A large number of spectral features is due to surface losses, which visualize the structure of the absorption coefficient in the excitation spectra. To separate this kind of structure from, e.g., that attributed to EE inelastic scattering it is promising to compare the excitation spectrum with the energy dependence of absorption or reflectivity. This is done in fig. 5, where the measured excitation spectrum and literature reflectivity [21] are shown together. A good agreement of the energy location of structure (minima of ~(h~) and maxima of R ( h v ) , which are close to the maxima of k ( h v)) is observed. Beyond the fundamental absorption edge ( E g = 5.5 eV for CaS and 5 eV for SrS) the structure in the excitation spectra is very sensitive to changes of the absorption coefficient. An increase of the absorption coefficient modifies the penetration depth of the exciting radiation leading to an increase of the surface losses. Besides the structure of the absorption, spectral features due to processes of EE scattering can also be observed. The steep rise of the luminescence quantum yield at 11-13 eV is attributed to these two effects: the decrease of the absorption coefficient and electron-hole pair
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multiplication. The energy of exciting photons in this range exceeds 2 Eg, so the energy of the created electron is sufficient to produce two electron-hole pairs. The rise of the quantum yield at 18 eV in SrS and at 20 eV in CaS has the same multiplication origin. The weak and narrow dips observed in the spectra at energy ranges from 24 to 32 eV in CaS and from 20 to 28 eV in SrS are due to the creation of core excitons. Their location is in good agreement with the calculations of the transitions 4p6-4p55s,4p54d in the Sr 2 + ion in the crystal field of SrS [21]. It is the first observation of core excitons in the luminescence excitation of alkali-earth sulphides with a resolution even better than in the reflection measurements [21]. Cooling of the samples results in the appearance of excitons near the fundamental absorption edge. The minima A - E in the excitation spectrum of SrS are attributed to them (fig. 6). The dips A 1, B 1, D and E have already been observed in reflection spectra [21] and were attributed to s-excitons with n = 1 at the X (A 1 and B1) and F points (D and E). Features A 2 and B 2 may be due to excitons with n = 2 ( X ; - X 3 ) . The following parameters can be derived from the model of Wannier excitons for A features: Eg = 5.08 eV, R y * = 0.31 eV, ~ = 1.99; for B features: Eg = 5.19 eV, R y * = 0.28 eV, ~ = 1.9. the value of Eg at the X point is in good agreement with the absorption results [26] and E = 0.11 eV with the theoretical s p i n - o r b i t splitting in SrS [26]. According to the calculations in ref. [23] the width of the valence band at the X point exceeds the spin-orbit splitting and equals 0.5-1.0 eV. Thus, the separation # between excitons X 4 - X 3 and X s - X s should be at least 0.5 eV. As the transition X 4 - X 3 is dipole-forbidden, the respective exciton should be of p-type. Intensities in III. VUV/SOFT X-RAY SPECTROSCOPY
ll2
v.V. Mikhailin / Excitation of seconda
the series of p-type Wannier excitons are given by I - ( I / n 3 - I / n 5) for n >~ 2, the second and the third members of the series having similar intensities. So it is natural to attribute the minima C 2 and C 3 to p excitons with n = 2 and 3, respectively. Then we obtain Eg = 5.69 eV, R y * = 3.6 eV, /~=2.33. Thus the width of the valence band at the X point of the first Brillouin zone in the SrS crystal is 0.56 eV.
5. Luminescence
excitation
of alkali-earth
sulphates
Sulphates of Ca, Sr and Ba in the fundamental absorption range have scarcely been studied. The large number of communications devoted to sulphates that is published annually generally deals with their application to ionizing radiation dosimetry. The values of the energy bandgaps for these compounds cited in the literature have a wide spread: from 4.5 to 12 eV. Due to t h e complex composition and low symmetry of the crystalline structure energy band diagrams for sulphates have not been calculated. On the other hand, the complex anion SO 2 has been thoroughly studied both theoreti4oo-
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cally (see, e.g., ref. [27]) and experimentally (e.g., ref. [28]) in terms of molecular orbitals. Excitation spectra in a wide range have been measured in ref. [1]. We have already mentioned that the luminescence quantum yield can provide information on the band structure. In luminescence excitation of sulphates in the energy regions 10-20 eV and 32-36 eV a set of minima is observed which is almost unaffected by cation substitution (fig. 7). They can thus be attributed to transitions in the complex anion SO42-. The dips at 32 and 34 eV, which are especially notable at LNT, are regarded as transitions from 2S core levels of oxygen. The transitions from core levels of cations appear in the excitation spectra of sulphates starting with 16 eV for Ba compounds, 18 eV for Sr and 25 eV for Ca. The onset of these transitions shows up as a set of narrow dips attributed to the creation of core excitons. They are observed in the energy interval 5 - 7 eV. The energy location of such excitons for the same cation is determined by the nearest neighbours. The contribution of transitions from core levels to the formation of ~(h~,) is not limited by the appearance of core excitons: they can also transform the general tendency of ~(h~): (1) At energies exceeding the cation core states spectral features are often observed similar to those that show immediately above the fundamental absorption edge. A dip at 27 eV in the excitation spectrum of SrSO4-Dy in these terms reproduces a deep trough at 8.8 eV. (2) At exciting photons energies h v ~ Ecore + Eg a steep rise of ~ ( h v ) is often observed due to both multiphcation of primary electrons with energies exceeding E~.,,r~+ Eg and a decrease of k ( h v ) resulting from the exhausting of transitions from core levels leading to deeper penetration of the exciting radiation into the crystal. We have proceeded in our consideration to the energy region where the main contribution to the formation of ~(h~,) is made by inelastic scattering of EE with energies sufficient to decay with the production of secondary EE. When the photon energy is large enough to create more than one EE (electron-hole pair of exciton) a step-wise rise of o ( h u ) can be observed. We think that this mechanism accounts for the rise of ~/(hu) which is observed above 23 eV in C a S O 4 - D y and 22 eV in SrSO 4 Dy. In BaSO 4 Dy the situation is complicated by the contribution of transitions from Ba 2+ core levels that fall into the same energy region, which disguises the appearance of multiplication. In all cases the multiplication starts at energies not less than 2Eg but the explicit energy location and the shape of the multiplication threshold is determined by a set of factors. It is evident, however, that exact assignment is impossible in the absence of values for the main optical constants of the compound. It was therefore necessary
113
V. V. Mikhailin / Excitation of secondal3' processes in the V U V range
to evaluate the energy band gap for sulphates. We tried to do this on the basis of the excitation and diffuse-reflection spectra. In the excitation spectra of all sulphates studied, irrespective of cation or type of luminescence detected (impurity or self-activated), a sharp trough falling almost to zero is observed at 8-9 eV. This depression separates the regions of exciting photon energies where the sensitivity of ~(h~,) to the substitution of the impurity is different: for h~, < 8-9 eV the shape of the spectrum is different and for h u > 8-9 eV similar for different luminescent centers. In diffuse-reflection spectra in the same energy range a decrease of Rdiff is observed as usually happens when we pass from the transparency region to fundamental absorption. Summing up all these facts we can state that the energy band gap of Ca, Sr and Ba sulphates falls into the region 8.5-10 eV. Besides, the shape of the rise at 10 eV suggest that in sulphates doped with rare-earth elements the decisive role in the excitation energy transfer to the luminescent centers is played by separated electron-hole pairs, while excitons are not efficient in this (the creation of excitons is attributed to the region of the pronounced trough). Such a mechanism of energy transfer to a luminescent center results in a shift of the multiplications threshold to energies exceeding 2 E g - a fact that was observed experimentally.
6. L u m i n e s c e n c e e x c i t a t i o n o f h a l i d e s of B a and S r
The luminescence quantum yield of barium and strontium halides in the energy range 6-35 eV has been measured using the SR of the "Pakhra" synchrotron of the Lebedev Institute [2]. The samples studied are known as X-ray phosphors. To understand the processes which occur in X-ray phosphors we must know the fundamental mechanisms of their excitation. Investigations in the VUV provide information about the intermediate stages of relaxation of X-ray excitations. Fig. 8 shows the excitation spectra of fluorides and chlorides of barium and strontium doped with Eu as well as those of the dihalides BaFC1 and SrFC1 (pure and Eu-doped), measured relative to sodium salicylate. The energy band gap of BaF 2 equals 10.5 eV, that of BaC12 8.5 eV. All these compounds have a high luminescence quantum yield at excitation energies below the fundamental absorption edge. Above this edge the ~ behaviour is different in different haiides. In BaC12-Eu, BaF2-Eu, pure BaFC1, SrC12 Eu and pure SrFC1 a notable depression of the quantum yield is observed directly above Eg, which can be attributed to a large thermalization length of electrons and holes in the process of hot relaxation. At energies above 2 Eg there are several maxima (in BaC12, e.g., peaking at 18 and 30 eV) corresponding to the creation of low-energy
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electrons with a small thermalization length. The maximum at 30 eV in BaC12 is due to the participation of 2+ (binding energy 17 eV) in the the core level Basp multiplication. The core levels also show up in the excitation spectra as shallow dips due to surface losses at the respective energies. The second group of spectra is more monotonous, with a quantum yield greater than in the spectra of the first group. One of the possible reasons for such a behaviour is the absence of migration losses; this can occur if the thermalization length of electrons and holes in the mixed crystals is small. If this is the case, the quantum yield for mixed crystals in the far VUV should be high. The intensity of X-ray luminescence of BaFC1-Eu is about an order of magnitude greater than that of BaCI2-Eu and BaF2-Eu. At an excitation energy of - 35 eV the luminescence intensity of BaFC1-Eu is 2.4 times that of BaC12-Eu and 7.3 times that of BaF2-Eu. Such intensity ratios indicate that the efficiency of X-ray excitation is closely related to the migration losses in the VUV.
References
[1] V.N. Meleshkin et al., Trudy Fiz. Inst. Akad. Nauk SSSR 80 (1975) 140 [English transl.: The Lebedev Institute Physics Series, vol. 80, ed., N.G. Basov (Consultants Bureau, New York, 1976) p. 139]. [2] V.I. Alekseyev et al., Preprint Lebedev Institute, Moscow, no. 20 (1986). [3] A.V. Kozhevnikov et al., Pis'ma ZhTF (1984) 677. [4] S.N. Ivanov et al., Izv. AN SSSR, Ser. Fiz. 41 (1977) 1326. III. VUV/SOFT X-RAY SPECTROSCOPY
114
V. V. Mikhailin / Excitation of secondary processes #1 the VU V range
[5] A.N. Belskiy et al., Vopr. Atom. Nauki i Tekhniki, ser. Obstchaya i Yadernaya Fiz. 4 (33) (1985) 80. [6] E.R. Ilmas and Ch. B. Lushchik, Optika i Spektroskopiya 18 (1965) 453. [7] A.N. Vasil'ev, V.V. Mikhailin and I.V. Ovtchinnikova, Izv. AN SSSR, Ser. Fiz. 49, no. 10 (1985). [8] M.A. Elango et al., Izv. AN SSSR, Ser. Fiz. 41 (1977) 1314. [9] Ch. Acherman et al., Phys. Stat. Sol (b) 74 (1976) 579. [10] E.D. Aluker, D.Yu. Lusis and S.A. Chernov, Electronic excitations and radioluminescence of alkali-earth crystals (Zinatne, Riga, 1979) in Russian. [11] V.V. Antonov-Romanovskiy, Kinetics of photoluminescence of phosphors (Nauka, Moscow, 1966) in Russian. [12] A.N. Vasil'ev and V.V. Mikhailin, Abstracts 6th All-Union Conf. on The physics of VUV (Izd. MSU, Moscow, 1982) p. 146. [13] A.N. Vasil'ev and V.V. Mikhailin, Izv. AH SSSR, Set. Fiz. 50 (1986) 537. [14] A.M. Gurvich et al., Zh. Prikl. Spectrosk. 23 (1975) 158. [15] S.N. Ivanov, V.V. Mikhailin and A.N. Vasil'ev, Abstracts Int. Conf. on Luminescence, West Berlin (1981) p. 183. [16] Yu.M. Aleksandrov et al., Trudy Inst. Fiz. Akad. Nauk Est. SSR 53 (1982) 31.
[17] Yu.M. Aleksandrov et al., Pis'ma ZhTF 7 (1981) 343. [18] Ch.B. Lushchik et al., Trudy Inst. Fiz. Akad. Nauk Est. SSR 47 (1977) 59. [19] T.N. Kyarner et al., Fiz. Tver. Tela (Leningrad) 22 (1980) 1178. [20] A.N. Belskiy et al., Preprint Dep. of Phys., MSU (1985) p. 28. [21] Y. Kaneko, K. Morimoto and T. Koda, J. Phys. Soc. Jpn. (1983) 4385. [22] Ye. V. Stepanova et al., Abstracts 7th All-Union Conf. on The Physics of VUV, Riga (1986) p. 19. [23] A. Hasegawa and A. Yanase, J. Phys. C 13 (1980) 1995. [24] V.V. Mikhailin, E. Koch and M. Skibowski, Proc. 4th Int. Conf. on Vacuum Ultraviolet Radiation Physics (1974) p. 401. [25] V. Mikhailin, in: Luminescence of Crystals, Molecules, and Solutions, ed., F. Williams (Plenum, New York, 1973) p. 269. [26] R.J. Zolweg, Phys. Rev. 111 (1958) 113. [27] H. Adachi and K. Taniguchi, J. Phys. Soc. Jpn. 49 (1980) 1944. [28] V.G. Sholokh, Thesis, Minsk (1985). [29] B.I. Nefedov et al., Izv. AN SSSR~ Ser. Fiz. 38 (1974) 448.
107
Section IlL VUV and soft X-ray spectroscopy E X C I T A T I O N O F S E C O N D A R Y P R O C E S S E S IN T H E V A C U U M U L T R A V I O L E T R A N G E V.V. M I K H A I L I N
Moscow State University,Moscow, USSR
The paper is a survey of the activity of the laboratory of synchrotron radiation, Moscow University, in the application of SR to the investigation of luminescence excitation of insulators: Mg and Be oxides, Ca, Sr and Ba sulphides, sulphates and halides and of the extrinsic photoeffect of MgO. General tendencies of the luminescence quantum yield behaviour are considered. The contribution of the transitions from core levels to the formation of the quantum yield is discussed.
1. Introduction This paper is a survey of the activity of the laboratory of synchrotron radiation, Physical Faculty of Moscow State University in the application of synchrotron radiation (SR) to the investigation of optical properties of insulators. The staff of the laboratory perform experiments at SR beam lines of several machines: synchrotrons S-60 [1] and Pakhra [2] of the Lebedev Institute, Moscow, Sirius of the Polytechnical Institute, Tomsk [3], storage rings of the Institute of Nuclear Physics, Novosibirsk [4] and the dedicated SR storage ring Siberia-1 of Kurchatov Institute of Atomic Energy, Moscow [5]. Solid-state spectroscopy has benefited much from the application of SR, namely in the VUV and XUV ranges experimental spectra have become more detailed and reliable. Research in this field contributes to our understanding of the electronic structure of solids and fundamental processes in them.
2. Quantum yield of secondary processes In this paper we shall deal mainly with luminescence excitation and shall also touch on the photoemission of wide band gap insulators. The luminescence quantum yield for a fixed energy of exciting photons is related to the following stages of the photon energy transfer: a) absorption of the exciting photon: the absorption coefficient k(h ~), providing information on primary electronic excitations (EE), shows up in the excitation spectrum due to surface losses; b) inelastic scattering of primary EE resulting in the creation of secondary EE [6]; c) transfer of the energy of EE (either electron-hole pairs or excitons) to luminescent centers. Reflection and absorption spectra originate from the first stage of photon absorption, thus, they provide 0168-9002/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
information only on the primary excitations. Besides, the determination of optical constants of powder materials (a number of the substances important for industrial applications are accessible only in powder form) is a task which cannot yet be solved by means of absorption and reflection spectroscopy. Thus, the study of luminescence excitation not only yields information complementary to the data provided by absorption and reflection, but in several cases is a unique method of investigation of EE evolution. We shall consider at first general tendencies of the luminescence quantum yield behaviour [7]. In the energy range of exciting photons limited by the fundamental absorption edge at the low-energy side and the onset of photon multiplication at the high-energy side two main tendencies are observed: (1) a decrease of the luminescence quantum yield with the energy of exciting photons in the samples with predominantly excitonic mechanism of energy transfer to luminescent centers; (2) the opposite tendency, i.e. an increase of the quantum yield in the case of recombination luminescence. The same feature is observed for the photoeffect quantum yield. When the energy of primary EE becomes sufficient to create secondary ones, the quantum yield should show a rise; however, incorporation of such processes as reflection, surface and migration losses, etc. complicates the picture, resulting in a variety of spectra for different phosphors. To study the effect of photon multiplication it is necessary to select it from the contributions of other processes to the measured spectra. Surface losses, e.g., are well described by a simple diffusion theory with a large EE recombination rate at the surface [8,9]. The luminescence quantum yield ~/(hu) is then related to the frequency of the incident light as 1 - R(h~')
7l(hu) Here
X + k(hu)L~lv(hu). ~v(hv) is the luminescence quantum yield III. VUV/SOFT X-RAY SPECTROSCOPY
108
v.v. Mikhailin / Excitation of secondao' processes in the VUV range
originating only from volume processes; k and R are absorption and reflection coefficients, respectively; L is the effective diffusion length. We shall assume that L is independent of the photon energy, this assumption is true for a fast relaxation of EE, when the thermalization length for electrons and holes is less than the diffusion length of thermalized excitations. Surface losses cannot account for the monotonic decrease or increase of the quantum yield which is uncorrelated to the absorption coefficient in the range preceding the onset of photon multiplication. Such a luminescence intensity dependence on the exciting photon energy cannot be explained on the basis of a simple band diagram for a phosphor, since the equations contain only the concentrations of electrons and holes, which are related to the intensity of the photon flux, but not to the energy of the photons creating them. In a series of papers [10-12] devoted to this problem the idea of "genetical" recombination was suggested taking into account the correlation between the electron and the hole created by the same photon. Here we shall consider correlations of this kind for a phosphor described by the following model. An electron e and a hole h created by the absorption of an exciting photon thermalize with a mean thermalization length l which depends on the energy of the incident photon. In the process of scattering they emit predominantly optical phonons (it is assumed that the energies of the electron and the hole are not sufficient yet to create secondary EE in the crystal). One of the following events can then take place: the electron and the hole can bind into an exciton e + h---, ex with further emission of the energy in the emission band of a free or self-trapped exciton or the exciton energy may be transferred to a luminescent center; successive recombination at a positive center e + c + ---, c 0, h + c o ---, c* with further emission from the excited state of the center c* in the respective emission band or radiation-
less relaxation of the center; the electron can be captured by a trap b with further thermal release from the trap: e+b~b
,
b
~b+e.
Fig. 1 shows the quantum yield for the excitonic channel of recombination versus the energy of the exciting photons, the intensity of exciting radiation and temperature, calculated with rather simple assumptions ( E 0 is the photon energy for which the thermalization length of the electron and the hole equals the radius of the recombination sphere). The curves evidently show that low intensities of excitation are characterized by a stronger dependence of ~,, on the energy of exciting photons: the quantum yield steeply decreases when the thermalization lengths of e and h exceed the recombination radius, and the number of e - h pairs with an infinite thermalization length increases. With increasing intensity even the electron which escaped from the recombination sphere has the possibility of recombining with a hole created by the absorption of another photon, so the depth of the depression in ~v decreases. When we take into consideration the temperature dependence of the probability W T for an electron to be released from a trap with activation energy %, the temperature of the sample will also influence the depth of the depression in the spectrum. At low temperatures electrons spend more time in the traps, their diffusion length decreases and the excitation spectrum becomes flatter. In the framework of the discussed model the recombination quantum yield equals 1 - ~v, it increases with the energy of the incident photon. In our model we did not consider the energy range where the multiplication of electronic excitations occurs. For simplicity let us start with crystals with a narrow valence band. At the energy above the threshold of EE multiplication electron-electron collisions are
C/
!
O
Fig. 1. Quantum yield for the excitonic channel of recombination vs the energy of exciting photons h~, the intensity of exciting radiation I, the temperature T and the concentration of c centers. Calculations are presented for k T = 0.08 (a) and C 1 / n 2o = 10- 3 (b).
V. V. Mikhailin / Excitation of seconda~ processes in the VUV range
109
:~, arb. unit~
Ot I 0
0
2
0
2
~
hv/Eg
I0
20
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Fig. 2. Quantum yield vs the energy of exciting photons. (a-c) Theoretical calculations for E0 =10Eg (a), Eo = 0.8Eg (b) and Eo = 0.15Eg (c); (d-f) experimental quantum yield of KI-TI [15] (d), MgO-A1 [16] (e) and BeO [17] (f).
accompanied by the phonon mechanism of EE energy loss mainly due to interaction with optical phonons h f2. Luminescence quantum yields calculated on the basis of a simple model [7,13] for three cases with different migration losses in the range from Eg to 2Eg (curves a-c) are plotted in fig. 2. The calculations were made with the assumption of comparable electron-electron and electron-phonon interactions in the region about 2.5Eg. When the interaction with phonons is weaker spectral features are sharper at energies larger than 2Eg and maxima shift towards lower energies. The depth of the trough at 3Eg is correlated to that at 2 Eg. This can be explained by the fact that immediately after 2Eg electrons and holes with low energy are created and therefore their thermalization length is small. The energy of the exciting quanta increasing up to 3Eg, secondary electrons are distributed in the interval Eg-2Eg, their mean thermalization length l increase leading to the reduction of the quantum yield due to migration losses. If we increase the energy of exciting photons further, secondary EE would also be able to produce new EE, this will result in a linear growth (above (4-5)Eg) of the quantum yield [14]. The theoretical curves (a-c) in fig. 2 can be compared with measured luminescence excitation spectra (d-f) of KI-T1 [15], MgO-A1 [16] and BeO [17]. The spectrum d is characterized by a step-wise growth of the quantum yield without notable depressions due to migration losses. The observed spectral features are mainly due to the structure of the absorption coefficient. Spectral features about 2Eg, which are observed at the background of a steep ~/ rise, are attributed to transitions from core levels. The main tendencies in the spectra a and d are similar: a monotone growth of ~l(hu) showing steps at energies that are multiples of Eg. The curve b can be illustrated by the excitation
spectrum of MgO-A1 (curve e). In this spectrum, besides features attributed to surface losses, a depression of the quantum yield in the range 8-19 eV is observed and another at energies of 26-32 eV. The appearance of the second trough is in good agreement with the proposed scheme of EE multiplication. A still deeper depression attributed to migration losses is observed in the excitation spectrum of BeO (curve f); this spectrum correlates with curve c. We tried to show how the proposed models can be applied to treat the measured luminescence excitation spectra of a series of phosphors. Now we shall proceed to a more detailed analysis of our experimental results.
3. Luminescence excitation and photoemission of MgO Radiation-resistant MgO single crystals have been extensively studied by a number of authors, so the stored information makes this wide band gap insulator a convenient model system for the investigation of EE multiplication. Impurity luminescence of MgO-A1, peaking at 5.3 eV, appears when electrons recombine with holes captured by associations of AI 3+ ions with cation vacancies [18,19]. The reported optical measurements were performed at the S-60 synchrotron of the Lebedev Institute [1]. The experimental setup was based on a normal incidence monochromator using the Wadsworth mounting without entrance silt. Photoemission spectra were measured at the "Sirius" synchrotron of the Polytechnical Institute, Tomsk [3]. All measurements were performed relative to the reference signal from the luminescent sodium salicylate. The results presented were obtained at room temperature. In fig. 3a reflection spectrum of a MgO single crystal III. VUV/SOFT X-RAY SPECTROSCOPY
110
V. V. Mikhailin / Excitation of secondary processes in the VUV range
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Fig. 3. (a) Reflection spectrum R(h~) of MgO single crystal (1) and absorption spectrum k(h~) derived from reflectivity data using Kramers-Kronig relations (2). (b) 5.3 eV luminescence excitation spectrum of MgO-AI (1) and excitation spectrum of the extrinsic photoeffect of MgO [3] (2).
in the range 5-25 eV is presented, the angle of incidence was 10 o. In the same figure an absorption spectrum is shown derived from reflectivity data using K r a m e r s - K r o n i g relations. In the fundamental absorption region ( 1 0 s - 2 × 1 0 6 cm 1) anion excitons are created at energies of about 7.58 eV, above Eg separated electrons and holes are produced. In the energy range 7.7-25 eV the holes are created in the valence band (Ell , oxygen levels), at 25-30 eV at the L l levels of oxygen. In the luminescence excitation spectrum steep minima are observed corresponding to the maxima of the reflection and absorption spectra. As was mentioned above, these minima should be attributed to radiationless recombination of electrons and holes at the surface. Besides spectral features due to this effect and that due to selective reflection we observe additional features: the quantum yield decreases with the energy of exciting photons in the ranges 7.6-19 and 26-31 eV and increases at energies from 19.5 to 26 eV and above 32 eV. It is natural to attribute the steep rise of the excitation efficiency to the effect of EE multiplication; depressions of 7/(h v), as was discussed earlier, can be interpreted in terms of a thermalization length increase with the energy of exciting photons. In the presence of any radiationless recombination channels
this effect results in the reduction of the excitation efficiency (see fig. 2). Fig. 3 also shows the excitation spectrum of the extrinsic photoeffect of MgO [3]. A series of maxima in this spectrum coincides with the maxima but not with the minima of the absorption. " M o d u l a t i o n s " by the absorption coefficient are superimposed on the rise of the electron emission efficiency in the energy ranges 10-19 and 22-27 eV and a notable decrease of the number of electrons escaping from the crystal at energies of 19.5-21.5 eV. The energy distribution of the emitted electrons, measured in ref. [3], shows that at h~,= 18.8 eV and 20 eV we deal mainly with fast photoelectrons; increasing the energy h J, up to 24.5 eV results in a decrease of the number of fast photoelectrons, while slow photoelectrons appear with energies of 1 2 eV. The shift of the extrinsic photoeffect excitation spectrum in comparison with the recombination luminescence excitation of MgO characterizes the electron affinity and equals about 2 eV. Consideration of the photoelectric measurements indicates that the first threshold of EE multiplication in MgO is at 19.5 eV, as was mentioned in ref. [16].
4. L u m i n e s c e n c e
excitation
of sulphides of alkali-earth
metals
We present luminescence excitation spectra measured in the energy range 3-35 eV at 300 K for sulphides doped with Ce (4 × 10 -4 g/g). The quantum yield was determined relative to sodium salicylate (fig. 4) [20]. A brief literature survey should be made on the
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Fig. 4. Luminescence excitation spectra at 300 K of CaS-Ce (1), SrS Ce (2) and core excitons in SrS-Ce (3).
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Fig. 5. Correlation of spectral features in luminescence excitation (solid line) and reflection (dashed line) [21] spectra of BaS. s8 objects of research. There are many papers dealing with alkali-earth sulphides. They have a rock salt structure, but unlike alkali halide crystals, their chemical bonding has a substantial contribution of covalency, which increases with the atomic number of the cation. Reflection spectra of single crystals have been measured in ref. [21]. The same authors investigated core states of Ca, Sr and Ba cations. The energy-band structure of CaS and SrS has been calculated in refs. [22] and [23], respectively. Stationary luminescence excitation spectra of these compounds in a wide spectral range have been measured for the first time in refs. [24,25]. As in the case of MgO, the accessible data on sulphides of alkaliearth metals makes them a convenient object for the study of EE dynamics in partially covalent compounds. The main tendency of */(hu) in the measured spectra is its growth with exciting photon energies. This growth, however, is not monotonous. A large number of spectral features is due to surface losses, which visualize the structure of the absorption coefficient in the excitation spectra. To separate this kind of structure from, e.g., that attributed to EE inelastic scattering it is promising to compare the excitation spectrum with the energy dependence of absorption or reflectivity. This is done in fig. 5, where the measured excitation spectrum and literature reflectivity [21] are shown together. A good agreement of the energy location of structure (minima of ~(h~) and maxima of R ( h v ) , which are close to the maxima of k ( h v)) is observed. Beyond the fundamental absorption edge ( E g = 5.5 eV for CaS and 5 eV for SrS) the structure in the excitation spectra is very sensitive to changes of the absorption coefficient. An increase of the absorption coefficient modifies the penetration depth of the exciting radiation leading to an increase of the surface losses. Besides the structure of the absorption, spectral features due to processes of EE scattering can also be observed. The steep rise of the luminescence quantum yield at 11-13 eV is attributed to these two effects: the decrease of the absorption coefficient and electron-hole pair
'1 t I I I £0
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hT(ev) Fig. 6. Luminescence excitation spectrum of SrS-Ce at 80 K and calculated band structure of SrS from ref. [23].
multiplication. The energy of exciting photons in this range exceeds 2 Eg, so the energy of the created electron is sufficient to produce two electron-hole pairs. The rise of the quantum yield at 18 eV in SrS and at 20 eV in CaS has the same multiplication origin. The weak and narrow dips observed in the spectra at energy ranges from 24 to 32 eV in CaS and from 20 to 28 eV in SrS are due to the creation of core excitons. Their location is in good agreement with the calculations of the transitions 4p6-4p55s,4p54d in the Sr 2 + ion in the crystal field of SrS [21]. It is the first observation of core excitons in the luminescence excitation of alkali-earth sulphides with a resolution even better than in the reflection measurements [21]. Cooling of the samples results in the appearance of excitons near the fundamental absorption edge. The minima A - E in the excitation spectrum of SrS are attributed to them (fig. 6). The dips A 1, B 1, D and E have already been observed in reflection spectra [21] and were attributed to s-excitons with n = 1 at the X (A 1 and B1) and F points (D and E). Features A 2 and B 2 may be due to excitons with n = 2 ( X ; - X 3 ) . The following parameters can be derived from the model of Wannier excitons for A features: Eg = 5.08 eV, R y * = 0.31 eV, ~ = 1.99; for B features: Eg = 5.19 eV, R y * = 0.28 eV, ~ = 1.9. the value of Eg at the X point is in good agreement with the absorption results [26] and E = 0.11 eV with the theoretical s p i n - o r b i t splitting in SrS [26]. According to the calculations in ref. [23] the width of the valence band at the X point exceeds the spin-orbit splitting and equals 0.5-1.0 eV. Thus, the separation # between excitons X 4 - X 3 and X s - X s should be at least 0.5 eV. As the transition X 4 - X 3 is dipole-forbidden, the respective exciton should be of p-type. Intensities in III. VUV/SOFT X-RAY SPECTROSCOPY
ll2
v.V. Mikhailin / Excitation of seconda
the series of p-type Wannier excitons are given by I - ( I / n 3 - I / n 5) for n >~ 2, the second and the third members of the series having similar intensities. So it is natural to attribute the minima C 2 and C 3 to p excitons with n = 2 and 3, respectively. Then we obtain Eg = 5.69 eV, R y * = 3.6 eV, /~=2.33. Thus the width of the valence band at the X point of the first Brillouin zone in the SrS crystal is 0.56 eV.
5. Luminescence
excitation
of alkali-earth
sulphates
Sulphates of Ca, Sr and Ba in the fundamental absorption range have scarcely been studied. The large number of communications devoted to sulphates that is published annually generally deals with their application to ionizing radiation dosimetry. The values of the energy bandgaps for these compounds cited in the literature have a wide spread: from 4.5 to 12 eV. Due to t h e complex composition and low symmetry of the crystalline structure energy band diagrams for sulphates have not been calculated. On the other hand, the complex anion SO 2 has been thoroughly studied both theoreti4oo-
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cally (see, e.g., ref. [27]) and experimentally (e.g., ref. [28]) in terms of molecular orbitals. Excitation spectra in a wide range have been measured in ref. [1]. We have already mentioned that the luminescence quantum yield can provide information on the band structure. In luminescence excitation of sulphates in the energy regions 10-20 eV and 32-36 eV a set of minima is observed which is almost unaffected by cation substitution (fig. 7). They can thus be attributed to transitions in the complex anion SO42-. The dips at 32 and 34 eV, which are especially notable at LNT, are regarded as transitions from 2S core levels of oxygen. The transitions from core levels of cations appear in the excitation spectra of sulphates starting with 16 eV for Ba compounds, 18 eV for Sr and 25 eV for Ca. The onset of these transitions shows up as a set of narrow dips attributed to the creation of core excitons. They are observed in the energy interval 5 - 7 eV. The energy location of such excitons for the same cation is determined by the nearest neighbours. The contribution of transitions from core levels to the formation of ~(h~,) is not limited by the appearance of core excitons: they can also transform the general tendency of ~(h~): (1) At energies exceeding the cation core states spectral features are often observed similar to those that show immediately above the fundamental absorption edge. A dip at 27 eV in the excitation spectrum of SrSO4-Dy in these terms reproduces a deep trough at 8.8 eV. (2) At exciting photons energies h v ~ Ecore + Eg a steep rise of ~ ( h v ) is often observed due to both multiphcation of primary electrons with energies exceeding E~.,,r~+ Eg and a decrease of k ( h v ) resulting from the exhausting of transitions from core levels leading to deeper penetration of the exciting radiation into the crystal. We have proceeded in our consideration to the energy region where the main contribution to the formation of ~(h~,) is made by inelastic scattering of EE with energies sufficient to decay with the production of secondary EE. When the photon energy is large enough to create more than one EE (electron-hole pair of exciton) a step-wise rise of o ( h u ) can be observed. We think that this mechanism accounts for the rise of ~/(hu) which is observed above 23 eV in C a S O 4 - D y and 22 eV in SrSO 4 Dy. In BaSO 4 Dy the situation is complicated by the contribution of transitions from Ba 2+ core levels that fall into the same energy region, which disguises the appearance of multiplication. In all cases the multiplication starts at energies not less than 2Eg but the explicit energy location and the shape of the multiplication threshold is determined by a set of factors. It is evident, however, that exact assignment is impossible in the absence of values for the main optical constants of the compound. It was therefore necessary
113
V. V. Mikhailin / Excitation of secondal3' processes in the V U V range
to evaluate the energy band gap for sulphates. We tried to do this on the basis of the excitation and diffuse-reflection spectra. In the excitation spectra of all sulphates studied, irrespective of cation or type of luminescence detected (impurity or self-activated), a sharp trough falling almost to zero is observed at 8-9 eV. This depression separates the regions of exciting photon energies where the sensitivity of ~(h~,) to the substitution of the impurity is different: for h~, < 8-9 eV the shape of the spectrum is different and for h u > 8-9 eV similar for different luminescent centers. In diffuse-reflection spectra in the same energy range a decrease of Rdiff is observed as usually happens when we pass from the transparency region to fundamental absorption. Summing up all these facts we can state that the energy band gap of Ca, Sr and Ba sulphates falls into the region 8.5-10 eV. Besides, the shape of the rise at 10 eV suggest that in sulphates doped with rare-earth elements the decisive role in the excitation energy transfer to the luminescent centers is played by separated electron-hole pairs, while excitons are not efficient in this (the creation of excitons is attributed to the region of the pronounced trough). Such a mechanism of energy transfer to a luminescent center results in a shift of the multiplications threshold to energies exceeding 2 E g - a fact that was observed experimentally.
6. L u m i n e s c e n c e e x c i t a t i o n o f h a l i d e s of B a and S r
The luminescence quantum yield of barium and strontium halides in the energy range 6-35 eV has been measured using the SR of the "Pakhra" synchrotron of the Lebedev Institute [2]. The samples studied are known as X-ray phosphors. To understand the processes which occur in X-ray phosphors we must know the fundamental mechanisms of their excitation. Investigations in the VUV provide information about the intermediate stages of relaxation of X-ray excitations. Fig. 8 shows the excitation spectra of fluorides and chlorides of barium and strontium doped with Eu as well as those of the dihalides BaFC1 and SrFC1 (pure and Eu-doped), measured relative to sodium salicylate. The energy band gap of BaF 2 equals 10.5 eV, that of BaC12 8.5 eV. All these compounds have a high luminescence quantum yield at excitation energies below the fundamental absorption edge. Above this edge the ~ behaviour is different in different haiides. In BaC12-Eu, BaF2-Eu, pure BaFC1, SrC12 Eu and pure SrFC1 a notable depression of the quantum yield is observed directly above Eg, which can be attributed to a large thermalization length of electrons and holes in the process of hot relaxation. At energies above 2 Eg there are several maxima (in BaC12, e.g., peaking at 18 and 30 eV) corresponding to the creation of low-energy
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so (ev)
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electrons with a small thermalization length. The maximum at 30 eV in BaC12 is due to the participation of 2+ (binding energy 17 eV) in the the core level Basp multiplication. The core levels also show up in the excitation spectra as shallow dips due to surface losses at the respective energies. The second group of spectra is more monotonous, with a quantum yield greater than in the spectra of the first group. One of the possible reasons for such a behaviour is the absence of migration losses; this can occur if the thermalization length of electrons and holes in the mixed crystals is small. If this is the case, the quantum yield for mixed crystals in the far VUV should be high. The intensity of X-ray luminescence of BaFC1-Eu is about an order of magnitude greater than that of BaCI2-Eu and BaF2-Eu. At an excitation energy of - 35 eV the luminescence intensity of BaFC1-Eu is 2.4 times that of BaC12-Eu and 7.3 times that of BaF2-Eu. Such intensity ratios indicate that the efficiency of X-ray excitation is closely related to the migration losses in the VUV.
References
[1] V.N. Meleshkin et al., Trudy Fiz. Inst. Akad. Nauk SSSR 80 (1975) 140 [English transl.: The Lebedev Institute Physics Series, vol. 80, ed., N.G. Basov (Consultants Bureau, New York, 1976) p. 139]. [2] V.I. Alekseyev et al., Preprint Lebedev Institute, Moscow, no. 20 (1986). [3] A.V. Kozhevnikov et al., Pis'ma ZhTF (1984) 677. [4] S.N. Ivanov et al., Izv. AN SSSR, Ser. Fiz. 41 (1977) 1326. III. VUV/SOFT X-RAY SPECTROSCOPY
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V. V. Mikhailin / Excitation of secondary processes #1 the VU V range
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