Multiplication Of Electronic Excitations Research Articles

Formation of defect triplets by synchrotron radiation in alkali halide crystals at 8K

The formation of spatially correlated defects (F—VK–I) and (F–VK-H triplets) was revealed in KBr crystals irradiated by 6–25 eV photons under the conditions of multiplication of electronic excitations. The efficiency of F–VK–I creation ν is high when one exciting photon forms two electron-hole pairs. The value of ν for F–VK—H triplets is high at the decay of cation excitons.

Luminescence quantum yield and multiplication of electronic excitations in the corundum crystals

The excitation spectra of luminescence in irradiated and nonirradiated corundum crystals are investigated by means of highly polarised synchrotron radiation in 5 to 30 eV region. In the fundamental absorption region the double-exciton peaks are observed in the region 8.5-9.2 eV for irradiated (especially by neutrons) crystals at temperature 90 K. At 9.5 eV sharp drops appeared, in the luminescence spectrum space which were interpreted as nonirradiative near-surface recombination, the probability of which sharply increased at high absorption coefficients. The enhancement of the luminescence efficiency in the high-energy region was connected with the decay effect of electron excitations as well as with the influence of volumetric excitations leading to the enhancement of recombination glow.

Spectral-kinetic study of self-trapping and multiplication of electronic excitations in Al2O3 crystals

The luminescence spectra and the excitation spectra of intrinsic (7.6 and 3.77 eV) and extrinsic emissions for α-Al2O3, Al2O3:Sc and Al2O3:Ga crystals have been measured by means of synchrotron radiation of 4–40 eV at 8 and 80 K. The luminescence of self-trapped excitons (7.6 eV) can be excited at the direct photocreation of excitons or at the recombination of electrons with unrelaxed holes. Fast intraband luminescence (τ<2 ns) has been detected in a spectral region from 2.5 to 6.5 eV. The absorption of one photon of 25–37 eV leads to the formation of two or three electron-hole pairs causing a sharp increase of the efficiency of extrinsic emission of Sc3+ (5.6 eV), Ga3+ (4.6 eV) and F+ centers (3.85 eV).

Excitations in halogen-containing aluminosilicate optical ceramics

The reflection spectra and the excitation spectra of various emissions have been measured for Na6Al6Si6O24(NaI)2x and Na6Al6Si6O24(NaBr)2x sodalite optical ceramics (x≅0.8) by means of VUV radiation of 5–30 eV at 8 K. The analysis of these spectra revealed two groups of electronic excitations (EEs). The photons of hν>8.5 eV generate EEs of an aluminosilicate carcass, while 5.5 to 8.0 eV photons cause the excitation of I− and Br− ions in the cages of an aluminosilicate matrix. The luminescence of Na4I, Na4Br, Na3I and Na3Br centers has been detected at the direct optical excitation of these centers as well as at the formation of EEs of the aluminosilicate carcass. The processes of creation, relaxation and multiplication of EEs as well as the process of VUV photochromism in sodalites are discussed.

Luminescence Excitation of Y3Al5O12 Monocrystals in the Vacuum Ultraviolet Region

Luminescence excitation spectra of Y 3 Al 5 O 12 single crystals in the region from 4 to 30 eV at tentperatures of 300 and 90 K were investigated using synchrotron radiation. In the region of fundamental absorption, excitation bands corresponding to excitonic (S ex ) and recombination (S b + e) states as well as an UV band stipulated by doped Nd 3+ ions were revealed. In the high energy region (hv > 25 eV), peculiarities of the multiplication of electronic excitations in both pure crystals and those doped by different concentrations of Nd 3+ are discussed.

Excitonic and recombination processes in CaWO4 and CdWO4 scintillators under synchrotron irradiation

Excitation spectra of intrinsic (IL) and extrinsic luminescence (EL) from CaWO4 and CdWO4 crystals, measured using synchrotron radiation (4–30 eV) at 8 K, have been analysed. In CaWO4 the IL (2.9 eV) is efficiently excited by photons of 5.5–7.0 eV at the creation of molecular excitons. The EL is efficiently excited in interband transitions (hν>6.8 eV). The energy threshold for the multiplication of electronic excitations is Et≈15 eV in CaWO4 and Et≈10 eV CdWO4. In CdWO4, a fraction of the molecular excitons undergoes autoionization due to an overlap with the continuum of the interband transitions. Fast emission (<2 ns) is found at 80 K under the pulse excitation by electrons with an energy of 300 keV. The continuous temperature-independent emission of CaWO4 crystals in the region of 2–5 eV is interpreted as an intraband luminescence. The emission at 5.5 eV is ascribed to a metastable molecular excitons.

Creation of groups of spatially correlated defects in a KBr crystal at 8 K

The creation spectrum of triplets of spatially correlated defects (F centre + self-trapped hole, centre + interstitial halogen ion, I centre) was measured for the first time in a KBr crystal at 8 K, using synchrotron radiation of 6-25 eV and highly sensitive luminescent methods. The spectrum of spatially correlated F and H centre creation by synchrotron radiation at 8 K was measured as well. The efficiency of triplet creation is especially high at crystal irradiation by photons under the conditions of multiplication of electronic excitations when the absorption of one photon leads, respectively, to the formation of an electron-hole (e-h) pair and a secondary exciton (15.0-16.5 eV), or two e-h pairs (16.7-19.0 eV), or to the formation of a cation exciton (19.5-20.5 eV). It is shown that the formation of an triplet is caused by the recharging of a primary F-H pair by an e-h pair or by a free exciton.

Photoelectron scattering in CO doped solid Ar

Inelastic photoelectron scattering in CO doped solid Ar has been investigated using the method of luminescence excitation spectroscopy. The threshold energies for multiplication of electronic excitations under selective excitation by synchrotron radiation have been measured. For specially selected radiative decay channels, different kinds of photoelectron scattering are observed. The observation of multiple electronic excitation is of particular interest.

Multiplication of electronic excitations and formation of radiation defects in alkali halides

The spectrum of F center creation has been measured for KBr:Tl using synchrotron radiation (12–32 eV). The creation efficiency of a stable F center is especially high at 15–17 and 28–30 eV where the absorption of one photon leads to the formation either both of an electron-hole pair and an exciton or two electron-hole pairs and an exciton. In these cases there are favourable conditions for stabilization of mobile radiation defects due to the association of interstitial halogen atoms, holes and cation vacancies.

Time resolved luminescence spectroscopy of wide bandgap insulators

This is a review of the recent activity of a collaboration on the basis of an INTAS grant ‘The luminescence process — a study of the excitation and radiation dynamics of materials using synchrotron radiation’. For the following crystals (CeF 3, LaF 3:Ce, PWO, LSO:Ce, CsI, RbI, KI, CsF, CsCl, BaF 2, CsPbCl 3, etc.) in the UV, VUV, XUV, and X-ray regions the sequential steps of the energy conversion of a high-energy photon into a luminescence quantum have been studied: (i) absorption; (ii) multiplication of electronic excitations; (iii) migration in the process of thermalisation; (iv) emission center excitation.

Cross-relaxational multiplication of electronic excitations of rare-earth ions

Cross-relaxational multiplication of electronic excitations of rare-earth ions

Mechanisms of intrinsic and impurity luminescence excitation by synchrotron radiation in wide-gap oxides

The excitation spectra of intrinsic and impurity luminescence have been measured in the spectral region of 12 to 30 eV for SrO, CaO and YAlO s crystals at 8 K. Processes of exciton selftrapping as well as electron-hole and excitonic mechanisms of multiplication of electronic excitations have been analyzed for wide-gap oxides (BeO, MgO, CaO, SrO, Y 2O 3, YAlO 3).

Secondary excitons in alkali halide crystals.

The peculiarities of multiplication of electronic excitations have been investigated in wide-gap alkali halide crystals where, in contrast to semiconductors, free and self-trapped excitons as well as free and self-trapped holes coexist. In particular, the luminescent methods for separation and quantitative investigation of the formation processes of secondary excitons have been elaborated. These methods are based on the investigation of the dependence of the intensity ratio for two different emissions from the same crystal as a function of the energy of exciting photons. The excitation spectra of triplet ($\ensuremath{\pi}$) and singlet ($\ensuremath{\sigma}$) emissions of self-trapped excitons as well as of impurity luminescence have been measured, using synchrotron radiation of 12 to 32 eV, for high-purity KI, KBr, and doped KI:Na, KI:Tl, KBr:I, KBr:Tl crystals at 8 K. The analysis of the intensity ratio spectra for two emissions allowed us to separate the regions of secondary triplet exciton formation in KI (13-15 and 26.5-29 eV) and KBr (15-17 and 28-30 eV). The threshold photon energy for the formation of a secondary exciton has been experimentally determined to be equal to 13.1\ifmmode\pm\else\textpm\fi{}0.2 eV in KI and 15.2\ifmmode\pm\else\textpm\fi{}0.2 eV in KBr. The probability of secondary exciton formation by hot photoelectrons reaches the main maximum at 0.5-0.7 eV above the value of threshold energy. The energy transport to impurity centers by free triplet secondary excitons has been revealed in KI:Na and KBr:I crystals. The decay of $3{p}^{5}4s$ cation excitons in KI and KBr, generated by 20-eV photons, leads to the formation of a double amount of electron-hole pairs.

Multiplication of anion and cation electronic excitations in alkali halides

Abstract Excitation spectra of intrinsic emissions have been measured for NaCl, RbBr and CsBr crystals at 8 K. The excitonic and electron-hole mechanisms of multiplication of electronic excitations have been detected. The processes of radiative decay of cation electronic excitations and the non-radiative decay with the formation of a double amount of anion electronic excitations will be discussed.

Multiplication of anion and cation electronic excitations in luminescent wide-gap ionic crystals

The spectra of intrinsic luminescence excitation by synchrotron radiation (6–32 eV) at 8 K have been analyzed for NaCl, KCl, RbCl, KBr, RbBr, CsBr, MgO, CaO and YalO 3 crystals. In all crystalsv (except MgO and CaO) the process of multiplication of electronic excitations (MEE) causes a sharp increase of the intensity of self-trapped exciton emission, but leads (in KBr and NaCl) to the decrease of intra-band luminescence efficiency. The analysis of the intensity ratio spectra for two components of exciton emission allows us to separate the process of secondary exciton creation by hot photolectrons (NaCl, KBr, YAlO 3). The threshold energies of excitonic and electron-hole mechanisms of MEE are compared for a number of alkali halides.

Creation of stable Frenkel defects by vacuum uv radiation in KBr crystals under conditions of multiplication of electronic excitations.

The spectrum of F-center creation has been measured for KBr:Tl by means of synchrotron radiation (12--24 eV). Analysis of the spectrum reveals the decay of cation excitons (19.7--20.3 eV) with the formation of a double amount of electron-hole pairs. The creation efficiency of a stable F center is especially high at 15--17 eV where the absorption of one photon leads to the formation both of an electron-hole pair and an exciton. In this case there are favorable conditions for stabilization of mobile radiation defects due to the association of interstitial halogen atoms, holes, and cation vacancies.

Multiplication of electronic excitations in CaO and YAlO3 crystals with free and self-trapped excitons

Using synchrotron radiation of 6-32 eV the reflection spectrum and excitation spectra for 5.7, 4.6 and 2.8 eV emissions have been measured for a freshly cleaved CaO crystal at LHeT. In CaO, indirect band-to-band transitions take place in the region of hv>or=6.4 eV. The excitation spectra for intrinsic emissions of 5.9 and 4.2 eV were measured in a YAlO3 crystal at LHeT as well. As in many other dielectrics the electron-hole mechanism of multiplication of electronic excitations connected with the creation of secondary electron-hole pairs by hot photoelectrons also occurs in CaO and YAlO3 crystals. The creation of secondary excitons by hot photoelectrons has been detected in YAlO3 for the first time. The excitonic mechanism of multiplication of electronic excitations has a high efficiency in crystals with self-trapping excitons of small radii.

Multiplication mechanisms of electronic excitations in KBr and KBr:Tl crystals.

The reflection spectrum and excitation spectra of self-trapped exciton emissions have been measured in the spectral region from 6.5 to 30 eV for a freshly cleaved KBr crystal at liquid-helium temperature. The following mechanisms of multiplication of electronic excitations have been distinguished by using luminescence methods: (i) formation of secondary excitons by hot photoelectrons; (ii) formation of secondary electron-hole pairs by hot photoelectrons; (iii) formation of electron-hole pairs due to nonradiative Auger decay of cation electronic excitations. The analysis of the spectrum of the intensity ratio of \ensuremath{\pi} and \ensuremath{\sigma} emissions of self-trapped excitons allowed us to obtain quantitative data on exciton formation by photoelectrons. The excitation spectra for the T and X Jahn-Teller components of ${\mathrm{Tl}}^{+}$-center emission have been measured in a KBr:Tl crystal at liquid-helium temperature in the region from 6.5 to 30 eV. A sharp maximum at 15 eV in the intensity ratio of T and X emissions is connected with the excitation of ${\mathrm{Tl}}^{+}$ centers by hot photoelectrons.

Multiplication of anion and cation electronic excitations in wide-gap KCl and CsCl crystals

Luminescence of self-trapped excitons, extrinsic recombinational luminescence and crossluminescence on the excitation by synchrotron radiation (7.7–30eV) have been investigated for KCl and CsCl crystals at 6–80 K. The following mechanisms of multiplication of electronic excitation are discussed: (i) decay of cation excitons with the formation of a double amount of anion excitons (KCl); (ii) photoionization of anions and cations with the subsequent formation of secondary excitons and electron-hole pairs by fast electrons.

Electronic properties of crystalline quartz excited by photons in the 5–25 eV range

The electronic excitation multiplication process and photoelectron emission in crystalline quartz were investigated and an estimation of the energy gap is presented.