Sergei Basun

The analysis of thermoluminescence glow curves

Information about the behavior of traps in a luminescent material is usually derived by fitting the glow curves in the thermoluminescence spectrum of the material to a general formula. From the fit one seeks to obtain values for the depth of the traps, the frequency factors governing the release of electrons from the traps, and some indication of the rates of trapping and retrapping. This study investigates how successful this fitting process is in providing reliable values for the trap parameters. The relevant rate equations are used to numerically generate simulated glow curves for specific values of trap parameters. These glow curves are then analyzed by the usual fitting process, and values for the trap parameters are derived from the fitting process. We comment on the comparison between the derived parameter values and the correct values. From these comparisons we attempt to obtain some useful insights to assist in the interpretation of experimentally observed thermoluminescence spectra.

Photoionization related phenomena in doped insulators: The role of inversion site symmetry of impurity centers

Abstract The light induced ionization of impurity centers and defects in insulating crystals results in many phenomena such as charge transfer optical transitions in absorption spectra, photoconductivity, the photovoltaic effect, optical bleaching, and photochemical persistent spectral hole burning. This paper considers specific effects in photoionization related phenomena which are closely connected with inversion symmetry of impurity centers. These recently observed effects include (a) the drastic dependence of the photoionization energy on impurity sites lacking inversion symmetry (Li2Ge7O15:Cr3+), (b) the direct dependence of the sign of the photovoltaic effect on the polarity of impurity sites in doped pyroelectric crystals (LiNbO3:Cr3+), and (c) the anomalous broadening of photochemical persistent spectral holes in zero-phonon spectra of inversionless impurity centers (CaS:Eu).

Photoionization spectral hole-burning and inversion symmetry of sites in doped insulators

The linear Stark shift of optical frequencies in impurity centers lacking inversion site symmetry results in various external electric field effects observed in persistent spectral hole-burning (PSHB) phenomena. This paper reviews some new effects of inversionless symmetry of centers in internal electric fields observed in PSHB of doped insulators with photoionization hole burning. (1) In multisite R-line ( 4 A 2 - 2 E) spectra of Cr 3 + ions, substituting Ge 4 + in the inversionless position in the crystal lattice of Li 2 Ge 7 O 15 (LGO), the two-photon self-gated photoionization PSHB was observed in the R-spectra of one site only. This drastic site dependence of PSHB demonstrates extremely strong(∼ eV) relative shifts of the position within the host energy gap of the 4 A 2 , 2 E levels of Cr 3 + ions, located at complex centers. These centers differ in the nature and the position of the nearby charge-compensating defect, where the Coulomb field produces different large linear Stark shifts of the electronic states of Cr 3 + . (2) Photon-gated PSHB in f-d spectra of Eu 2 + multisites in CaS crystal was observed with unusually broad hole wings in the case of lines belonging to perturbed inversionless sites. This efficient off-resonant PSHB is due to strong linear Stark shifts of the f-d transition frequencies of Eu 2+ ions in time-varying Coulomb fields resulting from photoionization and recharging of Eu 2+ -Eu 3+ ions during the hole-burning process in optically excited crystals.

Non-resonant hole burning in CaS:Eu by photoinduced recharging of Eu ions

Abstract Persistent spectral hole burning has been observed on the zero-phonon lines of the main and several perturbed sites of Eu 2+ in CaS: Eu single crystals. Hole burning occurs by two-step photoionization, can be strongly gated with IR irradiation, and takes place by excited-state absorption from the metastable 4f 6 5d excited state to the conduction band followed by electron transport to Eu 3+ centers which are the dominant traps. Unusually broad ~ 100 GHz wings were observed in hole spectrum for perturbed Eu 2+ sites. This is explained by a new off-resonant hole-burning mechanism due to the linear Stark effect of inversionless Eu 2+ sites in time-varying internal Coulomb fields produced by dynamical recharging of Eu ions in the course of hole-burning process.

Photoconductivity of titanium doped insulating oxide crystals

Abstract Photoconductivity of Ti 3+ in Al 2 O 3 , Y 3 Al 5 O 12 and MgAl 2 O 4 were studied by one-step and two-step ionization at room and liquid-nitrogen temperatures. By virtue of its selectivity, two-step photoconductivity can give more accurate ionization thresholds than the one-step process. The thresholds obtained agree with the predicted values based on a simple model.

Charge exchange processes between impurity ions and the host crystal in wide band-gap crystals

Abstract The point of this article is to investigate under what conditions the sum of a donor transition of an impurity ion, Mn+ → M(n+1)+ + e(CB), and an acceptor transition M(n+1)+ + e(VB) → Mn+ is equal to the band gap of the host. Our recent work on Ti3+, Ti4+ in aluminate host crystals provides the best of the few examples where both a donor and an acceptor transition are known.

New spectroscopic effects of ferroelectric phase transition in Li2Ge7O15 crystals doped with 3d3-ions

Abstract In Li2Ge7O15(LGO) crystals doped with Cr3+ and Mn4+ ions the transformation of luminescence spectra in the region of R-lines at ferroelectric phase transition D2h → C2x in LGO was used to determine the point group symmetry and microstructure of 3d3-centers. In experiments on pseudo-Stark splitting of R-lines in external electric field, the phase transition induced polarization of 3d3-ions was observed, the induced dipole moments being directed perpendicular to ferroelectric axis of C2x phase.

Nonequilibrium phonons in single crystal ruby fibers

Abstract The dynamical behavior of 29 cm−1 phonons in single crystal fibers of Al2O2:Cr3+ (ruby) at 2 K has been investigated. The fiber diameters are of the order of the phonon mean free path and transport is dominated by surface reflections and scattering and by the acoustic mismatch at the crystal interfaces.