Transient optical absorption and luminescence in Li2B4O7 lithium tetraborate

This paper reports on a study of the transient optical absorption exhibited by Li₂B₄O₇ (LTB) in the visible and UV spectral regions. Using absorption optical spectroscopy with nanosecond time resolution, it is established that the transient optical absorption (TOA) in these crystals originates from optical transitions in hole centers and that the kinetics of the optical-density relaxation is controlled by interdefect tunneling recombination, which involves these hole centers and electronic Li⁰ centers representing neutral lithium atoms. At 290 K, the Li⁰ centers migrate in a thermally stimulated, one-dimensional manner, without carrier ejection into the conduction or valence band. The kinetics of the pulsed LTB cathodoluminescence is shown to be controlled by a relaxation process connected with tunneling electron transfer from a deep center to a small hole polaron migrating nearby, a process followed by the formation of a self-trapped exciton (STE) in an excited state. Radiative annihilation of the STE accounts for the characteristic σ-polarized LTB luminescence at 3.6 eV, whose kinetics is rate-limited by the tunneling electron transfer.     

Self-Trapped Excitons in Ionic-Covalent Silver Halide Crystals and Nanostructures: High-Frequency EPR, ESE, ENDOR and ODMR Studies

Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag²⁺ ion inside of a (AgCl₆)^4? complex as a result of the Jahn–Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron–nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn–Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra.     

Triplet-triplet transfer of electronic excitation energy in vapors of organic molecules

The rate constants of the triplet-triplet (T-T) transfer of energy of the electronic excitation obtained in the gas phase for two sets of donor-acceptor pairs with various spectral characteristics are compared with those predicted by the theories of nonradiative transitions. The rate constants K _TT and efficiencies β_DA of the T-T transfer are estimated from quenching of the phosphorescence of biacetyl by aromatic hydrocarbons (anthracene, 9-methylanthracene, 9,10-dimethylanthracene, 9-phenylanthracene, pyrene, and chrysene) and from quenching of the delayed fluorescence of benzophenone, 4-phenylbenzophenone, anthraquinone, carbazole, and naphthalene by biacetyl, as well as from the sensitization of phosphorescence of biacetyl by these compounds. The causes of the broad variations of β_DA in the gas phase from 0.35 (biacetyl-anthracene) to 10^?4 (biacetyl-chrysene) are discussed for the studied donor-acceptor pairs, which differ in the exothermicity of the process and in the configuration of the lowest triplet levels of the molecules participating in the transfer. The dependences of the transfer rate constants K _TT on the free energy change ΔG (the range from 0.4 to ?1.7 eV) are analyzed. It is shown that, in gas-phase systems, K _TT increase with the exothermicity of the transfer, attain a highest value, and then decrease in the range of negative ?ΔG. The observed differences in the experimental values of β_DA are discussed in terms of changes in the free energy ΔG of the process, in the energy of rearrangement of the nuclear configuration of the molecules during the transfer, and in the matrix elements of the interaction.     

Transient optical absorption of hole polarons in ADP (NH4H2PO4) and KDP (KH2PO4) crystals

A study of transient optical absorption of the ADP (NH₄H₂PO₄) and KDP (KH₂PO₄) nonlinear crystals in the visible and UV spectral regions is reported. Measurements made by absorption optical spectroscopy with nanosecond-time resolution established that the transient optical absorption (TOA) of these crystals originates from optical transitions in the hole A and B radicals and the optical-density relaxation kinetics is rate-controlled by interdefect tunneling recombination, which involves these hole centers and the electronic H⁰ centers representing neutral hydrogen atoms. At 290 K, hole polarons and the H⁰ centers undergo thermally stimulated migration, which is not accompanied by carrier ejection into the conduction or valence band. The slow components of the TOA kinetics with characteristic times from a few tens of milliseconds to a few seconds can be assigned to diffusion-controlled annihilation of hydrogen vacancies associated with impurity or structural defects.     

Levelcrossing Experimente im Tm I-Spektrum

A semi phenomenological dynamical theory for the rotations of the BO₆ octahedra in ABO₃ perovskites is proposed, which accounts for the soft mode and the central peak forT≧T _c observed by neutron scattering in SrTiO₃. The neutron scattering cross section is discussed. The EPR line width is predicted to diverge as ?^?ν(1–2η) and as ?^?ν(2–2η) for three- and two dimensional correlations with ?=(T-T _c )/T _c . The theory is generalized to other structural transitions.     

Direct observation of the optical anisotropy of C70 fullerene molecules in the Shpol’skii spectra

Linear optical absorption and emission spectra of C₇₀ fullerene molecules in single-crystal toluene are investigated. It is established that the lines of purely electronic S ₀-S ₁ transitions are significantly polarized. The degree of linear polarization of the spectral lines depends on the position of the fullerene molecule in the toluene matrix and can be as high as 100%. The polarization characteristics of the lines can be understood in the context of a model in which the S ₀ → S ₁ electronic transition is represented by the excitation of a planar oscillator whose axis is oriented along the principal axis of the C₇₀ molecule. The relationship between the polarization of the spectral lines and the position of the fullerene molecule in the matrix is consistent with the conclusions drawn from a theoretical analysis of different configurations possible upon the embedding of C₇₀ molecules into crystalline toluene.     

Color centers in heavily irradiated CsI(Tl) crystals

The absorption and luminescence properties of CsI(Tl) crystals colored by irradiation are studied by the method of the time-resolved spectroscopy. The scheme of the electron transitions in CsI(Tl) crystal is suggested to explain the appearance of the color centers under exposure to the near-UV light. It is established that either of the two types activator color centers holds the charge carrier with opposite sign. The model of the hole Tl²⁺v_c⁻ activator color center is suggested. According to the model the positive charge of Tl²⁺ ion is compensated by the negative charge of a close cation vacancy v_c⁻. The color center emission reveals in the cathode-luminescence spectrum of the colored CsI(Tl) crystal. The high-dose irradiation of CsI(Tl) crystal results in the reduction of the decay time of the near-thallium self-trapped excitons (STE) emission. The decay kinetics of Tl²⁺v_c⁻ emission contains the time components typical for the decay kinetics of near-thallium STE emission. The reason of the observed effects is the energy transfer from the near-thallium STE excitons to the color centers via the inductive-resonant mechanism.     

Electronic Raman scattering from inter-valence band transition in photo-excited GaP and GaAs1−xPx crystals

Raman scattering from photo-created free carriers in undoped GaP and GaAs_1?xP_x (x = 0.85, 0.73 and 0.66) under high excitation intensity has been studied. Two new Raman bands have been observed and assigned to electronic transitions from the split-off hole band to the heavy hole band and from the light hole band to the heavy hole band. The spin-orbit splitting energies in these crystals have been determined from the analysis of observed Raman bands, and compared with other experimental values.     

Luminescence of impurity-bound excitons in corundum

The luminescence characteristics of M ³⁺: Al₂O₃ crystals, where M ³⁺ stands for an isoelectronic cation impurity with a filled electron shell, namely, Sc³⁺, Y³⁺, or La³⁺, were studied. The luminescence of excitons bound (BE) to these impurities was detected. The position of the BE energy states at the long-wavelength absorption edge as a function of the M ³⁺ ionic radius was established. The energies of the long-wavelength BE creation threshold and of the maximum of the BE luminescence band were approximated empirically by third-order polynomials of the Toyozawa polynomial type, which describes electron-phonon interaction. The energy and spatial structure of the BE was found to be similar to that of a self-trapped exciton (STE). The BE and STE states are separated by an energy barrier, and energy transfer from the STE to BE is frozen at low temperatures.     

Absorption spectra of self-trapped exciton in condensed rare gases

Optical absorption from the self-trapped exciton (STE) to its higher excited states has been observed in solid Ar, Kr, and Xe. A new absorption peak of Ar1₂ is found. Comparison of solid, liquid and gas phase data provides evidence of the similarity of level structures of Ar1₂ and STE. Tentative assignments of the observed transitions are also made.     

Two hole spectra and valence states in chromium and chromium silicides

Auger spectra for L₃M₂₃V and L₃V V transitions involving, respectively, one and two valence holes in the final state, have been measured for Cr and CrSi₂ using both X-ray photons and electrons as ionization source. Careful subtraction of the energy losses from the raw data permits determination of the lineshape of the Auger spectra. The valence hole spectral functions derived from the L₃M₂₃V transitions are compared with valence band spectra obtained by X-ray photoemission. The comparison provides direct evidence of the importance of multiplet coupling between the 3p and 3d holes in the final state. Results for the spectral function of two valence holes are consistent with the outcome of band structure calculations, although some correlation effects seem to be present.     

Carbon K-shell excitation spectra of linear and branched alkanes

The electron energy loss spectra of ethane, propane, n-butane, n-pentane, n-hexane, isobutane, isopentane and neopentane in the region of carbon K-shell excitation have been recorded under dipole-dominated conditions (2.8 ke V impact energy, small angle). The spectra are dominated by transitions to unoccupied valence π¹(CH₂, CH₃) and σ¹(C-C) levels. Additional weak features are assigned to Rydberg transitions. The position of the main continuum feature in each spectrum is consistent with the predictions of an empirical relationship with bond length. Systematic variations of spectral intensities are observed which support our assignments. The dominant feature in the K-shell spectrum of ethane, which was previously assigned to C 1s → 3p Rydberg transitions, is reassigned to excitation to a 3p/π¹(CH₃ ), mixed Rydberg/valence orbital (of antibonding σ-¹(C-H) character), in comparison to the other alkane spectra. An improved calibration value of 290.74(5) eV for the energy of the C 1s → π¹ transition in CO₂ is also obtained.     

Postcollision interaction upon electron excitation of the 5sns 1 S 0 and 5snd 3 D j levels of the cadmium atom

The optical excitation functions (OEFs) for two series of spectral lines of the Cd atom originating from the 5sns ¹ S ₀ (n = 6?11) and 5snd ³ D _{1, 2, 3} (n = 5 and 6) levels excited by an ultramonoenergetic (ΔE _1/2 < 0.05 eV) electron beam with energies exceeding the single ionization threshold are presented. In the energy range from 10.8 to 12.9 eV, the energy dependences of the excitation cross sections of the studied spectral transitions exhibit the effect of postcollision interaction of slow scattered and fast emitted electrons. This process leads to an additional population of the initial levels of the spectral transitions due to the capture of a scattered electron into an excited atomic level. The energies and widths of the electronic decay of autoionizing states are estimated in the classical approach by two methods, namely, by the least squares method and by direct calculation. Calculations are performed using approximate formulas valid for different relations between the postcollision shifts of the OEF maxima and the binding energies of the atomic levels. The terms of the autoionizing atomic states responsible for the maxima observed in the OEFs of the spectral transitions are determined.     

A new analysis and additional measurements of the millimeter and submillimeter spectrum of methanol

Over 100 new spectral lines of gas-phase methanol in the millimeter and submillimeter region (viz. 150–1000 GHz) corresponding to rotational transitions in the two lowest torsional states have been measured and assigned. Some of these lines have been combined with spectral lines measured by previous investigators below 1000 GHz into a global data set consisting of 470 lines involving transitions with J ≤ 8 and v_t (torsional quantum number) ≤ 2, and then analyzed via a nonlinear least-squares procedure. The analysis involves supplementing the standard IAM Hamiltonian with a number of new distortion and “interaction” constants. Direct diagonalization has been employed in preference to an earlier perturbation scheme. The quality of the least-squares fit compares favorably with the previous quantum mechanical approach.     

Theoretical investigations on the high light yield of the LuI3:Ce scintillator

The extremely high scintillation efficiency of lutetium iodide doped by cerium is explained as a result of at least three factors controlling the energy transfer from the host matrix to activator. We propose and theoretically validate the possibility of a new channel of energy transfer to excitons and directly to cerium, namely the Auger process when Lu 4f hole relaxes to the valence band hole with simultaneous creation of additional exciton or excitation of cerium. This process should be efficient in LuI₃, and inefficient in LuCl₃. To justify this channel, we perform calculations of density of states using a periodic plane-wave density functional approach. The second factor is the increase of the efficiency of valence hole capture by cerium in the row LuCl₃-LuBr₃-LuI₃. The third one is the increase of the efficiency of energy transfer from self-trapped excitons to cerium ions in the same row. The latter two factors are verified by cluster ab initio calculations. We estimate either the relaxation of these excitations and barriers for the diffusion of self-trapped holes (STH) and self-trapped exciton (STE). The performed estimations theoretically justify the high LuI₃:Ce³⁺ scintillator yield.     

Analysis of the ν6 band of 12CH3D at 8.6 μm

The ν₆(E) fundamental vibration-rotation band of monodeuteromethane (¹²CH₃D) has been recorded in the spectral range 1033–1270 cm^?1 with a resolution of approximately 0.04 cm^?1. Of the 669 transitions with J′ ≤ 17 identified, 633 have been retained for the determination of the rotational levels in the upper state v₆ = 1. The Coriolis interaction between the v₆ = 1(E) and v₃ = 1 (A₁) vibrational states of ¹²CH₃D results in large A₁A₂ splittings of levels with v₆ = 1 and |K ? l₆| = 0 or 3; the mixing in K and l₆ also gives rise to some ten forbidden transitions observed in the spectra. These effects have been very well explained within the formulation based on the contact transformation method. Values of 15 molecular structure constants of the v₆ = 1 state have been determined from a least-squares analysis of the 633 retained transitions. These constants can be used to estimate values of the upper-state energies up to fourth order, and through them the spectral positions of the 633 retained transitions are reproduced with an overall standard deviation of 0.013 cm^?1, which is within experimental uncertainties.     

Luminescence of trivalent praseodymium in oxides and fluorides

Three types of Pr-activated phosphors are studied: those with predominant 5d-4f transitions, those with transitions from the ³ P ₀-level, and those with successive transitions from the ¹ S _0? and ₃ P ₀-levels of 4f-configuration. The theoretical-model analysis of a photon cascade emission (PCE) is carried out. The basic properties of host matrixes necessary for PCE detection are ascertained. The spectral and kinetic (luminescence decay) characteristics of a number of Pr-activated oxides and fluorides are measured. SrAlF₃:Pr is found to exhibit the best luminescent properties among the phosphors studied. The host matrixes for the continued search for the PCE effect are suggested.     

Diode laser spectrum of the ν6 band of CH3I using a novel étalon as a calibration scale

The ΔK = Δl = +1 transitions of the ν₆ band of the CH₃I molecule were measured by a tunable diode laser spectrometer. A newly designed Fabry-Perot étalon with a free spectral range of 0.00945 cm^?1 was used to calibrate the line position very precisely. The present data were analyzed together with the previously reported transition wavenumbers, and precise molecular constants have been obtained.     

Negative U centers, percolation, and the insulator-metal transition in high-T c superconductors

The mechanism of formation of ?U centers in high-T _c superconductors (HTSCs) is considered. It is shown that the transition from the insulator to the metallic state on doping passes through a certain range of dopant concentrations in which it becomes possible for local transitions of singlet electron pairs to occur from oxygen ions to two neighboring cations (a ?U center), while single-electron transitions are still forbidden. Conduction arises in such systems at a concentration of ?U centers exceeding the percolation threshold for the orbitals of singlet hole pairs. A phase diagram constructed on the basis of the proposed model for the HTSC compounds of the Ln-214 class is in complete agreement with experiment. The mechanisms of formation and relaxation of free hole carriers are considered. It is shown that a distinctive feature of the normal state of HTSCs is the dominant contribution of electron-electron scattering to the charge carrier relaxation processes. It is concluded from the analysis presented that HTSCs comprise a special class of solids in which a nonstandard mechanism of superconductivity, different from the BCS mechanism, is realized.     

The central role of oxygen on H-irradiated Lu2SiO5 luminescence

The behavior of self-trapped defects (STDs) in ion-beam irradiated Lu₂SiO₅ (LSO) crystal has been investigated via temperature-dependent radioluminescence (RL) measurements. Production of oxygen vacancies is the major effect of H⁺ irradiation on luminescencent properties of this phosphor. Luminescence centers for self-trapped exciton (STE) and self-trapped hole emission are assigned to oxygen vacancies and oxygen ions, respectively. Ion-induced structural damage modifies the thermal stability of the STDs and creates perturbed STEs. A striking effect of ion irradiation is the approximate factor-of-two enhancement of STE RL intensity that results from implantation of only a thin (∼250 nm) surface layer of LSO. This enhancement is attributed to ion-beam modification of a surface dead layer.