The effect of pressure has been studied on the thermoluminescence and photoluminescence of a well... more The effect of pressure has been studied on the thermoluminescence and photoluminescence of a well-characterized sample of ZnS:Mn:Cu:Cl. The thermoluminescence was particularly revealing. Initially two traps were present. As they increased in depth with increasing pressure new shallower traps appeared. The pressure dependence of four traps was established. By comparison with pressure measurements on ZnS doped only with Mn+2 and only with Cu+ and Cl−, we established that the deepest trap is associated with defects introduced by Mn+2, while the two traps next lower in energy are associated with defects introduced by Cl−. The shallowest trap could not be assigned to a specific defect. The photoluminescence emission (from the 4T1, to the 6A1 energy level of Mn+2) shifted to lower energy at a rate identical to that previously reported for a less well-characterized ZnS:Mn sample. The shift could be described quantitatively in terms of changes in the ligand field parameters.
Raman spectroscopy of the CH stretching modes was used to investigate the molecular level respons... more Raman spectroscopy of the CH stretching modes was used to investigate the molecular level response of hexahydro‐1,3,5‐trinitro‐s‐triazine (RDX) to shock compression. Single crystals of [111], [210], and [100] orientation were compressed under stepwise shock loading to peak stresses from 3.0 to 5.5 GPa. Similar frequency shifts were observed for all three orientations below 3.5 GPa. Significant spectral changes were observed in crystals shocked above 4.5 GPa. These changes were consistent with the α−γ phase transition observed under static compression. No apparent dependence on crystal orientation was observed to 5.5 GPa. The phase transition had an incubation time of about 100 ns when RDX was shocked to 5.5 GPa peak stress.
In this paper we discuss recent applications of luminescence spectroscopy to resolve problems con... more In this paper we discuss recent applications of luminescence spectroscopy to resolve problems concerning phosphors in crystals or dissolved in polymers. On the first two examples we treat intramolecular excitations for molecules dissolved in polymers and explain mechanisms of energy dissipation due to changes of molecular conformation. The second set of examples involve ZnS phosphors with various dopants. We establish the characteristics of the donor and acceptor states via photoluminescence, and the trap characteristics by means of thermoluminescence. We treat very briefly thermoluminescence in crystalline coronene.
Abstract Charge carrier photogeneration in crystalline phenanthrene has been studied by monitorin... more Abstract Charge carrier photogeneration in crystalline phenanthrene has been studied by monitoring the UV-induced thermoluminescence (UVTL) as a function of external electric field for the excitation quanta at energies lower than the band gap. The electric field effect on the UVTL is discussed in terms of Poole-Frenkel- and Onsager-type dissociation of the defect-trapped geminate electron-hole pairs produced under low-temperature excitation. Quantitative agreement with the experiment is obtained with the Onsager-type model, indicating that the average distance between hole and electron in the trapped geminate pairs amounts to 7 nm.
Abstract Photoluminescence measurements as a function of pressure were made on ZnS and ZnxCd1-xS ... more Abstract Photoluminescence measurements as a function of pressure were made on ZnS and ZnxCd1-xS (x = 0.937) samples doped with Cu+, Cl-, and Al3+. A limited amount of thermoluminescence and optical absorption data aided in the interpretation. In contrast to the ZnS:Cu:Cl studied earlier, these materials were light yellow in color and could be excited by the 442-nm as well as the 325-nm line of a He-Cd laser. When excited by the 325-nm line into the conduction band the emission peaks shifted to higher energy at a rate which could be related to the increase of the energy gap with pressure? Optical absorption data combined with the excitation spectra of Hoshina and Kawai demonstrated that for the ZnxCd1-xS phosphor 442-nm excitation was via an exciton band apparently pinned to the conduction band. For ZnS the 442-nm excitation was from a complex acceptor band to the conduction band. The peak shift with pressure indicated that the upper states of this band were no longer pinned to the valence band.
Abstract Light-induced phosphorescence of 4-N,N-dimethylaminobenzonitrile (DMABN) dissolved in so... more Abstract Light-induced phosphorescence of 4-N,N-dimethylaminobenzonitrile (DMABN) dissolved in solid polymer has been observed for the first time. This unusual phosphorescence develops after continuous illumination of the sample at room temperature. The phosphorescence follows an initiation period the duration of which depends strongly on the exciting light intensity, concentration of solute in the polymer, pressure and the previous treatment of the sample. Under continuous illumination, the overall shape of the phosphorescence intensity curve versus time depends also on the exciting light intensity and concentration. At pressures higher than 13 kbar light-induced phosphorescence is no longer observed.
Abstract To understand pressure effects on dimer structure stability, Raman and FTIR spectroscopi... more Abstract To understand pressure effects on dimer structure stability, Raman and FTIR spectroscopies were used to examine changes in H-bonded dimers of benzoic acid (BA). Experiments were performed on single crystals compressed to 33 GPa in a diamond anvil cell (DAC). Several changes in Raman spectra were observed in the range 6–8 GPa indicating modification in the dimer structure suggesting the lowering of molecular symmetry. Pressure increase above 15 GPa induced strong luminescence and a gradual change of the crystal color from white to yellow/brownish. FTIR measurements on the sample released from 33 GPa indicated formation of a new compound. It is proposed that molecules of this compound are composed of the hydroxyl group associated with alcohol, carbonyl group associated with ketone, and the sp3 hydrocarbon groups. This study demonstrates that sufficient high pressure compression and subsequent decompression can lead to significant changes in the H-bonded dimer structure, including the breaking of bonds and formation of new chemical compound.
Optical spectroscopy methods were used to examine the effect of nonhydrostaticity on the electron... more Optical spectroscopy methods were used to examine the effect of nonhydrostaticity on the electronic structure of anthracene single crystals compressed statically to 9 GPa. Two pressure-transmitting media, nitrogen (hydrostatic) and water (nonhydrostatic above approximately 5.5 GPa), were utilized. It was found that nonhydrostatic compression generates several new features both in the absorption and fluorescence spectra: (i) formation of new absorption and fluorescence bands, (ii) deviations in pressure shift of fluorescence peaks, (iii) extensive broadening of vibrational peaks, and (iv) irreversible changes in the spectra shape upon pressure unloading. Furthermore, the time-resolved fluorescence decay curves measured at the wavelength corresponding to the new fluorescence band show clear initial increase. These new features are accompanied by inhomogeneous color changes and macroscopic lines on the (001) plane of the crystal. All of the changes are discussed and correlated with microscopic transformations in the crystal. It is demonstrated that nonhydrostatic compression in anthracene crystal introduces inelastic changes in the form of dislocations along [110] and [110] directions. These dislocations lead to the development of dimeric structures and, consequently, to various changes in the electronic response of the compressed anthracene crystal.
The effect of pressure has been studied on the thermoluminescence and photoluminescence of a well... more The effect of pressure has been studied on the thermoluminescence and photoluminescence of a well-characterized sample of ZnS:Mn:Cu:Cl. The thermoluminescence was particularly revealing. Initially two traps were present. As they increased in depth with increasing pressure new shallower traps appeared. The pressure dependence of four traps was established. By comparison with pressure measurements on ZnS doped only with Mn+2 and only with Cu+ and Cl−, we established that the deepest trap is associated with defects introduced by Mn+2, while the two traps next lower in energy are associated with defects introduced by Cl−. The shallowest trap could not be assigned to a specific defect. The photoluminescence emission (from the 4T1, to the 6A1 energy level of Mn+2) shifted to lower energy at a rate identical to that previously reported for a less well-characterized ZnS:Mn sample. The shift could be described quantitatively in terms of changes in the ligand field parameters.
Raman spectroscopy of the CH stretching modes was used to investigate the molecular level respons... more Raman spectroscopy of the CH stretching modes was used to investigate the molecular level response of hexahydro‐1,3,5‐trinitro‐s‐triazine (RDX) to shock compression. Single crystals of [111], [210], and [100] orientation were compressed under stepwise shock loading to peak stresses from 3.0 to 5.5 GPa. Similar frequency shifts were observed for all three orientations below 3.5 GPa. Significant spectral changes were observed in crystals shocked above 4.5 GPa. These changes were consistent with the α−γ phase transition observed under static compression. No apparent dependence on crystal orientation was observed to 5.5 GPa. The phase transition had an incubation time of about 100 ns when RDX was shocked to 5.5 GPa peak stress.
In this paper we discuss recent applications of luminescence spectroscopy to resolve problems con... more In this paper we discuss recent applications of luminescence spectroscopy to resolve problems concerning phosphors in crystals or dissolved in polymers. On the first two examples we treat intramolecular excitations for molecules dissolved in polymers and explain mechanisms of energy dissipation due to changes of molecular conformation. The second set of examples involve ZnS phosphors with various dopants. We establish the characteristics of the donor and acceptor states via photoluminescence, and the trap characteristics by means of thermoluminescence. We treat very briefly thermoluminescence in crystalline coronene.
Abstract Charge carrier photogeneration in crystalline phenanthrene has been studied by monitorin... more Abstract Charge carrier photogeneration in crystalline phenanthrene has been studied by monitoring the UV-induced thermoluminescence (UVTL) as a function of external electric field for the excitation quanta at energies lower than the band gap. The electric field effect on the UVTL is discussed in terms of Poole-Frenkel- and Onsager-type dissociation of the defect-trapped geminate electron-hole pairs produced under low-temperature excitation. Quantitative agreement with the experiment is obtained with the Onsager-type model, indicating that the average distance between hole and electron in the trapped geminate pairs amounts to 7 nm.
Abstract Photoluminescence measurements as a function of pressure were made on ZnS and ZnxCd1-xS ... more Abstract Photoluminescence measurements as a function of pressure were made on ZnS and ZnxCd1-xS (x = 0.937) samples doped with Cu+, Cl-, and Al3+. A limited amount of thermoluminescence and optical absorption data aided in the interpretation. In contrast to the ZnS:Cu:Cl studied earlier, these materials were light yellow in color and could be excited by the 442-nm as well as the 325-nm line of a He-Cd laser. When excited by the 325-nm line into the conduction band the emission peaks shifted to higher energy at a rate which could be related to the increase of the energy gap with pressure? Optical absorption data combined with the excitation spectra of Hoshina and Kawai demonstrated that for the ZnxCd1-xS phosphor 442-nm excitation was via an exciton band apparently pinned to the conduction band. For ZnS the 442-nm excitation was from a complex acceptor band to the conduction band. The peak shift with pressure indicated that the upper states of this band were no longer pinned to the valence band.
Abstract Light-induced phosphorescence of 4-N,N-dimethylaminobenzonitrile (DMABN) dissolved in so... more Abstract Light-induced phosphorescence of 4-N,N-dimethylaminobenzonitrile (DMABN) dissolved in solid polymer has been observed for the first time. This unusual phosphorescence develops after continuous illumination of the sample at room temperature. The phosphorescence follows an initiation period the duration of which depends strongly on the exciting light intensity, concentration of solute in the polymer, pressure and the previous treatment of the sample. Under continuous illumination, the overall shape of the phosphorescence intensity curve versus time depends also on the exciting light intensity and concentration. At pressures higher than 13 kbar light-induced phosphorescence is no longer observed.
Abstract To understand pressure effects on dimer structure stability, Raman and FTIR spectroscopi... more Abstract To understand pressure effects on dimer structure stability, Raman and FTIR spectroscopies were used to examine changes in H-bonded dimers of benzoic acid (BA). Experiments were performed on single crystals compressed to 33 GPa in a diamond anvil cell (DAC). Several changes in Raman spectra were observed in the range 6–8 GPa indicating modification in the dimer structure suggesting the lowering of molecular symmetry. Pressure increase above 15 GPa induced strong luminescence and a gradual change of the crystal color from white to yellow/brownish. FTIR measurements on the sample released from 33 GPa indicated formation of a new compound. It is proposed that molecules of this compound are composed of the hydroxyl group associated with alcohol, carbonyl group associated with ketone, and the sp3 hydrocarbon groups. This study demonstrates that sufficient high pressure compression and subsequent decompression can lead to significant changes in the H-bonded dimer structure, including the breaking of bonds and formation of new chemical compound.
Optical spectroscopy methods were used to examine the effect of nonhydrostaticity on the electron... more Optical spectroscopy methods were used to examine the effect of nonhydrostaticity on the electronic structure of anthracene single crystals compressed statically to 9 GPa. Two pressure-transmitting media, nitrogen (hydrostatic) and water (nonhydrostatic above approximately 5.5 GPa), were utilized. It was found that nonhydrostatic compression generates several new features both in the absorption and fluorescence spectra: (i) formation of new absorption and fluorescence bands, (ii) deviations in pressure shift of fluorescence peaks, (iii) extensive broadening of vibrational peaks, and (iv) irreversible changes in the spectra shape upon pressure unloading. Furthermore, the time-resolved fluorescence decay curves measured at the wavelength corresponding to the new fluorescence band show clear initial increase. These new features are accompanied by inhomogeneous color changes and macroscopic lines on the (001) plane of the crystal. All of the changes are discussed and correlated with microscopic transformations in the crystal. It is demonstrated that nonhydrostatic compression in anthracene crystal introduces inelastic changes in the form of dislocations along [110] and [110] directions. These dislocations lead to the development of dimeric structures and, consequently, to various changes in the electronic response of the compressed anthracene crystal.
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Papers by Zbigniew Dreger