F. Fermi

Temperature Dependence of Exciton Lifetimes in GaAs/AlGaAs Quantum Well Structures

The photoluminescence decay times in GaAs/AlGaAs single quantum well structures with layer thickness between 20 A and 80 A have been investigated in the temperature range from 4 K up to room temperature. It is shown that the increase in the PL decay time usually ascribed to radiative free-excitons recombination is present even in conditions where other mechanisms such as recombination of localized excitons and free carriers, exciton ionization and thermal activation of nonradiative processes are effective.

Absorption and luminescence spectroscopy of Eu3+ in lead silicate glasses

Abstract The absorption and luminiscence spectra of a lead silicate glass containing Eu3+ as impurity have been measured. The doping ion was found to occupy two classes of non-centrosymmetric sites. The intensities of the 4f6→4f6 transitions are perturbed by the interaction with low-lying charge-transfer states. The quantum yield of the luminescence from the 5D0 level of Eu3+ is usually low, probably because of the presence of a strong ion-phonon coupling.

Spectral and time decay analysis of the photoluminescence induced by Suzuki-like occlusions in KCl:Pb

Abstract The nucleation kinetics of Suzuki-like phase occlusions at the expenses of I.V. dipoles, are studied in KCL:Pb by means of correlated measurements of emission and excitation spectra (taken at 15 and 78 K), radiative lifetime (78–300 K) and ionic thermoconductivity for lead concentrations ranging from 13 to 100 ppm.

VUV excitation of luminescence in pure and Tl+ doped KI

Abstract Excitation spectra of pure and Tl+ doped KI have been measured at room temperature (RT) and liquid nitrogen temperature (LNT) between 13 and 30 eV. Energy transfer between the host lattice ions and the Tl+ impurities, and intrinsic recombinaton of electron-hole pairs, have been studied at different temperatures and excitation energies. Energy transfer has been observed only at RT while at LNT the intrinsic recombination, Vk centers-electrons, is the dominant process.

Luminescence in pure and Tl+ doped alkali halides, excited by synchrotron radiation☆

Abstract Vacuum ultraviolet excitation of intrinsic luminescence and of the Tl + emission in alkali halides yields information on energy transfer processes. 7

A study of the absorption and emission of U(VI) in some glassy matrices

Abstract The absorption and emission spectra of xSiO 2 /yNa 2 O/zPbO glasses doped with U(VI) show the influence of the lead oxide on the formation of the UO 2 2+ ion, that can exist only when PbO is absent or present at most in a mole percentage about 29%. This is due to a destruction of the random three-dimensional network by PbO and to the consequent steric impossibility for the hexavalent uranium to form the UO 2 2+ linear ion.

Effect of the α-F̃∗ center interaction on the luminescence lifetime of the F center

Abstract The presence of α and F ′ centers in a F coloured KCl crystal produces an increase of the lifetime of the luminescence excited with F light. One can exclude that the increase of the luminescence lifetime arises from the contribution of an F ′ excited state. The increased lifetime is studied at temperatures lower than 80°K and for some α center concentrations as a function of the initial F center concentration. The results are possibly due to the perturbation of the wavefunction of the F excited state by the α centers present after the F → F ′ conversion.

Spatial and temporal plasma inhomogeneity effects on the luminescence of highly excited GaAs/AlGaAs multiple quantum well structures

SummaryThe effects on the luminescence lineshape of the spatial and temporal inhomogeneities of the electron-hole plasma generated by strong laser pulses in GaAs/AlGaAs multiple quantum well structures are experimentally investigated. It is found that inhomogeneities strongly affect the luminescence spectra and must be taken into account for a correct interpretation of the experimental data. The lineshape of the luminescence spectra is analysed using a statistical model that includes, in a simple way, the spatial and temporal inhomogeneities of the laser pulses used for excitation. A fair agreement is obtained with the experimental data; at the same time the values of the fit parameters turn out to be rather sensitive to the model assumed for describing the inhomogeneities. Therefore, caution has to be used when inferring plasma properties, such as the carrier density and temperature, from a lineshape analysis.RiassuntoSi presenta, in questo lavoro, uno studio sperimentale degli effetti delle disomogeneità spaziali e temporali nei plasmi di elettroni e lacune generati da impulsi laser potenti in strutture a buche quantiche di GaAs/AlGaAs. Si trova che tali disomogeneità modificano fortemente gli spettri di fotoluminescenza e devono quindi essere considerate per una corretta interpretazione dei dati sperimentali. La forma di riga degli spettri di luminescenza viene analizzata mediante un modello statistico che include, in modo semplice, le disomogeneità spaziali e temporali degli impulsi laser usati per l’eccitazione. Si ottiene un accordo soddisfacente con i dati sperimentali e si trova che i valori dei parametri ottenuti dal fit dipendono in modo sensibile dal modello assunto per la descrizione delle disomogeneità. Grande cautela deve essere quindi usata nel dedurre le proprietà del plasma fotogenerato a partire da una analisi delle forme di riga.

Time-resolved photoluminescence of GaAs/GaAlAs M.Q.W. structures under picosecond excitation

Abstract Low temperature photoluminescence of GaAs/GaAlAs multiple quantum well structures has been investigated by means of time decay measurements at a given emission energy and time resolved spectra after picosecond excitation. Two typical decay times of order 300 ps and 1.5 ns have been detected in the emission from the quantum wells. Moreover time-resolved spectra clearly indicate that this emission results from possibly four contributions. The assignment to different decay channels together with a similar analysis for the emission from the GaAs buffer layer is presented and discussed.

Intrinsic luminescence in CdF2 crystals

Abstract The luminescence stimulated in pure CdF 2 crystals by photons with energy higher than 6.5 eV is studied in the temperature range 10K–100K. The emission spectrum, extending from 300nm to 500nm, is independent on the temperature and excitation energy. The excitation spectrum up to 41 eV shows a maximum at about 7.2 eV and a shoulder at 7.6 eV. The dependence of the intensity on the temperature and the decay time of the luminescence are also examined.