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The emission bands ␣ E ͑4.2 eV͒ and ␤ ͑3.1 eV͒ have been investigated in a variety of as-grown natural silica types. We report experimental results on the stationary photoluminescence spectra and their relationship with the absorption band B 2␤ ͑5.15 eV͒, on the temperature dependence of their competition, on their excitation pathway and their kinetic decay ensuing a pulse excitation. We use our results to characterize the electronic transitions involved and the intersystem crossing process linking the two excited states. Our results evidence the role played by the conformational disorder. In particular, we make use of a model in which the interconversion processes between conformational substates are effective at high temperature (TϾ200 K) and become frozen at low temperature (TϽ120 K). Finally we discuss our results on the basis of a structural model ascribing these optical activities to an oxygen deficient center related to a Ge impurity.

We report experimental results on the time decay of photoluminescence at 4.2 eV in Ge-doped silica. This optical emission is assigned to a singlet-singlet transition between electronic states localized on an oxygen deficiency nearby a Ge atom and its radiative decay rate is in competition with an intersystem crossing mechanism that populates an excited triplet state. We investigate the dependence of the lifetime of this photoluminescence on the temperature, in the 6 -295 K range, and on the excitation energy, in the ultraviolet and vacuum ultraviolet region. The mean value of the decay time decreases on increasing the temperature, in agreement with the phonon-assisted nature of the intersystem crossing process whose activation energy is estimated ϳ90 meV. In particular, at temperatures higher than 145 K, the time decay of the 4.2 eV emission deviates from a single exponential law and depends on the excitation energy, suggesting the presence of a distribution of intersystem crossing rates. These features are ascribed to the structural heterogeneity of vitreous matrix embedding the optical active point defects.

The decay of transient nutations has been experimentally investigated in S =spin systems at microwave frequency: E' centers in silica and [A104] centers in quartz have been studied. We have found that the damping is well described by a single exponential decay function, as expected from a Tl-T2 model (Bloch model). However, the agreement is only qualitative. In fact the measured decay rate I is faster than expected and depends on the driving-field amplitude: it tends to the Bloch value I z = -'T2 in the low-power limit and becomes faster and faster on increasing the input power. In all the cases examined the power dependence of the decay rate is fit well by a simple linear dependence of I on the induced Rabi frequency y. The observed power dependence of I cannot be ascribed to the inhomogeneity of g over the sample volume nor to the radiation damping, since both eff'ects are negligible in our experiments. Other mechanisms, which can, in principle, yield a g dependence of I, e.g. , the direct interaction of the driving field with structural two-level systems or the spreading of the spin-field coupling constant, are not compatible with the experimental conditions. So, our results suggest that the homogeneous dephasing time of each isochromat contains an intrinsic term and a y-dependent one. The latter may originate in a field-induced enhancement of the hyperfine or dipolar interaction; however, neither of these mechanisms completely fits the experimental features. The relationship with the decay properties of other coherent regimes is also discussed.

The anomalous behavior of transient nutations is experimentally investigated in a set of two-level (Sϭ 1 2 ) spin systems differing only in spin concentration. Our results show that the non-Bloch power dependence of the decay rate of transient nutations is a concentration-dependent effect, which is more and more pronounced in more and more concentrated samples. The experimental results are interpreted in the framework of the recent theory by Shakhmuratov et al. ͓Phys. Rev. Lett. 79, 2963 ͑1997͔͒ and support the hypothesis that the anomalous decay of transient nutations in solids originates from radiation-induced changes of the dipolar field, rather than from residual fluctuations of the nominally coherent field. ͓S1050-2947͑99͒03905-0͔ PACS number͑s͒: 42.50. Md, 76.30.Mi, 76.90.ϩd

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The article describes an experimental method that allows to estimate the inhomogeneous and homogeneous linewidths of the photoluminescence band of a point defect in an amorphous solid. We performed low temperature time-resolved luminescence measurements on two defects chosen as model systems for our analysis: extrinsic Oxygen Deficient Centers (ODC(II)) in amorphous silica and F + 3 centers in crystalline Lithium Fluoride. Measurements evidence that only defects embedded in the amorphous matrix feature a dependence of the radiative decay lifetime on the emission energy and a time dependence of the first moment of the emission band. A theoretical model is developed to link these properties to the structural disorder typical of amorphous solids. Specifically, the observations on ODC(II) are interpreted by introducing a gaussian statistical distribution of the zero phonon line energy position. Comparison with the results obtained on F + 3 crystalline defects strongly confirms the validity of the model. By analyzing experimental data within this frame, we obtain separate estimations of the homogenous and inhomogeneous contributions to the measured total linewidth of ODC(II), which results to be mostly inhomogeneous.