Slow Lifetime Research Articles

Radiative and nonradiative processes in strain-free AlxGa1−xN films studied by time-resolved photoluminescence and positron annihilation techniques

Radiative and nonradiative processes in nearly strain-free AlxGa1−xN alloys were studied by means of steady-state and time-resolved (TR) photoluminescence (PL) spectroscopy, and the results were connected with that of positron annihilation measurement. The results of steady-state optical reflectance and PL measurements gave the bowing parameter b of approximately −0.82 eV. Values of the full width at half maximum (FWHM) of the near-band-edge PL peak nearly agreed with those predicted by the classical alloy broadening model. However, the Stokes-type shifts (SS) were as large as 100–250 meV and both SS and FWHM of the PL increased with the increase in x for x⩽0.7. Simultaneously, the luminescence redshift due to the increase in temperature T from 8 to 300 K decreased with increasing x and approached zero for x=0.5. These results indicated the presence of compositional fluctuation forming weakly bound states in the alloys, and the localized excitons tended to delocalize with the increase in T. The TRPL signals showed a biexponential decay at low temperature, and the slower component became longer with the increase in x (over 40 ns for x=0.49). Simultaneously, density or size of cation vacancies (VIII) and relative intensity of the deep-level emission over that of the near-band-edge one at 300 K increased as x increased to x=0.7. Consequently, certain trapping mechanisms associated with VIII where suggested, and excitons were then detrapped and transferred to the localized states before the radiative decay at low temperature; the increase in the slower lifetime and its dominance over the entire TRPL signal intensity with increasing x may reflect the increase of the depth and concentration of the trapping level. As the temperature was increased, the TRPL signal became single exponential due to the increasing dominance of nonradiative recombination processes in the free states, resulting in lower internal quantum efficiency (ηint) with increasing x for x⩽0.7. Therefore, realization of AlGaN-based efficient deep-UV light emitters requires further reduction of the nonradiative defect density as well as the VIII-related trap density.

Effect of Inhibiting Oxyanions on the Localized Corrosion Susceptibility of Waste Package Container Materials

AbstractThe DOE is currently preparing a license application for the permanent disposal of high level radioactive waste at Yucca Mountain, Nevada. The proposed design of waste packages for the disposal of high level radioactive waste consists of an outer container made of Alloy 22, a corrosionresistant Ni-Cr-Mo-W alloy, surrounding an inner container made of Type 316 nuclear grade stainless steel. Under conditions where passivity is maintained, the uniform corrosion rate of Alloy 22 is slow and long waste package lifetimes are projected. However, the initiation of localized corrosion such as pitting or crevice corrosion may decrease waste package lifetimes. In this study the crevice corrosion susceptibility of Alloy22 was determined in chloride solutions with additions of oxyanions that are present in the groundwater at the potential repository site. When present in sufficient concentrations relative to chloride, nitrate, carbonate, bicarbonate, and sulfate inhibited pitting and crevice corrosion of Alloy 22.

Temperature independent Er 3+ photoluminescence lifetime in a-Si:H and a-SiO x:H

The photoluminescence (PL) lifetime of Er 3+ in a-Si:H<Er> and a-SiO x :H<Er> was measured between 15 and 300 K in a set of samples containing ∼1 at.% Er and up to ∼10 at.% O. The room temperature PL intensity increased and the temperature quenching decreased with O content. The maximum PL intensity at 15 K, however, is obtained from samples with no intentional oxygen added. The PL lifetimes were obtained using the quadrature frequency resolved spectroscopy (QFRS) technique. The QFRS signal was well fitted supposing two lifetimes, the fast decay in the 20–150 μs range and the slow decay in the 200–830 μs range, consistently increasing with the O content of the samples. For all samples both the fast and the slow lifetimes did not depend on the temperature within experimental incertitude. Our results are interpreted supposing two different lattice sites for Er 3+ in the hosts. Moreover, the de-excitation of the Er 3+ ions by multiple phonon emission is negligible in this class of materials.

The temperature dependence of free volume in polymethylpentene by positron annihilation

Positron lifetime measurements in polymethylpentene were performed in the temperature range 8.3–299 K. A conventional fast–slow coincidence lifetime spectrometer with plastic scintillators were used. All the measurements were made in vacuum. Additional measurements (X-ray diffraction, differential scanning calorimetry and mechanical spectroscopy) were also performed to establish properties of samples. Mean free volume radii and fractional free volume in the investigated samples were estimated from the positron lifetimes.

Time-Resolved Gain Dynamics in Silicon Nanocrystals

AbstractTime-resolved variable stripe length (VSL) experiments on a set of silicon nanocrystal waveguides obtained by plasma enhanced chemical vapor deposition (PECVD) have revealed a fast recombination dynamics (20 ns) related to population inversion under 6 ns optical pumping at 355 nm. Modal gain values about 10 cm-1 have been measured at 760 nm by VSL technique for the fast recombination component while optical losses about 15 cm-1 are measured for the integrated signal in the slow (lifetime of about 10 μs) recombination tail. Threshold behavior in the emission intensity together with a pumping length and pumping power dependence of both the intensity and the time duration of the fast recombination component has been observed. These results are explained within an effective four level model to describe the strong competition among different Auger processes and stimulated emission.

Spatial distribution of defects in ultra-fine grained copper prepared by high-pressure torsion

A study of ultra-fine grained copper produced by high-pressure torsion is presented. Positron annihilation spectroscopies (slow positron implantation, lifetime and Doppler broadening spectroscopies), transmission electron microscopy, X-ray diffraction spectroscopy, and microhardness measurements have been employed in order to identify defects and their spatial distribution, i.e. lateral distribution and depth profile, in the specimens. Two types of defects could be identified: dislocations in the distorted regions along grain boundaries and microvoids of size of 3–4 vacancies inside the grains. No change in grain size as a function of the distance from the center of the sample disk was observed. The mean coherent domain size close to the surface was found to be 80 ± 20 nm and it slightly increases with depth. The concentration of microvoids has been found to decrease with depth and it slightly varies with the distance from the center of the disk, being lowest in the center. No position dependence of the concentration of dislocations was observed.

Correlation between Photoinhibition Sensitivity and the Rates and Relative Extents of Xanthophyll Cycle De-epoxidation in Chlorina Mutants of Barley (Hordeum vulgare L.)

We compared photoinhibition sensitivity to high irradiance (HI) in wild-type barley (wt) and both its chlorina f104-nuclear gene mutant, that restricts chlorophyll (Chl) a and Chl b synthesis, and its f2-nuclear gene mutant, that inhibits all Chl b synthesis. Both Fv/Fm and ΦPS2 decreased more significantly in f2 than f104 and wt with duration of HI exposure. Chl degraded more rapidly in the f2 than in either f104 or wt. Most sensitivity to photoinhibition was exhibited for f2, whereas there was little difference in response to HI between the f104 and wt. The highest de-epoxidation (DES) value at every time point of exposure to HI was measured for f2, whereas the wt had the lowest value among the three strains. There were two lifetime components resolved for the conversion of violaxanthin (V) to zeaxanthin plus antheraxanthin (Z + A). The most rapid lifetime was around 6 min and the slower lifetime was >140 min, in both the mutants and wt. However, the wt and f104 both displayed larger amplitudes of both de-epoxidation lifetimes than f2. The difference between the final de-epoxidation state (DES = [Z + A]/[V + A + Z]) in the light compared to the dark expressed as ΔDES for wt, f104, and f2 was 0.630, 0.623, and 0.420, respectively. The slow lifetime component and overall larger ΔDES in the wt and f104 correlated with more photoprotection, as indicated by relatively higher Fv/Fm and ΦPS2, compared to the f2. Hence the photoprotection against photoinhibition has no relationship with the absolute DES value, but there is a strong relationship with de-epoxidation rate and relative extent or ΔDES.

Consecutive and Parallel Dissociation of Energy-Selected Co(CO)3NO+ Ions

Photoelectron photoion coincidence (PEPICO) spectroscopy has been used to investigate the dissociation dynamics of the cobalt tricarbonyl nitrosyl ion, Co(CO)3NO+. The ionization energy of Co(CO)3NO was measured from the threshold photoelectron spectrum to be 8.33 ± 0.04 eV. The dissociation of the molecular ion proceeds by two sequential carbonyl-loss steps and then a parallel carbonyl- or nitrosyl-loss step. The first two reactions were observed to be slow (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and the relative ion abundances (breakdown curves), 0 K onsets for the following fragment ions were determined: Co(CO)2NO+, 9.28 ± 0.02 eV; CoCONO+, 10.43 ± 0.02 eV; CoNO+, 12.00 ± 0.02 eV; CoCO+, 12.07 ± 0.02 eV. Combining these onsets with the experimental adiabatic ionization energy of Co(CO)3NO+, the three cobalt−carbonyl bond energies in Co(CO)xNO+ (x = 1−3) were determined along with the cobalt-nitrosyl bond energy in CoCONO+. Using a literature value for the [Co−CO]+ bond energy, the 0 K heats of formation of the above-mentioned molecular and fragment ions and of the neutral compound, Co(CO)3NO, were determined. Using these thermochemical data, the cobalt−nitrosyl bond energies were also derived for Co(CO)3NO+ and CoNO+. The latter is in good agreement with a theoretical literature value, while the first one explains why there is no observable NO loss from the molecular ion, Co(CO)3NO+. Room-temperature values of the heats of formation are also given using the calculated harmonic frequencies.

Effect of surfactant Sb on carrier lifetime in GaInP epilayers

Samples of Ga0.52In0.48P grown on (001) GaAs with small amounts of surfactant Sb were investigated using time-resolved photoluminescence. All samples show a luminescence that may be fit to a two-stage exponential decay with a fast and a slow lifetime. For growth without Sb (Sb/III(v)=0), the sample shows a strong CuPtB ordering and a fast component lifetime of 7 ns. As the Sb concentration is increased, the degree of order is reduced, with a consequent increase in band gap energy. In the highest band gap material, produced at Sb/III(v)=0.016, the fast lifetime is 2.9 ns, an order of magnitude larger than published values for GaInP disordered by misorienting the substrate. Increasing the Sb further causes the band gap energy to decrease due to the onset of composition modulation. At Sb/III(v)=0.064, the fast component lifetime decreases to 0.79 ns. Samples grown with Sb/III(v)&amp;gt;0.016 show a lifetime that depends on energy and is fit well by a model of localized excitons.

The dissociation kinetics of energy-selected CpMn(CO)(3)(+) ions studied by threshold photoelectron-photoion coincidence spectroscopy.

Threshold photoelectron-photoion coincidence spectroscopy has been used to investigate the dissociation kinetics of the cyclopentadienyl manganese tricarbonyl ion, CpMn(CO)(3)(+). The ionization energy of CpMn(CO)(3) was measured from the threshold photoelectron spectrum to be 7.69 +/- 0.02 eV. The dissociation of the CpMn(CO)(3)(+) ion proceeds by the sequential loss of three CO molecules. The first and third CO loss reactions were observed to be slow (lifetimes in the microsecond range). By simulating the resulting asymmetric time-of-flight peak shapes and breakdown diagram, 0 K onsets for three product ions were determined to be 8.80 +/- 0.04, 9.43 +/- 0.04, and 10.51 +/- 0.06 eV, respectively. Combined with the adiabatic ionization energy, the three successive Mn-CO bond energies in the CpMn(CO)(3)(+) were found to be alternating with values of 1.11 +/- 0.04, 0.63 +/- 0.04, and 1.08 +/- 0.06 eV, respectively. Using a scaled theoretical Cp-Mn(+) bond energy of 3.10 +/- 0.10 eV and the combined results from theory and experiment, the 298 K gas-phase heat of formation of CpMn(CO)(3) is suggested to be -419 +/- 15 kJ/mol. Based on this value, the 298 K heats of formation of CpMn(CO)(3)(+), CpMn(CO)(2)(+), CpMnCO(+), and CpMn(+) are 325 +/- 15, 546 +/- 15, 719 +/- 15, and 938 +/- 15 kJ/mol, respectively. By scaling theoretical calculated neutral bond energies with the experimental information derived in this study, the successive Mn-CO bond energies were estimated to be 1.88, 1.10, and 1.03 eV, respectively, while the Cp-Mn bond energy was found to be 2.16 eV. Comparison between the quantum chemical calculations and experimental values shows that the loss of CO groups follows the lowest energy adiabatic path, in which electronic spin on the metal center is not conserved.

Ultrafast spectroscopy studies on the mechanism of electron transfer and energy conversion in the isolated pseudo ginseng, water hyacinth and spinach chloroplasts

The spectroscopy characteristics and the fluorescence lifetime for the chloroplasts isolated from the pseudo ginseng, water hyacinth and spinach plant leaves have been studied by absorption spectra, low temperature steady-state fluorescence spectroscopy and single photon counting measurement under the same conditions and by the same methods. The similarity of the absorption spectra for the chloroplasts at room temperature suggests that different plants can efficiently absorb light of the same wavelength. The fluorescence decays in PS II measured at the natural QA state for the chloroplasts have been fitted by a three-exponential kinetic model. The three fluorescence lifetimes are 30, 274 and 805 ps for the pseudo ginseng chloroplast; 138, 521 and 1494 ps for the water hyacinth chloroplast; 197, 465 and 1459 ps for the spinach chloroplast, respectively. The slow lifetime fluorescence component is assigned to a collection of associated light harvesting Chl a/b proteins, the fast lifetime component to the reaction center of PS II and the middle lifetime component to the delay fluorescence of recombination of P+ 680 and Pheo-. The excitation energy conversion efficiency(η) in PS II RC is defined and calculated on the basis of the 20 ps electron transfer time constant model, 60%, 87% and 91% for the pseudo ginseng, water hyacinth and spinach chloroplasts, respectively. This interesting result is in unconformity with what is assumed to be 100% efficiency in PS II RC. Our result in this work stands in line with the 20 ps electron transfer time constant in PS II rather sound and the water hyacinth plant grows slower than the spinach plant does as envisaged on the efficiency. But, our results predict that those plants can perform highly efficient transfer of photo-excitation energy from the light-harvesting pigment system to the reaction center (closely to 100%). The conclusion contained in this paper reveals the plant growth characteristics expressed in the primary processes of photosynthesis and a relationship between a plant growing rate and its spectroscopy characteristics and fluorescence lifetimes, namely, the slower a plant grows, the less excitation energy conversation efficiency used might be anticipated.

S–T conversion induced by external magnetic field in gaseous oxalylfluoride excited to the 7151-level of the à 1Au state

Fluorescence decay of gaseous oxalylfluoride (COF)2 excited to the à 1Au(7151) level was measured as a function of gas pressure in absence and in presence of magnetic field, B=0.3 T. On excitation to this level, the dynamics in both zero and nonzero fields may be described in the intermediate-molecule limit, with the fluorescence exhibiting biexponential and triexponential decay, respectively. The fast component decay rate constant kf=(5.31±0.22)⋅107 s−1 is independent on the (COF)2 gas pressure or magnetic field strength, while that of the first slow component depends on both. The second slow component lifetime is independent of the field strength, while dependent on the gas pressure. The Stern–Volmer dependence, measured with a field in the 0.3–12 mTorr pressure range, is nonlinear. The results obtained were explained by the indirect (electronic- and nuclear-spin-decoupling) mechanism, proposed earlier.

Oxygen sensing based on lifetime of photoexcited triplet state of platinum porphyrin-polystyrene film using time-resolved spectroscopy

Optical oxygen-sensing systems based on the quenching of the photoexcited triplet state of platinum porphyrins—platinum octaethylporphyrin (PtOEP) and platinum tetrakis(pentafluorophenyl)porphyrin (PtTFPP)—in polystyrene (PS) using two different time-resolved spectroscopies (luminescence lifetime measurement and diffuse reflectance laser flash photolysis) have been developed. Using both spectroscopies, the same values of Stern-Volmer constant KSV and quenching rate constant kq (KSV = kqτ0) are obtained. The decays of the luminescence and triplet-triplet reflectance of the platinum porphyrins in PS consisted of two components (faster and slower lifetimes) in the absence and presence of oxygen. For both faster and slower components the lifetime decreases with increasing oxygen concentration. For both components a Stern-Volmer plot of the platinum porphyrin-PS films exhibits linearity. However, kq of the faster component is larger than that of the slower component (for PtOEP, three times larger; for PtTFPP, 40 times larger), indicating that two different oxygen-accessible sites exist in the platinum porphyrin-PS films. The faster and slower components are related to oxygen-accessible sites on the surface and in the bulk of the platinum porphyrin films respectively. Concerning the fractional contributions of each lifetime component, the contribution of the faster component is greater than that of the slower component, indicating that the sensing site on the surface is important for optical sensing. The contribution of different oxygen-accessible sites in platinum porphyrin-PS films for oxygen sensing is clarified by these techniques.

Optical oxygen sensing based on triplet-triplet decay of platinum octaethylporphyrin-polystyrene film using time-resolved spectroscopy by laser flash photolysis

Optical oxygen sensing system based on the quenching of the photoexcited triplet state of platinum octaethylporphyrin, PtOEP in polystyrene (PS) by means of time-resolved spectroscopy using laser flash photolysis has been developed. The decays of the triplet-triplet reflectance of PtOEP in PS consisted of two components (faster and slower lifetimes) in the absence and presence of oxygen. In both of faster and slower components, the lifetime decreases with increasing oxygen concentration and a Stern-Volmer plot of PtOEP-PS film exhibits linearity. However, kq in faster component is larger than that of slower component, indicating that two different oxygen-accessible sites exist in PtOEP -PS film. The faster and slower components are attributed t o oxygen-accessible sites on the surface and bulk of PtOEP film, respectively. The contribution of different oxygen-accessible sit es in PtOEP-PS film for oxygen sensing is clarified by this technique.

Magnetic and microwave field effects for single rotational levels of the -band of oxalylfluoride in cooled jet conditions

Fluorescence intensity and decay in oxalylfluoride vapors ((COF)2), excited to single rotational levels (SRLs) of the 1 Au(00) state of the à 1Au←à 1Ag transition, were measured as a function of an external magnetic field. On excitation to these levels, dynamics in zero field may be described in the small-molecule limit, with fluorescence exhibiting an almost exponential decay. However, at increased field strength B the initial fluorescence decay becomes faster, the decay profile becoming biexponential at higher fields. Thus, a magnetic field-induced change of dynamics occurs in the à 1Au state, from that of a small molecule, to the intermediate case. The decay rate constant of the fast component was measured for different SRLs, being independent on the magnetic field strength, while the slow component lifetime is field dependent, increasing at higher fields. Both the fast and slow decay lifetimes depend on the studied SRL. At higher fields, the slow component amplitude decreases, while that of the fast component increases with subsequent saturation at high fields. Halfwidth value (B1/2) of the field dependence of the slow component amplitude increases linearly with (2J′+1)−1. Structure of the OD EPR spectrum of (COF)2 excited to the J′=4 level was resolved. Experimental data are interpreted using the indirect mechanism theory in the low level density limit.

The effect of Glu75 of staphylococcal nuclease on enzyme activity, protein stability and protein unfolding.

Staphylococcal nuclease mutants, E57G and E75G, were generated. A comparison of the kinetic parameters both for mutants and wild-type protein shows that the Michaelis constants (Km) were almost identical for the wild-type protein and E57G mutant. An approximately 30-fold decrease in Km compared with the wild-type protein was observed for the E75G mutant. The turnover numbers for the enzyme (kcat) were higher with both the wild-type protein and the E57G mutant (3.88 +/- 0.21 x 103 s-1 and 3.71 +/- 0.28 x 103 s-1) than with the E75G mutant (3.04 +/- 0.02 x 102 s-1). The results of thermal denaturation with differential scanning microcalorimetry indicate that the excess calorimetric enthalpy of denaturations, DeltaHcal, was almost identical for the wild-type protein and E57G mutant (84.1 +/- 6.2 kcal.mol-1 and 79.3 +/- 7.1 kcal.mol-1, respectively). An approximately twofold decrease in DeltaHcal compared with the wild-type protein was observed for the E75G mutant (42.7 +/- 5.5 kcal.mol-1). These outcomes imply that Glu at position 75 plays a significant role in maintaining enzyme activity and protein stability. Further study of the unfolding of the wild-type protein and E75G mutant was conducted by using time-resolved fluorescence with a picosecond laser pulse. Two fluorescent lifetimes were found in the subnanosecond time range. The faster lifetime (tau2) did not generally vary with either pH or the concentration of guanidinium hydrochloride (GdmHCl) in the wild-type protein and the E75G mutant. The slow lifetime (tau1), however, did vary with these parameters and was faster as the protein is unfolded by either pH or GdmHCl denaturation. The midpoints of the transition for tau1 are pH 3.5 and 5.8 for the wild-type protein and E75G mutant, respectively, and the GdmHCl concentrations are 1.1 m and 0.6 m for the wild-type protein and E75G mutant, respectively. Parallel steady-state fluorescence measurements have also been carried out and the results are in general agreement with the time-resolved fluorescence experiments, indicating that Glu at position 75 plays an important role in protein unfolding.

Silicon Quantum Dots in Silica

AbstractSilicon ions were implanted into fused silica substrates at doses of 1×1021, 2×1021, 5×1021, and 1×1022 ions/cm3. The implanted substrates were annealed at 1100°C for one hour in a reducing atmosphere (95% Ar+5% H2). Optical absorption spectra recorded after the annealing treatment showed absorption onsets at 3.86, 3.73, 2.86 and 2.52 eV for substrates implanted with 1×1021, 2×1021, 5×1021, and 1×1022 ions/cm3, respectively. Static photoluminescence (PL) measurements indicated red emission between 1.72 and 1.61 nm with a slightly increasing red shift with ion dose. Time resolved PL at room temperature revealed slow (∼50 μs) and fast (×20 μs) lifetimes which increased with decreasing temperature. TEM studies showed that the particle size increased with increasing ion dose. Typical particle sizes ranged between 2 and 5 nm indicating quantum confinement of the exciton, which can account for the blue shift in the absorption edge with decreasing ion dose. However, the maxima in the PL spectra for all ion doses are relatively independent of the ion dose and are strongly shifted from the absorption spectra. This suggests that radiative recombination occurs from a common luminescent center, possibly a surface or interfacial state in thexs SiOx, layer surrounding the nanocrystal.

Investigation of band bending in silicon by slow positron lifetime measurements

A novel approach to the analysis of positron lifetime data in depth sensitive studies using a pulsed positron beam is applied to model the diffusion of positrons in silicon. By examining only the observable lifetime parameters, inhomogeneous effects can be studied without the need to solve the time dependent diffusion equation. In particular, we study the effect of band bending near the sample surface, which creates an internal electric field. We present our first results of this analysis on p- and n-type bulk silicon samples. No band-bending could be measured for samples covered with a natural oxide. After surface treatment with HF(aq), p-type samples exhibited band bending, but the results for n-type samples were not significantly affected. In only one case, a p-type Si(B) sample with acceptor concentration c a = 10 20 cm −3 and with a H terminated surface, is evidence of Fermi level pinning observed.

The Early Stages of Photosystem II Assembly Monitored by Measurements of Fluorescence Lifetime, Fluorescence Induction and Isoelectric Focusing of Chlorophyll-Proteins in Barley Etiochloroplasts

The relationship between functional and structural aspects of PSII formation during greening of etiolated barley leaves has been investigated using fluorescence lifetime measurements, fluorescence kinetics analysis and analysis of chlorophyll-protein complexes by IEF-PAGE. Two phases of different character could be distinguished in the course of the greening process in dark-grown plants. An early phase covering the first 3-4 h after the onset of illumination and a late phase covering the subsequent greening. During the first phase the formation of PSII reaction centers and their minor antenna components was observed as manifested by the IEF-PAGE polypeptide pattern. This was accompanied by shortening of the slow and middle components of the fluorescence lifetime, as well as by the rapid drop in the amplitude of the slow component. A room temperature emission band at 676 nm was associated with uncoupled chlorophyll and with the slow fluorescence lifetime component during the first hours of greening. During the late greening phase peripheral light-harvesting complexes of PSII were formed concomitantly to an increase in lifetime and amplitude of the fast component and to a further decrease in the lifetime of the middle component. The gradual increase in PSII complexity during both phases of greening was also manifested by changes in proportion and kinetic properties of PSII a and PSII^ units. Similar changes in fluorescence lifetime components as in the late greening of dark-grown plants were also observed in intermittentlight plants during continuous greening associated with the development of PSII antenna. The relationships between fluorescence lifetime characteristics and development of PSII are discussed in terms of a stepwise mechanism invoving a first step of Chi integration into small size PSII units followed by progressive increase of antenna size.

Time Decay Characteristics of the Yb3+- Related 0.98 μm Emissions in Porous Silicon

AbstractEnhanced emissions at 0.98 μm of Yb3+ ions (2F5/2 → 2F7/2) incorporated in porous silicon are obtained when the host porous silicon is pre-annealed in an O2 containing atmosphere. Time decay characteristics are measured for the Yb 3+-related 0.98 μm emissions as well as for the emissions of these host porous silicon between 20 K and 300 K. The decay of the Yb3+ peak at 20 K is characterized with a fast (˜ 30 μs) and a slow decay time (≤ ˜ 400 μs), whereas it is fitted with one slow decay time (˜ 390 μs) at 300 K. The results are explained in terms of two optical centers: one has a fast fluorescent lifetime with a large temperature quenching, and the other has a slow fluorescent lifetime which is nearly constant between 20 K and 300 K.