Singlet oxygen lifetime dependence on photosensitizer concentration in lipid films

It was shown that lipids substantially influence singlet oxygen lifetime. Question arises whether photosensitizers triplet states and excitation energy transfer to oxygen are also affected by lipids. In this contribution, the influence of lipids on excitation energy transfer from lipophilic photosensitizer tetraphenylporphyrin (TPP) to oxygen was investigated in bulk lipids and dry lipid films.Two components of TPP triplets decays were identified: quenching by oxygen which does not depend on TPP concentration and triplet-triplet annihilation. Rather long lifetimes of the TPP triplets around are due to low solubility and diffusion coefficient of oxygen in the lipid. They are also reflected in low signal of singlet oxygen luminescence. Kinetics of the singlet oxygen luminescence follow convolutions of two exponential decays: rise-time independent on concentration and well corresponding to the short component of TPP triplet decay and decay time decreasing from 14 to with increasing TPP concentration due to quenching of singlet oxygen by TPP.     

Field-induced negative differential spin lifetime in silicon

We show that the electric-field-induced thermal asymmetry between the electron and lattice systems in pure silicon substantially impacts the identity of the dominant spin relaxation mechanism. Comparison of empirical results from long-distance spin transport devices with detailed Monte?Carlo simulations confirms a strong spin depolarization beyond what is expected from the standard Elliott-Yafet theory even at low temperatures. The enhanced spin-flip mechanism is attributed to phonon emission processes during which electrons are scattered between conduction band valleys that reside on different crystal axes. This leads to anomalous behavior, where (beyond a critical field) reduction of the transit time between spin-injector and spin-detector is accompanied by a counterintuitive reduction in spin polarization and an apparent negative spin lifetime.     

Vibrational lifetime of bond-center hydrogen in crystalline silicon

The lifetime of the stretch mode of bond-center hydrogen in crystalline silicon is measured to be T1 = 7.8+/-0.2 ps with time-resolved, transient bleaching spectroscopy. The low-temperature spectral width of the absorption line due to the stretch mode converges towards its natural width for decreasing hydrogen concentration C(H), and nearly coincides with the natural width for C(H) approximately 1 ppm. The lifetimes of the Si-H stretch modes of selected hydrogen-related defects are estimated from their spectral widths and shown to range from 1.6 to more than 37 ps.     

Positron lifetime measurements inn- andp-type silicon

Positron lifetime measurements were carried out in four different samples of silicon, namelyn-type (P-doped) 75 Ωcm,n-type (Sb-doped) 0.018 Ωcm,p-type (B-doped) 60 Ωcm andp-type (B-doped) 0.02 Ωcm. The measurements were made at room temperature and at 77K. A positron lifetime of τ₁=(230±2) psec was found for all samples, independant of dopant or temperature. Paper A 17 presented at 3 rd Internat'l Conf. Positron Annihilation. Otaniemi, Finland (August 1973).     

Frequency dependence of electron lifetime in one-dimensional metals

It is shown that the same approximations which give a nearly frequency independent electron lifetime in 3D-metals lead to a strong ω-dependence of τ for 1D-metals. τ(ω) is discussed in a simple parametrized form and the optical properties derived from it are compared with experimental data for KCP.     

Energy dependence of the quasiparticle lifetime in a 2DES

We present a novel technique for measuring the lifetime of quasiparticle excitations of a 2DES by investigating the tunnelling into a quantum dot from a 2DES over an extended range of energy from the Fermi energy to the sub-band edge. We find that the lifetime τ_qp, of a quasihole excitation, caused by removing an electron from a 2DES state with energy below the Fermi energy E_F, has the form τ_qp=α/, where α is a constant of order unity.     

Isotope effects in the EPR spectrum of a tin-vacancy pair in silicon

The S = 1 EPR spectrum for a tin-vacancy pair in silicon reveals easily detected isotope shifts in the fine structure splitting D for the various nuclear isotopes of tin (σD/D = + 1.0 × 10^?4 per unit mass) as well as well as those for the six near neighbor silicon atoms (σD/D = ?0.67 × 10^?4 per unit mass). These are attributed to the different vibrational amplitudes vs isotopic mass.     

Fluorescence lifetime imaging of oxygen in living cells

The usefulness of the fluorescent probe ruthenium tris(2,2′-dipyridyl) dichloride hydrate (RTDP) for the quantitative imaging of oxygen in single cells was investigated utilizing fluorescence lifetime imaging. The results indicate that the fluorescence behavior of RTDP in the presence of oxygen can be described by the Stem-Volmer equation. This shows that fluorescence quenching by oxygen is a dynamic quenching process. In addition, it was demonstrated that the fluorescence lifetime of RTDP is insensitive to pH, ion concentration, and cellular contents. This implies that a simple calibration procedure in buffers can be used to quantify oxygen concentrations within cells. First fluorescence imaging experiments on J774 macrophages show a nonuniform fluorescence intensity and a uniform fluorescence lifetime image. This indicates that the RTDP is heterogeneously partitioned throughout the cells, while the oxygen concentration is constant.     

Isotope dependence of widths of hole analog states

The isotope dependence of the widths of the hole analog states is studied for the Zr, Mo and Sn isotopes. In order to calculate the proton escape widths, we propose a microscopic model, in which the coupling of a neutron hole state to the isobaric analog state of the core is considered. It is found that the proton escape widths carry the observed isotope dependence of the widths of the hole analog states and that the isotope dependence of the proton escape widths is caused by two effects: the isotope dependence of the maximum available energy for a decaying proton, and that of the occupation probabilities of the excess neutron orbits in the parent state.     

New oxygen donors in silicon

Oxygen donor traps and oxygen-related precipitates are investigated by deep level transient spectroscopy (DLTS) and transmission electron microscopy (TEM). The so-called New Donors (ND's) occur after thermal treatments in the temperature range of 650 °–800 ° C. They have a continuous distribution of trap states with respect to energy in the band gap of Si. The concentration of the trap states increases towards the conduction band edge. The precipitates observed are mainly platelets and ribbon-like defects. The formation and annihilation kinetics of ND traps and oxygen-related precipitates are correlated. An SiO _x Interface Model is proposed to explain the origin and the donor-like behavior of the ND traps. The ND trap spectrum is composed of two different types of trap states: interface states at the surface of precipitates and bound states in the Coulombic wells of a fixed positive charge which is located in the SiO _x precipitates.     

Thermal configurations of oxygen in silicon

The central position and the infrared absorption coefficient of the 9 m band of Si samples were measured with Fourier transform infrared spectroscopy (FTIR) at temperatures from T=77 K to 775 K. The infrared absorption coefficients were corrected by considering background absorption and free carrier absorption calculated from the increased free carrier concentration and from the resistivity determined from Hall effect measurements. We found the central position of the 9 m band to shift to longer wavelengths with increasing temperature. The concentration [O_i] of interstitial oxygen is almost constant for T<600 K, but decreased rapidly for T>600 K. These results verified there are two types of thermal configurations of oxygen in silicon: The bonded Si₂O configuration with a binding energy E _b0.8 to 1.0 eV at T77 K to 600 K, and the Si₂O configuration coexists with a quasi-free interstitial oxygen (QFIO) state for T>600 K. The lattice potential barrier E _L, which retards QFIO atoms from migrating in the lattice, is estimated to be 1.5 to 1.6 eV. From these configurations the anomalous diffusivity of oxygen in silicon can be explained quite well.     

The effect of thermal treatment on the charge-carrier lifetime in nickel-doped silicon

This work is aimed at studying the electrophysical and recombination properties of slowly and rapidly cooled n-and p-type silicon doped by nickel during thermal treatment. It is shown that the changes in the lifetime in slowly and rapidly cooled n-Si<P, Ni> under isochronous thermal treatment depend on the “nickel + trace impurity” complexes. An increase in the lifetime observed in p-Si<B, Ni> as compared with the reference silicon is caused by the presence of a carrier-capture level. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 57–61, February, 2006.     

Temperature dependence of positron lifetime in GaAs crystals with defects

Positron lifetime has been measured as a function of temperature in Sidoped GaAs single crystals subjected to various heat treatments. Defects produced by these heat treatments trap positrons. In all the GaAs samples containing defects positron lifetime was found to decrease with temperature in the range from 375 K to 16 K. The decrease is explained as due to the decrease in the trapping rate. The trapping rate is mainly controlled by the diffusion of the positron to the trap. The diffusion constant is determined mainly by the scattering from charged Si impurities.     

Amorphous hydrogenated silicon studied by positron lifetime spectroscopy

Hydrogenated amorphous silicon (a-Si:H) prepared by dc glow discharge in silane was investigated by positron lifetime measurements at room temperature. The lifetime spectrum shows considerably longer lifetimes than in simultaneously measured Si single crystals. The dominant component with the time constant ₂=402 ps is discussed thoroughly in conjunction with positron trapping at microvoids containing more than 10 to 15 vacancies. Positron trapping at H-saturated dangling bonds cannot be ruled out. The long-lived component with ₃=1800 ps (I ₃=0.06) indicates positronium formation at larger voids.