János Erostyák

Anodic oxidation of p- and p+-type porous silicon: surface structural transformations and oxide formation

The transformations induced by the anodic oxidation process on the pore structure and surface states of p-type and p+-type porous silicon have been studied by Rutherford backscattering spectroscopy, local vibrational mode and electron paramagnetic resonance spectroscopy. Surprisingly the anodic oxidation does not lead to a significant formation of silicon oxide in p-type layers; the oxygen is mainly incorporated in silicon back-bonded configurations without perturbation of the initial H surface passivation. On the contrary, in p+-type samples, a uniform oxide layer of a thickness up to 45 A is formed from the very first stages of anodic oxidation. The influence and correlations between the surface structure and the PL efficiency are discussed.

Time-resolved study of intramolecular energy transfer in Eu3+, Tb3+/β-diketone/o-phenanthroline complexes in aqueous micellar solutions

Abstract We found that by adding Phen to the Eu3+/TTA complexes, the luminescence intensity of Eu3+ can be increased very much. The Phen is bounded to the complexes and can be found not only in the first coordination sphere. It is an efficient shield against the quenching processes and opens a new IMET channel towards the Eu3+. Concerning the ligands, only the S1 excited state plays a role in the IMET.

Phase-fluorimetry study on dielectric relaxation of human serum albumin.

The dielectric relaxation (DR) of human serum albumin (HSA) was studied by the method of phase-fluorometry. The protein environment of the single tryptophan in HSA shows a relatively low-speed DR of sub-ns characteristic time. This relaxation can be measured as a decaying red-shift of the time-resolved fluorescence emission spectra. The details of calculations of time-emission matrices (TEM) and comparison to the fluorescence data of the reference solution of N-acetyl-L-tryptophanamide (NATA) are also presented.

Amorphous carbon luminescence; excitation and emission in a broad energy range

Abstract Photoluminescence excitation (PLE) spectra at photon energies of 2.47 to 5.63 eV have been obtained for hydrogenated amorphous carbon samples (a-C:H) deposited by radio frequency glow-discharge decomposition of methane at 0 V self bias. A peak in the PLE spectra was observed at ∼3.0 eV excitation energy, above which the luminescence intensity slowly decreases with photon energy. Photoluminescence (PL) spectra with a peak at 2.34 eV show similar asymmetric line-shape and peak energy when the excitation photon energy decreases from 5.6 eV to 2.5 eV. The highest photon energy which excites the characteristic PL band is 5.63 eV. Our results demonstrate that light-emitting electron hole pairs remain strongly localized even at excitation energies much higher than the optical gap. A new localization mechanism based on symmetry differences between σ and π bonds is proposed.

The isomerization reaction of electronically excited (dibenzylmethine) borondifluoride in n-alcohols. Study of microscopic friction and polarity

Abstract Excited state isomerization reaction of a polar and neutral molecule (dibenzylmethine) borondifluoride in solution has been studied using picosecond spectroscopy. The observed isomerization kinetics is clearly non-Kramers type: the hydrodynamic viscosity overestimates the friction experienced by the reaction at high viscosities. Correcting the hydrodynamic friction for relative size effects of the solute and the solvent according to Giere—Wirtz and Dote—Kivelson—Schwartz results in acceptable well and barrier frequencies but underestimation of the rates at high viscosities remains. Using rotation correlation times as a measure of friction and the Kramers—Hubbard model results in much better agreement between the experiment and theory but an exceedingly high value of barrier frequency is obtained. We believe that frequency dependence of frictional effects is important in isomerization of DBMBF2 in n-alcohols solutions at high values of friction.

Sensitized luminescence of the Eu3+/La3+/cinnamic acid mixed complex: comparison to the Eu3+/Gd3+/cinnamic acid mixed complex

Abstract The photoluminescence properties of europium and lanthanum mixed complexes with cinnamic acid are described. The maximum emission intensity is found to be at about 80% Eu 3+ /20% La 3+ content at room temperature. The roles of intramolecular and intermolecular energy transfer processes are discussed. Comparison to photoluminescence properties of Eu 3+ /Gd 3+ /cinnamic acid mixed complexes is given. A slight photochemical degradation of the samples is found, the degree of which depends on the Eu 3+ /La 3+ ratio.

Sensitized luminescence of Eu3+/Gd3+/cinnamic acid mixed complex

Abstract The photoluminescence of europium and gadolinium mixed complexes with cinnamic acid is investigated at room temperature. The maximum emission intensity is found to be at 70% Eu 3+ /30% Gd 3+ content. The probable ways of energy transfer processes are discussed on the basis of spectroscopic data. Photochemical degradation of the sample is found, the degree of which depends on the Eu 3+ /Gd 3+ ratio.

TIME-RESOLVED LUMINESCENCE OF EU(III)/THENOYLTRIFLUOROACETONE/SURFACTANT SYSTEMS IN AQUEOUS SOLUTIONS

Abstract Temporal developing of photoemission of Europium(III)-Thenoyltrifluoroacetone complex in micelle solutions was investigated. From steady-state fluorescence and phosphorescence excitation, steady-state and time-resolved emission spectra and decay curves it was found that not only 5D0 triplet level but 5D1 triplet level of Europium(III) also plays role in the energy transfer from the ligand to the Europium(III) ion.

Stark spectroscopy measurements of electron density of ablative discharges in Teflon-(CF2)n capillaries

The peak electron density (Ne ) in the ablative dense plasma produced by the pulsed discharge through a Teflon-(CF2 )n capillary has been measured using Starks spectroscopy of the atom emission lines in the range Ne = 1016 -1018 cm-3 . The time-resolved profiles of F(I) (683.43, 685.60, 687.02, 690.25, 690.98 nm) lines were detected for the different discharge energy densities and compared with the theoretical data. The results show that the method can be used for the diagnostic of plasmas with peak electron density up to about 3 × 1017 cm-3 , which corresponds to a discharge energy density of 9 kJ cm-3 .

Phase-fluorometry study on dielectric relaxation of acrylodan-labeled human serum albumin

Dielectric relaxation (DR) of acrylodan-labeled human serum albumin (HSA/AC) was studied by phase-fluorometry. A non-monoexponential behavior of both the total fluorescence--and the DR decays has been found. The protein environment of the fluorescent marker shows DR times ranging from the pico to nanosecond timescale. In fluorescence emission decays measured on the red side of the fluorescence spectrum a time constant (<10 ps) affected by a negative preexponential was found supporting the existence of DR of the excited states.