Raman scattering study of toluene vibrational relaxation in benzene-toluene liquid system

The polarizes and depolarized profiles of the 520 cm^?1 Raman line of toluene in benzene solutions have been measured as a function of concentration. The vibrational correlation functions were obtained by Fourier inversion of Raman band contours. The concentration dependence of the experimental vibrational correlation time has been compared with that computed from the Fischer-Laubereau vibrational dephasing model modified for use in binary mixtures.     

Surface photovoltage spectroscopy and surface piezoelectric effect in GaAs

“Real” (111) surfaces of n-type GaAs were investigated employing surface photovoltage spectroscopy and the surface piezoelectric effect. Surface states at the energy position E_c ? E_t ? 0.72 eV were found on both the Ga and the As surfaces. Both types of surfaces exhibited a barrier of about 0.55 V. No variations in the surface barrier or the energy position of the surface states were observed in various ambients at atmospheric pressure (dry air, wet air, ammonia and ozone). However, the capture cross-section of the surface states for electrons, as determined from the surface piezoelectric effect transients (of the order of 10^?13 cm²), was found to be sensitive to the ambient. It decreased in wet air and increased in ozone. This effect was more pronounced on the As than on the Ga surfaces. Additional surface states were found to be present in the energy region of 0.9 to 1.0 eV, below the bottom of the conduction band. However, their exact energy positions could not be determined due to interference caused by the carrier trapping of the surface states at E_c ? E_t ? 0.72 eV.     

Designing strategy to enhance mode selectivity of higher-output oxide-confined vertical-cavity surface-emitting lasers

Performance of modern oxide-confined (OC) vertical-cavity surface-emitting diode lasers (VCSEL s) is more sensitive to the construction details than in the case of other VCSEL s. In particular, a stable single-fundamental-mode operation is difficult to be achieved in these VCSEL s especially in higher-output large-size continuous-wave (cw) operating devices at higher temperatures. In the present paper, an operation of OC VCSEL s has been investigated with the aid of the comprehensive fully self-consistent model using the (GaIn)(NAs)/GaAs quantum-well VCSEL with two oxide apertures as a typical example. A new approach is proposed to enhance cw RT single-fundamental-mode low-threshold operation in higher-output OC VCSEL s. One of their oxide apertures should be shifted to the node position of the resonator standing wave where it is working as the electrical aperture only. Then diameters of both apertures may be changed independently giving an additional degree of freedom for VCSEL designing, which enables their optimisation. While the larger-diameter optical aperture placed in the anti-node position creates an efficient radial waveguiding effect, the smaller-diameter electrical aperture enhances a more uniform current injection into the VCSEL active region. Due to combining influence of both the apertures, the single-fundamental-mode operation is predicted in a large device with the 10-m-diameter active region even for 80 K active-region temperature increase over RT of the ambient. An impact of intentional detuning at room-temperature (RT) of VCSEL active-region gain spectrum towards shorter wavelengths with respect to the resonator mode improves mode selectivity is also analysed. PACS  42.55.Px; 02.60.Cb; 85.60.Bt     

Ergodic Properties of the Quantum Geodesic Flow on Tori

We study ergodic averages for a class of pseudodifferential operators on the flatN-dimensional torus with respect to the Schrödinger evolution. The later can be consider a quantization of the geodesic flow on . We prove that, up to semi-classically negligible corrections, such ergodic averages are translationally invariant operators.Mathematics Subject Classifications (2000) 58J50, 58J40, 81S10.     

Determination of nonsteroidal antiinflammatory drugs in water samples using liquid chromatography coupled with diode-array detector and mass spectrometry

An analytical method for the determination of trace levels of six different nonsteroidal antiinflammatory drugs (NSAIDs) in water samples has been developed and validated. Environmentally relevant pharmaceuticals were chosen according to human consumption in Poland. Final analysis of the target compounds was performed by RP LC-diode-array detection-MS, whereas sample preparation included an SPE step. For this SPE step, a number of packing materials, such as LiChrolut RP-18, calixarene, Strata-X, BAKERBOND Narc-2, BAKERBOND Polar Plus, BAKERBOND styrene divinylbenzene-1, and Discovery DSC-18, were used, and their respective advantages and disadvantages in this study were discussed. The RP-18 phase was found to be the most retentive for all analytes. The detection limits for compounds in surface waters were varied from 0.005 for diflunisal to 0.095 microg/L for ibuprofen. The average recoveries of NSAIDs from the surface water samples ranged from 80 up to 103%. RSD value is relatively low (from 4% for fenoprofen up to 8% for ibuprofen). The performance of the method was tested with several environmental water samples.     

Solid-state structure of polypeptide-based rod-coil block copolymers: Folding of helices

This work compares the solid-state structures of films made from a polystyrene-poly(Z-L-lysine) (1) and a polystyrene-poly(-benzyl-L-glutamate) (2) block copolymer, both having virtually the same numbers of repeating units and block length ratios. Small-angle X-ray scattering (SAXS) revealed a hexagonal-in-undulated lamellar morphology for both films. The long-period and the thickness of layers obtained for 2 were by a factor of three smaller as compared to 1, indicating that PBLGlu helices were folded twice, whereas PZLLys helices were fully stretched. Another difference shows up in the packing of helices, the level of ordering being considerably lower in 2. This might be due to spatial restrictions in the proper alignment of back-folded helical segments.     

Counter-Ion Influence on Polypyrrole Potentiometric pH Sensitivity

The pH sensitivity of conducting polymer films is an important issue from the sensor design point of view. The doping and supporting electrolyte anions effect on the potentiometric sensitivity and response time of polypyrrole (PPy) electrodes towards changes of solution pH were studied. It was found that (i) the response of PPy doped by easily exchangeable common anions (Cl^–, NO₃ ^–, ClO₄ ^–) in their solutions (KCl, KNO₃, NaClO₄) is slow. In contrast, (ii) polypyrrole films deposited in the presence of weak acid anions (phthalates, oxalates, salicylates) were characterised by instantaneous responses in the above mentioned solutions. On the basis of electrochemical experiments (open circuit potential vs. time dependencies, cyclic voltammetry, EQCM), the observed differences were attributed to different mechanisms of pH sensitivity of tested films. The long response times are related to the incorporation of the solution ions into the film in order to compensate charges created due to protonation. On the other hand, if the ion-exchange is hindered as in the case of (ii), instantaneous open circuit responses are observed due to polarisation of the oxidised polymer layer, analogously to the metal electrode. Moreover, for these films the internal pH buffering within the polymer membrane will weaken the pH change effect.The mechanisms were confirmed in the course of studying the pH effect in solutions containing anions easily (KCl, NaClO₄, KNO₃) or hardly exchangeable with polypyrrole (K₂SO₄, sodium poly(4-styrenesulphonate) solutions) acidified with H₂SO₄.