Pavel Engst

Diode laser application for research of molecular ions

Abstract We have developed several methods of diode laser spectroscopy and have applied them for high-resolution spectroscopy and kinetic studies of molecular ions. For these purposes we have used a special high-voltage modulation technique for generation of ions in the plasma of a gas discharge. This modulator enables the studies of the formation and decay processes of ions. The modulator can also alternately switch the polarity of a high voltage and can be used for experimental measurement of velocity modulation of ions in the gas discharge. It was found that this spectrometer can be successfully used in the following regions: • -velocity modulation spectroscopy of molecular ions (ArD+, SD−), • -distinction between the velocity modulation signal of cation and anion (due to the reverse phase of both signals), • -measurement of Doppler shifts of ions (information about the velocity and mobilities of ions), • -ion kinetics (measurement of times of formation and decay of ions in the gas discharge).

Influence of protonation on the reactions of triplet-state sulfonated chloro-aluminium(III) phthalocyanine with dioxygen

The kinetics of the reactions of sulfonated chloro-aluminium(III) phthalocyanine (AISPC) in the excited state have been studied by laser flash photolysis. Two different protonated forms of this substance were identified in the triplet state 3ππ*, AISPC and AISPCH+, differing in their lifetimes, τ≈ 290 and 90 µs, respectively, and rate of quenching by dioxygen [ko2f=(1.6 ± 0.2)× 109 dm3 mol–1 s–1, ko2s=(6.6 ± 0.7)× 107 dm3 mol–1 s–1, respectively]. It follows from the reaction kinetics that the establishment of equilibrium between the two forms AISPC and AISPCH+ in the 3ππ* state is insufficiently mobile compared with the rate of quenching by dioxygen.

Fast degradation of fullerenes by ultraviolet laser radiation

The first observation of fullerene C60 ultraviolet photolysis in hexane solution was published two years ago [1]. Similar further experiments realized with an ultraviolet lamp and solar light gave inconsistent results with ambiguous interpretations. We report the unexpectedly fast and efficient degradation of the fullerenes in n-hexane solutions, induced by an XeCl-excimer laser. Well-defined experimental conditions and good reproducibility in these experiments allow us to estimate the minimal value of the quantum yield of fullerene photolysis.

Quenching of the Triplet State of Metallophthalocyanines by Dioxygen in the Presence of Bovine Serum Albumin

Sulfonated chloro-aluminium phthalocyanine (A1SPC), excited by laser flash to the 3 * state, is quenched by dioxygen in a reaction with the rate constant k, = 1.7Xl09dm3 mol 1 s-1. In the presence of bovine serum albumin (BSA), the kinetics become biphasic: The faster process is identical with that in the absence of albumin, the slower one (k2 = 1.9Xl08dm3 mol1 s-1) corresponds to quenching of the excited phthalocyanine-BSA adduct by dioxygen. In the absence of oxygen, BSA increases the lifetime of the 3 * state from 440 ps to 1160 //s. The calculated A1SPC : BSA ratio in the adduct is 2:1. Analogous behaviour was observed with sulfonated zinc phthalocyanine and sulfonated free ligand.

Light-emitting Si prepared by laser annealing of a-Si:H

Abstract We present time-resolved reflectivity, photoluminescence, dark conductivity and morphology studies of light-emitting Si prepared by pulsed XeCl laser irradiation of amorphous hydrogenated silicon (a-Si:H) deposited on a silica substrate by glow discharge deposition. Laser-induced melting and recrystallization of the a-Si:H layers lead to visible room temperature photoluminescence, accompanied by an increase in dark conductivity by more than three orders of magnitude. We investigate the influence of the number of applied laser pulses on the properties of the processed layers.

Urban Air Pollution and Its Photochemistry Studied by Laser Spectroscopic Methods

This work compares approaches both of mathematical and physical modelling of pollutant dispersion in simulated atmospheric boundary layer (ABL) with results of remote sensing of atmospheric pollutants. Measurements were performed over a highway outside a city and in an urban street canyon with extensive traffic under different meteorological conditions (autumn versus summer period). Time-resolved spatial distributions of pollutants (NO2 and O3) were measured by the combined DIAL (differential absorption light detection and ranging)/SODAR (sound detection and ranging) method and using spot analyzers appropriately located on the leeward and windward sides near the urban street canyon bottom. Qualitative agreement was found between the results obtained by remote sensing in the real atmosphere and those obtained by physical modelling in the simulated atmosphere of a wind tunnel for the autumn period. On the other hand, the analysis of the monitoring results and outputs of the physical modelling shows disagreement for the summer period. Besides neglecting the thermal effect during the sunny period, chemical reactions or photochemical processes taking place in the street canyon can affect the dispersion and distribution of pollutants very significantly. To improve the description of the system investigated, the Computational Fluid Dynamics (CFD) environment was tested for a basic implementation of photochemical reactions into the commonly used mathematical models of turbulence and dispersion processes as well.

MOTION OF SPECIES IN A LASER-ABLATED Y-BA-CU-O PLASMA PLUME STUDIED BY TIME-DEPENDENT ATTENUATION OF A PROBE BEAM

The time dependence of attenuation of the He‐Ne and CO2 laser beams passing through a plume produced by the excimer laser ablation of Y‐Ba‐Cu‐O in the air is characterized by two maxima. The first maximum is apparently caused by a change in the refractive index at the front of the shock wave and is propagated with a velocity of 360 m s−1 comparable with the speed of sound at room temperature and it is independent on laser energy. The initial velocity for the second maximum is ∼20 m s−1 and can be connected with scattering on species ablated from the superconductor surface. The velocity of ablated species increases linearly with laser energy; the change is 81 m s−1 J−1.

CFD modelling for atmospheric pollutants/aerosols studies within the complex terrains of urban areas and industrial sites

Computational fluid dynamic (CFD) modelling of pollution dispersion and chemical conversion to aerosol particles in the atmospheric boundary layer (ABL) has been studied. The investigation focused on the numerical modelling above complex orographic terrains of urban areas and industrial sites including the dispersion of toxic substances in the air as a result of accidents. A finite-rate model of chemical reactions, including the turbulence chemistry for modelling the reaction between nitric acid and ammonia, has been applied. As supporting experiments, online monitoring of the spatial distribution of pollutants and aerosols has been performed above real complex areas. Minimal detectable concentrations 8 μg m–3 (SO2), 20 μg m–3 (NO2), 2 μg m–3 (O3) and minimal detectable absorptivity 5 × 10–7 cm–1 (aerosols) have been reached.

Pulsed laser photolysis of dirhenium decacarbonyl at 308 nm

Abstract Dirhenium decacarbonyl was photolyzed in the gas phase by excimer laser radiation at 308 nm. The products of the photolysis were probed by the time-resolved UV/VIS emission on the nanosecond time scale. Only the emission from the excited rhenium atoms was observed. The Re(a6D sol9 2 ) state was identified as a photoproduct. From the quadratic dependence of the emission signal it follows that a sequential mechanism of the photolysis takes place and an effective saturation occurs in the fluency range 25–170 mJ cmt-1. A significant quenching of the Re(z6Poj)states was found. The collisional deexcitation of an excited Re(CO) or Re2 species can be responsible for the quenching. The atomic resonance contributes to the population of excited states above the one-photon energy.