Pavel Kubát

Degradation of pyrene by UV radiation

Abstract The photodegradation of pyrene (Py) in the presence of acetic acid (AA) occurs through a radical mechanism and is faster than in organic solvents (methanol). PyH (absorption maximum in the UV/VIS spectra λmax=395xa0nm in AA, lifetime τ=∼2xa0μs in oxygen saturated solution) was identified as an intermediate in the photodegradation in AA. Absorption of a photon by ground state of Py also leads to the formation of excited states (1Py* and 3Py), which deexcite to the ground state without chemical changes. Singlet oxygen 1O2 formed by energy transfer from 3Py to dissolved oxygen does not react with Py. PyH− and PyH+ were not found following excitation under the given conditions. The rate of photodegradation can be considerably increased by addition of water and hydrogen peroxide. The accelerating effect of hydrogen peroxide consists in photolytic formation of reactive hydroxyl radicals, which attack the pyrene nucleus.

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).

The Role of the Oxygen Molecule in the Photolysis of Fullerenes

Abstract Experiments designed to clarify the role of molecular oxygen in the photolysis of fullerene are described. The formation of oxygen-fullerene adducts, both in fullerene solutions in various solvents and in micro-crystallites formed by evaporating these solutions, and their photolysis was studied under various experimental conditions. The results confirm that the role of oxygen in the photolysis process in these systems is ambivalent and highly dependent on the conditions under which the irradiation of the fullerenes occurs (wavelength and intensity of the photolyzing radiation, character of the solvent molecules, presence of additional molecules in the solvent-fullerene-oxygen system, etc.).

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.

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.