Petr Bitala

Desorption/ablation of lithium fluoride induced by extreme ultraviolet laser radiation

Abstract The availability of reliable modeling tools and input data required for the prediction of surface removal rate from the lithium fluoride targets irradiated by the intense photon beams is essential for many practical aspects. This study is motivated by the practical implementation of soft X-ray (SXR) or extreme ultraviolet (XUV) lasers for the pulsed ablation and thin film deposition. Specifically, it is focused on quantitative description of XUV laser-induced desorption/ablation from lithium fluoride, which is a reference large band-gap dielectric material with ionic crystalline structure. Computational framework was proposed and employed here for the reconstruction of plume expansion dynamics induced by the irradiation of lithium fluoride targets. The morphology of experimentally observed desorption/ablation craters were reproduced using idealized representation (two-zone approximation) of the laser fluence profile. The calculation of desorption/ablation rate was performed using one-dimensional thermomechanic model (XUV-ABLATOR code) taking into account laser heating and surface evaporation of the lithium fluoride target occurring on a nanosecond timescale. This step was followed by the application of two-dimensional hydrodynamic solver for description of laser-produced plasma plume expansion dynamics. The calculated plume lengths determined by numerical simulations were compared with a simple adiabatic expansion (blast-wave) model.

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.

A new line-shape asymmetry model for wavelength modulation spectroscopy in gaseous flows

Abstract This communication reports technical notes on the development and application of an automated line-shape fitting procedure for wavelength modulation spectroscopy (WMS). Near-infrared transitions of carbon dioxide (CO2) around 1573 nm were measured in vertical cold (nonreacting) flow of CO2 at atmospheric pressure using WMS with demodulation at second harmonic frequency. Semi-empirical model based on the set of so-called Gabor functions was developed and parameters of Lorentzian line-shape profile and its asymmetry resulting from simultaneous frequency and amplitude response of the current-modulated semiconductor laser were determined. Nonlinear least-square fitting procedure employing differential evolution algorithm was successfully utilized for performing this task. Line-shape fitting procedure enabling efficient signal de-noising and background subtraction of wavelength modulation spectra was implemented into an open-source code.

Thin graphite membranes for laser photoacoustic spectroscopy

The mechanical behavior of thin suspended graphite membranes (thickness of about 100 nm) has been investigated by a method utilizing Laser Photoacoustic Absorption Spectroscopy (PAS). Membranes prepared by multiple mechanical cleavage of highly ordered pyrolytic graphite (HOPG) were attached to a glass window with circular opening (diameter 4 mm). The movement of the membranes was induced by pressure changes inside a photoacoustic cell and detected via a laser beam reflected from the membranes onto a position sensitive detector. The movement of graphite membranes has been compared to the movement of a membrane of a condenser microphone. Several membranes have been tested, differing in method of preparation and/or in design. The pressure induced movement of the membranes is influenced by mechanical properties of the membranes itself, as well as by the acoustic properties of the photoacoustic cell. The free volume on the sides of the membranes had the most significant influence on its movement. On the other hand, metallization by thin layer of gold (70 nm) had negligible effect. The sensitivity of membranes utilized as an optical microphone employed in PAS method has been tested. Using methanol vapor as testing gas, we have found that the sensitivity of the membranes is comparable with the sensitivity of a top class microphone and is expected to be further enhanced by reducing the thickness of the membrane, by increasing its diameter or different (e.g. cantilever like) geometry.

Material properties of lithium fluoride for predicting XUV laser ablation rate and threshold fluence

This paper deals with prediction of extreme ultraviolet (XUV) laser ablation of lithium fluoride at nanosecond timescales. Material properties of lithium fluoride were determined based on bibliographic survey. These data are necessary for theoretical estimation of surface removal rate in relevance to XUV laser desorption/ablation process. Parameters of XUV radiation pulses generated by the Prague capillary-discharge laser (CDL) desktop system were assumed in this context. Prediction of ablation curve and threshold laser fluence for lithium fluoride was performed employing XUV-ABLATOR code. Quasi-random sampling approach was used for evaluating its predictive capabilities in the means of variance and stability of model outputs in expected range of uncertainties. These results were compared to experimental data observed previously.

CO2 laser photoacoustic spectrometry: sensitivity and drift analysis

Trace gas measurements were performed by the eddy correlation technique. The time domain stability criterion for laminar and turbulent flows measured in the street canyon model was determined. The evaluation of the instrument performance was done by the concentration measurements (CH3OH, C2H5OH). The Allan and Hadamard variance methods were used for stability analysis. The dependence of variances on different degrees of flow turbulence was evaluated. The influence of turbulence on the optimal averaging time for minimum detectable concentrations has been studied. The stability analysis of experimental set up consisting of the CO2 laser photoacoustic detection and the simulated atmosphere in a wind tunnel was performed for different sample concentrations and flows. The sensitivity and stability analysis were determined by 1000 s, 2000 s and 10 000 s measurements.