Laboratory spectroscopic studies of reactive intermediates relevant to the combustion

The goal of this thesis is to describe studies about molecular species invertigated by spectroscopic methods which are relevant to the safety sciences – it means first of all chemistry of combustion. The first chapter called “Microwave spectroscopy of the H212C16O” deals with molecule of formaldehyde studied by microwave spectroscopic technique, namely rotational transitions within excited vibrational states in the millimeter-wave and submillimeter-wave region. This work has been motivated by the atmospheric and astrophysical importance of formaldehyde (H2CO). The main aid of this study is to complete the already existing list of rotational transitions within the ground vibrational state by a list of transitions within the four first excited 21, 31, 41 and 61 vibrational states, to help the detections of this species by microwave or millimeterwave techniques. In the second chapter called “Photoacoustic spectroscopy of the C7H14O2” are interpretated absorption spectra of molecule of n-pethylacetate studied by photoacoustic detection (PAS) for concentration analysis. The absorption spectra of gaseous n-penthylacetate has been investigated by means of Fourier transform infrared spectroscopy (FTIR) as well as CO2-laser photoacoustic spectroscopy (PAS) for simulation of the dispersion of a nerve agent (sarin) within a modeled atmospheric boundary layer. Three CO2-laser emission lines has been used for photoacoustic detection of n-penthylacetate with detection limit in the range of 1-3 ppm. And the last chapter called “Infrared spectroscopy of XCN+” deals with molecular ions from CN• radical family, especially halogen-cyanide radical cations XCN+ (X = Br, I, Cl) studied by use of tunable diode-laser (TDL) infrared spectroscopic technique. Radical cation ICN+ has been studied theoretically and also experimentally by means of this method as well as by means of the FTIR spectroscopy in the discharge plasma as a simulation of exothermic medium. Series of the lines with periodic wavenumber distances have been detected. It is supposed that they indicate new generated species that has been proved by Loomis Wood analysis. Furthermore the FTIR spectra have been recorded and individual bands have been assigned to determinate the chemical composition of the newly generated species at discharge conditions.Cílem této disertace je characteritika molekulárních iontů významných z hlediska bepečnostních věd, což znamená především chemie hoření, vyšetřovaných pomocí vybraných spektroskopických metod. První kapitola nazvaná “Microwave spectroscopy of the H212C16O” se zabývá mikrovlnnými spektry molekuly formaldehydu, přesněji rotačními přechody v rámci excitovaných vibračních stavů v milimetrové a submilimetrové oblasti elektromagnetického spectra. Tato studie byla motivována atmosférickou a astrofyzikální důležitostí formaldehydu (H2CO) a jejím hlavním cílem bylo doplnění již existujícího přehledu rotačních přechodů v rámci základního vibračního stavu molekuly o zcela nový soupis přechodů v rámci čtyř prvních excitovaných vibračních stavů 21, 31, 41 a 61 k zlepšení detekce této spécie pomocí metod mikrovlnné či milimetrové spektroskopie. Ve druhé kapitole nazvané “Photoacoustic spectroscopy of the C7H14O2” jsou interpretována apbsorpční spectra molekuly n-pentylacetátu zkoumaná metodou optoakustické spektroskopické detekce (PAS) ke koncentrační analýze. Absorpční spektra plynného n-pentylacetátu byla zkoumána infračervenou spektroskopií s Fourierovou transformací (FTIR) a zároveň CO2-laser optoakustickou spektroskopií (PAS) k přímé kvantitativní spektroskopické detekci stopových množství plynu a následné využití této metody při experimentech fyzikálních simulací šíření zástupce nervového plynu (sarinu) uvnitř modelované mezní vrstvy atmosféry v nízkorychlostním aerodynamickém tunelu. A poslední kapitola nazvaná “Infrared spectroscopy of XCN+” se zabývá studiem molekulárních iontů z řady CN• radikálů, především halogen-kyanidových radikálových kationtů XCN+ (X = Br, I, Cl) s použitím metod laditelné laser-diodové (TDL) infračervené spektroskopie. Radikálový kation ICN+ byl studován teoreticky a experimentálně pomocí této metody, stejně tak pomocí FTIR spektroskopie, v nízkoteplotním výbojovém plazmatu jako simulace exotermického prostředí. Byla detekována řada linií s pravidelným vlnočtovým rozestupem, které pravděpodobně indikují nové spécie vznikající ve výboji, což bylo prověřeno analýzou v programu LoomisWood. Dále byla zaznamenána FTIR spektra a byly přiřazeny jednotlivé pásy k přesnějšímu určení chemického složení nově generovaných spécií ve výbojových podmínkách.030 - Katedra požární ochranyvyhově

Long-lived transient anion of c-C₄F₈O

We report partial cross sections for electron attachment to c-C4F8O, a gas with promising technological applications in free-electron-rich environments. The dissociative electron attachment leads to a number of anionic fragments resulting from complex bond-breaking and bond-forming processes. However, the anion with the highest abundance is the non-dissociated (transient) parent anion which is formed around 0.9 eV electron energy. Its lifetime reaches tens of microseconds. We discuss the origin of this long lifetime, the anion’s strong interactions with other molecules, and the consequences for electron-scavenging properties of c-C4F8O in denser environments, in particular for its use in mixtures with CO2 and N

Partial cross sections for dissociative electron attachment to tetrahydrofuran reveal a dynamics-driven rich fragmentation pattern

We report partial absolute cross sections for dissociative electron attachment (DEA) to tetrahydrofuran C₄H₈O. The high sensitivity of the present setup, quantitative DEA spectrometer with time-of-flight analyzer, led to identification of a number of previously unreported fragments (CH₂⁻, OH⁻, CHO⁻, C₄H₅O⁻, C₃H₃⁻) thus revealing complex dissociation dynamics of the involved resonant states. The chemical composition of fragment anions was assigned by experiments with completely deuterated tetrahydrofuran. We also show that the dominant heavy anionic fragment with m/z = 41 is C₃H₅⁻ anion rather than C₂HO⁻ as reported in the previous studies, in spite of the fact that formation of the latter one is energetically more favorable. The dynamics on the resonant state is thus more decisive for the reaction outcome than the asymptotic energetics. The assignment of resonances and their dynamic pathways is discussed by using comparison with recent complementary experiments that characterize electronic states of neutral and ionic tetrahydrofuran

Electron attachment properties of c-C₄F₈O in different environments

The electron attachment properties of octafluorotetrahydrofuran (c-C₄F₈O) are investigated using two complementary experimental setups. The attachment and ionization cross sections of c-C₄F₈O are measured using an electron beam experiment. The effective ionization rate coefficient, electron drift velocity and electron diffusion coefficient in c-C₄F₈O diluted to concentrations lower than 0.6% in the buffer gases N₂, CO₂ and Ar, are measured using a pulsed Townsend experiment. A kinetic model is proposed, which combines the results of the two experiments

Laserová spektrometrie aplikovaná při studiu distribuce sekundárních škodlivin v simulované mezní vrstvě atmosféry

Import 05/10/2007Prezenční546 - Institut environmentálního inženýrstv

Dissociative electron attachment to methylacetylene and dimethylacetylene: Symmetry versus proximity

We have measured absolute dissociative electron attachment (DEA) cross sections in methylacetylene (propyne, C₃H₄) and dimethylacetylene (but-2-yne, C₄H₆). The main feature in the low-energy DEA spectrum is a π* shape resonance giving rise to fragments at 3.4 eV in C₃H₄ and 4.0 eV in C₄H₆. The process C₃H₄+e→ C₃H₃  −+H proceeds via abstraction of the acetylenic hydrogen which is mediated by effective vibronic coupling. The abstraction of methyl group hydrogen, which does not require symmetry lowering of the transient negative ion, was not observed; the spatial separation of the bond from the resonance decreases the cross section dramatically. The presence of the methyl group further influences the DEA cross sections via change of the resonance's position and via blocking one or both of the DEA active sites in acetylene. No cleavage of the C-methyl bond has been observed, and reasons for this effect are discussed. Additionally, several higher-lying resonances (6 to 15 eV) leading to production of an H⁻ fragment were observed and assigned

Experimental evidence on the mechanism of dissociative electron attachment to formic acid

Two mechanisms for dissociative electron attachment in HCOOH, the formation of HCOO−+H, were proposed in the literature: (i) via a direct electron attachment into a σ∗ resonance, augmented by dipole binding of the incident electron [G. A. Gallup et al., Phys. Rev. A 79, 042701 (2009)], and (ii) with the 1.8 eV π∗ resonance as a doorway state, linked to the products by symmetry lowering–distortion of the temporary anion, primarily the CH bond, from the planar symmetry [T. N. Rescigno et al., Phys. Rev. Lett. 96, 213201 (2006)]. The later mechanism implies a reduction of the cross section upon deuteration of the hydrogen bonded to the C atom, whereas the former mechanism would leave the cross section unaffected. Our experimental absolute cross sections for the four isotopomers of formic acid show that deuteration on the C atom reduces the cross section value only marginally (by 12%) compared to deuteration on the O atom (reduction by a factor of 16), and thus favor mechanism (i)

Optoacoustic Detection of Gases and Possibilities of Its Utilization in Modelling of Hazardous Gas Pollutant Releases

Tento článek popisuje možnosti využití metody CO2 laserové optoakustické detekce plynů pro včasné odhalení úniku nebezpečné látky. Cílem tohoto příspěvku je popsat možnosti využití této metody pro fyzikální modelování šíření nebezpečných plynných látek v aerodynamickém tunelu. K tomuto účelu byl vybrán jednoduchý model komínu, který byl umístěn ve středu zkušební plochy aerodynamického tunelu, a jako modelové plyny byly použity metanol a trichloretylen. Naměřená data byla vyhodnocena pomocí statistického softwaru Trilobyte QC.Expert 2.5 s ohledem na posouzení opakovatelnosti stanovení.This article describes possibilities of utilization of the method of CO2 laser optoacoustic detection of gases for the timely detection of a hazardous substance release. The aim of this contribution is to describe the possibilities of utilization of this method for the physical modelling of spread of hazardous gaseous substances in an aerodynamic tunnel. For this purpose, a simple chimney model placed in the centre of aerodynamic tunnel test area was selected and methanol and trichloretylene were used as model gases. Measured data were evaluated by means of statistical software Trilobyte QC.Expert 2.5 with regard to the assessment of determination repeatability

Electron attachment properties of c-C4F8O in different environments

ISSN:0022-3727ISSN:1361-646

Allan variance for optimal signal averaging-monitoring by diode–laser and CO2 laser photo-acoustic spectroscopy

The concept of the Allan variance has been utilized to test the detection abilities of two powerful experimental methods for trace gas monitoring: IR diode–laser and CO2 laser photo-acoustic spectroscopy. The detection of stable molecules (OCS, allene, CH3OH), and unstable species (ozone, short-lived free radical CN and molecular ion ArD+) have been compared. An approach for studies of the influence of the reactivity on the optimal averaging time for the minimum detectable concentration is demonstrated