Stanislaw Wlodek

Signal-shape analysis of a thermally cycled tin-oxide gas sensor

Abstract The conductance versus time responses of a thermally cycled Figaro #813 gas sensor have been measured for clean laboratory air with the addition of several gases (hydrogen, propane, isopropanol and carbon monoxide) and their bicomponent mixtures. Output sensor signals are represented as linear combinations of two or more Gaussian functions. It is found that the positions of the maxima of these functions are characteristic features of a gas, providing a good criterion for the selective detection of inflammable or poisonous gases. The potential application of sensor signal deconvolution with Gaussian functions for quantitative analysis of two-component gas mixtures and its limitations are demonstrated.

The structure of C60Mu and other fullerenyl radicals

Abstract The structures of the radicals exo-C 60 Mu, endo-C 60 Mu, and (Mu@C 60 ) are discussed, with particular reference to the spin distribution of the unpaired electron in the most stable isomer (exo). The magnitude of the measured muon hyperfine interaction indicates partial spin delocalization in this radical. This is supported by semi-empirical calculations on C 60 H. Calculations also show that there is a significant energy barrier between the endo radical and (Mu@C 60 ), supporting the assignment of the pseudo-free muonium signal to the latter structure.

Spin relaxation of muonated radicals in the gas phase

We report on recent results obtained for longitudinal field (T1) spin relaxation of the muonium-substituted (“muonated”) free radicals MuCO, MuC2F4, MuC2H3F, and MuC4H8 (t-butyl), comparing with results reported earlier for MuC2H4 (and MuC2D4). Some comparison with transverse field (T2) data is also given. These data are fit to a phenomenological model based on NMR theory of spin relaxation in gases. The parameters of these fits are presented and discussed.

Muon investigations of fullerenyl radicals

Four of the five possible isomeric C70Mu radicals have been detected by transversefieldμSR in a C70 powder sample at 298 K. Their assignment is based on the results of semi-empirical MNDO calculations. There are significant changes in intensity and lineshape of the signals at low temperature. The first μSR spectrum of a fullerenyl radical in solution has been obtained.

Kinetic model of thermally cycled tin oxide gas sensor

Abstract The equation describing changes of gas sensor conductance with sensor temperature and reducing gas concentration within a thermal cycle has been derived from simple kinetic expressions for basic gas/ surface interactions. Output signals of a thermally cycled Figaro #813 gas sensor for carbon monoxide, hydrogen and propane have been subsequently fitted to the derived equation. The satisfactory agreement obtained between theoretical and experimental responses implies that the shape of the output signal for a thermally cycled tin oxide gas sensor can be understood in terms of a pure kinetic model including three basic processes: adsorption, desorption and catalysed oxidation.

Detection of an α-Muonium-substituted methyl radical

Reported here is the first observation of anα-Muonium-substituted methyl radical. The key to the success of this experiment was the use of a more stable precursor, trimethylsilyldiazomethane, in lieu of the prototypical diazomethane. The radical, CHMuSi(CH3)3 was observed by TF-μSR, with Aμ=187.6(2) MHz. This result is compared to literature values for the proton analogue and the isotope effect is discussed.

μSR Investigation of ethyl radicals adsorbed on silica

AbstractμSR spectra of the ethyl radical adsorbed on porous silica were observed in transverse and in longitudinal magnetic fields in the temperature range 190–298 K. The line widths reflect the dynamic partial averaging of the hyperfine anisotropy due to reorientation and surface diffusion.

Level crossing resonance due to chlorine nuclei in a free radical

Muonium adds to allyl chloride, CH2=CHCH2Cl, to form two radicals: MuCH2CHCH2Cl (main product) and CH2CHMuCH2Cl (minor product). Both radicals were fully characterized byμSR andμLCR. In the main product, the LCR lines due to the35Cl and37Cl nuclei were observed. Also, the temperature dependence of various hyperfine coupling constants (hfc) indicates that both Mu and Cl eclipse the unpaired electronp2-orbital in the minimum energy conformation. For the fragment-CH2Cl, the presence of Mu in theβ′-position is found to affect significantly the hfc of Cl in theβ-position; an internal rotational barrier of 12 kJ mol−1 was estimated using a simpleV2 torsional potential.

Conformational studies of thiyl and selenenyl radicals

Muonium adds to the unsaturated bond in 1,3-dithiolane-2-thione or -2-selenone to form a chalcogen-centered radical. The muon hyperfine coupling constants (hfc) were measured for both compounds over a range of temperatures. In solution, the temperature dependence of the muon hfc indicates that for the thiyl radical the preferred conformation corresponds to the muon eclipsing the unpaired electronpz-orbital, while there is practically free rotation in the case of the selenenyl radical in the temperature range studied. In pure thione the hfc values show a remarkable discontinuity at the melting point. The two radical signals seen in the solid are interpreted as due to the presence of two dominant crystal orientations in the samples studied. In pure selenone data were obtained only in the solid. For both systems, the solid phase results show that interaction between the muon and the unpaired electron spin is enhanced compared to the liquid phase and/or the solution.