The plasma etching of silicon and silicon dioxide in CF4‐O2 mixtures has been studied as a function of feed‐gas composition in a 13.56‐MHz plasmagenerated in a radial‐flow reactor at 200 W and 0.35 Torr. Conversion of CF4 to stable products (CO, CO2, COF2, and SiF4) and the concentration of free F atoms ([F]) in the plasma were measured using a number of different diagnostics. The rate of etching, the concentration of F atoms, and the intensity of emission from electronically excited F atoms (3s 2 P–3p 2 P° transition at 703.7 nm) each exhibit a maximum value as a function of feed‐gas composition ([O2]); these respective maxima occur at distinct oxygen concentrations. For SiO2, the variation in etching rate with [O2] is accounted for by a proportional variation in [F], the active etchant. The etching of silicon also occurs by a reaction with F atoms, but oxygen competes with F for active surface sites. A quantitative model which takes oxygen adsorption into account is used to relate the etch rate to [F]. The initial increase of [F] with [O2] is accounted for by a sequence of reactions initiating with the production of CF3 radicals by electron impact and followed by a reaction of CF3 with oxygen. When [O2] exceeds ∼23% (under the present discharge conditions), [F] decreases due, probably, to a decrease in electron energy with an increase of oxygen in the feed gas.

Plasma etching of Si and SiO2—The effect of oxygen additions to CF4 plasmas

The nature of the charge carriers in ‘conducting’ polymers is of considerable interest at present1,2, largely on the basis of the technological potential of these materials for use as the semiconducting layer in field-effect transistors (FETs) and the emissive layer in light-emitting diodes3 (LEDs). One of the main outstanding questions concerns the relative importance of intra- versus inter-chain charge transfer in determining the overall rate of charge transport. Here we apply the pulse-radiolysis time-resolved microwave conductivity technique4 to dilute solutions of a soluble dialkoxy derivative of the semiconducting polymer poly(phenylene vinylene), PPV, by which means we determine the one-dimensional intra-chain mobilities of electrons and holes on isolated polymer chains free from inter-chain interactions. The values so obtained—0.5 and 0.2 cm2 V−1 s−1 respectively—are considerably larger than the mobilities measured previously for bulk PPV-based materials5,6,7,8,9. This suggests that considerable improvement in the performance characteristics (in particular switching time and maximum current) of organic FET and LED devices should be possible if material purity and structural order can be better controlled.

Highly mobile electrons and holes on isolated chains of the semiconducting polymer poly(phenylene vinylene)

Photocross-linking is a useful technique for the partial definition of the nucleic acid-protein interface of nucleoprotein complexes. It can be accomplished by one or two photon excitations of wild-type nucleoprotein complexes or by one photon excitation of nucleoprotein complexes bearing one or more substitutions with photoreactive chromophores. Chromophores that have been incorporated into nucleic acids for this purpose include aryl azides, 5-azidouracil, 8-azidoadenine, 8-azidoguanine, 4-thiouracil, 5-bromouracil, 5-iodouracil, and 5-iodocytosine. The various techniques and chromophores are described and compared, with attention to the photochemical mechanism.

Photocross-Linking of Nucleic Acids to Associated Proteins

An assessment of high-temperature mass spectrometry and of sources of inaccuracy is made. Experimental, calculated, and estimated cross-sections for ionizalion of atoms and inorganic molecules typically present in high-temperature vapors are summarized. Experimental cross-sections determined for some 56 atoms are generally close to theoretically calculated values, especially when excitation-autoionization is taken into account. Absolute or relative cross-sections for formation of parent ions were measured for ca. 100 molecules. These include homonuclear diatomic and polyatomic molecules, oxides, chalcogenides, halides, and hydroxides. Additivity of atomic cross-sections supplemented by empirical corrections provides fair estimates of molecular cross-sections. Causes of uncertainty are differences in interatomic distances and in shapes of potential energy curves (surfaces) of neutral molecules and of molecular ions and tendency toward dissociative ionization in certain types of molecules. Various mass spectrometric procedures are described that render the accuracy of measured thermodynamic properties of materials largely independent of ionization cross-sections. This accuracy is comparable with that of other techniques applicable under the conditions of interest, but often only the mass spectrometric procedure is appropriate at high temperatures.

High-temperature mass spectrometry: Instrumental techniques, ionization cross-sections, pressure measurements, and thermodynamic data (IUPAC Technical Report)

Environmental persistence of organic compounds estimated from structure-reactivity and linear free-energy relationships. Unsaturated aliphatics

This is a compilation of ionization and appearance potentials of positive ions published from 1955 through June 1966. The compilation lists the ion formed, the parent species from which it was formed, the other products of the process, the threshold energy for the formation of this ion, and the method by which this data was obtained. Where feasible, the heat of formation at 298 K of the positive ion has been computed for each entry using auxiliary thermochemical data. From these computed values “best” values have been chosen.

Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions

Three sources of quantitative data for methyl-substituted allyl cations are available experimentally: Gas phase heats of formation, rotational barriers in solution, and rates of solvolysis. NMR chemical shifts also provide an index of charge distri- bution. This paper draws all these lines together in comparison with results of STO-3G ab initio molecular orbital calculations performed on planar and perpendicular methyl-substituted allyl cations. Sequential substitution of the terminal positions by one, two, three, and four methyl groups stabilizes allyl cations electronically by 17, 15, 13, and I1 kcal/mol, respectively; a methyl group on the central carbon has a much smaller stabilizing effect (-5 kcal/mol). The steric strain for the first endo- methyl group was determined to be 3 kcal/mol, whereas a second endo-methyl experiences a somewhat greater strain of 5 kcal/mol. Allyl solvolysis rates correlate well with gas phase allyl cation stability data, but there is a reduction in magnitude due to solvation. The calculated rotational barriers also are 4-1 2 kcal/mol higher than the activation free enthalpies deter- mined in superacid solution; the difference provides further evidence for differential solvation effects-the more highly delo- calized planar forms are solvated to a lesser extent than their rotational transition states. The rotational barrier of the parent allyl cation is predicted to be 34 kcal/mol in the gas phase but to decrease to 23.7 f 2 kcal/mol in superacid solution.

https://epub.ub.uni-muenchen.de/3863/1/016.pdf

K. Draxl

Papers

Ionization Potentials, Appearance Potentials, and Heats of Formation of Gaseous Positive Ions

Methyl-Substituted Allyl Cations. A Comparison of Experimental Stability, Rotational Barrier, and Solvolysis Data with ab Initio Calculations