ISSN:0144-235X    

DOI:10.1080/01442359609353171

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Spatially dispersed spectra of ions, electrons or photons are traditionally measured by scanning the spectra across a narrow slit behind which is a detector. However, the efficiency increase offered by arrays of detectors is so large that their development is increasingly demanding the attention of spectrometer manufacturers. One-dimensional arrays of independent detectors (discrete electrode arrays) offer the highest data accumulation rate as detection can occur simultaneously at many sites, but a high resolution array of this type requires much associated electronics and this has limited the size and the market of such devices. The design and performance issues relating to discrete electrode arrays are discussed and a new high resolution array with all electronics integrated on a single silicon chip developed at Aberystwyth is described. A familiarity with silicon technology is not required by the reader. It is shown that integration brings not only advantages of scale but also of performance.

ISSN:0144-235X    

DOI:10.1080/01442359609353172

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

High pressure up to 50 kbar generated in a diamond anvil cell has been applied to the investigation of organic molecules in their phosphorescent triplet state. Experimental techniques include specifically adapted optically detected magnetic resonance (Odmr) and optical spectroscopy. This paper reviews some of the high points of our results. On the microscopic scale there is no isotropic compression. Pressure enhances the interaction of a chromophore with its neighbours in a few specific modes according to crystalline packing. These interactions may shift the energy levels of the chromophore, or enhance the dynamical processes in the crystal. A variety of pressure-induced effects may be rationalized on this basis.

ISSN:0144-235X    

DOI:10.1080/01442359609353173

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Charge transfer cross-sections in the collision of a doubly-charged nitrogen ion and a helium atom are evaluated with a close-coupling formalism. The charge transfer produces N+(3P), N+(1D) and He+ions via the2Σ+and2π molecular states of NHe2+. We describe a method for obtaining a particular diabatic representation from a set of adiabatic dipole moments. Comparison with Landau-Zener model calculations is made and the importance of inner couplings for the production of charge transfer states is demonstrated. Collisional rate coefficients are also obtained. At 104K, the rate coefficient for charge transfer into the ground N+(3P) state is about 1·3 × 10−10cm3s−1and for charge transfer into the metastable N+(1D) state 1·8 × 10−11em3s−1.

ISSN:0144-235X    

DOI:10.1080/01442359609353174

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Excitation of15NO molecules into the B2π state by collisional intramolecular energy transfer from the metastable a4π state was studied under single-collision conditions. Passing a beam of15NO (a4π) through a cell filled with argon at a few mTorr, B2π → ×2π emission was observed. It originated from only three specific B state levels, namely2π1/2,J= 21·5 and2π3/2,J= 21·5 inv= 2 and2π1/2,J= 10·5 inv= 3. This selectivity is characteristic of a gateway-type process mediated by spectroscopic perturbations. The analysis showed that in the last case there is a direct spin-orbit interaction of the emitting B state level with a well-specified level of NO(a). The other two perturbations are caused by interactions of the respective NO(B) levels with uniquely identifiable levels of the short-lived NO(b4Σ−) state, which in turn are collisionally populated from a broad range of NO(a4π) levels. This latter mechanism is thus a novel type of gateway process. The results explain the anomalous spectral distribution observed in an earlier isotopic study of the15N16O afterglow, which showed an enhanced population of the vibrational levels B,v= 2 and 3 [14].

ISSN:0144-235X    

DOI:10.1080/01442359609353175

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A reproducible current-voltage curve of pure water, obtained with the dropping mercury electrode (DME), contains a 1-2 V wide region where no faradaic processes take place. This region consists of three distinct parts according to different orientations of water molecules and their interactions with the mercury surface. On irradiation of the DME by UV and visible light a cathodic photocurrent is produced in the negative part of the nonfaradaic region, which increases with increasing voltage. The photocurrent appears in water in the absence of scavengers of hydrated electrons, and its magnitude is of the same order as that of the current due to photoemission of electrons into solutions of electrolytes containing scavengers. Additions of electrolytes decrease and ultimately suppress this kind of photocurrent. We explain the photocurrent by the effect of an electric field of the extended electrode double layer on the return of hydrated electrons, generated by photoemission, back to the electrode. Retarded by electrostatic repulsion from the negatively-charged electrode, some of the hydrated electrons undergo a relatively slow reaction with water, which under those conditions acts as a weak electron scavenger.

ISSN:0144-235X    

DOI:10.1080/01442359609353176

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Electron impact ionization and dissociation studies of C60and C70carried out in our laboratory over the past three years have revealed many exciting and novel features of this new class of molecules (clusters). The most salient results are summarized in this paper, including first a description of the crossed beams apparatus and the various mass spectrometric (two-sector field) techniques used. This is followed by a discussion of the production of parent and fragment ions (with up to eight charges) via electron impact ionization of C60and C70including results on mass spectral fragmentation patterns and measured and calculated absolute ionization cross-section functions. The experimental results concerning the energetics of produced parent and fragment ions are presented in the next section and measured appearance energies and breakdown curves are analysed in the frame of various theoretical concepts (Rrkm, Fhbt) thereby allowing us to derive the corresponding binding energies. The last section is devoted to (i) the different aspects of the stability of singly- and multiply-charged fullerene ions (i.e. the quantitative study of various unimolecular decay channels such as monomer evaporation, sequential decay series, charge separation reactions, etc.) and, based on the measured properties of these decay reactions, to (ii) a discussion of the possible decay mechanisms.

ISSN:0144-235X    

DOI:10.1080/01442359609353177

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Formation of negative ions under low energy electron impact (0-15 eV) is studied from C6F6and C6F5Cl at different stages of aggregation: gas phase molecules, free molecular clusters and condensed molecules. The gas phase experiments (molecules, clusters) are performed in a crossed beam apparatus and the condensed phase experiments in a UHV apparatus where the molecules are deposited in definite amounts on a cold (30 K) metallic substrate. In each case the relative cross-section for the formation of a particular anion as a function of the electron energy is recorded mass spectrometrically. Under single-collision conditions, both molecules yield parent anions from a narrow resonance at zero eV and various fragment ions from further resonances at higher energies. In clusters the low energy attachment feature broadens considerably. This is the result of effective intermolecular relaxation which strongly competes with autodetachment. Mono-meric anions are also generated from clusters in considerable amounts at higher energies (> 3 eV) via inelastic scattering from one molecule and evaporative capture of the slowed-down electron by another molecule of the same cluster (self-scavenging). In C6F5Cl clusters, dissociative attachment is strongly quenched with the exception of a new intense resonance at 6·5 eV yielding Cl−and C6F5−. At this energy, desorption of Cl−from condensed C6F6Cl also has a pronounced maximum. The 6·5 eV feature is interpreted as a core excited open channel resonance which is converted into a closed channel (Feshbach) resonance when coupled to environmental molecules, i.e. any degree of aggregation strongly quenches autodetachment in favour of dissociation.

ISSN:0144-235X    

DOI:10.1080/01442359609353178

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The question of negative differential capacitance (C) at electrified interfaces is critically reviewed. The importance of the thermodynamic constraints and the physical differences between systems under charge and potential control is emphasized. For a system under potential control,Ccan never be negative, while no such general constraint is mandated for systems under charge control. However, the appearance of a negativeCdomain in isolated systems can be limited by a phase transition to a non-uniform state. The occurrence of such transitions depends on the ‘stiffness’ of the interface. Theoretical calculations that predictC< 0 are analysed. We investigate a series of electromechanical analogues of the existing models of double layer, illustrating the appearance ofC< 0 in ‘rigid’ systems under equilibrium conditions, and the transition to the non-uniform state in systems with moderate ‘surface tension’, narrowing theC< 0 domain. The effect of intrinsic non-homogeneity is also addressed. The appearance of domains whereCis negative under charge control implies the occurrence of surface phase transitions under potential control. The characteristics of phase transitions in isolated and open systems, and the relationship between them, are considered. Some biophysical implications are discussed.

ISSN:0144-235X    

DOI:10.1080/01442359609353179

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Gas-phase reactivity of negative ions with fully halogenated alkanes is reviewed, restricted to rate constant measurements under thermal conditions. Reactivity generally increases upon substitution by heavier halogens. Perfluorocarbons (PFCs) are generally unreactive with negative ions, but the highly reactive O−anion reacts with the larger PFCs through multiple product channels. The trifluoromethyl halide series CF3X (X = Cl, Br, I) has been well-studied, and the reactions tend to be fast when nondissociative electron transfer to the halocarbon would be exothermic, regardless of the actual resulting product channels. For many of the reactions of CF3X and other perhaloalkanes, reactivity correlates with both the electron affinities and the electron attachment rates of the perhaloalkanes, though notable exceptions exist. This and other evidence suggests that many of the reactions are initiated by electron transfer to the halocarbon, which tends to explain why the reactivity of CF3X generally increases in the order Cl < Br < I for second-order reactions. The same reactivity trend in third-order association reactions is explained by the corresponding trends in CF3X dipole moment and polarizability. The results of several studies show that negative ion reactions are not a significant removal process for halocarbons in the atmosphere.

ISSN:0144-235X    

DOI:10.1080/01442359609353180

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor