Conventional Ions Research Articles

Sectorized Hamming Concentric Circular Arrays for Stratospheric Platforms Cellular Design

Recently, stratospheric platform communicat ions system (SPs) has gained great interest due to its superior performance co mpared to conventional communicat ions systems. Th is paper addresses one of the major performance keys in SPs which is the cellu lar design based on adaptive concentric circu lar arrays (CCA). The proposed design technique aims to provide circular cells at any elevation angle to overco me the cell flattening and broadening which result in ellipsoidal cell shape. The cell footprint is controlled by adjusting the beamwidths which can be established by sectorizing the CCA and tapering the current amplitudes of the effective sectors by Hamming function while the others are muted and the resulted array is called Sectorized Hamming tapered CCA (SHTCCA). In this array, each sector has an angular width of 90 o and some and two opposite sectors are fully fed while the others has less number of quarter- circular arrays. The tapering of the active sectors results in lower sidelobe levels which is a paramount improvement for the cellu lar systems. In addition, the SHTCCA is analy zed and optimized to prov ide the desired beamwidths at any elevation angle that are needed to design circular cell footprint.

Electrochemical incorporation of graphene oxide into conducting polymer films

In this paper we report a simple, one-step method for direct electrochemical incorporation of graphene oxide (GO) into conducting polymer films without the use of any additional dopants. Polypyrrole (PPy) and poly(3,4-ethylenedioxythiophene) (PEDOT) were successfully electropolymerized in as-prepared aqueous GO dispersions using potentiostatic polymerization. When dispersed in water, the GO flakes carry a negative surface charge which allows them to serve as counter ions and hence be incorporated into the polymer films during electropolymerization. The incorporation of GO was verified using XPS whereas vibrational spectroscopy was used to confirm the successful formation of the two conducting polymers. The surface morphologies of the polymer–GO films were investigated with SEM. The incorporation of GO into PPy resulted in a distinctly different and more porous surface morphology than that observed in PPy films synthesized in the presence of conventional counter ions. The PEDOT–GO film, on the other hand, had a very smooth surface morphology similar to that of PEDOT films polymerized in the presence of other polyanions. As these composite films are aimed for electrochemical device applications, their redox behavior and electrochemical capacitance properties were also investigated using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The polymer–GO films exhibited good electrochemical reversibility during cycling in 0.1M KCl. The PPy films exhibited high redox capacitance regardless of the counter ion used in electropolymerization whereas PEDOT–GO had lower redox capacitance than PEDOT–PSS.

Nanohybrids of Silver Particles Immobilized on Silicate Platelet for Infected Wound Healing

Silver nanoparticles supported on nanoscale silicate platelets (AgNP/NSP) possess interesting properties, including a large surface area and high biocide effectiveness. The nanohybrid of AgNP/NSP at a weight ratio 7/93 contains 5-nm Ag particles supported on the surface of platelets with dimensions of approximately 80×80×1 nm3. The nanohybrid expresses a trend of lower cytotoxicity at the concentration of 8.75 ppm Ag and low genotoxicity. Compared with conventional silver ions and the organically dispersed AgNPs, the nanohybrid promotes wound healing. We investigated overall wound healing by using acute burn and excision wound healing models. Tests on both infected wound models of mice were compared among the AgNP/NSP, polymer-dispersed AgNPs, the commercially available Aquacel, and silver sulfadiazine. The AgNP/NSP nanohybrid was superior for wound appearance, but had similar wound healing rates, vascular endothelial growth factor (VEGF)-A levels and transforming growth factor (TGF)-β1 expressions to Aquacel and silver sulfadiazine.

Substrate dependent stability of conducting polymer coatings on medical electrodes

Conducting polymer (CP) coatings on medical electrodes have the potential to provide superior performance when compared to conventional metallic electrodes, but their stability is strongly dependant on the substrate properties. The aim of this study was to examine the effect of laser roughening of underlying platinum (Pt) electrode surfaces on the mechanical, electrical and biological performance of CP coatings. In addition, the impact of dopant type on electrical performance and stability was assessed. The CP poly(ethylene dioxythiophene) (PEDOT) was coated on Pt microelectrode arrays, with three conventional dopant ions. The in vitro electrical characteristics were assessed by cyclic voltammetry and biphasic stimulation. Results showed that laser roughening of the underlying substrate did not affect the charge injection limit of the coated material, but significantly improved the passive stability and chronic stimulation lifetime without failure of the coating. Accelerated material ageing and long-term biphasic stimulus studies determined that some PEDOT variants experienced delamination within as little as 10 days when the underlying Pt was smooth, but laser roughening to produce a surface index of 2.5 improved stability, such that more than 1.3 billion stimulation cycles could be applied without evidence of failure. PEDOT doped with paratoluene sulfonate (PEDOT/pTS) was found to be the most stable CP on roughened Pt, and presented a surface topography which encouraged neural cell attachment.

High solid and high performance UV cured waterborne polyurethanes

High solid and thermally stable waterborne polyurethanes (WPUs) were designed and synthesized by introducing imide groups into the hard segment and locating the anionic groups at the chain termini (terminal ions). It was found that the terminal ions gave rise to a much finer dispersion, higher dispersion stability and viscosity, whereas the conventional hard ions (those containing ionic groups in the hard segments) produced greater water swelling. Also, the terminal ions lead to a high-solid content (50%) at a low ionic content (2%), due to the effective exposure of the ionic groups to the particle surface, owing to the high mobility of the ionic groups and low free energy of the chain ends. When imide groups replaced urethane groups in the hard segment, remarkably enhanced mechanical, dynamic mechanical properties and thermal stability were obtained.

Application of Cluster Boron Implantation to pMOSFETs

We applied B18HX+ as an alternation of B+ or BF2+ to the implantation for source-drain extension in pMOSFETs corresponding to 65 nm and 28 nm technology nodes. We could obtain identical or better characteristics compared to the cases of conventional ions. In addition, we found from blank wafer that larger impact damage to Si atoms in B18HX+ implantation leads to more advantageous Rs−Xj in activation processing with only millisecond annealing.

Removing Organic Compounds from Aqueous Medium via Wet Peroxidation by Gold Catalysts

A new heterogeneous Fenton-like system, consisting of supported Au catalysts and hydrogen peroxide, was proved to be effective in removing low level organic compounds (ca. 100 ppm) such as phenol, ethanol, formaldehyde, and acetone in aqueous solution. Among all gold catalysts the Au/ hydroxyapatite (Au/HAp) exhibits the highest activity, and even better than the conventional iron ions exchanged zeolite (Fe/ ZSM-5) catalyst. In particular, unlike the limited operational pH range (pH: 2 approximately 5) for the other heterogeneous Fenton catalysts such as Fe/ZSM-5, Au/HAp shows higher stability even in strong acid solution (pH approximately 2), due to almost no leaching of active metal from supports into solution. It can be potentially applied in treating the industrial wastewaters with strong acidity and purifying drinking water. In addition, in the case of complete oxidation of phenol, a plausible route was suggested for deep understanding of this process.

Distonoid Ions

By Yates, Bouma, and Radom's definition, distonic radical ions are those formally arising by ionization of diradicals or zwitterionic molecules (including ylides). These ions differ, therefore, from conventional radical ions by displaying the charge site and unpaired electron site (spin) localized mandatorily on separate atoms or group of atoms; that is, these sites are separated in all of their major resonance forms. Many conventional radical ions with a major resonance form in which charge and spin sites reside formally on the same atom or group of atoms display, however, high degree of discretionary (non-mandatory) charge-spin separation. By analogy with the metal/metalloid terminology, we propose that these distonic-like radical ions be classified as distonoid ions. Radical ions would, therefore, be divided into three sub-classes: conventional, distonic, and distonoid ions. B3LYP/6-311 + G(d,p) calculations for a proof-of-principle set of radical cations are used to demonstrate the existence of many types of distonoid ions with a high degree of discretionary charge-spin separation. Reliable calculations are indispensable for probing distonoid ions, since an ion that was expected to be distonoid (by the analysis of its resonance forms) is shown by the calculations to display a characteristic conventional-ion electronic distribution. Similarly to many distonic radical ions, and in sharp contrast to a conventional radical ion (ionized 1,4-dioxane), the gas-phase intrinsic bimolecular reactivity with selective neutrals of a representative distonoid ion, ionized 2-methylene 1,3-dioxolane, is found to include dual ion-radical type reactions.

Synergistic flotation of U(VI)–alizarin complex with some diamines followed by spectrophotometric determination of U(VI) using 4,4′-diaminophenylmethane

In this work, synergistic flotation of U(VI)–alizarin complex at the presence of some diamine compounds was firstly investigated by the spectrophotometric method. The flotation process was carried out on aliquots of 100 ml of U(VI) solutions containing alizarin and the diamine at pH of 5.00 using n-heptane. The floated layer was then dissolved in acetonitrile and its absorbance was measured. Since the synergistic effect of 4,4′-diaminophenylmethane (dapm) was much more than the others, it was used for the determination of U(VI) by this method. Beer's law was obeyed ( λ max = 591 nm) in the range of 5 × 10 −7 to 1 × 10 −5 mol l −1 with the apparent molar absorptivity of 1.12 × 10 6 l mol −1 cm −1, and a correlation coefficient of 0.9991. The detection limit ( n = 7) was 1 × 10 −7 mol l −1, and the R.S.D. ( n = 7) obtained for 4 × 10 −6 mol l −1 of U(VI) was 2.23%. Except that only a few analogous cations, which could be masked by EDTA, no interference was observed at the presence of various conventional ions, even at high concentrations. The reliability and applicability of the method were confirmed using some geological standard reference materials and spiked synthetic water samples, respectively.

Catalysis by Gold: Isolated Surface Au3+ Ions are Active Sites for Selective Hydrogenation of 1,3‐Butadiene over Au/ZrO2 Catalysts

In recent years, there has been an increasing interest in catalysis by nanosized gold catalysts, which are active in reactions such as CO oxidation, propylene epoxidation, hydrogenation of unsaturated compounds, vinyl chloride synthesis, and others. 5,11] Despite such a diverse spectrum of reactions, efforts have been mainly devoted to elucidating the catalytically active sites in the oxidation reactions. Little attention has been paid to the nature of active gold, especially cationic gold species, in the other reactions such as hydrogenation of unsaturated compounds. This is surprising, since catalytic hydrogenation is one category of the best studied and industrially most important reactions on conventional transition metal catalysts, 19] and Au ions in solution can activate carbon– carbon double and triple bonds and are finding applications as efficient homogeneous catalysts in many organic reactions. 21] Hydrogenation of 1,3-butadiene has long been a model reaction for the study of active sites on conventional metal catalysts. 19] A few preliminary investigations on the hydrogenation of 1,3-butadiene with supported gold catalysts showed that small gold nanoparticles were active in the reaction, although theoretical calculations seem to suggest that the surface of metallic gold is chemically inert for hydrogen activation. By using Au/ZrO2 catalysts with very low loadings of Au (< 0.1%), we have now discovered that isolated surface-type Au ions are actually the active sites for the selective hydrogenation of 1,3-butadiene. The activity and selectivity of these Au ions are comparable to those of conventional precious transition metals such as Pd and Pt. This finding that low loadings of Au ions are more active for the hydrogenation reaction than high loadings of Au nanoparticles could raise intensive interest in exploring the potential of surface-type Au ions in heterogeneous catalytic reactions. The use of isolated and heterogenized Au ions in catalysis and related materials may be the key to using the noble gold in areas usually reserved for conventional precious metal catalysts, for example, to significantly improve the economics and reduce the use of other precious metals. Figure 1 shows the effect of gold loading on 1,3-butadiene conversion over Au/ZrO2 catalysts calcined at 200 8C. No butane (i.e., the product of complete hydrogenation) but only butenes were detected as the hydrogenation products over all

Molecule-specific imaging with mass spectrometry and a buckminsterfullerene probe: application to characterizing solid-phase synthesized combinatorial libraries.

We employ a newly developed buckminsterfullerene (C(60)) primary ion beam with time-of-flight secondary ion mass spectrometry to create molecule-specific images of resin particles employed in the solid-phase synthesis of peptide combinatorial libraries. This new cluster ion source, when operated at an incident energy of 20 keV, is remarkably effective at desorbing small peptides directly from a polymer surface and opens new possibilities for characterizing large arrays of diverse sets of molecules. In addition, the C(60) ion beam may be focused to a spot of 1.5 microm in diameter, enabling molecule-specific images of single 100 microm resin particles to be acquired. We report three significant aspects associated with utilizing the C(60) projectile that show how this technology can be taken to a more advanced level, especially when compared to results obtained with more conventional atomic primary ions. First, the useful yield of molecular ions is generally observed to be enhanced by at least 3 orders of magnitude over those previously possible. Second, the energy dissipation process associated with the C(60) impact is most efficient at desorbing molecules on soft substrates such as polymer surfaces rather than harder substrates such as metals or semiconductors. Third, there is a greatly reduced tendency for insulating surfaces to build up excess charge, obviating the need for charge compensation. Using a small five-member peptide library as a model, we show that by utilizing the focusing properties of the C(60) beam, it is possible to assay the surface composition of 100-microm polymer beads at a rate of up to 10 particles/s. Moreover, even at the picomole level, there are enough sequence ions in the mass spectrum to determine a unique composition. The results illustrate the ability to quickly assay large libraries without the use of tags and suggest the strategy may be applicable to a range of high-throughput experiments.

“Fast excitation” cid in a quadrupole ion trap mass spectrometer

Collision-induced dissociation (CID) in a quadrupole ion trap mass spectrometer is usually performed by applying a small amplitude excitation voltage at the same secular frequency as the ion of interest. Here we disclose studies examining the use of large amplitude voltage excitations (applied for short periods of time) to cause fragmentation of the ions of interest. This process has been examined using leucine enkephalin as the model compound and the motion of the ions within the ion trap simulated using ITSIM. The resulting fragmentation information obtained is identical with that observed by conventional resonance excitation CID. “Fast excitation” CID deposits (as determined by the intensity ratio of the a 4/b 4 ion of leucine enkephalin) approximately the same amount of internal energy into an ion as conventional resonance excitation CID where the excitation signal is applied for much longer periods of time. The major difference between the two excitation techniques is the higher rate of excitation (gain in kinetic energy) between successive collisions with helium atoms with “fast excitation” CID as opposed to the conventional resonance excitation CID. With conventional resonance excitation CID ions fragment while the excitation voltage is still being applied whereas for “fast excitation” CID a higher proportion of the ions fragment in the ion cooling time following the excitation pulse. The fragmentation of the (M + 17H) 17+ of horse heart myoglobin is also shown to illustrate the application of “fast excitation” CID to proteins.

Charge state separation for protein applications using a quadrupole time-of-flight mass spectrometer.

A novel method for separating ions according to their charge state using a quadrupole time-of-flight mass spectrometer is presented. The benefits of charge state separation are particularly apparent in protein identification applications at low femtomole concentration levels, where in conventional TOF MS spectra peptide ions are often lost in a sea of chemical noise. When doubly and triply charged tryptic peptide ions need to be filtered from singly charged background ions, the latter are suppressed by two to three orders of magnitude, while from 10-50% of multiply charged ions remain. The suppression of chemical noise reduces the need for chromatography and can make this experimental approach the electrospray equivalent of conventional MALDI peptide maps. If unambiguous identification cannot be achieved, MS/MS experiments are performed on the precursor ions identified through charge separation, while the previously described Q2-trapping duty cycle enhancement is tuned for approximately 1.4 of the precursor m/z to enhance intensities of ions with m/z values above that of the precursor. The resulting product ion spectra contain few fragments of impurities and provide quick and unambiguous identification through database search. The multiple charge separation technique requires minimal tuning and may become a useful tool for analysis of complex mixtures.

Ambident reactivity and characterization of small ionized carbenes

The gas phase reactions of five ionized carbenes, HCOH + 1, HCNH 2 + 2, CH 3COH + 3, HOCOH + 4 and HOCNH 2 + 5 with different molecules are studied by FT-ICR mass spectrometry. Interaction between an ionized carbene and a molecule can yield two kinds of stable adducts, as expected from the electronic structure of the carbene radical cations, explaining the ambident reactivity of these ions. The first kind of adduct corresponds to H-bonded species (hydrogen-bridged radical cations), the second to covalent structures. Since interconversion between these adducts is generally slow, each kind of adduct leads to a particular set of reactions. The H-bonded species can be involved in the protonation of the neutral as well as in the catalyzed interconversion between the carbene and its conventional radical cation counterpart. The covalent adducts, formed by reaction of ionized carbenes with methanal and alkenes, are β-distonic ions. Reactions with labeled propene show that the so formed distonic ions either dissociate by simple cleavage or undergo rearrangements and H-exchange after isomerization into conventional ions by 1,4-H transfers. Cyclopropane gives a characteristic reaction of the carbene structure: addition yields a γ-distonic ion which loses ethylene. Finally, H , I and SCH 3 abstraction from appropriate neutrals confirms a radical reactivity of the carbenic carbon.

In-situ determination of dissolved iron production in recent marine sediments

Fe-DGT (Diffusive Gradient in Thin-films), as a measure of in-situ net dissolved Fe production, together with conventional pore water ions, were determined in the upper 40 cm of organic rich sediments from Loch Duich, Scotland. There was an increase in net Fe production towards the base of the apparent Fe-R zone, which was interpreted as either being due to rapidly decreasing dissolved Fe removal and/or to an increase in sulphate reducing microniches towards the base of the Fe-R zone. A sharp Fe-DGT minimum, due to non-steady state Fe removal processes, was found at the boundary between the Fe-R and S-R zones. The Fe-DGT profile showed clear evidence of decreasing dissolved Fe production in the upper layers of the S-R zone. A comparison of the calculated net rate of iron pyritisation (0.23 moles Fe m–2 yr–1) with the net production rate (2.0 moles Fe m–2 yr–1), as measured by Fe-DGT, showed that there was significant recycling of iron before final burial in the sediment as pyrite. The Fe-DGT profiles provide in-situ data which is consistent with previous interpretations of iron production and recycling in marine pore waters obtained by less direct means.

Dehalogenation of protonated C-halogeno-1,2,4-triazoles: synthesis of new heterocyclic carbenic and ylid radical cations and contrasting behaviour of collision gases

C(3,5)-halogeno-1,2,4-triazoles, protonated under chemical ionization conditions, are found to undergo easy dehalogenation upon 8 keV collisional activation conditions, provided the collision gas is oxygen, not helium. The ions produced under these reactions are demonstrated to be five-membered cyclic carbenic ions or ylid ions, isomers of more conventional molecular ions of 1,2,4-triazoles. The same unconventional radical cations can also be produced in the low kinetic energy regime (∼20–30 eV) if the halogen is bromine, not chlorine. These conclusions were derived from tandem mass spectrometric measurements (collisional activation, neutralization–reionization, and specific ion molecule reactions) performed on a hybrid tandem mass spectrometer of sector–quadrupole–sector configuration. Quantum chemical calculations using the density functional theory (DFT) at the B3LYP/6-31G(d,p) + ZPE level were also carried out on the protonated C-halogenated-1,2,4-triazoles in both singlet and triplet states and their fragmentation products (halogen = Cl and Br). Calculated results suggest that the dehalogenation, occurring when oxygen gas was employed, is likely to arise from an excitation of protonated species into their lowest-lying triplet state prior to dissociation. Ionization energies and proton affinities of triazoles were also evaluated.

Effects of Neutral Bases on the Isomerization of Conventional Radical Cations CH3X•+ to Their Distonic Isomers •CH2X+H (X = F, OH, NH2): Proton-Transport Catalysis and Other Mechanisms

High level ab initio theory is used to investigate the effects of the neutral bases HF, H2O, and NH3 on the mechanisms and barriers for isomerization of the conventional radical cations CH3X•+ (X = F, OH, and NH2) to their corresponding distonic isomers •CH2X+H. It is found that the isomerization mechanism is determined largely by the relative proton affinities of the base and the parent radical •CH2X. If the proton affinity of the base is substantially lower than the proton affinity at either C or X of •CH2X, the barrier is lowered but remains positive relative to separated base plus CH3X•+. If the proton affinity of the base lies between that at C and X, the barrier becomes negative and the base successfully catalyzes the isomerization of CH3X•+ to •CH2X+H. In fact, the barrier is found to be negative even in cases where the proton affinity of the base is lower than the lower proton affinity site of •CH2X, provided that this proton affinity difference is not too large. If the proton affinity of the base is higher than that at both C and X, the barrier to rearrangement is lowered even further. However, intermolecular proton transfer from the ion to the base rather than intramolecular proton migration is then the lower energy process. An alternative isomerization mechanism for the [CH3OH/base]•+ systems is also detailed in which the base remains bound to the hydroxyl hydrogen throughout. The barriers for this so-called “spectator” mechanism are found to be higher than those for the interconversion of the isolated conventional and distonic ions. A rationalization based on the nature of the intervening ion−base complexes is presented.

Glancing-angle ion bombardment for modification and monitoring of semiconductor surfaces

Using glancing-angle ion bombardment for surface modification rather than conventional near-normal incidence ions has the advantages of reducing damage and implantation projected ranges, reducing channeling, reducing sputtering, and preferentially removing surface asperities leading to flat surfaces. The effects of bombardment conditions on the surface morphology and perfection of GaAs (001), InP (001), and Si (001) surfaces are reported. Air-exposed surfaces were cleaned and smoothened to near atomic flatness without damage under optimal conditions. Sputtering yield, measured using film thicknesses and changes in reflection high-energy electron diffraction oscillations, decreased with decreasing incidence angle. The low sputtering yield and minimal damage make a glancing-angle geometry ideal for real-time characterization by ion scattering spectroscopy. Surface composition measurements on single monolayers of InAs on GaAs showed that the glancing-angle Ar beam did not measurably change the In coverage over relatively long times. A new ion beam monitoring technique was also developed that utilizes the advantages of glancing-angle ions. Specular scattering of 3 keV He ions was observed for incidence angles of 2–6° from GaAs (001). Oscillation in the specularly scattered ion current during GaAs growth were observed with periods corresponding to monolayer growth times. The oscillations allow a simple quantitative interpretation based on scattering by adatoms and step edges.

Multiple stage pentaquadrupole mass spectrometry for generation and characterization of gas-phase ionic species. The case of the PyC 2H 5+· isomers

Eleven isomers with the PyC2H 5 (+·) composition, which include three conventional (1-3) and eight distonic radical cations (4-11), have been generated and in most cases successfully characterized in the gas phase via tandem-in-space multiple-stage pentaquadrupole MS(2) and MS(3) experiments. The three conventional radical cations, that is, the ionized ethylpyridines C2H5-C5H4N(+·) (1-3), were generated via direct 70-eV electron ionization of the neutrals, whereas sequences of chemical ionization and collision-induced dissociation (CID) or mass-selected ion-molecule reactions were used to generate the distonic ions H2C(·)-C5H4N(+)-CH3 (4-6), CH3-C5H4N(+)-CH 2 (·) (7-9), C5H5N(+)-CH2CH 2 (·) (10), and C5H5N(+)-CH(·)-CH3 (11). Unique features of the low-energy (15-eV) CID and ion-molecule reaction chemistry with the diradical oxygen molecule of the isomers were used for their structural characterization. All the ion-molecule reaction products of a mass-selected ion, each associated with its corresponding CID fragments, were collected in a single three-dimensional mass spectrum. Ab initio calculations at the ROMP2/6-31G(d, p)//6-31G(d, p)+ZPE level of theory were performed to estimate the energetics involved in interconversions within the PyC2H5 (+·) system, which provided theoretical support for facile 4⇌7 interconversion evidenced in both CID and ion-molecule reaction experiments. The ab initio spin densities for the a-distonic ions 4-9 and 11 were found to be largely on the methylene or methyne formal radical sites, which thus ruled out substantial odd-spin derealization throughout the neighboring pyridine ring. However, only 8 and 9 (and 10) react extensively with oxygen by radical coupling, hence high spin densities on the radical site of the distonic ions do not necessarily lead to radical coupling reaction with oxygen. The very typical "spatially separated" ab initio charge and spin densities of 4-11 were used to classify them as distonic ions, whereas 1-3 show, as expected, "localized" electronic structures characteristic of conventional radical ions.

Ions and the landau critical velocity in He II

Investigations of the motion of conventional negative ions (electron bubbles) in He II under pressures above 11 bar have provided the only means of measuring the Landau critical velocity for roton creation,vL, and for studying supercritical dissipation at higher velocities. Earlier work on roton creation is reviewed and it is pointed out that there is still no generally agreed explanation of the fact that the rotons seem to be emitted from the moving ionin pairs; nor is it known why the matrix element characterising the pair emission process should decrease rapidly with pressure. The possibility of studying these phenomena through use either of the “fast ion” (whose nature remains unknown), or of selected ions from the large variety of species that can be injected into He II by the recently developed technique of laser ablation, is discussed.