Fluorine desorption

In Ref. 13 the cross section value for electron induced fluorine desorption was calculated from the time dependence of the fluorine... 

Product desorption is a crucial step in the etch process. A free radical can react rapidly with a solid surface, but unless the product species has a reasonable vapor pressure so that desorption occurs, no etching takes place. For instance, when an aluminum surface is exposed to fluorine atoms, the atoms adsorb and react to form AIF3. However, the vapor pressure of AIF3 is 1 torr at 1240 C thus etching is precluded at ambient temperatures. 

An electrochemical cell [93,94] was used to obtain an efficient anodic deposition of no carrier added F-fluoride solubilized in the target water. The radioisotope is electrochemically adsorbed on the anode (glassy carbon electrode) and can be easily dried. An opposite electrical field releases the radionuclide directly into a solution of a phase transfer catalyst in dipolar aprotic solvents. The nucleophilic fluorination can be performed simultaneously if the electrochemically and thermally induced desorption of radioactivity is done in the presence of the precursor. However, the yields remain poor (3 % in the electrochemical n.c.a [ F]fluorination of anisole). 

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has contributed remarkably to unravelling the termination and initiation steps of the styrene/CO copolymerisation catalysed by the highly active bis-chelated complex [Pd(bipy)2](Pp5)2 in TFE [40]. Chain-end group analysis of the material produced in the absence of BQ showed that the termination by P-H elimination is accompanied by three different initiators two palladium alkyls from Pd-H formed by reaction of the precursor with CO and water (a and b) and a palladium carboalkoxy species formed by reaction of the precursor with the fluorinated alcohol and CO (c) (Chart 7.4). The suppression of the chain-transfer by alcoholysis was proposed to be responsible for the enhanced stability of the palladium acyl intermediates and hence for the high molecular weight of the copolymers produced. 

Of course this need not necessarily be the case, as in any mechanism where species are attached to an anode, changes in their electronic make-up, resulting from hydrogen replacement by fluorine, might be expected to cause variations in the adsorption forces with possible desorption into the electrolyte as a consequence. 

In the argument against the ECbECN concept, much, perhaps too much, store has been placed by critics on observations which seem to undermine the zipper mechanism in the apparent belief that this mechanism is axiomatic with the ECbECN as a whole. However, as has been shown, changing physico-chemical properties of progressively fluorinated products inevitably result in different adsorptive forces for these compounds, with possible desorption from the electrode as a consequence this situation could apply equally well with products formed via a carbocation mechanism as with radically generated compounds, even though, as originally stated, the implication is otherwise. 

Desorption and diffusion of unused fluorine and gaseous byproducts.6... 

An interesting result was the appearance of F+ in the residual gas analysis when the electron beam in AES was placed on the sample, suggesting easy desorption and a very unstable surface. In fact, when electron beam currents were not minimized, the fluorine frequently was desorbed completely, and did not appear in the Auger spectrum. 

When a cesium atom is adsorbed on a tungsten surface, level A is higher than level D (Fig. 6) and the desorption of the cesium is in ionic form, provided that no external electric fields are used that will force atoms to evaporate. The potential curves of Fig. 6 are completely comparable to the formation of the ionic molecule of CsF from the atoms of cesium and fluorine (Fig. 7). 

The fluorinated carbon-coated AAO film has an interesting adsorption characteristic that has not been reported so far. Figure 3.12 shows N2 adsorption/desorption isotherms at -196°C for the pristine carbon-coated AAO film and the films fluorinated at different temperatures [119]. The isotherm of the pristine film is characterized by the presence of a sharp rise and a hysteresis in a high relative pressure range. Such a steep increase can be ascribed to the capillary condensation of N2 gas into the nanochannels of the AAO films, that is, the inner space of the nanotubes embedded in the AAO films. The amount of N2 adsorbed by the condensation into the fluorinated channels is lower than that of the pristine one. Moreover, the amount drastically decreases with an increase in the severity of fluorination. Since TEM observation revealed that the inner structure of the fluorinated CNTs was not different from that of the pristine nanotubes, the reason why the N2 isotherm was so changed as in Figure 3.12 cannot be attributed to the alteration of the pore texture upon the... 

FIGURE 3.12 N2 adsorption/desorption isotherms at -196°C for the pristine carbon-coated AAO film and the films fluorinated at different temperatures (100°C and 200°C). 

Due to the multiple desorption products, the etching of the surface with halogen appears to be quite complex. A multi-step reaction mechanism has been suggested to account for the SiCl2 desorption species. In the case of fluorine atom adsorption, F atom abstraction and dissociative chemisorption mechanisms have been suggested. In order to account for the complex surface reactions, more studies are needed. 

Catalyst Fluorination. The adsorption and desorption record and calculated data for a silica-alumina catalyst of low alumina content and a median pore radius of 78 A. were obtained. This indicated a fairly wide distribution of pore sizes in this material. 

In neither case was it possible to propose definitive mechanisms due to the complexity of the systems in the 7-alumina study, it is suggested that adsorption-desorption processes are slow relative to rapid dismutation between two adsorbed species [105], while from the chromia study mono-molecular halogen exchange reactions with metal halide surface sites are indicated [38], The latter mechanism is reminiscent of the halogen exchange model proposed [95] for C2 CFCs on fluorinated chromia. 

Often soft ionization techniqnes such as fast-atom bombardment (FAB) and matrix-assisted laser desorption (MALDI) greatly increase the utility of mass spectrometry in the analysis of fluorinated compounds. Using a commercial matrix-assisted laser desorption time-of-flight mass spectrometer, molecular ions of a polyelectrolyte,... 

For the Ti02 system the work by Knotek, Jones and Rehn [86] showed that the desorption by ESD and also PSD of H, OH and F following water exposure (with residual fluorine) has major thresholds at the O 2s and F 2s core levels, consistent with the applicability of Auger decay model also to covalent che-... 

The objectives of the laboratory studies were (1) to study the behavior of volatile technetium compounds produced by direct fluorination of UO with elemental fluorine (2) to study the technetium sorption characteristics of MgF and (3) to determine the feasibility of repeated regeneration of MgF by desorption of technetium with fluorine. 

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