Emittance, surface

Carbon Dioxide The contribution to the emissivity of a gas containing CO9 depends on gas temperature Tc, on the CO9 partial pressure-beam length product p L and, to a much lesser extent, on the total pressure P. Constants for use in evaluating at a total pressure of 101.3 kPa (1 atm) are given in Table 5-8 (more on this later). The gas absorptivity Ot equals the emissivity when the absorbing gas and the emitter are at the same temperature. When the emitter surface temperature is Ti, Ot is (Tc/Ti)° times , evaluated using Table 5-8 at T instead of Tc and at p LTi/Tc instead of Line broadening, due to... 

Finally, we can also find in the literature arrangements where the working electrode is also the emitter part of the transducer, normally named as sonotrode [22] or sonoelectrode [41]. Some authors have used only the main emitter surface as electrode [42], see Fig. 4.2b, and other authors have used the fully surface tip as working electrode [43], see Fig. 4.2c. In theory, this arrangement assures that all the specific effects derived from the ultrasound field propagation are directly focused on the surface electrode. Not only the shorted-lived bubbles non-uniformly collapse on the electrode surface but also the electrode surface itself oscillates. This provides additional effects which have been specifically used in the nanoparticles preparation. 

Sometimes, FI mass spectra show signals due to reactions of the analyte with the emitter surface or between molecules adsorbed to that surface. In case of acetone for example, it was demonstrated that [M+H]" quasimolecular ions are produced mainly by a field-induced proton-transfer reaction in the physically adsorbed layer. [59] The mechanism of this field-induced reaction depends on the existence of tautomeric structures of the neutral molecule. Besides the [M+H] quasimolecular ions, [M-H] radicals are formed ... 

Furthermore, the radicals formed upon field-induced hydrogen abstraction can lead to polymerization products on the emitter surface. The mechanism of this field polymerization helped to elucidate the phenomenon of activation of field emitters, i.e., the growth of microneedles on the emitter surface under the conditions of field ionization of certain polar organic compounds. [59]... 

Multiply charged ions of minor abundance are frequently observed in FI and FD mass spectra. Their increased abundance as compared to El spectra can be rationalized by either of the following two-step processes i) Post-ionization of gaseous M ions can occur due to the probability for an M ion to suffer a second or even third ionization while drifting away from the emitter surface. [69,70] Especially ions generated in locations not in line-of-sight to the counter electrode pass numerous whiskers on their first 10-100 pm of flight ... 

The surface states observed by field-emission spectroscopy have a direct relation to the process in STM. As we have discussed in the Introduction, field emission is a tunneling phenomenon. The Bardeen theory of tunneling (1960) is also applicable (Penn and Plummer, 1974). Because the outgoing wave is a structureless plane wave, as a direct consequence of the Bardeen theory, the tunneling current is proportional to the density of states near the emitter surface. The observed enhancement factor on W(IOO), W(110), and Mo(IOO) over the free-electron Fermi-gas behavior implies that at those surfaces, near the Fermi level, the LDOS at the surface is dominated by surface states. In other words, most of the surface densities of states are from the surface states rather than from the bulk wavefunctions. This point is further verified by photoemission experiments and first-principles calculations of the electronic structure of these surfaces. 

There are basically two kinds of experiments which can be used for studying the mechanisms of the field ionization process near a field ion emitter surface and also the field ion image formation process. They are the measurement of field ion current as functions of tip voltage, tip temperature, and other experimental parameters, and the measurement of the ion energy distribution. 

Fig. 2.29 Recovery of field ion current after the tungsten emitter surface is pulse field evaporated to deplete all the field adsorbed image gas atoms at the surface. Zero time refers to the time when the surface is completely depleted of field...

In many field emission and field ionization experiments, field strength is a basic parameter which has to be known accurately before a lot of experimental data can be interpreted properly. Determination of field strength at the field emitter surface and field distribution above the field emitter surface in field electron and field ion emission, however, is not an easy task because of the complicated geometry of the tip. In field emission, the validity of the Fowler-Norheim theory has been established experimentally to within about 15%, and the current density as a function of the field has been tabulated.26 Thus it is possible to determine the field strength simply from the field emission current density. The field strength so determined cannot of course be more accurate than 15%. 

In field ion microscopy, one would also like to know how the field distributes itself above an emitter surface. This information is important in the quantitative interpretation of many field ion emission phenomena and experiments. It is also important in calculating the ion trajectory to enable a proper aiming in an atom-probe analysis. Unfortunately, not only does each tip have its own particular shape, but the presence of lattice steps also complicates the situation immensely. There are so far no reliable calculations for the field distribution above an emitter surface, nor for predictng the ion trajectory, nor yet for where the probe-hole... 

Imaging atom-probes, although severely limited in mass resolution, are very useful where one seeks information about the spatial distribution of chemical species on the emitter surface as well as in the bulk. They find many applications in studies of metallurgical problems,55 in studies of chemisorptions and surface reactions56 and oxidation of metals,57 etc., as will be discussed in later chapters. In using imaging atom-probes, it is important to select the system carefully and intelligently so that mass overlap of different elements can be avoided. 

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