Sputter desorption

The exact process by which sputtering/desorption (steps 3 and 4 as mentioned earlier) occurs is, however, still an area under study with several substrate-specific models having been presented. These models, which can also be kinetically assisted, are as follows ... 

An introduction to the principles behind SPI-SALI, this ankle presents a theoretical discussion of why SPI-SALI is much less fragmenting than MPI-SALI. Examples are shown which describe the additional fragmentation induced by the desorption beam—in this case ESD is compared to ion sputtering. The main focus of the article is the advantages of SPI-SALI for surface analysis of bulk organic polymers. 

By bombarding a surface consisting of species A with primary ions, the surface coverage of A is reduced. Particles of A can he removed hy desorption, hy driving them into a deeper layer or, for molecular species, hy fragmentation. The ratio of the number of sputtered particles to the number of primary ions is given by the disappearance yield Y (A) ... 

Some particles sputtered from the surface are neutral whereas others are charged. Molecular particles can be emitted either as intact molecules or fragmented. The probability of the desorption of A into the emission channel X is given by the transformation probability P (A -> X ) ... 

On a third sample, thermal desorption of carbon monoxide was carried out before and after annealing to 760 K and after sputtering into the region where the Ti" " was a minimum (l.e., about 180 seconds in Fig. 3). The TDS results are summarized in Table I. The last column gives the Integrated peak desorption area for carbon monoxide. 

The SIMS system is mounted on a UHV spectrometer which also has XPS, UPS, LEED and thermal desorption capabilities ( ). Heating is achieved by electron bombardment from a filament mounted on the manipulator behind the sample. Cooling is achieved by circulating liquid N2 or He. Temperatures of 25K can be reached. The samples used, Ni(lOO), Cu(17%) Ni(83%) (100) and (111) and Ag(lll) were oriented within 1 and cleaned in situ by standard heating and Ar ion sputtering procedures. 

As evident from Scheme 7.13, most modern ionisation techniques have been used for TLC-MS, and no single ionisation method is used exclusively with TLC-MS. Various ionisation methods may be applied that avoid the need to evaporate the sample into an El or Cl source these are based in particular on sputtering (FAB, SIMS) or laser desorption. Several sputtering methods of ionisation do not require the use of a liquid matrix, e.g. TLC-SIMS [797], Recent developments include the use of matrix-assisted laser desorption ionisation (MALDI) and surface-assisted laser desorption ionisation (SALDI). It is obvious that TLC-MS is complemented with TLC-MS11 [800] and TLC-HRMS techniques. Table 7.82 lists the general characteristics of TLC-MS. 

Unfortunately, some of the analyzed molecules, as most biologically related molecules (e.g., amino acids), are solids with extremely low vapor pressures at room temperature and rapidly decompose when they are heated. For these molecules, which cannot be thermally vaporized, laser ablation or desorption have been alternatively used to produce neutral species in the gas phase.Both methodologies refer to laser-induced particle removal (laser sputtering) from a surface under the two extremes of massive and negligible rates of surface erosion, respectively. 

Sample treatment Distance calibration Field desorption, annealing, sputtering Annealing, sputtering, cleaving a single crystal... 

Thus as a starting point for understanding the bombardment process we have developed a classical dynamics procedure to model the motion of atomic nuclei. The predictions of the classical model for the observables can be compared to the data from sputtering, spectrometry (SIMS), fast atom bombardment mass spectrometry (FABMS), and plasma desorption mass spectrometry (PDMS) experiments. In the circumstances where there is favorable agreement between the results from the classical model and experimental data It can be concluded that collision cascades are Important. The classical model then can be used to look at the microscopic processes which are not accessible from experiments In order to give us further insight into the ejection mechanisms. 

Although the FABMS (Fast Atom Bombardment Mass Spectrometry) technique has only been in use for a few short years (since 1981), it traces its roots back well over a century (1). It has been observed that the bombardment of a surface by energetic ions produces the desorption of atoms and molecules from the surface of the target. This process, known as Sputtering, produces a yield (number of atoms sputtered per incident ion) which generally increases with the energy, the mass and the incidence angle of bombardment (2). 

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