Xe bombardment

PL spectra of Si-implanted samples before and after Xe bombardment are shown in Fig. 2. In the PL spectrum of the as-implanted sample the broad band peaked at about 670 nm was observed. Under irradiation with Xe ions the intensity of the band varied with the dose non-monotonously (Fig. 2) maintaining the position of its maximum. Initially the irradiation causes the intensity to decrease, however further bombardment results in the growth of PL. Passivation quenched the 670 tun PL band, but simultaneously more intensive PL peak at -780 nm appeared (Fig. 2). The lowest SHI dose provided marked growth of the PL intensity However, further bombardment led to continuous diminishing of the peak (Fig. 3). 

General. Melting point (mp) was collected on Yanagimoto micro melting point apparatus. Optical rotation was measured with a JASCO DIP-140. UV and IR spectra were recorded on Shimadzu UV-160A spectrometer and a Perkin-Elmer 1760-X infiared fourier transform spectrometer, respectively. MS )ectra were obtained with JEOL JMX-HX 100 and JMA-DA 5000 spectrometers under Xe bombardment (6.0 keV). NMR spectra were measured with a JOEL EX-270 instrument in CD3OD with TMS as an internal standard (270 MHz for H and 67.5 MHz for "C). 

The partial positive ion FAB mass spectrum (obtained by Xe bombardment) is shown in Fig. 2 and incorporates an abundant protonated molecular ion at m/z 1040. Three clearly defined series of diagnostic fragment ions, corresponding to the general structures (1) (N-terminal), (2) (N-terminal) and (3) (C-terminal) identify the sequence as PhCO-Ala-Phe-Val-Ile-Asp-Asp-Glu-Gln (or /.yo-Gln). The negative ion FAB spectrum showed an abundant [M —H] ion at m/z 1038 and the series of ions corresponding to structure (4) was able to confirm the sequence of the six C-terminal amino acids. 

I.I. Electron Bombardment Plasma Sources. These gas-feed sources generally employ Ar or Xe at relatively low vapour pressures (10 3mbar). A heated cathode is a common electron source, and these are accelerated towards an anode to give them... 

Figure 4.4 Left sputter yields of copper under bombardment with Ne, Ar, Kr and Xe ions as a function of energy (data from [2]). Right, relative sputter yields of polycrystalline copper as a function of incident angle measured from the surface normal. The primary ion energy is 1.05 keV. The dashed line represents 1/cos (adapted from Oechsner [8]).

The terms surface ionization mass spectrometry (SIMS) and ion sputtering are often used when accelerated atoms such as Xe or ions such as Ar+ strike a surface causing ionization of the material on the surface. The surface can be solid or liquid in the form of a solution or a suspension in the solvent. In this section, the terms fast atom bombardment (FAB) and fast ion bombardment (FIB) will be used. 

The fast atom bombardment ionization (FAB) technique is a soft ionization method, typically requiring the use of a direct insertion probe for sample introduction in which a high energy beam of Xe atoms, Cs+ ions, or massive glycerol-NH4+ clusters sputter the sample and matrix from the probe surface (Figure 8). 

The secondary-electron yield is almost independent of energy (see Fig. 6), but depends strongly upon the ionization potential of the bombarding ion. Figure 6 shows that the yield for He is 0.24 while for Xe, it is less than 0.02. 

This bombardment results in the transfer of energy from the Xe atoms to the matrix, leading to the breaking of intermolecular bonds and the desorption of the analyte (usually as an ion) into the gas phase. Unlike EI, FAB can also be used to generate negatively charged ions. 

Mass spectrometry may also be used to sequence polypeptides of 25 residues or fewer. Here ionization is accomplished by fast atom bombardment (FAB) and mass analysis carried out by two coupled spectrometers (tandem mass spectrometers). In FAB, the macromolecule sample is dissolved in a viscous, nonvolatile solvent such as glycerol and directed into a mass spectrometer with a stream of neutral atoms such as Ar or Xe. FAB generates predominately the protonated molecular species of the macromolecule (M + H)"+. 

In the SIMS a primary noble gas atom or ion (e.g. Ar°, Ar+, Xe°, Xe+) beam is bombarded on the sample in ultra-high vacuum, penetrating to a depth of 30-100 A. The kinetic energy of the particle is assumed to dissipate via a collision cascade process, which causes the emission of electrons, neutral species and secondary ions, the yields of which vary with polymer surface composition and obviates the possibility of quantitative SIMS informa-... 

Ion Bombardment Conditions. A base pressure of 10 9 Torr is maintained in this chamber the noble gas pressure (He, Xe) rises to about 10"7 Torr during the ion bombardment. The ion beam is rastered on (1.5 x 1.5) mm2 areas at normal incidence 4He-2 keV ions are used for the ISS analysis when Xe-4 keV ions are used for SIMS. The incident ion current is measured with the aid of a moveable Faraday cup. Since the investigated samples are electrical insulators, charge neutralization is performed with low energy electrons ( 10 eV) emitted from a heated W filament. 

Fig. 1 shows HREM image of as-implanted Si02 layers after Xe ion bombardment. The pictures exhibit 3-4 nm-size dark spots. An increase in the irradiation dose resulted in the growth of number and size of the spots. In small part of them lattice fringes may be distinguished, indicating their crystal character. The interplanar spacing matched those of silicon. 

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