Post-ionization, SNMS laser

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

SNMS sensitivity depends on the efficiency of the ionization process. SNs are post-ionized (to SN" ) either hy electron impact (El) with electrons from a hroad electron (e-)heam or a high-frequency (HF-) plasma (i.e. an e-gas), or, most efficiently, hy photons from a laser. In particular, the photoionization process enables adjustment of the fragmentation rate of sputtered molecules by varying the laser intensity, pulse width, and/or wavelength. 

Surface analysis by non-resonant (NR-) laser-SNMS [3.102-3.106] has been used to improve ionization efficiency while retaining the advantages of probing the neutral component. In NR-laser-SNMS, an intense laser beam is used to ionize, non-selec-tively, all atoms and molecules within the volume intersected by the laser beam (Eig. 3.40b). With sufficient laser power density it is possible to saturate the ionization process. Eor NR-laser-SNMS adequate power densities are typically achieved in a small volume only at the focus of the laser beam. This limits sensitivity and leads to problems with quantification, because of the differences between the effective ionization volumes of different elements. The non-resonant post-ionization technique provides rapid, multi-element, and molecular survey measurements with significantly improved ionization efficiency over SIMS, although it still suffers from isoba-ric interferences. 

SNMS ions neutrals ZZ sputtered neutrals (post ionized by e-beam or laser) > 3 nm 5 pm (+) > P gg-1 high depth resolution elemental information poor sensitivity... 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

The principles of ion sources which use a primary ion beam for sputtering of solid material on sample surface in a high vacuum ion source of a secondary ion mass spectrometer or a sputtered neutral mass spectrometer are shown in Figure 2.30a and Figure 2.30b, respectively. Whereas in SIMS the positive or negative secondary ions formed after primary ion bombardment are analyzed, in SNMS the secondary sputtered ions are suppressed by a repeUer voltage and the sputtered neutrals which are post-ionized either in an argon plasma ( plasma SNMS ), by electron impact ionization ( e-beam SNMS ) or laser post-ionization are nsed for the surface analysis (for details of the ionization mechanisms see references 122-124). 

An average sensitivity of up to 10 for all elements is achieved by using SNMS. Higher sensitivities are possible with the laser post-ionization schemes. 

Laser SNMS requires the operation with properly selected duty cycles that control the delay times between the primary ion pulse, a pulsed extraction voltage for separating the secondary ions from post-ionized neutrals, and the firing of the postionizing laser pulse. Such duty cycles have, in addition, to be synchronized with the stepwise motion of the pulsed primary ion beam across the sample surface in the microprobe mode of laser SNMS. The selection of appropriate duration and decay times of the ion and laser pulses, of the laser intensity, and beam shape is important to make the photoion yields independent on the sputtered particle velocities. The detection volume must be matched to the entrance ion optics of the TOP such that it becomes independent of the individual ionization process. Usually, laser intensities in the range from 10 to lO Wcm are applied. While the particle density in the detection volume is monitored at small laser intensities, the particle flux is measured at high photon densities. 

A high percentage of the sputtered secondary particles are neutral and must be post-ionized for mass spectroscopy analysis (SNMS) (62). Post-ionization can be achieved by electron impact in a plasma or by an electron beam. Alternatively, resonant and nonresonant laser ionization can be applied. Applications of SNMS for catalyst characterization have still not been reported. 

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