Laser lateral resolution

There also will be improvements in instrumentadon and software to decrease data acquisidon time. Changes can be made to improve lateral spatial resolution. For example, if the probe monochromator is replaced by a tunable dye laser spadal resolutions down to about 10 pm can be achieved. 

The application of interference techniques overcomes the limitations exerted by the large optical wavelengths. With commercial phase-measurement interference microscopes (PMIM), a surface resolution of the order of 0.6 nm can be achieved [33, 34]. In a microscope a laser beam is both reflected from the sample surface and from a semitransparent smooth reference surface (Fig. 3). The interference pattern is recorded on an area detector and modulated via the piezo-electric driven reference surface. The modulated interference pattern is fed into a computer to generate a two-dimensional phase map which is converted into a height level contour map of the sample surface. While the lateral resolution (typically of the... 

SSMS can be classified among the milliprobe techniques (Figure 8.3), i.e. it is a unique link between microprobe techniques and macroanalytical methods that are characterised by poor lateral and in-depth resolutions (as in OES), or that have no lateral resolution whatsoever (as in NAA). Also, the achievable precision and accuracy are poor, because of the irreproducible behaviour of the r.f. spark. Whereas analysis of metals, semiconductors and minerals is relatively simple and the procedures have become standardised, the analysis of nonconducting materials is more complex and generally requires addition of a conducting powder (e.g. graphite) to the sample [359]. Detection limits are affected by the dilution, and trace contamination from the added components is possible. These problems can be overcome by the use of lasers [360]. Coupled with isotope dilution, a precision of 5% can be attained for SSMS. 

The laser source is a Nd YAG pulsed laser, operating at 1064 nm, which delivers about 10 mJ on the sample surface, in 8 ns. The spatial lateral resolution of the LIBS measurements corresponds to the dimensions of the micro-crater left by the laser on the sample surface. The same dimensions are also a measurement of the damage induced on the pigment. A computer-enhanced enlargement of a typical laser crater is shown in Figure 2 its diameter does not exceed 10 pm, which is practically invisible at naked eye. The reduced size of the crater also allows for a high spatial resolution of the LIBS analysis. 

Of all of the machines used for microanalysis LAMMA seems to be the most problematic. A laser beam is used to disintegrate a spot in the sample, and the material emitted is then analyzed in a mass spectrometer. It has similar lateral resolution to PIXE, and like SIMS can be used to distinguish between isotopes of the same element. It has, however, proved very difficult to quantify, and is destructive to the specimen. One recent investigation (13) ofthe distribution of stable isotopes of calcium, magnesium, and potassium in Norway spruce used three microprobes EDAX at 0.3 pm lateral resolution isotope specific point analysis, using LAMMA at 1.5 pm lateral resolution and isotope specific imaging using SIMS at 1-3 pm lateral resolution. 

In the past, for the analysis of thin sections of tissues or thin transparent foils a laser ionization ion source in the transmission or reflection mode of laser irradiation (using e.g., the LAMMA 500 or LAMMA 1000, respectively) was employed. The lateral resolution observed in the transmission mode was about 1 (im and in the reflection mode 5-20 gm. 

The application of a laser in mass spectrometers offers the possibility of a direct microlocal analysis with a lateral resolution down to the (tm range. The capability of near-field LA-ICP-MS (NF-LA-ICP-MS)65 created in the author s laboratory provides local information on inhomogeneous element distribution on solid sample surfaces with a spatial resolution in the sub-p,m range. 

Figure 1.15 shows the lateral and depth resolution achievable with the three mass spectrometric techniques described in this section. As can be seen, the depth resolution obtained with the GD techniques is similar to that with dynamic SIMS (with the additional advantage of less matrix effects in the GD sources). However, the lateral resolution obtained with SIMS is much better because the primary ion beam in SIMS is highly focused whereas in a GD the limitations in the source design make it necessary to sputter a sample area with a diameter of 14 mm. On the other hand, the depth resolution obtained with techniques based on lasers is not yet as good as with SIMS or GDs. 

Several other microanalytical methods in common use potentially have application on soil and sediments section samples. Laser-ablation inductively coupled plasma mass spectrometery (LA-ICP-MS) has been used on soil thin-sections from a controlled field experiment (21) but required special resins in the preparation. There is presently (May 2006) no reported use of this method on archaeological soil samples. Likewise, for extremely fine-resolution studies (i.e. <10 pm) with low minimum detection limits and despite difficult calibration, secondary ion microscopy (SIMS) has a potential role in examining archaeological soil thin sections. At even higher lateral resolutions ( 100 nm) Auger electron spectroscopy (AES) could also be considered for surface (<5 nm deep) analyses. At present however, the use of these methods in soil systems is limited. SIMS has been focused on biochemical applications (22), whereas AES... 

Laser microprobe mass analyzers permit mass spectrometric analysis of very small volumes (0.01-1 pm3) of thin Sections. The method is based on laser induced ion production from a microvolume and analysis of the evaporated ions in a time-of-flight mass-spectrometer. The technique allows detection of all elements and isotopes with a sensitivity approaching the ppm range and an extremely low limit of detection 10 15 to 10-20 g. Transmission type instruments such as the LAMMA 500 are designed for the analysis of particles of 3 pm in diam. The lateral resolution is about 0.5-1 pm. Because the area to be analyzed is selected by an optical microscope, distribution of chemical constituents can be precisely correlated with morphologic structures (Hillenkamp et al., 1982 39), Simons, 198440), Kaufmann, 1984)41 >. 

Thin Sections or pieces of the rock about 1 mm in size or less can be also analyzed with this instrument at glancing incident irradiation, if the site to be analyzed is close to the surface of the section or the edge of a fraction. However in this mode of operation, mass resolution is severely reduced. Nevertheless ions can be identified because of their low mass numbers and the very low background in the spectra, by known cluster patterns or by comparison with reference spectra obtained with reduced laser irradiation. The spectra have shown to be highly reproducible. For more precise bulk. sample analysis, other instruments such as the LAMMA 1000 are more suited. Their lateral resolution is 1-3 pm in diameter and the depth of analysis is typically 0.1 pm. 

The scanning laser spot technique allows the lateral resolution of the photoelec-trochemical properties of the semiconductor and permits the visualization of the defects (Furtak and Parkinson, 1980). Figure 10.30 shows the results of sweeping and laser spot over a step in InSe. A step is a recombination center and the recombination of holes and electrons increases. 

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