Reflection detector techniques

Each of the methods described in Sect. 2.2.1—2.2.5 requires an accurate absolute determination of the exposure to derive sticking probabilities and this is a notoriously difficult parameter to determine quantitatively. The reflection detector techniques, which involve a direct measurement of the reflection coefficient for incident particles at a solid surface, avoid this difficulty and are therefore considerably more reliable for systems where the sticking coefficient is high ( 0.05). 

King and Wells [205] developed a molecular beam technique specifically for the determination of absolute sticking probabilities on single 

The technique has also been developed by Madey [206] in a simpler form, readily added to stainless steel systems, in which a non-collimated flux of gas is provided by an orifice or capillary array. Here, only a fraction, f, of the gas emerging from the orifice is incident on the crystal, f being determined by the orifice dimensions and the orifice-to-crystal distance. The sticking probability is obtained from an expression similar to eqn. (16), viz. 

A clear advantage of molecular beam techniques is the relative ease of independently controlling the beam temperature, the incidence angle and the crystal temperature, providing a means of studying adsorption dynamics. 

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Fig. 7. The experimental arrangement for the reflection detector technique for metal films as used by Horgan and King [76],...

Fig. 8. Molecular beam reflection detector technique as used by King and Wells [205].

Reflectance-Absorbance A reflection sampling technique used on thin films coated on shiny metal surfaces. The infrared beam passes through the film, reflects from the metal, then passes through the film a second time before reaching the detector. This technique is also known as double-transmission. ... 

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

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... 

Fig. 1—Profile measurement technique of Champper 2000+. A surface measurement is made with a linearly polarized laser beam that passes to translation stage which contains a penta-prism. The beam then passes through a Nomarski prism which shears the beam into two orthogonally polarized beam components. They recombine at the Nomarski prism. The polarization state of the recombined beam includes the phase information from the two reflected beams. The beam then passes to the nonpolarizing beam splitter which directs the beam to a polarizing beam splitter. This polarizing beam splitter splits the two reflected components to detectors A and B, respectively. The surface height difference at the two focal spots is directly related to the phase difference between the two reflected beams, and is proportional to the voltage difference between the two detectors. Each measurement point yields the local surface slope [7].

For materials which are available not in the form of substantial individual crystals but as powders, the technique pioneered by Debye and Scherrer is employed (Moore, 1972). The powder is placed into a thin-walled glass capillary or deposited as a thin film, and the sample is placed in the X-ray beam. Within the powder there are a very large number of small crystals of the substance under examination, and therefore all possible crystal orientations occur at random. Hence for each value of d some of the crystallites are correctly oriented to fulfil the Bragg condition. The reflections are recorded as lines by means of a film or detector from their positions, the d values are obtained (Mackay Mackay, 1972). 

The development of hydrodynamic techniques which allow the direct measurement of interfacial fluxes and interfacial concentrations is likely to be a key trend of future work in this area. Suitable detectors for local interfacial or near-interfacial measurements include spectroscopic probes, such as total internal reflection fluorometry [88-90], surface second-harmonic generation [91], probe beam deflection [92], and spatially resolved UV-visible absorption spectroscopy [93]. Additionally, building on the ideas in MEMED, submicrometer or nanometer scale electrodes may prove to be relatively noninvasive probes of interfacial concentrations in other hydrodynamic systems. The construction and application of electrodes of this size is now becoming more widespread and general [94-96]. 

FTIR instrumentation is mature. A typical routine mid-IR spectrometer has KBr optics, best resolution of around 1cm-1, and a room temperature DTGS detector. Noise levels below 0.1 % T peak-to-peak can be achieved in a few seconds. The sample compartment will accommodate a variety of sampling accessories such as those for ATR (attenuated total reflection) and diffuse reflection. At present, IR spectra can be obtained with fast and very fast FTIR interferometers with microscopes, in reflection and microreflection, in diffusion, at very low or very high temperatures, in dilute solutions, etc. Hyphenated IR techniques such as PyFTIR, TG-FTIR, GC-FTIR, HPLC-FTIR and SEC-FTIR (Chapter 7) can simplify many problems and streamline the selection process by doing multiple analyses with one sampling. Solvent absorbance limits flow-through IR spectroscopy cells so as to make them impractical for polymer analysis. Advanced FTIR... 

Electrically modulated infrared spectroscopy, ( EMIRS). In all three external reflectance approaches the signal processing technique serves two purposes (a) to remove the contributions to the reflected ray that do not change, e.g. the detector response, the source emission envelope, the solvent,... 

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