Laser reflectance

Bar code scanners, for example, exploit the directionality of a laser beam. The laser light reflects more strongly from white stripes than from black. Because of the directionality of the laser beam, the reflected laser beam faithfully mirrors the pattern of the bar code. A sensor reads these variations and converts the light pattern into an eiectronic representation of the bar code. The sensor transmits the electronic representation to a computer. 

Alvaro, M., Atienzar, P., Bourdelande, J.L., and Garda, H. (2004) An organically modified single wall carbon nanotube containing a pyrene chromophore Fluorescence and diffuse reflectance laser flash photolysis study. Chem. Phys. Lett. 384, 119-123. 

McCaffery etal. discuss the use of a low-resolution Raman spectrometer to directly monitor a batch mini-emulsion polymerization.41 While this kind of equipment is unlikely to be installed in an industrial facility, the article raises several important points. In order to compensate for laser-power fluctuations, a functional group present in both the reactants and the product, the phenyl ring in styrene, was used as an internal standard. Since internal standards cannot be added to industrial reactions, this approach can be quite helpful. However, scientists must be certain that the internal standard will remain unchanged by the reaction and that changes in its signal only reflect laser-power fluctuations. 

Neri, L. M., Cinti, C., Santi, S., Marchisio, M., Capatini, S., and Maraldi, N. M. (1997) Enhanced resolution of specific chromosome and nuclear regions by reflectance laser scanning confocal microscopy. Histochem. Cell. Biol. 107, 97-104. 

Diffuse-Reflectance Laser-Hash Photolysis, in Photochemistry in Condensed Phases Kelly, G.P., Wilkinson, F. Scaiano, J.C. Ed. CRC Press, (1989). 

Diffuse Reflectance Laser Flash Photolysis of Thermomechanical Pulp... 

The observable parameter in diffuse reflectance laser flash photolysis experiments is the fractional reflectance change AJ(t)/J0 defined as in Equation 1. 

Figure 1. Schematic Diagram of the Apparatus and Optical Arrangement used for Diffuse Reflectance Laser Flash Photolysis Experiments.

Independent diffuse reflectance laser flash measurements have been performed to characterize formation of the cation radical and the chemical events leading to permanent changes. 

Gopidas, K. R. Kamat, P. V. George, M. V. Photochemical processes on oxide surfaces. A diffuse reflectance laser flash photolysis study, Mol. Cryst. Liq. Cryst. 1990, 183, 403. 

Figure 2 Studying membrane fusion with supported bilayers. A supported bilayer is suspended from a quartz substrate (top, gray background) and illuminated by the evanescent wave of a totally internally reflected laser beam (angled cylinders red). A membrane vesicle is observed to approach, hemifuse, and then fully fuse with the supported membrane. Vesicle contents, lipids, or proteins may be labeled fluorescently to monitor this process.

After aligning the laser, the mirror position is turned such that the reflected laser light is directed onto the photodiode (Fig. 2.10a). A maximized sum signal shows a good position (Fig. 2.9). Subsequently, the spot is centered on the photodiode by moving the diode in the vertical and lateral direction such that the corresponding difference signals (see Fig. 2.9) are 0. 

Standard CM-AFM set-up A CM cantilever with a spring constant ofca. 0.3 N/m, or preferably smaller, is inserted into the cantilever holder, the laser is aligned, and the photodiode is moved such that the laser spot is centered. The alignment of the laser must be carried out carefully in order to ensure that the spot is located very close to the end of the cantilever. This can be verified by studying the shape of the reflected laser beam using a piece of paper (Chap. 2). 

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