Film, ultraviolet reflecting

The focusing of radiation within the instrument was formerly done by means of lenses, but these suffer from chromatic aberration and particularly in respect of the relationship between the visible and ultraviolet parts of the spectrum. Focusing is now usually carried out by means of suitably curved mirrors having a reflecting surface coated with aluminium which is protected by a silica film. 

Reflectance spectroscopy in the infrared and visible ultraviolet regions provides information on electronic states in the interphase. The external reflectance spectroscopy of the pure metal electrode at a variable potential (in the region of the minimal faradaic current) is also termed electroreflectance . Its importance at present is decreased by the fact that no satisfactory theory has so far been developed. The application of reflectance spectroscopy in the ultraviolet and visible regions is based on a study of the electronic spectra of adsorbed substances and oxide films on electrodes. 

The high rate of dimer formation in frozen solution reflects the necessity for a preferred orientation of the two thymine molecules concerned, but it is still considered that water plays an important part. Thymine in dried films,32,34 and on filter paper33 can be converted to dimer by ultraviolet light, but the maximum conversions are much lower, 1732-55%34 (depending upon the humidity34). 

In UV reflected photography, a light source emitting ultraviolet light is directed at the subject which then reflects this radiation into the camera. Visible radiation from the room or reflected from the subject will be absorbed by an ultraviolet transmission filter over the camera lens thereby preventing any visible light from reaching the film. 

In UV fluorescence photography, the fluorescence of a substance excited by UV illumination is captured. The source of ultraviolet radiation filtered with an ultraviolet transmission filter, or excitation filter, is aimed at the subject in a completely darkened room. The subject reflects the ultraviolet light, but can also emit a visible fluorescence. The ultraviolet light is then prevented from reaching the film by a barrier filter that only allows visible light to be transmitted to the film. 

The effect of ultraviolet irradiation in air on the wettability of thin films of amorphous polymers has been studied. With poly(vinyl chloride), poly(methyl methacrylate), poly(n-butyl methacrylate), poly (ethylene terephthalate), and polystyrene the changes in contact angles for various liquids with irradiation time are a function of the nature of the polymer. A detailed study of polystyrene by this technique and attenuated total reflectance spectra, both of which are sensitive to changes in the surface layers, indicates that the contact angle method is one of the most sensitive tools for the study of polymer photooxidation in its early stages. The method is useful in following specific processes and in indicating solvents to be used in the separation and isolation of photooxidation products. 

An empirical relationship has been shown between the contact angles for wettability of a polymer film and the degree to which photooxidation products have accumulated in the surface layers of the film. Changes in wettability of polymer films during photooxidation are markedly dependent on the nature of the polymer. In the detection and identification of the earliest processes and products of surface photooxidation, the wettability method is far more sensitive than the infrared transmission or attenuated reflectance spectra and is about as sensitive but more specific than the ultraviolet transmission spectrum. Contact angle measurements themselves can be used as leads in the selection of solvents for the separation and identification of photooxidation products formed in the surface layers of a polymer film and are potentially useful in establishment of rates of specific processes. 

A surface mirror for the UV is an aluminium mirror which is deposited very fast in a very good vacuum to prevent detrimental oxidation, and is then coated immediately with a protective magnesium fluoride film [75-83] sometimes LiF is also used. This film combination gives excellent reflectance in the ultraviolet region of200 nm up to 400 nm, as can be seen in Fig.12. 

The spectral reflectance clearly shows the improvement resulting from the interference effect of the MgF2 protection film in the ultraviolet. Such mirrors also show... 

Heath and coworkers have recently reported the observation of a reversible, room-temperature metal-non-metal transition in organically-functionalized silver particle monolayers. A useful parameter for characterizing these monolayers is the quantity [d/D], where d is the interparticle separation, as measured between particle centers, and D is the particle diameter (c.f. Fig. 4). The ultraviolet-visible reflectance spectrum from a Langmuir layer of 40 A diameter silver particles, collected in-situ as the film is compressed, is shown in Fig. 13. Upon initial compression, the film becomes more reflective (see (1), (2), and (3) in Fig. 13). The final reflectance spectrum is similar to that reported for thin, metallic silver films ((4) and (5) in Fig. 13) and indicates conclusively that the Langmuir film has finally become metallic. 

Spectra. A Perkin-Elmer Hitachi model 200 spectrophotometer was ised to record all ultraviolet spectra. The infrared spectra of the films were obtained using a Nicolet FTIR-T199. The transmission spectra of films were obtained from samples moistened in tetrachloroethylene (2) and mounted between NaCl plates. Attennuated total reflection (ATR) spectra were taken by placing the exposed side of the film in contact with a germanium crystal at a angle of incidence. Fluorescence from film surfaces were measured using a Perkin-Elmer MPF-i iiB fluorescence spectrometer. The excitation beam (3 0 nm, slit, i+nm) was incident on the film at and the emission (iiOO-500 nm) was measured at 90 to the excitation beam. 

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