Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

58 EXPERIMENTAL METHODS FOR SLOW REACTIONSmethod of filtering out unwanted radiation. These regions are given in Table 7.Conversely, these decide what material may be used for the RV and lenses for aparticular experiment. Commercial glass filters are also available.Various chemical filters and their useful regions and stability are shown in Table8. Fuller details are given in refs. 9 and 22c and also by Nicholas and P~llak'~'.Some chemical filters can be unsatisfactory since their properties are time-dependentsimply due to decomposition. An alternative to these are interference filters.Two partially reflecting surfaces produced by vacuum deposition of metallic filmson glass plates are separated by a thin transparent spacer with a thickness of theorder of the wavelength of the radiation, such that multiple reflection occurs betweenthe two mirrors, which are arranged so that only the required radiation istransmitted. The unwanted radiation is reflected away. These have very stablefiltering properties. Filters are commercially available, for the UV, visible and IRwavelengths.(6) Variable monochromatic filter for the visible spectrumThe Christiansen filter gives radiation of different wavelengths in the visibleregion depending on the temperat~re'~~. It is based on the principle that a finelydivided solid suspended in a liquid will have a maximum transmission where therefractive indices of the liquid and solid coincide. The refractive indices vary at adifferent rate with temperature and hence the maximum transmission positionalso varies as a function of temperature. This is shown in Fig. 45 for powderedcrown glass and methyl ben~oate'~'. The filter itself is shown in Fig. 46. G, andG, are plate-glass plates, L, and L, are water-air lenses and F holds the mixtureof glass and ester. Other ranges may be obtainedg by using different materials for F.Fig. 45. Maximum transmission wavelength variation with temperature. From ref. 156.