Tables 1 also show that radiation attenuation parameters R, n, k, a, ax and xa all decreased with wavelength between 0.3 to 0.8 gm and then increased with wavelength beyond these wavelengths for these films. The parameters were found to increase with increasing thickness (Tables 2). In general, these parameters were high in the UV and NIR region but low in the visible region. This seems to confirm the fact that these films are opaque to UV and NIR radiation, but are transparent in the visible.

Again, for Snl2 film of thickness 11.7 x 10-9m, the absorption coefficients are around 6.2 to 3.2 x 106m-1 in the UV and from 1.1 to 3.3 x106m-1 in the NIR. The values of absorption coefficient for MnBr2 and FeCl2 are similar. Absorption peaks were found to occur around 325nm for Snl2, 323 for mnBr2 and 420 forFeCl2. the values of the absorption coefficient in the visible are as low as 0.6 to 2.2 x 106m-1 for Snl2 film, and 0.7 to 2.8 x 106m-1 for MnBr2 films. For FeCl2 film, it is from 0.12 to 5 .3 x 106 m-1. The optical constants n and k increased markedly with wavelength, especially around the absorption peaks. Fig. 4 shows a sample of variation of n and k with wavelength for these films. The figure shows that both n and k decreased exponentially with wavelength in the visible region. Hence these films behave like

transparent insulators [63]. The shape of the transmittance/absorbance curves (fig.2) and the values of the transmission parameters and attenuation parameters show that halides of manganese exhibit properties that would make them better candidates for visible transmitting films. These films have broader-band high UV absorption. This means that all UV radiation will be absorbed by such films. They also have high visible transmission which means that they will allow high level of visible transmission. Their NIR absorption is also high enough to shut off high temperature radiation.

The UV and NIR absorption of Tin halide films (fig.1) are not high enough to cause complete extinction of these radiations. Although these seem to have improved visible transmittance, their UV and NIR absorption would allow substantial high energy and thermal radiation to be admitted into buildings interiors. This may cause unnecessary overheating which will be unwanted in cases where very low in door temperature is the emphasis. When moderate temperature is the demand, Sn halide films will be deal. On the other hand, Fe halide have complete near UV absorption over a very narrow band. Also, their visible transmittance starts later in the spectral region well into the visible (i. e. from about 550 nm as shown in fig.3). Also, the visible transmittances are not as high as those of Sn or Mn halides. Thus, these films might allow some unwanted radiation into the building from the NIR and also suffer from loss of visible radiation. Hence ‘day light’ level might be low for these films. They might therefore be useful in situation where day lighting is not of great importance and where moderate temperature is the emphasis.

Table 3 shows that maximum visible transmission decrease with thickness only for films of the same type. No such variation exists between thickness and transmission for all the films viewed together (Table 4). These result shows that both the materials concerned and the thickness of film play important roles in determining the optical properties of films. It was also observed that thickness affect both the wavelength of the onset of absorption and the wavelength of absorption threshold. Hence thicker films have lower onset of absorption but higher fundamental or threshold wavelength. Thinner films have higher onset of absorption, but lower threshold wave length as shown in Fig. 1-3 and in the table 5. Thus the wavelength of onset of absorption decreases and shifts well into the UV region with increasing thickness. These relationships between thickness and onset of absorption and fundamental wavelength exist only for films of the same materials and not for all the materials viewed together. It was also observed that Fe halide films and even Mn halide films exhibit colour changes with respect to transmitted and reflected lights. These films might therefore be also useful as phototropic materials [3, 62-64].

3. Conclusion

Analysis of the optical parameters of Sn, Mn and Fe halides show that these films are transparent in the visible but opaque in the UV and NIR regions. Such films can be used as optical shutters to UV and NIR radiation but as transmitters to visible radiation. These films are therefore designated in this work as visible transmitting films VTF. They could be used as selective coatings for building windows. Such windows would be capable of shutting off high energy (UV) and high thermal (NIR) radiation but admit only visible radiation for “daylighting”. Window with these properties could be used in warm climate where emphasis is on warding off UV and IR radiation and admitting only visible radiation into buildings. This will create a comfortable cool and conducive indoor temperature environment. Thus a kind of natural air-conditioning can be achieved by using these VTF coated windows. Fe and Mn

halide films which also exhibit colour changes with respect to transmitted and reflected lights could be useful as phototropic materials.

Table 1. Variation Of Optical Properties With Wavelength Of Radiation And Photon Energy For Tf (Sni2) Films Photon Wavelength and Energy Radiation Transmission Parameters Radiation Extinction Optical Parameters (nm) Hv (eV) T (%) T x106 (m-1) A(m-1) a x 106 (m-1) R x 106 (m-1) K x 10-1 (m-1) n x 10-1 (m-1) ax x 10-3 Ta x 1015 (m-2) 325 3.800 29.0 2.9 0.62 6.2 8.16 1.65 0.97 9 .84 1.72 330 3.761 27.0 2.7 0.58 5.8 7.24 1.52 0.92 9 .66 1.67 350 3.546 35.0 3.5 0.46 4.6 6.45 1.28 0.77 7 .66 1.66 400 3.103 47.0 4.7 0.35 3.5 5.27 1.11 0.67 5 .83 1.65 450 2.758 56.0 5.6 0.22 2.2 4.28 0.97 0.46 3 .58 1.21 500 2.483 62.5 6.3 0.21 2.1 3.68 0.84 0.50 3 .50 1.32 550 2.257 66.5 6.0 0.17 1.7 3.28 0.74 0.45 2 .83 1.14 600 2.069 70.0 7.0 0.15 1.5 2.99 0.72 0.43 2 .50 1.05 650 1.909 76.0 7.6 0.12 1.2 2.40 0.62 0.37 2 .00 0.91 700 1.773 80.0 8.0 0.10 1.0 2.00 0.56 0.34 1 67 0.80 750 1.655 84.0 8.4 0.08 0.8 1.59 0.48 0.29 1 .33 0.67 770 1.600 82.0 8.5 0.07 0.7 1.50 0.47 0.28 1 .26 0.65 800 1.552 86.0 8.6 0.07 0.7 1.40 0.45 0.27 1 17 0.60 820 1.510 88.0 8.8 0.06 0.6 1.19 0.39 0.24 1 .00 0.53 850 1.460 78.0 7.8 0.11 1.1 2.20 0.74 0.45 1 .83 0.86 900 1.356 65.0 6.5 0.18 1.8 3.48 1.29 0.74 3 .00 1.17 950 1.300 62.5 6.3 0.31 3.1 3.68 1.59 0.95 3 .50 1.32 1000 1.242 47.0 4.7 0.33 3.3 5.27 2.63 1.58 5 .49 1.55

Table 2. Variation of optical properties with thickness for films at 600nm Film Thickne ss Maximum Radiation Transmission Parameters at 829 nm Radiation Extinction Optical Parameters (t x10- T (%) T x A a x 106 R x 106 K x 10-1 n x 107 ax x Ta x 9m) 108 (m-1) (m-1) (m-1) (m-1) (m-1) (m-1) 10-3 1015 (m-2) 3.20 99.0 9.9 0.005 0.05 0.10 0.024 0.08 0.016 0.05 4.23 98.0 9.8 0.010 0.10 0.20 0.48 0.11 0.42 0.10 5.15 96.0 9.6 0.02 0.2 0.40 0.10 0.19 0.10 0.19 7.61 95.0 9.5 0.025 0.25 0.50 0.12 0.16 0.19 0.24 8.68 94.0 9.4 0.03 0.3 0.59 0.14 0.17 0.26 0.28 10.22 90.0 9.0 0.05 0.5 1.00 0.24 0.23 0.51 0.45 11.67 88.0 8.8 0.06 0.6 1.29 0.28 0.25 0.70 0.53 14.20 86.0 8.8 0.10 1.0 1.99 0.48 0.34 1.42 0.88 15.40 76.0 7.6 0.12 1.2 2.38 0.57 0.37 1.86 0.91 16.65 70.0 7.0 0.15 1.5 2.99 0.72 0.43 2.50 0.05

Table 3: Variation of maximum visible transmission with thickness for a film of the same type FILM THICKNESS (X 10-9M) MAXIMUM VISIBLE TRANSMISSION (%) Snl2-10 15.46 76 -19 14.20 86 -5 11.67 88 -14 10.22 90 -3 8.68 94

Table 4: Variation of maximum visible transmission. Thickness for all the species considered together. Film Thickness (x 10-9m) Maximum visible Transmission (%) FeCl2 5.90 76 MnBr2 6.13 70 FeBr2 7.50 74 Snl2 11.67 88 Fel2 13.51 47 Mnl2 16.51 56

Table 5: Variation of thickness with wavelength of onset of absorption and absorption threshold (fundamental wavelenght for, SnI2, MnBr2 and FeBr2 films. Film Thickness (x 10-9m) Wavelength of Onset of Absorption(nm)(X0) Fundamental Wavelength of Maximum Absorption(nm).(X0) Snl2-3 8.68 310 322 -14 10.22 309 324 -5 11.76 307 326 -10 16.65 306 327 MnBr2-6 6.13 311 325 -11 10.82 308 326 -16 13.35 307 326 -20 15.79 305 327 FeCl2-8 2.45 338 394 -4 7.50 336 392 -12 12.25 335 393 -7 17.38 333 395

Wavelength (A) Fig 1. Spectral Transmittance /Absorbance for Snl2 Films

g 20 s 4=1 40 60

MnBr2-20

MnBrr16

MnBr2-11

350 400 500 600 700 800 Wavelength (A). Fig 2. Spectral Transmittance /Absorbance for MnBr2 Films

Fig 3. Spectral Transmittance /Absorbance for FeCl 2 Films

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