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Cesium Bromide films grown of glass substrates by thermal evaporation showed interesting optical properties. The UV-visible absorption spectra showed peaks which showed red shift with time. Structural and morphological studies suggested decrease in grain size with time which was unusual. Theoretical simulation shows the optical behaviour to be due to surface plasmon resonance resulting from Cesium clyindrical rods embedded in the films. Recent interest in AlkaliHalides is due to the appearance of Surface Plasmon Resonance (SPR) peaks in the UV-visible region of its absorption spectrum, when noble and other metal nano-particle are embedded in it [11]. However, we have noticed SPR peaks developing in cesium iodide [12] and cesium bromide (present work) thin films even with aging. We believe this to be a direct proof of the idea put forward by Emel'yanov and others [13,14]. They claimed that the defect (such as F-centers) in alkali halides when provided the right conditions 2 would lead to metal cluster formations. We believe it is these alkali clusters that form the metal nanoparticles responsible for SPR formation. In the present work we report formation of Cesium cluster in CsBr thin films due to ageing effect and their effect on optical properties. ExperimentalCesium Bromide thin films were deposited by thermal evaporation on microscopy glass substrates. The starting material (powder) was obtained from HiMedia (Mumbai) and was of 99.98% purity. Films depositons were carried out at room temperature with vacuum better than 1.4 × 10 −5 Torr. The thickness of the films were monitered with the help of a Digital Thickness Moniter (DTM-106). One part of the films were kept in desiccators and taken out only for characterization while the second part were kept outside, exposed to open air. This was done to compare the ageing effect of these films maintained in different conditions. The structural and optical characterization the samples were done with Philips PW 3020 diffractometer and Systronics double beam UV-visible spectrophotometer (2202). Surface morphology and elemental composition of films were examined with Field-Emission Scaning Electron Microscopy (FE-SEM) FEI-Quanta 200F and the Energy Dispersive Analysis of Xrays (EDX) system attached with it. Results and Discussion Optical StudiesThe absorption spectra of Cesium Bromide films showed thickness dependence with films thicker than 550 nm showing intense absorbance in the visible region. We believe that these peaks are due to surface plasmon resonance (SPR). SPR peaks results from the interaction between metal conduction electron and the electric field component of incident electromagnetic radiation [15]. This interaction leads to collective oscillations of free electrons on the metal surface if the metal particle's dimensions are smaller than or comparable to the incident wave-

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