Irradiation induced changes in the complex dielectric spectra of single crystal Si are investigated with spectroscopic ellipsometry. The analysis of microstructural variations resulting from ion irradiation is carried out to establish the existence of a well defined amorphization threshold. Increase in fluence leads to amorphization of the crystal resulting in an order-disorder transition as inferred from peak to peak height of the first derivative spectra. The amorphous volume fraction deduced from a microstructure model using effective medium approximation is found to have a power law behavior. The variations in the critical point (CP) structure in the energy range 1.5 to 5 eV is studied as a function of fluence ranging from 6×10~(13) to 1×10~(17) Ar~(+) ions cm~(-2). The changes in the nature of the CPs is examined in detail by line shape analysis of the second derivative of the optical spectra above and below the amorphization threshold. Irradiation induced structural disorder and its effect on the interband optical transition, particularly the E_(1) critical point at 3.4 eV is analyzed in detail. The E_(1) structure is fitted in the complete fluence range with a 2D CP line shape with a large excitonic phase angle. A red shift of the E_(1) critical point, decrease in its amplitude, increase in excitonic phase angle and broadening of the line shape are observed with increasing fluence. All these parameters of the E_(1) CP show distinct behavior above and below the amorphization threshold.
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