Raman spectra of graphite were measured under Ne+, Kr+ and Xe+ ions irradia
tion at ion flux of 1.7X10(15) ions/m(2) at room temperature. The peak inte
nsity ratio of the disorder-induced peak with respect to the Raman active E
-2g2, mode peak of the first order Raman spectrum is proportional to the sq
uare root of the ion irradiation time in the beginning of the irradiation.
However, the ratio saturated around 150 s for each ion irradiation. The sat
uration was explained by creation of vacancy cluster due to the overlap of
the irradiation-induced cascades for the direction parallel to the surface.
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