  The effect of plasma irradiation is studied systematically on a 4H polytype
(002) oriented ${\rm CdI_2}$ stoichiometric film having compressive residual
stress. Plasma irradiation was found to change the orientation to (110) of the
film at certain moderate irradiation distances. A linear decrease in grain size
and residual stress was observed with decreasing irradiation distance (or
increasing ion energy) consistent with both structural and morphological
observations. The direct optical energy gap ${\rm E_g}$ was found to increase
linearly at the rate ${\rm 15\mu eV/atm}$ with the compressive stress. The
combined data of present compressive stress and from earlier reported tensile
stress show a consistent trend of ${\rm E_g}$ change with stress. The
iodine-iodine distance in the unit cell could be responsible for the observed
change in ${\rm E_g}$ with stress.
