A glow discharge device is constructed and operated by using Ar gas. Paschen curve is analyzed at different pressures and inter-electrode distances. The experimental results show that the discharge is optimized at 5 cm inter-electrode distance. The I-V characteristic of the glow discharge at different pressures is studied and the Zinc selenide (ZnSe) thin film is treated. The effect of exposure to the Ar plasma immersion-ion-implantation at different inter-electrode spacing (5, 8.5, 11.5 and 14.5 cm) on the microstructure parameters and energy gap of ZnSe thin film are investigated. It is observed that, both crystallite size and lattice strains are increased by increasing the inter-electrode distances. The values of absorbance in strong absorption region are reduced with increasing the inter-electrode spacing. In addition, the optical band gap $E_g^{opt}$ has been determined in terms of absorbance curves and found to be increased with the Paschen's minimum, when inter-electrode spacing increases.