We present an electrical and spectroscopic characterization of Acetylene plasma used for deposition of different forms of hydrogenated amorphous carbon (a-C:H) films at different discharge powers. Asymmetric radio frequency capacitively coupled plasma source driven by 13.56 MHz used for generation of the discharge. Fast Fourier transformation of the experimentally measured discharge current waveforms shows several harmonics of the driving frequency, the number and amplitude of these harmonics depends on the experimental conditions. The plasma series resonance (PSR) oscillations generated such harmonics leading to an enhancement of ionization process. Optical emission spectroscopy results showed strong emissions intensities from C4H2+ which varied with both power and pressure, the presence of these emissions attributed to the PSR effect. The formation and transformation of the films from polymeric a-C:H to graphitic a-C:H at different discharge powers was confirmed by Raman, FTIR and optical absorption spectroscopy. For the first time we constructed a ternary phase diagram of the a-C:H films based on the relative intensity ratio of [I(C4H2+)/I(CH)] to provide quantitative information about the types of the deposited films. We pointed out that the C4H2+ ionic species play the major role in the films formation and transformation process.