Hydrogenated amorphous carbon (a-C:H) films were deposited on-double-side polished germanium substrates by RF plasma-assisted chemical vapour deposition method using benzene as a precursor, and their optical properties were investigated in the wavelength range from 700 nm to 10 mum. In particular, we used a dispersion-function-based non-linear regression to fit the reflectance in the range from 700 to 1800 nm and the baselines of the infrared transmittance in the range from 1000 to 5000 cm(-1), respectively. Optical constants determined in both ranges, together with their respective thickness values, showed overall consistency. As a function of self-bias, we found that the refractive index increased while the band gap decreased, which was indicative of film densification and increase of sp(2)-bonded carbons. Detailed information on the self-bias-dependent evolution of microscopic bonding structure in a-C:H films was revealed via the quantitative vibration-absorption spectra in the mid-infrared region, which was obtained after taking Urbach-tail-like electronic absorption into consideration. More specifically, it was found that as self-bias was increased hydrogen content decreased, olefinic sp(2)-bonding decreased while aromatic sp(2)-bonding increased, and more carbon bonding was distorted due to increased cross-linking. Maximum sp(2)-bonding was observed between -300 and -400 V of self-bias.