The soot formation process in the high temperature - high pressure pyrolysis of ethylene has been studied in a conventional diaphragm-type shock tube behind reflected shock waves. The investigated ranges of temperature and pressure were 1600-2200 K and 9-13 bar, respectively. Light scattering/extinction measurements in the visible, and extinction measurements in the near infrared have been analysed to characterise the soot formation process. Pyrolytic products, formed prior to soot particles, have been evidenced by continuous absorption measurements, with high time/spectral resolution, in the W-visible range. The absorption spectra have been analysed according to the Tauc optical band-gap model. In the investigated experimental conditions, the characteristic optical band-gaps of aromatic structures with 2-4 rings at most have been observed before soot-inception. After the soot-inception, a sharp decrease of the band-gap has been revealed, indicating a high degree of aromaticity of soot particles.