Deconvolution of the absorption spectra of S4; a) for the original spectrum, b), c) and d) for exposures to UV doses of 2.9 × 10²²,6.3 × 10²² and 1.0 × 10²³ (eV cm^-2), respectively. The inset in the panel b) shows that the rise might be due to the existence of P4 and P5 (small new band at 4.6 μm^-1).