A comparative study of the desorption efficiencies in the UV irradiation of molecular van der Waals films above and below the ablation threshold

Abstract

In the irradiation of thick films of aromatics (C6H5Cl and C6H5CH3 enriched with dopants of varying volatilities), the attainment of the threshold is shown to result in qualitatively different ejection characteristics. In particular, the comparison of the desorption efficiencies either of species premixed in the film or of photoproducts formed by the irradiation shows that below the threshold only highly volatile species desorb. In contrast, above the threshold, even highly involatile species are found to be ejected efficiently. The efficient ejection of these species cannot be accounted for by a change in the absorbed energy. Instead, the operation of a non-thermal ejection mechanism is strongly indicated. The results are consistent with the delineation drawn by molecular dynamics simulations [12] for surface vaporization at fluences below the ablation threshold and ejection as a result of pressure buildup above it.