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Summary form only given. In the past decade photorefractive keratectomy (PRK), i.e. surgery of the cornea using ArF excimer laser radiation, has become a very popular clinical procedure for changing the refractivity of the human eye. It has currently been accepted that PRK proceeds via a thermochemical mechanism, although a photochemical pathway (so-called ablated photodecomposition) was also discussed in the literature. In our previous work it was shown that the peptide bond is likely the main target in collagen and its scission could be the most important photochemical reaction taking place on 193 nm laser irradiation of human cornea. The present work is devoted to laser photochemistry of dipeptides-model collagen components. Using 193 nm laser irradiation and HPLC analysis we have determined and compared the quantum yields of photodecomposition and peptide bond scission for a number of aromatic and aliphatic dipeptides in neutral Ar-saturated aqueous solution.