Context. The presence of dust can strongly affect the chemical composition of the interstellar medium. We model the chemistry in photodissociation regions (PDRs) using both gas-phase and dust-phase chemical reactions. Aims. Our aim is to determine the chemical compositions of the interstellar medium (gas/dust/ice) in regions with distinct (molecular) gas densities that are exposed to radiation fields with different intensities. Methods. We have significantly improved the Meijerink PDR code by including 3050 new gas-phase chemical reactions and also by implementing surface chemistry. In particular, we have included 117 chemical reactions occurring on grain surfaces covering different processes, such as adsorption, thermal desorption, chemical desorption, two-body reactions, photo processes, and cosmic-ray processes on dust grains. Results. We obtain abundances for different gas and solid species as a function of visual extinction, depending on the density and radiation field. We also analyse the rates of the formation of CO₂ and H₂O ices in different environments. In addition, we study how chemistry is affected by the presence/absence of ice mantles (bare dust or icy dust) and the impact of considering different desorption probabilities. Conclusions. The type of substrate (bare dust or icy dust) and the probability of desorption can significantly alter the chemistry occurring on grain surfaces, leading to differences of several orders of magnitude in the abundances of gas-phase species, such as CO, H₂CO, and CH₃OH. The type of substrate, together with the density and intensity of the radiation field, also determine the threshold extinction to form ices of CO₂ and H₂O. We also conclude that H₂CO and CH₃OH are mainly released into the gas phase of low, far-ultraviolet illuminated PDRs through chemical desorption upon two-body surface reactions, rather than through photodesorption.

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Context. The ultraluminous infrared galaxy Mrk 231, which shows signs of both black hole accretion and star formation, exhibits very strong water rotational lines between λ = 200-670 μm, comparable to the strength of the CO rotational lines. High-redshift quasars also show similar CO and H 2O line properties, while starburst galaxies, such as M 82, lack these very strong H 2O lines in the same wavelength range, but do show strong CO lines. Aims. We explore the possibility of enhancing the gas phase H 2O abundance in X-ray exposed environments, using bare interstellar carbonaceous dust grains as a catalyst. Cloud-cloud collisions cause C and J shocks, and strip the grains of their ice layers. The internal UV field created by X-rays from the accreting black hole does not allow reforming the ice. Methods. We determined the formation rates of both OH and H 2O on dust grains, with temperatures T dust = 10-60 K, using both Monte Carlo and rate equation method simulations. The acquired formation rates were added to our X-ray chemistry code, which allowed us to calculate the thermal and chemical structure of the interstellar medium near an active galactic nucleus. Results. We derive analytic expressions for the formation of OH and H 2O on bare dust grains as a catalyst. Oxygen atoms arriving on the dust are released into the gas phase in the form of OH and H 2O. The efficiencies of this conversion due to the chemistry occurring on dust are near 30 percent for oxygen converted into OH and 60 percent for oxygen converted into H 2O between T dust = 15-40 K. At higher temperatures, the efficiencies decline rapidly. When the gas is mostly atomic, molecule formation on dust is dominant over the gas-phase route, which is then quenched by the low H 2 abundance. Here, it is possible to enhance the warm (T > 200 K) water abundance by an order of magnitude in X-ray exposed environments. This helps explain the observed bright water lines in nearby and high-redshift ULIRGs and quasars.@en