Experimental and Theoretical Study of the Chemical Network of the Hydrogenation of NO on Interstellar Dust Grains

Nitrogen monoxide (NO) is observed in the gas phase of molecular clouds. It may accrete on dust grains, and there its hydrogenation should lead to hydroxylamine (NH2OH), the same way that CO is transformed into methanol (CH3OH) on the surface of dust grains. NO hydrogenation has been said barrier-less, whereas CO hydrogenation proceed through quantum tunneling, and is thus slower. However, CH3OH is widely observed and is considered as a proxy of complex organic molecules while hydroxylamine remains undetected. We aim at studying, analyzing, and understanding the chemical network of NO hydrogenation on cold surfaces. Experiments are carried out using a new ultrahigh vacuum (UHV) setup named VENUS. NO molecules and H atoms are codeposited on a golden mirror at different temperatures. Infrared spectroscopy as well as temperature-programmed desorption (TPD) are used to follow the NO reactivity, with both H and D, and in presence or absence of water substrate. Quantum calculations on water ice cluster models are computed separately. During the hydrogenation of NO, 10 reactions proceed concurrently. They are identified and constrained by changing physical conditions in experiments or in calculations. Among them, we demonstrate that the HNO + H addition reaction has a barrier which is probably crossed via quantum tunneling at 10 K. Moreover, abstraction reactions are occurring although they are limited by H and 0 bonding with their environment. Chemical desorption should occur especially in the absence of water which enhances the total production of hydroxylamine. The chemical network of the hydrogenation of NO has been reinvestigated. Each of the 10 reactions are sorted by efficiency. We exclude the possibility of a chemical loop between NO and HNO, especially in the presence of water. Therefore, hydroxylamine remains the main product of the hydrogenation of NO on grains, and the question of its nondetection in ices or in the gas phase, specifically in shocked regions where ice mantles should be sputtered, is still open.