No abstract

OH radicals may play an important role in reactions that take place on interstellar icy grain surfaces, due to their high reactivity. Unfortunately, laboratory experiments aiming at simulating such reactions are hindered by the high reactivity of these species. Indeed, a method to isolate and further study the reactivity of OH radicals is still missing. This paper presents a study we carried out to determine the best conditions to isolate OH radicals in water ice. OH radicals were produced using a microwave discharge from either pure gaseous water or a gaseous mixture containing diluted water. Species thus formed were further condensed on a cold mirror at temperatures ranging from 3.5 to 30 K. The samples were then probed with a Fourier transform infrared spectrometer. The dilution of water into a rare gas (He, Ne) was found to be the best way to form an ice containing a high proportion of OH radicals, especially at low temperatures. These conditions, namely low temperatures and low concentrations, allow for the cooling of species formed during the discharge and prevent radical recombination.

The solid phase reaction between NO and non‐energetic OH radicals was investigated using helium as a dilution medium at 3 K. The species generated were characterized by in situ Fourier transform infrared (FTIR) spectroscopy. The choice of solid phase experiment was motivated by its potential applications in the field of atmospheric chemistry and astrochemistry. Unfortunately, under such conditions, broad absorption bands with overlapping features are observed, and as a consequence, the assignment of each signal to the newly formed specie is risky. This is the reason why, in order to attribute all the absorption bands observed in the solid phase FTIR spectra, similar experiments were carried out by replacing the dilution medium with neon whereas other experimental parameters were retained. As a result, all produced species are trapped in neon matrix showing narrow peaks so the elucidation of spectral signals is simple. Furthermore, the correlation of the spectra in solid phase and in neon matrix helps to avoid the intricacies in understanding the spectrum observed in solid phase reactivity. This approach allowed us to characterize the formation of nitrous acid (HONO), nitrogen dioxide (NO2), nitrosyl hydride (HNO), nitric acid (HNO3), nitrous oxide (N2O) and ozone (O3). The concentration effect of reagents on the end products was also investigated. Based on these experimental results, reaction pathways are proposed to explain the formation of all the observed products.

Reactions between dilute methane and nonenergetic hydroxyl radicals were carried out at 3.5 K. The temperature was kept low in order to characterize the stepwise reaction and prevent parasitic side reactions. The hydroxyl radicals originate from discharged H(2)O/He mixtures. The reactions were monitored in situ using a Fourier transform infrared spectrometer. The formation of CH(3) radicals was confirmed simultaneously with the formation of water ice. Subsequent recombination reactions lead to the formation of ethane (C(2)H(6)). Production of ethane and water ice occur preferentially to the formation of methanol.

In interstellar media, in which water is present as a major species, OH radicals can form following irradiation by energetic cosmic rays. Laboratory experiments have proved that when water ices are irradiated with energetic particles, OH-(H 2 O) are formed. Additionally, ground-state OH radicals may exist in the interstellar media, especially in dense interstellar clouds whose outer regions stop energetic particles. To this extent, the understanding of the reactivity of OH radicals in their ground state is of primary importance in the astrochemical context. We experimentally show that the co-injection of an OH radical beam with dilute water molecules leads to the formation of OH-(H 2 O) complexes on copper rhodiated mirrors maintained at 3.5 K, under a pressure of 10 −5 mbar.

No abstract