Production and isolation of OH radicals in water ice

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 NO and non-energetic OH radical reactivity: characterization and reaction scheme

“…The characterization of OH radical produced from the discharged H 2 O:He (10:100) mixture has been reported elsewhere (Zins et al 2011a). Fig.…”

“…All these attributions and the reaction pathways are described in more detail in our previous publication (Zins et al 2011a). When the water‐containing mixture is introduced in the device without any microwave discharge, water and carbon monoxide are observed, whereas carbon dioxide is not observed.…”

“…Prior discussing the different reaction pathways for the newly formed species, it is necessary to note that the microwave discharge of water leads to the formation H, O, H 2 , O 2 and H 2 O, besides OH radicals (Oba et al 2010b; Noble et al 2011). As a consequence, and even if the experimental conditions we used limit recombination reactions and favour the formation of hydroxyl radicals as the main specie (Zins et al 2011a), HO 2 is formed by the reaction between atomic oxygen and hydroxyl radical: HO 2 radicals may be involved in the reaction pathway with NO radicals which will be discussed later.…”

“…Recently, it has been experimentally proved that the hydroxyl radical is involved in the surface formation of several different interstellar species at low temperatures, such as CO 2 , HCOOH and H 2 CO 3 (Oba et al 2010a,c; Ioppolo et al 2011a,b; Noble et al 2011). An experimental method has been also proposed in order to isolate OH radicals in water ice, and its presence has been spectroscopically confirmed in our laboratory (Zins, Joshi & Krim 2011a).…”

“…Moreover, the low temperature facilitated to minimize the mobility of cold OH and NO radicals and further avoid the secondary reactions, such as OH combinations. The helium was chosen as a dilution medium for two reasons: (1) it allows the formation of non‐energetic OH radicals (Zins et al 2011a), and (2) it avoids the rare gas matrix formation thus further admitting a solid phase investigation. To lessen the anxiety in the interpretation of the spectra obtained in solid phase reactivity, similar studies using neon as a dilution medium was carried out where the species formed after reactivity study were trapped in neon matrix.…”