Role of OH radicals in the formation of oxygen molecules following vacuum ultraviolet photodissociation of amorphous solid water

Abstract

Photodesorption of O 2 ͑X 3 ⌺ g − ͒ and O 2 ͑a 1 ⌬ g ͒ from amorphous solid water at 90 K has been studied following photoexcitation within the first absorption band at 157 nm. Time-of-flight and rotational spectra of O 2 reveal the translational and internal energy distributions, from which production mechanisms are deduced. Exothermic and endothermic reactions of OH+ O͑ 3 P͒ are proposed as plausible formation mechanisms for O 2 ͑X 3 ⌺ g − and a 1 ⌬ g ͒. To examine the contribution of the O͑ 3 P͒ +O͑ 3 P͒ recombination reaction to the O 2 formation following 157 nm photolysis of amorphous solid water, O 2 products following 193 nm photodissociation of SO 2 adsorbed on amorphous solid water were also investigated.

O2 formation in cold environments

“…Extrapolating the stretched exponential decay from the [KKY/RKHS] simulations from recombination on the ASW surface (see Table 1) to 1 ms yields r OO = 1.2386 Å which corresponds to v = 2, consistent with experiment. 17 Formation of electronically excited O 2 ( 1 D g and 1 S + g ) has been found from OH + O( 3 P) involving a barrierless and an activated process, respectively. 17 The fraction of O 2 in its 1 D g state was 2.5%.…”

“…17 Formation of electronically excited O 2 ( 1 D g and 1 S + g ) has been found from OH + O( 3 P) involving a barrierless and an activated process, respectively. 17 The fraction of O 2 in its 1 D g state was 2.5%. 53 Given that the 3 S À g , 1 D g , and 1 S + g of O 2 dissociate to the same asymptote (O( 3 P) + O( 3 P)) it will be interesting to include electronically excited states in future work.…”

“…In modeling interstellar processes, the O + O -O 2 channel is sometimes included 5 and sometimes not. 15 Another, and probably the predominant, 16,17 O 2 -formation reaction is O + OH -O 2 + H. A more exhaustive list of O 2 -formation reactions, including O + OH -O 2 + H in the gas phase 18 or on the surface of grains, 19 can be found in the KIDA database. 20 It has been previously shown by experiment and simulations that migration of atomic species, such as oxygen, in ASW is readily possible on the sub-microsecond time scale over distances of several ten Angstroms.…”

“…[22][23][24] This pathway for the formation of molecular species from adsorbates is generally known as the Langmuir-Hinshelwood mechanism. 25 In laboratory experiments using amorphous solid water, formation of ground state 3 S À g (from OH + O( 3 P) with DH = À68 kJ mol À1 ) and excited state 1 D g O 2 (from OH + O( 3 P) with DH = 26 kJ mol À1 ) was found following electronic excitation of the ASW with 157 nm irradiation at 90 K. 17 This process for O 2 formation follows a nonequilibrium route, involves higher electronic states and differs from the processes considered in temperature-programmed desorption experiments. 2 The study of SO 2 photolysis at 193 nm adsorbed on ASW yields O 2 ( 3 S À g ) from recombination of O( 3 P) + O( 3 P) with DH = À498 kJ mol À1 .…”

“…2 The study of SO 2 photolysis at 193 nm adsorbed on ASW yields O 2 ( 3 S À g ) from recombination of O( 3 P) + O( 3 P) with DH = À498 kJ mol À1 . 17 Furthermore, a model for formation of O 2 following electronic excitation of ice by fast ions, low-energy electrons, and UV photons at low temperatures was presented. 26 In this model trapped O( 3 P) is the principal species considered for formation of O 2 at low doses.…”