Formation mechanisms of oxygen atoms in the O(D21) state from the 157nm photoirradiation of amorphous water ice at 90K

Abstract

Vacuum ultraviolet photolysis of water ice in the first absorption band was studied at 157 nm. Translational and internal energy distributions of the desorbed species, O͑ 1 D͒ and OH͑v =0,1͒, were directly measured with resonance-enhanced multiphoton ionization method. Two different mechanisms are discussed for desorption of electronically excited O͑ 1 D͒ atoms from the ice surface. One is unimolecular dissociation of H 2 O to H 2 +O͑ 1 D͒ as a primary photoprocess. The other is the surface recombination reaction of hot OH radicals that are produced from photodissociation of hydrogen peroxide as a secondary photoprocess. H 2 O 2 is one of the major photoproducts in the vacuum ultraviolet photolysis of water ice.

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

“…O͑ 1 D͒ atoms via reaction ͑12͒ were successfully detected following 157 nm photolysis of ASW, 16 but would play only a minor role in the present study, considering the small quantum yields ͑Յ0.01 for 145-185 nm͒ in the gas phase, 10 and high reactivity of O͑ 1 D͒ with parent H 2 O molecules by collisions with ASW to produce OH or H 2 O 2 . 44,45 Our previous study showed that O͑ 3 P͒ atoms were photodesorbed immediately after 157 nm irradiation started, and are mainly formed by recombination of two OH radicals moving on the ASW surface, i.e., reaction ͑13͒,…”

“…These OH͑v = 0 and 1,T trans = 7500 K͒ can proceed in endothermic reactions on ASW surface. 16 21 pointed out a possibility of the nonadiabatic transition for the OH+ O reaction. The potential energy surface of OH+ O reaction corresponding to the electronically excited 2AЈ state correlates only with the O 2 ͑a 1 ⌬ g ͒ + H product channel through the electronically excited state of the HO 2 ͑2AЈ͒ complex, but nonadiabatic electronic relaxation of this complex to the lowest 2AЉ state can provide O 2 ͑X 3 ⌺ g − ͒ + H product channel.…”

“…The translational and internal energy distributions of OH͑v =0,1͒ radicals and O͑ 1 D and 3 P͒ atoms were previously measured following 157 nm photodissociation of ASW. [15][16][17] For the O͑ 3 P͒ production, two different formation mechanisms were proposed: the exothermic recombination reaction of OH and the photodissociation of OH on the ASW surface, 17 OH + OH → H 2 O + O͑ 3 P͒, ⌬H = − 67 kJ mol −1 , ͑3͒…”

O(3PJ) formation and desorption by 157-nm photoirradiation of amorphous solid water

“…In the condensed phase with 7.9-eV photons, reaction (4) leads to O( 1 D) desorption with T trans = 800 K. 12 Quenching of these O( 1 D) atoms to the ground state is improbable because O( 1 D) readily reacts with H 2 O to form H 2 O 2 or 2 OH. 28 In the condensed phase, conservation of momentum can involve neighboring water molecules, state mixing can occur due to many-body interactions, and ionization energies can be lowered considerably relative to the gas phase.…”

“…Because photodissociation of water is such an important process in atmospheric, interstellar, and planetary chemistry, it has been studied extensively. [11][12][13][14][15][16] Even when the primary experimental goal is to measure photodesorption of water, a significant amount of photodissociation occurs and complicates the interpretation of measured water removal cross sections. 9,17,18 O( 3 P J ) formation by 157-nm irradiation of 1500 L amorphous solid ASW at 90 K has been measured previously by Hama et al, who found that four Maxwell-Boltzmann components with translational temperatures of 5000, 1300, 300, and 100 K fit their data.…”