Formation mechanisms of oxygen atoms in the O(PJ3) state from the 157nm photoirradiation of amorphous water ice at 90K

Abstract

Desorption of ground state O͑ 3 P J=2,1,0 ͒ atoms following the vacuum ultraviolet photolysis of water ice in the first absorption band was directly measured with resonance-enhanced multiphoton ionization ͑REMPI͒ method. Based on their translational energy distributions and evolution behavior, two different formation mechanisms are proposed: One is exothermic recombination reaction of OH radicals, OH + OH → H 2 O+O͑ 3 P J ͒ and the other is the photodissociation of OH radicals on the surface of amorphous solid water. The translational and internal energy distributions of OH radicals as well as the evolution behavior were also measured by REMPI to elucidate the roles of H 2 O 2 and OH in the O͑ 3 P J ͒ formation mechanisms.

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

“…Photodesorbed O͑ 3 P J=2,1,0 ͒ atoms were ionized by the 2 + 1 REMPI transition via the O͑ 3 P J -3 P J ͒ transition at 225.6-226.4 nm. 17 We tried to search for REMPI signals of oxygen molecules in the higher electronically excited states, i.e., b 1 ⌺ g + , c 1 ⌺ u − , and 5 ͟ g using the REMPI transitions reported by Morrill et al 32 and Slanger and Copeland, 33 but no evidence for these products was obtained.…”

“…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…”

“…For comparison purpose, Fig. 5͑a͒ includes the time evolution of photodesorbed O͑ 3 P 2 ͒ signal intensity that was recorded at the peak of TOF, i.e., t = 4.5 s. 17 The be- On the other hand, the O 2 ͑T trans = 250 K͒ component appeared promptly upon 157 nm irradiation of ASW, as shown in Fig. 5͑c͒.…”

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

“…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. 11 The 5000 K component, which was attributed to hydroxyl dissociation, increased significantly with 30 min of prior irradiation when compared to freshly dosed ASW. 11 When compared to the time-of-flight (TOF) spectrum of O( 3 P J ) from ASW, the TOF spectrum for O( 3 P J ) from H 2 O 2 was found to be roughly the same shape but twice as intense.…”

“…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.…”