Photodissociation of polycrystalline and amorphous water ice films at 157 and 193nm

Abstract

The photodissociation dynamics of amorphous solid water (ASW) films and polycrystalline ice (PCI) films at a substrate temperature of 100 K have been investigated by analyzing the time-of-flight (TOF) mass spectra of photofragment hydrogen atoms at 157 and 193 nm. For PCI films, the TOF spectrum recorded at 157 nm could be characterized by a combination of three different (fast, medium, and slow) Maxwell-Boltzmann energy distributions, while that measured at 193 nm can be fitted in terms of solely a fast component. For ASW films, the TOF spectra measured at 157 and 193 nm were both dominated by the slow component, indicating that the photofragment H atoms are accommodated to the substrate temperature by collisions. H atom formation at 193 nm is attributed to the photodissociation of water species on the ice surface, while at 157 nm it is ascribable to a mixture of surface and bulk photodissociations. Atmospheric implications in the high latitude mesopause region of the Earth are discussed.

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

“…Following the ultraviolet photolysis of H 2 O 2 in various rare-gas matrices at 7.5 K, a ͑H 2 O-O͒ complex was formed from the recombination of OH radicals, while the primary formations of H 2 O+O͑ 1 D͒ or O͑ 3 P J ͒ have not been established from the photodissociation of gaseous H 2 O 2 at 124-254 nm. 11,12 Experimental works on photon-or electron-stimulated desorption ͑ESD͒ of species from water ice have also been reported, [13][14][15][16][17][18][19][20] In the present study, using pulsed 157 nm ͑181 kcal/ mol͒ laser radiation the desorptions of electronically excited O͑ 1 D͒ atoms via the primary and secondary photoprocesses from ASW at 90 K have been directly confirmed by REMPI. Translationally and internally excited OH formation mechanisms were also discussed since hot OH is a plausible source for the oxygen atom.…”

“…10 In the present experiment, the contribution of the photolysis of OH͑ads͒ for O͑ 1 D͒ formation, reaction ͑26͒, would be small in fresh ASW and H 2 O 2 on ASW because it is unlikely that multiphoton process occurred with this low laser intensity of unfocused 157 nm laser at a fluence Ͻ0.1 mJ cm −2 pulse −1 , 18,54 and also the surfaces of ice were kept fresh for laser irradiation by exposing them with water or H 2 O 2 / H 2 O vapor to suppress the secondary photolysis of the accumulated products formed on the ice surface during the photolysis, 55,56 In fact, the secondary photolysis of OH͑ads͒ is attributed to O͑ 3 P J ͒ formation in the 157 nm photolysis of water ice where the TOF spectra of O͑ 3 P 2 ͒ for ASW after prolonged photoirradiation have a different translational distribution from those of fresh ASW and fresh H 2 O 2 on ASW. These results will be described in the following paper for our separate experiments.…”

Excited state dynamics of liquid water: Insight from the dissociation reaction following two-photon excitation

“…Only the first collision or two is likely to have enough energy to overcome the reaction barrier, even for a favorable impact parameter, because collisions rapidly dissipate excess energy from the H atom. 60 The approximately tetrahedral arrangement of liquid water preferentially aligns the dissociating O-H bond toward the O atom on the nearest neighboring water molecule. Thus, the most likely first collision has an unfavorable orientation for hydrogen abstraction that limits the yield of reaction ͑9͒.…”

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

“…193 nm irradiation of ASW produced no measurable REMPI signals of O 2 ͑X 3 ͚ g − and a 1 ⌬ g ͒ and OH at such low incident intensities, although H atom photofragments from dimerlike water molecules on ASW surface were observed following 193 nm photodissociation of ASW. 28,29 Photodesorbed O 2 ͑X 3 ͚ g − ͒ were ionized at a vertical distance of 2 mm from the ice surface by 2 + 1 REMPI process via the C 3 ⌸ g ͑vЈ =2͒ ← X 3 ͚ g ͑vЉ =0͒ at 283-289 nm produced by frequency doubling the output of a Nd 3+ : YAG ͑yttrium aluminum garnet͒ pumped dye laser ͑Lambda Physik, Göttingen, SCANmate͒, 30,31 and collected with a small mass spectrometer aligned perpendicular to the ice surface. Electronically excited O 2 ͑a 1 ⌬ g , v =0͒ photodesorbed from ASW has also been observed with the same experimental setup via the d 1 ⌸ g ͑vЈ =1͒ ← a 1 ⌬ g ͑vЉ =0͒ REMPI transitions at 329-332 nm.…”