Hot-atom mechanism in photodesorption of molecular oxygen from a stepped platinum (113) surface

Abstract

The photodesorption of oxygen admolecules was studied on a stepped Pt(113)ϭ(s)2(111) ϫ(001) surface with 193 nm irradiation at 110 K. Multidirectional desorptions were found to collimate at Ϯ12-20°and Ϯ45-49°off the surface normal and also along the surface normal in a plane along the trough. The first component is always dominant, and the weak second component only appears at higher oxygen coverages. The normally directed desorption is not significant. The translational energy of desorbing O 2 peaks around 15-20°and 50°, confirming the inclined desorptions. It is proposed that these inclined components are due to the desorption induced by the impact of oxygen admolecules with hot oxygen atoms from the photodissociation of adsorbed molecular oxygen, emitted along the trough. A simple cosine distribution was found to fit the thermal desorption from oxygen admolecules and also the recombinative desorption of oxygen adatoms. The 193 nm irradiation also produces additional, less tightly bound oxygen adatoms, which yield a desorption component collimated at 15°from the surface normal in the step-down direction.

The translational energy of desorbing products in NO and N2O decomposition on Pd (110)

“…10 In the latter, a hot atom emitted along the bond orientation of oxygen admolecules collides with neighboring O 2 ͑ad͒. The direction of resultant desorption is controlled by the momentum transfer from the hot atom and also from the repulsive force operative along the surface normal.…”

“…Furthermore, hot atoms or molecules produced in thermal-or photodissociation can participate in the reaction 6,7 and, in some cases, induces desorption inclined far from the site normal. [8][9][10] The latter desorption may occur whenever the dissociation of admolecules is able to emit translationally hot atoms or molecules.…”

Surface species studied by angle-resolved product desorption: Emission mechanism and spatial distribution

“…This provides supplemental evidence for the collimation angle shift from the site normal observed in thermal CO oxidation [21]. In the latter dissociative process, a simple momentum transfer mechanism is operative from the hot atom to the product leaving in an inclined way [17]. This mechanism has led to the first inclined fragment emission in the thermal dissociation of adsorbed molecules, in which the parallel momentum transfer is reproduced in a simpler way [22].…”

“…Instead, the molecules that receive momentum from the hot atom leave the surface. Such hot-atom-mediated desorption was examined in photoinduced emission of O 2 and CO 2 on stepped platinum surfaces as well as of O 2 on Ag(110) and noble gases on Pt(111) in the presence of O 2 (a) [14][15][16][17][18][19][20]. On platinum surfaces, oxygen ad-molecules lie parallel to the surface plane and are likely to be localized on the terrace edge at small coverage because of the high adsorption energy.…”

“…Thus, studies of such hot-atom-mediated desorption were not extended as ESDIAD work and were limited to only a few flat metal surfaces in which the hot atom does not desorb and, instead, the second species receiving the atom or its energy can leave the surface [13][14][15][16]. After detailed analysis of product desorption in ultra-violet photon-stimulated reactions of aligned O 2 on stepped Pt surfaces [17][18][19][20], it became clear that this hot-atom-mediated desorption exemplifies the typical relationship of the desorption mechanism toward the resultant spatial distributions of desorbing species, i.e., the structural information preserved in the distribution when the product is directly emitted from an associative process is different from that when a nascent fragment is first emitted parallel to the surface plane in a dissociative process [21]. In the former associative process, the collimation angle largely moves toward the local normal of the product formation site with increasing translational temperature [19].…”