| AVS 58th Annual International Symposium and Exhibition | |
| Surface Science Division | Monday Sessions |
| Session SS1-MoA |
| Session: | Selectivity and Reactivity of Chemisorbed Species |
| Presenter: | Ichiro Arakawa, Gakushuin University, Japan |
| Authors: | I. Arakawa, Gakushuin University, Japan D. Matsumoto, Gakushuin University, Japan S. Takekuma, Gakushuin University, Japan R. Tamura, Gakushuin University, Japan T. Miura, Gakushuin University, Japan |
| Correspondent: | Click to Email |
Photon stimulated dynamics, such as desorption, dissociation, or chemical synthesis, at the water cluster which contains organic molecules have been studied in conjunction with the photochemistry at ice particles in cosmic space and in the atmosphere. In our experimental study, the clusters were prepared on the surface of a solid rare gas, which was condensed on the copper substrate cooled by liquid helium in an ultra high vacuum chamber. Our method has the advantage in controlling the cluster size and of high density of specimens in comparison with a molecular beam experiment. The clusters on the solid rare gas were excited by vacuum ultra-violet light with a photon energy between 12 and 108 eV with a pulse width of 10 ns, which was generated by a laser plasma light source [1]. The mass spectrum of photo-desorbed ions was measured by a time-of-flight (TOF) method. There are variety of species in the photo-desorbed ions from the co-adsorbed system of water and methane; protonated water clusters, (H2O)nH+, methane clusters, (CH4)nCHk+, hetero-clusters of water and methane, (H2O)n(CH4)mCHk+, and synthesized species, methanol, CH3O+.
It was found that the presence of a water molecule in a cluster substantially enhanced, or was almost essential for, the desorption of any species, even for CH3+ and CH5+, observed in the spectrum. Dissociation of the water molecule plays a key role in the chemical reaction in the clusters. It was also found that the desorption yield of each species showed strong dependence on the composition and the size of the mother cluster on the substrate, which were controlled by the amount of adsorption of water and methane. Close and systematic investigation of their correlation has revealed the mother cluster which yields the each desorbed ions: (H2O)(CH4) clusters yield CH3+, CH4+, (H2O)CH3+, and CH3O+ while (H2O)(CH4)2 clusters yield CH5+, (H2O)CH3+, and C2H4+. These specific behaviors were also the case for the clusters of water and ethylene.
[1] T. Tachibana et al, Surf. Sci., 593, 264-268 (2005).