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system mentioned in section I, we note that this study probes only the initial stage of 02 chemisorption where only the a-state has been observed. The /3-state, which is observed at higher exposures (<2 langmuirs), is not studied here.Our results show that for 02 exposures <0.25 langmuir, oxygen chemisorbs in sites that are either on top of or in the topmost surface plane. For exposures between 0.25 and 2.0 langmuirs, the oxygen sites are below the outermost Ti layer. The amount of oxygen chemisorbing in the top sites at <0.25 langmuir corresponds to a coverage of 0.2 monolayer. Previous studies7'13,16,17 of this system have concluded that oxygen goes subsurface for exposures <2.0 langmuirs. The high sensitivity of DRS allows us to modify this conclusion such that oxygen adds in top layer sites <0.25 langmuirs and subsurface sites between 0.25 and 2.0 langmuirs.

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