Totir G G, Le Y, Osgood R M
Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA.
J Phys Chem B. 2005 May 5;109(17):8452-61. doi: 10.1021/jp045326z.
The adsorption, thermal chemistry, and photoreaction dynamics of methyl iodide on the (2x2) magnetite termination of natural single-crystal hematite have been investigated by time-of-flight quadrupole mass spectrometry (TOF-QMS), temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). The methyl iodide thermal desorption spectra, taken after dosing the (2x2) surface at 100 K with CH(3)I, show a multiple-peak coverage-dependent behavior, consistent with the presence of several distinct adsorbed phases, along with defect-mediated dissociative chemisorption in the first monolayer. At >1 ML, methyl iodide forms a metastable physisorbed second layer, which desorbs at 148 K, but at higher coverage converts to a layer, which desorbs at 170 K. In the presence of low-fluence-pulse irradiation at 248 nm, angle-resolved TOF-QMS measurements show that 1.6 and 0.3 eV CH(3) fragments are ejected from the adsorbate surface; these fragments originate from direct photodissociation and dissociative photoinduced electron transfer, respectively. These energetic photoejected fragments have characteristic angular distributions peaked at approximately 0 degree with respect to the surface normal. These results and the coverage-dependent relative intensities suggest that the predominant orientation in the first monolayer of the adsorbed CH(3)I is normal to the crystal plane.
利用飞行时间四极质谱（TOF-QMS）、程序升温脱附（TPD）和俄歇电子能谱（AES），研究了甲基碘在天然单晶赤铁矿（2x2）磁铁矿端面上的吸附、热化学和光反应动力学。在用CH(3)I在100K下对（2x2）表面进行剂量处理后得到的甲基碘热脱附谱显示出多峰覆盖依赖行为，这与几个不同吸附相的存在一致，同时在第一单层中存在缺陷介导的解离化学吸附。在>1 ML时，甲基碘形成一个亚稳态的物理吸附第二层，其在148K解吸，但在更高覆盖度下转变为一个在170K解吸的层。在248nm低通量脉冲辐照下，角分辨TOF-QMS测量表明，1.6和0.3 eV的CH(3)碎片从吸附物表面射出；这些碎片分别源于直接光解离和光诱导解离电子转移。这些高能光发射碎片具有相对于表面法线在约0度处达到峰值的特征角分布。这些结果以及覆盖度依赖的相对强度表明，吸附的CH(3)I在第一单层中的主要取向垂直于晶面。
