Chou L-C, Jang C-Y, Wu Y-H, Tsai W-C, Wang S-K, Chen J, Chang S-C, Liu C-C, Shai Y, Wen C-R
Department of Physics, National Cheng Kung University, Tainan 70101, Taiwan.
J Chem Phys. 2008 Dec 7;129(21):214104. doi: 10.1063/1.3026598.
Photon-exposure-dependent positive- and negative-ion photon-stimulated desorption (PSD) was proposed to study the photoreactions and obtain the photolysis cross sections of molecules adsorbed on a single-crystal surface by monochromatic soft x-ray photons with energy near the core level of adsorbate. The changes in the F(+) and F(-) PSD ion yields were measured from CF(3)Cl molecules adsorbed on Si(111)-7x7 at 30 K (CF(3)Cl dose=0.3x10(15) molecules/cm(2), approximately 0.75 monolayer) during irradiation of monochromatic soft x-ray photons near the F(1s) edge. The PSD ion yield data show the following characteristics: (a) The dissociation of adsorbed CF(3)Cl molecules is due to a combination of direct photodissociation via excitation of F(1s) core level and substrate-mediated dissociation [dissociative attachment and dipolar dissociation induced by the photoelectrons emitting from the silicon substrate]. (b) the F(+) ion desorption is associated with the bond breaking of the surface CF(3)Cl, CF(2)Cl, CFCl, and SiF species. (c) the F(-) yield is mainly due to DA and DD of the adsorbed CF(3)Cl molecules. (d) The surface SiF is formed by reaction of the surface Si atom with the neutral fluorine atom, F(+), or F(-) ion produced by scission of C-F bond of CF(3)Cl, CF(2)Cl, or CFCl species. A kinetic model was proposed for the explanation of the photolysis of this submonolayer CF(3)Cl-covered surface. Based on this model and the variation rates of the F(+)F(-) signals during fixed-energy monochromatic photon bombardment at 690.2 and 692.6 eV [near the F(1s) edge], the photolysis cross section was deduced as a function of energy.
提出了光致曝光依赖的正离子和负离子光激发脱附（PSD）方法，以研究光化学反应，并通过能量接近吸附质核心能级的单色软X射线光子，获得吸附在单晶表面的分子的光解截面。在30 K下，测量了吸附在Si(111)-7x7上的CF(3)Cl分子（CF(3)Cl剂量 = 0.3×10(15) 分子/cm(2)，约0.75单层）在F(1s)边缘附近的单色软X射线光子辐照期间F(+)和F(-) PSD离子产率的变化。PSD离子产率数据显示出以下特征：(a) 吸附的CF(3)Cl分子的解离是由于通过F(1s)核心能级激发的直接光解和底物介导的解离 [由从硅衬底发射的光电子诱导的解离附着和偶极解离] 的组合。(b) F(+)离子脱附与表面CF(3)Cl、CF(2)Cl、CFCl和SiF物种的键断裂有关。(c) F(-)产率主要归因于吸附的CF(3)Cl分子的解离附着（DA）和偶极解离（DD）。(d) 表面SiF是由表面Si原子与CF(3)Cl、CF(2)Cl或CFCl物种的C-F键断裂产生的中性氟原子、F(+)或F(-)离子反应形成的。提出了一个动力学模型来解释该亚单层CF(3)Cl覆盖表面的光解。基于该模型以及在690.2和692.6 eV [接近F(1s)边缘] 的固定能量单色光子轰击期间F(+)F(-)信号的变化率，推导出了光解截面作为能量的函数。
