Tegeder Petra, Balog Richard, Mason Nigel J, Illenberger Eugen
Freie Universität Berlin, Institut für Chemie-Physikalische und Theoretische Chemie, Takustrasse 3, D-14195 Berlin, Germany.
Phys Chem Chem Phys. 2005 Feb 21;7(4):685-90. doi: 10.1039/b416987e.
Electron stimulated desorption (ESD) of Cl(-) from condensed molecular chlorine in the energy range 0-15 eV is studied. Cl2 is deposited in either multilayer amounts directly on a cryogenically cooled gold crystal or in sub-monolayer quantities on rare gas films (Xe, Kr) or ammonia ice films. Cl(-) desorption from multilayer films shows an intense resonance peaking at 5.5 eV and a comparatively smaller feature at 3 eV in qualitative agreement with an earlier ESD experiment. The desorption signal is enhanced by about one order of magnitude when a 0.2 monolayer (ML) Cl2 is adsorbed on a multilayer rare gas film. In this case, the desorption signal shows two clearly separated resonances peaking at 2.5 and 5.5 eV closely resembling dissociative electron attachment (DEA) from gas phase Cl2. These resonances can be associated to the transitions Cl2(1Sigmag+) --> Cl2(-)(2Pig) and Cl2(1Sigmag+) --> Cl2(-)(2Piu), respectively, both final states representing core excited resonances. The shape of the resonance around 5.5 eV splits into different peaks when changing from grazing incidence of the electron beam to an impact angle of 45 degrees with respect to the surface normal. On the basis of the pronounced angular dependence of the Cl(-) intensity reported from gas phase DEA this observation is compatible with a situation in which the molecules are oriented along the surface normal. Compared to the noble gas films, ESD from sub-monolayer Cl2 on top of a multilayer NH3 film is suppressed while the overall shape of the yield function is approximately preserved. None of the present experiments show a Cl(-) desorption signal below 2 eV while the charging behaviour of the film indicates that electron attachment is still operative in this energy domain. This suggests that the transient anion in its electronic ground state (Cl2(-)#(2Sigmau+)) is still formed by low energy electron attachment but is subjected to effective energy dissipation creating either stabilized ions (Cl2(-)(2Sigmau+)) or fragment ions (Cl(-) (2P)) with insufficient kinetic energy to leave the surface.
研究了能量范围在0 - 15电子伏特时，从凝聚态分子氯中进行的Cl(-)的电子激发脱附（ESD）。Cl2以多层量直接沉积在低温冷却的金晶体上，或以亚单层量沉积在稀有气体薄膜（Xe、Kr）或氨冰薄膜上。多层薄膜的Cl(-)脱附显示出在5.5电子伏特处有一个强烈的共振峰，在3电子伏特处有一个相对较小的特征峰，这与早期的ESD实验在定性上是一致的。当0.2单层（ML）的Cl2吸附在多层稀有气体薄膜上时，脱附信号增强了约一个数量级。在这种情况下，脱附信号显示出两个明显分开的共振峰，分别在2.5和5.5电子伏特处达到峰值，与气相Cl2的解离电子附着（DEA）非常相似。这些共振可以分别与跃迁Cl2(1Sigmag+) --> Cl2(-)(2Pig)和Cl2(1Sigmag+) --> Cl2(-)(2Piu)相关联，这两个终态都代表核心激发共振。当从电子束的掠入射角改变为相对于表面法线45度的入射角时，5.5电子伏特左右的共振形状会分裂成不同的峰。基于气相DEA报道的Cl(-)强度明显的角度依赖性，这一观察结果与分子沿表面法线取向的情况相符。与稀有气体薄膜相比，多层NH3薄膜顶部亚单层Cl2的ESD受到抑制，而产率函数的整体形状大致保持不变。目前的实验均未显示出低于2电子伏特的Cl(-)脱附信号，而薄膜的充电行为表明电子附着在该能量域仍然有效。这表明处于电子基态的瞬态阴离子（Cl2(-)#(2Sigmau+)）仍然是由低能电子附着形成的，但会经历有效的能量耗散，从而产生稳定离子（Cl2(-)(2Sigmau+)）或动能不足以离开表面的碎片离子（Cl(-) (2P)）。
