Crossed molecular beams of gases have provided definitive information concerning the dynamics of chemical reactions. The results have, however, of necessity been averaged over collisions with impact parameters ranging from zero to infinity, thus obscuring the effect of this important variable. Here we employ a method through which impact parameter averaging is suppressed in a surface reaction. We aim a highly collimated reactive ‘projectile’ molecule along a surface at a stationary adsorbed ‘target’ molecule, with both the projectile and target being observed by Scanning Tunnelling Microscopy (STM). The projectile was CF₂ recoiling from electron-induced bond-breaking in chemisorbed CF₃ on Cu(110) at 4.6 K. The collimation of the resulting CF₂ ‘surface-molecular-beam’ restricted it to a lateral spread of ±1° as a consequence of its interaction with the Cu rows below. This collimation was modelled by molecular dynamics simulations. In the experiments the recoiling CF₂ projectile was aimed, successively, at impact parameters of b = 0 and +3.6 Å at a chemisorbed second CF₂, b = +1.8 Å at a chemisorbed I-atom, or b = −4.0, b = −0.4 and b = +3.2 Å at a chemisorbed vinyl radical. The pattern of reactive and non-reactive scattering was determined by STM. These collimated surface-molecular-beams have the potential to aim molecular projectiles with selected impact parameters at the many target species identifiable at a surface by STM.

中文翻译：

---

表面上电子诱导的分子解离导致在选定的冲击参数下发生反应性碰撞

交叉的气体分子束提供了有关化学反应动力学的确定信息。但是，结果必须在碰撞参数范围从零到无穷大的情况下取平均，从而使此重要变量的影响难以理解。在这里，我们采用一种方法来抑制表面反应中的平均冲击参数。我们的目标是在表面上固定吸附的“靶”分子上高度准直的反应性“弹丸”分子，并通过扫描隧道显微镜（STM）观察弹丸和靶标。弹丸是在4.6 K下化学吸附在Cu（110）上的CF _3中电子诱导的键断裂引起的CF ₂卷曲。所得CF ₂的准直“表面分子束”由于其与下面的Cu行相互作用而使其横向扩展限制为±1°。通过分子动力学模拟对该准直进行建模。在实验中的反冲CF ₂弹丸旨在，依次，在冲击参数b = 0，并且在3.6化学吸附第二CF ₂，b在α= 1.8化学吸附I-原子，或b = -4.0，b = −0.4和b在化学吸附的乙烯基上为+3.2Å。反应性和非反应性散射的图案通过STM确定。这些准直的表面分子束具有将具有选定冲击参数的分子射弹对准STM可在表面识别的许多目标物种的潜力。