Electron momentum spectroscopy with fixed-in-space molecules

通过近三十年的二元（e，2 e）能谱或电子动量谱（EMS）研究，证明了Bethe脊附近的电离反应是研究电子结构和电子关联的灵敏探针。特别是，在动量空间中观察单个分子轨道的能力是这种技术的显着特点。然而，EMS还没有达到充分利用其深入研究分子电子结构的能力的阶段。其原因可能有两个方面：（1）随着实验的复杂性增加，对二元（e，2 e）反应机制的更完整的了解越来越有必要，以及（2）目前的EMS实验测量气体分子所有取向的平均值，导致电子结构信息的巨大损失，特别是目标波函数的各向异性。然而，二元（e，2 e）反应的小反应截面阻碍了EMS的发展，在这种情况下，我们研制了一台电子-电子-碎片离子三重符合谱仪。该谱仪具有非常高的灵敏度的两个出射电子的能量和动量的同时检测，并进一步使我们能够进行（e，2 e）的实验与固定在空间分子的轴向反冲碎裂的基础上。利用多通道检测的优点。利用该谱仪，我们成功地研究了氢分子电离激发过程的碰撞动力学和氧分子的定向动量密度。这是第一次观察到分子框架（e，2 e）横截面。

Through the last three decades of studies of binary (e,2e) spectroscopy or electron momentum spectroscopy (EMS), it has been demonstrated that the ionization reaction near the Bethe ridge is a sensitive probe for electronic structure and electron correlation. In particular, the ability to look at individual molecular orbitals in momentum space is the remarkable feature of this technique. However, EMS has not yet reached the stage of full use of its ability for intensive investigation on electronic structure of molecules. The reason for this may have been twofold; (1) a more complete knowledge of the binary (e,2e) reaction mechanism is an ever-increasing necessity as sophistication of experiments increases, and (2) the present EMS experiments measure averages over all orientations of gaseous molecules, resulting in enormous loss of information on electronic structure, anisotropy of the target wavefunction in particular. However, small cross section involved in the binary (e,2e) reaction has hindered one from developing EMS satisfactorily.Under these circumstances, we have constructed an electron-electron-fragment ion triple coincidence spectrometer. The spectrometer features remarkably high sensitivity for the two outgoing electrons by simultaneous detection in energy and momentum, and further it enables us to carry out (e,2e) experiments with fixed-in-space molecules based on their axial recoil fragmentation. By taking advantage of multichannel detection in. the spectrometer, we have successfully applied it to studies on collision dynamics of ionization excitation processes of the hydrogen molecule and directional momentum densities of the oxygen molecule. These are the first observation of molecular frame (e,2e) cross sections.