Electron momentum spectroscopy with fixed-in-space molecules

近三十年来对二元(e，2e)谱或电子动量谱(EMS)的研究表明，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项技术的显著特征。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。这样做的原因可能有两个；(1)随着实验的复杂性的增加，对二元(e，2e)反应机理的更完整的了解是日益必要的，以及(2)目前的EMS实验测量了气体分子所有取向的平均值，导致关于电子结构的信息的巨大损失，特别是目标波函数的各向异性。然而，二元(e，2e)反应涉及的小截面阻碍了EMS的发展，在这种情况下，我们构建了一台电子-电子-碎片离子三符合谱仪。该光谱仪通过同时检测能量和动量，对两个出射电子具有极高的灵敏度，并使我们能够基于空间固定分子的轴向反冲碎裂来进行(e，2e)实验。通过利用中的多通道检测。该光谱仪已成功地应用于氢分子电离激发过程的碰撞动力学研究和氧分子定向动量密度的研究。这是首次观测到的分子骨架(e，2e)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.