Collaborative Research: Probing Attosecond Electron Correlation in Atoms

物质中电子动力学的时间尺度用原子时间单位表示，即24阿秒。虽然孤立的阿秒光子脉冲在2001年被证明，但很少有实验室能够拥有这种新光源，因为产生技术非常困难。此外，现有的阿秒源不够强，无法进行阿秒泵浦/阿秒探测实验。在过去的五年里，Chang的团队开发了一种技术，双光门控（DOG），它可以很容易地产生孤立的阿秒脉冲。它还允许阿秒脉冲能量的放大。在目前国家科学基金会的支持下，Chang的研究组和Hu的理论小组共同努力，利用基于DOG方案的阿秒源，在时域上理解和控制超快电子-电子相互作用。我们关注原子的自离，因为它是由电子相关控制的。本文以双电子系统氦为研究对象。与过去使用同步加速器光进行的光谱域实验不同，氦原子被孤立的阿秒XUV脉冲从基态泵送到连续态或双激发态。双激发态的瞬时启动的电子相关，它们随后的快速衰减和与连续态的干扰，然后用另一个延时阿秒XUV脉冲或用强的少周期近红外（NIR）激光脉冲探测。阿秒探测脉冲将“冻结”两个电子的运动，并实时“绘制”出超快电子相关动力学。强近红外场可以在一个光周期的一小部分内改变电子-电子相互作用，从而操纵和控制超快电子相关。实验与理论的合作使得量子力学从头计算的实验基准成为可能，为研究更复杂的原子和分子中的电子动力学奠定了基础。在现代化学、物理学和生物学中，电子-电子相互作用在许多重要的多体现象中起着至关重要的作用。该计划的广泛影响是双重的。首先，开发用于以前所未有的时间分辨率观察电子动力学的工具将导致对电子相关如何在分子结构形成中发挥作用的基本问题的新见解。这些见解将特别有助于化学家更好地理解化学反应中的电子相关性。其次，寻找利用外场控制电子动力学的技术可以在基本电子水平上显著推进复杂系统和化学反应的操纵技术。此外，正在开发的新型阿秒光源可以推动超高速自由空间通信和生物分子成像的革命。该计划支持三名学生在物理学最令人兴奋的前沿之一工作。他们被训练成为这个新研究领域的领导者和下一代技术的专家。实验是在佛罗里达阿秒科学技术（FAST）实验室进行的，该实验室是在中佛罗里达大学新成立的。张教授开设了一门阿秒光学和另一门阿秒物理课程，供使用阿秒设备进行实验室演示的本科生和研究生使用。

The time scale of electron dynamics in matter is represented by the atomic unit of time, which is 24 attoseconds. Although isolated attosecond photon pulses were demonstrated in 2001, few laboratories have been able to own the new light source because the generation technique is very difficult. Furthermore, the existing attosecond source is not strong enough for performing attosecond pump/attosecond probe experiments. In the last five years, Chang's group has developed a technique, Double Optical Gating (DOG), which can generate isolated attosecond pulses rather easily. It also allows the up-scaling of attosecond pulse energy. With the current support from the National Science Foundation, Chang's experimental group and Hu's theoretical group work together to understand and control ultrafast electron-electron interaction in the time domain by taking advantage of the attosecond source based on the DOG scheme. We focus on the autoionization of atoms as it is dominated by the electron correlation. As the starting point, the two-electron system, helium, is chosen as the target. Unlike spectral-domain experiments performed in the past using synchrotron light, helium atoms are pumped from the ground state into either continuum states or doubly-excited states by isolated attosecond XUV pulses. The instantly-initiated electron-correlations in the doubly-excited states, their subsequent fast decay and the interference with the continuum states, are then probed either by another time-delayed attosecond XUV pulse or by an intense few-cycle near infrared (NIR) laser pulse. The attosecond probe pulse would "freeze" the motion of the two electrons and "map-out" in real time the ultrafast electron-correlation dynamics. The strong NIR field can modify the electron-electron interactions within a fraction of an optical cycle in order to manipulate and control the ultrafast electron correlations. The experiment-theory collaboration allows experimental benchmarking of the quantum-mechanical ab initio calculations, laying a foundation for studying electron dynamics in more complex atoms and molecules. Electron-electron interactions play an essential role in a wide range of fundamentally important many-body phenomena in modern chemistry, physics and biology. The broad impacts of this program are two-fold. First of all, developing tools for observing electron dynamics with an unprecedented time resolution will lead to new insights into the fundamental questions of how electron correlation plays a role in molecular structure formation. Such insights will particularly help chemists to understand electron correlations in chemical reactions better. Secondly, finding techniques to control electron dynamics with external fields can significantly advance the technologies for manipulating complex systems and chemical reactions at the fundamental electronic level. Moreover, the new attosecond light source under development can spur a revolution in ultrafast free-space communication and biomolecule imaging. The program supports three students to work at one of the most exciting forefronts of the physics. They are trained to become leaders in this new research field and experts of the next generation technologies. The experiments are conducted at the Florida Attosecond Science and Technology (FAST) laboratory, which was newly established at the University of Central Florida. A course on attosecond optics and another one on attosecond physics are offered by Chang for undergraduate and graduate students who use the attosecond facility for lab demonstrations.