Warrick Erika R, Fidler Ashley P, Cao Wei, Bloch Etienne, Neumark Daniel M, Leone Stephen R
Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Faraday Discuss. 2018 Dec 13;212(0):157-174. doi: 10.1039/c8fd00074c.
Nonlinear multidimensional spectroscopy is ubiquitous in the optical and radio frequency regions as a powerful tool to access structure and dynamics. The extension of this technique into the extreme ultraviolet (XUV) region with attosecond pulses holds promise for probing electronic dynamics and correlations with unprecedented time and spatial resolution. In this work, we use noncollinear four-wave mixing of a weak XUV attosecond pulse train (11-17 eV) and few-femtosecond NIR pulses (800 nm) to spectroscopically and dynamically probe the dipole-forbidden double-well potential of the a'' 1∑+g electronic state of nitrogen. The results demonstrate optical coupling of the inner and outer limits of the initial XUV-prepared vibrational wave packet in the valence character b' 1∑+u state to the inner and outer wells, respectively, of the a'' 1∑+g double well state by 800 nm light. Two four-wave mixing schemes with different pulse timing sequences and noncollinear beam geometries are used (one NIR pulse collinear and one NIR pulse noncollinear versus both NIR pulses noncollinear to the XUV beam) to measure the a'' dark state energetic structure and to control the dynamical preparation and motion of a dark state wave packet by selective population of either the inner Rydberg or outer valence-character potential well. Experimental measurements of the a'' 1∑+g outer well vibrational spacing and anharmonicity closely match the values theoretically predicted for this previously unobserved state.
非线性多维光谱在光学和射频领域无处不在，是一种用于研究结构和动力学的强大工具。将该技术扩展到具有阿秒脉冲的极紫外（XUV）区域，有望以前所未有的时间和空间分辨率探测电子动力学和相关性。在这项工作中，我们利用弱XUV阿秒脉冲序列（11 - 17 eV）和飞秒近红外脉冲（800 nm）的非共线四波混频，对氮分子a'' 1∑+g电子态的偶极禁戒双阱势进行光谱和动力学探测。结果表明，通过800 nm光，初始XUV制备的价态b' 1∑+u态振动波包的内、外极限分别与a'' 1∑+g双阱态的内、外阱发生光学耦合。使用了两种具有不同脉冲时序和非共线光束几何结构的四波混频方案（一个近红外脉冲共线，一个近红外脉冲非共线，与两个近红外脉冲均与XUV光束非共线的情况相对比）来测量a''暗态的能量结构，并通过选择性填充内里德堡或外价态特征势阱来控制暗态波包的动力学制备和运动。对a'' 1∑+g外阱振动间距和非谐性的实验测量结果与理论预测的该先前未观测到的态的值紧密匹配。
