Kanai Tsuneto, Minemoto Shinichirou, Sakai Hirofumi
Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Nature. 2005 May 26;435(7041):470-4. doi: 10.1038/nature03577.
High-order harmonic generation (HHG) from atoms and molecules offers potential application as a coherent ultrashort radiation source in the extreme ultraviolet and soft X-ray regions. In the three-step model of HHG, an electron tunnels out from the atom and may recombine with the parent ion (emitting a high-energy photon) after undergoing laser-driven motion in the continuum. Aligned molecules can be used to study quantum phenomena in HHG associated with molecular symmetries; in particular, simultaneous observations of both ion yields and harmonic signals under the same conditions serve to disentangle the contributions from the ionization and recombination processes. Here we report evidence for quantum interference of electron de Broglie waves in the recombination process of HHG from aligned CO2 molecules. The interference takes place within a single molecule and within one optical cycle. Characteristic modulation patterns of the harmonic signals measured as a function of the pump-probe delay are explained with simple formulae determined by the valence orbital of the molecules. We propose that simultaneous observations of both ion yields and harmonic signals can serve as a new route to probe the instantaneous structure of molecular systems.
原子和分子的高次谐波产生（HHG）在极紫外和软X射线区域作为一种相干超短辐射源具有潜在应用价值。在HHG的三步模型中，一个电子从原子中隧穿出来，在连续态中经历激光驱动的运动后，可能会与母离子重新结合（发射一个高能光子）。取向排列的分子可用于研究HHG中与分子对称性相关的量子现象；特别是，在相同条件下同时观测离子产率和谐波信号有助于区分电离和复合过程的贡献。在此，我们报告了来自取向排列的CO₂分子的HHG复合过程中电子德布罗意波量子干涉的证据。这种干涉发生在单个分子内部以及一个光学周期内。作为泵浦 - 探测延迟函数测量的谐波信号的特征调制模式，可用由分子价轨道确定的简单公式来解释。我们提出，同时观测离子产率和谐波信号可作为探测分子系统瞬时结构的一条新途径。
