Xu Han, He Feng, Kielpinski D, Sang R T, Litvinyuk I V
Centre for Quantum Dynamics and Australian Attosecond Science Facility, Griffith University, Nathan, QLD 4111, Australia.
Key Laboratory for Laser Plasmas (Ministry of Education), and Department of Physics and Astronomy, IFSA Collaborative Innovation Center, SJTU, Shanghai 200240, People's Republic of China.
Sci Rep. 2015 Aug 28;5:13527. doi: 10.1038/srep13527.
When a diatomic molecule is ionized by an intense laser field, the ionization rate depends very strongly on the inter-nuclear separation. That dependence exhibits a pronounced maximum at the inter-nuclear separation known as the "critical distance". This phenomenon was first demonstrated theoretically in H2(+) and became known as "charge-resonance enhanced ionization" (CREI, in reference to a proposed physical mechanism) or simply "enhanced ionization"(EI). All theoretical models of this phenomenon predict a double-peak structure in the R-dependent ionization rate of H2(+). However, such double-peak structure has never been observed experimentally. It was even suggested that it is impossible to observe due to fast motion of the nuclear wavepackets. Here we report a few-cycle pump-probe experiment which clearly resolves that elusive double-peak structure. In the experiment, an expanding H2(+) ion produced by an intense pump pulse is probed by a much weaker probe pulse. The predicted double-peak structure is clearly seen in delay-dependent kinetic energy spectra of protons when pump and probe pulses are polarized parallel to each other. No structure is seen when the probe is polarized perpendicular to the pump.
当一个双原子分子被强激光场电离时，电离率强烈依赖于核间距。这种依赖关系在被称为“临界距离”的核间距处呈现出明显的最大值。这种现象最初在H₂⁺中得到理论证明，并被称为“电荷共振增强电离”（CREI，参考一种提出的物理机制）或简称为“增强电离”（EI）。该现象的所有理论模型都预测H₂⁺的电离率随核间距（R）变化呈现双峰结构。然而，这种双峰结构从未在实验中被观测到。甚至有人认为由于核波包的快速运动，不可能观测到。在此我们报道了一个几周期泵浦 - 探测实验，该实验清晰地分辨出了这种难以捉摸的双峰结构。在实验中，由强泵浦脉冲产生的正在膨胀的H₂⁺离子被一个弱得多的探测脉冲探测。当泵浦脉冲和探测脉冲相互平行偏振时，在质子的延迟相关动能谱中清晰地看到了预测的双峰结构。当探测脉冲垂直于泵浦脉冲偏振时，则看不到结构。
