[论文解读] Attosecond angular streaking and tunnelling time in atomic hydrogen

U. Satya Sainadh, Han Xu+10 more|arXiv (Cornell University)|Jul 18, 2017

Laser-Matter Interactions and Applications被引用 14

一句话总结

本研究首次对原子氢实施了阿秒角向条纹实验，采用实验激光参数的精确三维时间依赖薛定谔方程（3D-TDSE）模拟。实验结果与模拟在偏移角度上表现出极佳的一致性，结论为氢原子中的隧穿过程是瞬时的（上限为1.8 as），观测到的角向偏移完全由库仑散射引起，而非隧穿时间延迟。

ABSTRACT

Tunnelling, one of the key features of quantum mechanics, ignited an ongoing debate about the value, meaning and interpretation of 'tunnelling time'. Until recently the debate was purely theoretical, with the process considered to be instantaneous for all practical purposes. This changed with the development of ultrafast lasers and in particular, the 'attoclock' technique that is used to probe the attosecond dynamics of electrons. Although the initial attoclock measurements hinted at instantaneous tunnelling, later experiments contradicted those findings, claiming to have measured finite tunnelling times. In each case these measurements were performed with multi-electron atoms. Atomic hydrogen (H), the simplest atomic system with a single electron, can be 'exactly' (subject only to numerical limitations) modelled using numerical solutions of the 3D-TDSE with measured experimental parameters and acts as a convenient benchmark for both accurate experimental measurements and calculations. Here we report the first attoclock experiment performed on H and find that our experimentally determined offset angles are in excellent agreement with accurate 3D-TDSE simulations performed using our experimental pulse parameters. The same simulations with a short-range Yukawa potential result in zero offset angles for all intensities. We conclude that the offset angle measured in the attoclock experiments originates entirely from electron scattering by the long-range Coulomb potential with no contribution from tunnelling time delay. That conclusion is supported by empirical observation that the electron offset angles follow closely the simple formula for the deflection angle of electrons undergoing classical Rutherford scattering by the Coulomb potential. Thus we confirm that, in H, tunnelling is instantaneous (with an upperbound of 1.8 as) within our experimental and numerical uncertainty.

研究动机与目标

• 通过基准体系解决量子力学中隧穿时间长期存在的争议。
• 对最简单的单电子原子——原子氢，首次开展阿秒角向条纹实验。
• 检验隧穿时间延迟是否对阿秒时钟测量中观测到的电子角向偏移有贡献。
• 通过将实验数据与使用实测激光参数的精确3D-TDSE模拟进行对比，验证理论模型。
• 确定阿秒时钟实验中电子偏移角的起源——是隧穿延迟还是库仑散射。

提出的方法

• 利用少周期激光脉冲对原子氢进行阿秒角向条纹实验。
• 使用实验测得的激光参数（强度、偏振、持续时间）进行三维时间依赖薛定谔方程（3D-TDSE）模拟。
• 采用长程库仑势来模拟氢原子中的电子动力学。
• 在模拟中用短程杨氏势（Yukawa势）替代库仑势，以分离长程相互作用的作用。
• 将实验测得的偏移角度与模拟结果进行对比，评估一致性并推断角向偏移的起源。
• 应用经典卢瑟福散射公式预测偏转角度，并与实验和模拟数据进行对比。

实验结果

研究问题

• RQ1 原子氢中的隧穿过程是否表现出可测量的时间延迟，还是为瞬时过程？
• RQ2 阿秒时钟实验中观测到的电子偏移角的起源是什么——是隧穿时间延迟还是库仑散射？
• RQ3 使用实验激光参数的3D-TDSE模拟能否准确再现氢原子的实验阿秒时钟数据？
• RQ4 与库仑势相比，引入短程势（杨氏势）对预测偏移角度有何影响？
• RQ5 经典卢瑟福散射预测在多大程度上与实验中观测到的电子偏转角度相吻合？

主要发现

• 实验测得的原子氢偏移角度与使用库仑势的3D-TDSE模拟结果表现出极佳的一致性。
• 采用短程杨氏势的模拟在所有强度下均预测零偏移角度，表明长程库仑相互作用对观测到的角向偏移至关重要。
• 测得的偏移角度与经典卢瑟福散射公式高度吻合，证实库仑散射是角向偏移的主要起源。
• 数据排除了隧穿时间延迟在氢原子中产生显著贡献的可能性，任何此类延迟的上限为1.8 as。
• 结果支持结论：在实验和数值不确定度范围内，氢原子中的隧穿过程可视为有效瞬时。