Koval P, Wilken F, Bauer D, Keitel C H
Max-Planck-Institut für Kernphysik, Postfach 103980, 69029 Heidelberg, Germany.
Phys Rev Lett. 2007 Jan 26;98(4):043904. doi: 10.1103/PhysRevLett.98.043904. Epub 2007 Jan 24.
A second plateau in the harmonic spectra of laser-driven two-electron atoms is observed both in the numerical solution of a low-dimensional model helium atom and using an extended strong field approximation. It is shown that the harmonics well beyond the usual cutoff are due to the simultaneous recombination of the two electrons, which were emitted during different, previous half-cycles. The new cutoff is explained in terms of classical trajectories. Classical predictions and the time-frequency analysis of the ab initio quantum results are in excellent agreement. The mechanism corresponds to the inverse single photon double ionization process in the presence of a (low frequency) laser field.
在低维模型氦原子的数值解以及扩展的强场近似中，均观察到了激光驱动双电子原子谐波谱中的第二个平台。结果表明，远超出通常截止频率的谐波是由于两个电子同时复合产生的，这两个电子是在之前不同的半周期发射的。新的截止频率可以用经典轨迹来解释。经典预测与从头算量子结果的时频分析结果高度吻合。该机制对应于存在（低频）激光场时的逆单光子双电离过程。
