QST, SPring-8, Kouto 1-1-1, Sayo, Hyogo 679-5148, Japan.
UVSOR, IMS, Nishigo-Naka 38, Myodaiji, Okazaki, Aichi 444-8585, Japan.
Phys Rev Lett. 2018 Dec 28;121(26):263201. doi: 10.1103/PhysRevLett.121.263201.
We report an experimental and numerical study of the propagation of free-electron laser pulses (wavelength 24.3 nm) through helium gas. Ionization and excitation populates the He^{+} 4p state. Strong, directional emission was observed at wavelengths of 469, 164, 30.4, and 25.6 nm. We interpret the emissions at 469 and 164 nm as 4p-3s-2p cascade superfluorescence, that at 30.4 nm as yoked superfluorescence on the 2p-1s transition, and that at 25.6 nm as free-induction decay of the 3p state.
我们报告了自由电子激光脉冲（波长 24.3nm）通过氦气传播的实验和数值研究。电离和激发使 He^{+} 4p 态饱和。在 469、164、30.4 和 25.6nm 波长处观察到强定向发射。我们将 469nm 和 164nm 的发射解释为 4p-3s-2p 级联超荧光，30.4nm 的发射解释为 2p-1s 跃迁上的耦合超荧光，25.6nm 的发射解释为 3p 态的自由感应衰减。
