超荧光、自由感应衰减和四波混频：自由电子激光脉冲通过氦离子致密样品的传播
Superfluorescence, free-induction decay, and four-wave mixing: propagation of free-electron laser pulses through a dense sample of helium ions
摘要：
本文报道了波长为24.3 nm的自由电子激光脉冲在氦气中传输的实验和数值研究。电离和激发填充He $^{+}$ 4 $p$状态。在469 nm、164 nm、30.4 nm和24.5 nm波长处观察到强的定向发射。我们将469 nm和164 nm处的发射解释为4 $p$ -3 $s$ -2 $p$级联超荧光，将30.4 nm处的发射解释为2 $p$ -1 $s$跃迁的耦合超荧光，将25.6 nm处的发射解释为3 $p$态的自由诱导衰减。
英文摘要：
We report an experimental and numerical study of the propagation of free-electron laser pulses (wavelength 24.3 nm) through helium gas. Ionisation and excitation populates the He$^{+}$ 4$p$ state. Strong, directional emission was observed at wavelengths of 469 nm, 164 nm, 30.4 nm, and 24.5 nm. We interpret the emissions at 469 nm and 164 nm as 4$p$-3$s$-2$p$ cascade superfluorescence, that at 30.4 nm as yoked superfluorescence on the 2$p$-1$s$ transition, and that at 25.6 nm as free-induction decay of the 3$p$ state.
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