@article{oai:repo.qst.go.jp:00049464,
 author = {ハリーズ, ジェームズ and Iwayama, H and Kuma, S and Iizawa, M and Suzuki, N and Azuma, Y and Inoue, I and Owada, S and Togashi, T and Tono, K and Yabashi, M and Shigemasa, E and ハリーズ ジェームズ},
 issue = {26-28},
 journal = {physical review letters},
 month = {Dec},
 note = {We report an experimental and numerical study of the propagation of free-electron laser pulses (wavelength \\SI{24.3}{\
ano\\metre}) through helium gas. Ionisation and excitation populates the He$^{+}$~4$p$ state. Strong, directional emission was observed at wavelengths of \\SIlist{469;164;30.4;25.6}{\
ano\\metre}. We interpret the emissions at \\SIlist{469;164}{\
ano\\metre} as 4$p$-3$s$-2$p$ cascade superfluorescence, that at \\SI{30.4}{\
ano\\metre} as yoked superfluorescence on the 2$p$-1$s$ transition, and that at \\SI{25.6}{\
ano\\metre} as free-induction decay of the 3$p$ state.},
 pages = {263201-1--263201-6},
 title = {Superfluorescence, free-induction decay, and four-wave mixing: propagation of free-electron laser pulses through a dense sample of helium ions},
 volume = {121},
 year = {2018}
}