Journal Article Single-atom response of helium atoms to pulses from an EUV free-electron laser - implications for the subsequent development of superfluorescence

ハリーズ, ジェームズ  ,  Ohae, C  ,  Kuma, S  ,  Nakajima, K  ,  Togashi, T  ,  Miyamoto, Y  ,  Sasao, N  ,  Iwayama, H  ,  Nagasono, M  ,  Yabashi, M  ,  Shigemasa, E

94pp.063416-1 - 063416-9 , 2017-02 , American Physical Society
Using multilevel (161-level) numerical simulations beyond the rotating-wave approximation, we investigate the single-atom response of helium to intense pulses of radiation with properties characteristic of a SASE free-electron laser source, and central wavelengths resonant with 1snp excitation (n=3,4). The results show a complete breakdown of the two-level approximation, and suggest that under typical experimental conditions levels other than the resonant level are significantly populated. We discuss the implications for the subsequent development of superfluorescence (a collective effect) from the excited atomic medium, and compare the results to recent experiments.

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