Ultrafast Nonlinear Double Excitations of He in Intense EUV FEL Fields

Fushitani, Mizuho; Hikosaka, Y.; Matsuda, Akihiro; Liu, C.-N.; Morishita, Tōru; Shigemasa, E.; Hishikawa, Akiyoshi

doi:10.1051/epjconf/20134102009

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“…In particular, the measurement of the double-excitation spectrum [18] was one of the early ground-breaking applications of synchrotron radiation. The advent of XUV FEL sources has enabled the study of nonlinear effects in helium such as resonant multiphoton excitation [19,20] and superfluorescence at visible wavelengths [21]. While XUV amplified spontaneous emission from helium has not been observed before, superfluorescence from the 3a 1 P o doubly excited state was investigated theoretically in Ref.…”

Section: Introductionmentioning

confidence: 99%

XUV superfluorescence from helium gas in the paraxial three-dimensional approximation

et al. 2023

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We present the results of a theoretical study of XUV superfluorescence from doubly excited states of helium resonantly pumped by free-electron laser (FEL) pulses. Our model allows us to predict both the spectrum and angular distribution of emitted XUV radiation in a wide range of experimentally accessible parameters. This is achieved by going beyond two key deficiencies of most previous models: The one-dimensional treatment in space is upgraded to three dimensions with electromagnetic fields treated in the paraxial approximation and spontaneous emission is modeled by a recently developed approach that avoids the unrealistic delayed response but preserves the expected characteristics of the emitted field in the spontaneous emission limit. The case study of 3a 1 P o resonance in helium with 63.66 eV excitation energy is presented for realistic parameters of seeded light pulses from the FERMI FEL facility and a recently developed high-pressure gas cell. Results of numerical simulations show that both the spectral width and angular divergence of emitted radiation vary with gas pressure and pump pulse intensity in a complex way.

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