Enhancement of Above Threshold Ionization in Resonantly Excited Helium Nanodroplets

Clusters and nanodroplets hold the promise of enhancing high-order nonlinear optical effects due to their high local density. Here we explore the nonlinear optical response of He nanodroplets prepared in multiply excited states by irradiation with relatively intense XUV pulses.R.Michiels et al.Physical Review Letters 127 093201 (2021) 
Photoelectron spectra of He droplets

In this paper we report the observation of energetic electrons generated by above-threshold ionization (ATI) of helium (He) nanodroplets which are resonantly excited by ultrashort extreme ultraviolet (XUV) free-electron laser pulses and subsequently ionized by near-infrared (NIR) or near-ultraviolet (UV) pulses. The electron emission due to high-order ATI is enhanced by several orders of magnitude compared with He atoms. The crucial dependence of the ATI intensities with the number of excitations in the droplets suggests a local collective enhancement effect.
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Michiels R., Abu-samha M., Madsen L. B., Binz M., Bangert U., Bruder L., Duim R., Wituschek A., LaForge A. C., Squibb R. J., Feifel R., Callegari C., Di Fraia M., Danailov M., Manfredda M., Plekan O., Prince K. C., Rebernik P., Zangrando M., Stienkemeier F., Mudrich M. Physical Review Letters 127 093201 (2021)
doi: 10.1103/PhysRevLett.127.093201
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