ECAMP 15

We have detected self-amplified spontaneous emission (ASE) from He atoms in $3^+\ ^1P^o$ doubly excited state. This resonance with 63.66 eV excitation energy autoionizes within 80 fs but may also decay by spontaneously emitting a 40.75-eV photon to populate the 1s3s 1Se atomic state with $3\times 10^{-4}$ probability [1,2]. Despite such a small fluorescence branching ratio, our recent calculations in the paraxial approximation predicted strong ASE in the forward direction if a sufficiently dense and long column of helium gas is traversed by intense, resonantly tuned XUV light [3]. Indeed, stimulated amplification of the weak $3^+\ ^1P^o\rightarrow 1s3s\ ^1S^e$ fluorescence decay was observed at the EIS-TIMEX beamline using light pulses from the free electron laser (FEL-1) facility FERMI in Trieste, Italy. The helium gas column was a few mm long and the pressure went up to 100 mbar. The 50-fs long FEL pulses with few tens of $\mu J$ energy were focused to the $15 \times 26\ \mu$m$^2$ cross section in the center of an open-end glass micro-capillary. A maximum observed average conversion factor from the number of probe photons to the number of ASE photons was 4.1%, quite consistent with the theoretical prediction. Simultaneously with ASE, we have measured position-resolved VIS emission spectra collecting the light from different positions along the gas cell. The VIS signal was due to late transitions between singlet and triplet singly excited states in the atomic He, as well between the excited states in He+ ions. The intensity of VIS lines was found to diminish with a distance from the cell entrance as dictated by absorption of the FEL light along the cell. A weak correlation of the single-shot VIS spectra with the ASE emission was detected.\
[1] F. Penent et al., Phys. Rev. Lett. 86, 2758-2761 (2001).
[2] J. Soderstrom, Phys. Rev. A 77, 012513 (2008).
[3] Š. Krušič et al., Phys. Rev. A 107, 013113-1-13 (2023).