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Description
We have detected self-amplified spontaneous emission (ASE) from 3a $^1P^o$ doubly excited state (DES) in He. In an isolated atom, this autoionizing resonance features a small $5\times 10^{-4}$ branching for fluorescence decay, preferably populating the singlet 1s3s singly excited state. To locate FEL wavelengths resonant with DES, the microfluidic gas cell was modified by adding the two parallel 100 nm diameter Pt wires to measure the charges generated in the gas by single FEL pulses. The optimum voltage difference between the two wires 400 $\mu$m apart was 30-40 V, which secured reasonably high signal and still avoiding discharges. The collected charge signal followed Fano profiles of autoionizing DES states and enabled a direct measurement of FEL spectral shift between the ASE emission maximum and Fano maximum and verification of a weak ASE amplification of fluorescence emitted by the 4a $^1P^o$ DES. We believe that the lack of ASE in this case comes because of the emission wavelength, which is already quite close to the 1s-2p energy difference in He$^+$. Since these ions are abundantly created in the gas, the ASE signal from the $n\geq4$ resonances is strongly absorbed in a given length of the gas column (9 mm). On the other hand, we saw a significant ASE signal from the 3b $^1P^o$ resonance. This means that regarding the ASE, 100 times smaller oscillator strength of the $3b$ versus $3a$ $^1P^o$ series can compensated by longer DES lifetime and 100-times larger fluorescence branching ratio of the $3b$ state. We found also unexpectedly large ASE signal emitted from the $2a$ $^1P^o$ state. This is surprising because the $2a$ state features 4-times smaller fluorescence branching ratio than $3a$ and has a significantly shorter lifetime (10 fs) than FEL pulse duration (50 fs). For $2a$ resonance, we found a strong excimer effect on ASE meaning that the wavelength of ASE emission from 2a drifts with the target pressure. The closeness of He atoms in the target not only allows for the amplification of emitted light but also affects emission wavelength by shifting energies of the upper and lower state via potential energy dependence on interatomic distance. At a given gas pressure we have detected a typical atomic linear Raman dispersion of the XUV emission signal meaning that non-linear ASE process just amplifies emission of an isolated atom while preserving its properties such as chemical shift.