Speaker
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.
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