Amplification of Spontaneous Emission from Doubly Excited He Atoms

We have observed amplification of weak fluorescence from He atoms in the 3a1P0 autoionizing state, which decay by spontaneous emission of 40.74 eV photons in one out of 1600 cases. On average, a 4.1 +/-1.6% conversion of 63.67 eV photons was detected in the forward direction at 20  μuJ pump pulse energy and 40 mbar He gas pressure. A comparison with state-of-the-art theoretical simulations shows an unexpected spectral line shift at high gas pressure that is ascribed to transient molecular and electron scattering effects.

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