Amplification of Spontaneous Emission from Doubly Excited He Atoms

We have observed amplification of weak fluorescence from He atoms in the 3a <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mrow><a:mmultiscripts><a:mrow><a:msup><a:mrow><a:mi>P</a:mi></a:mrow><a:mrow><a:mi>o</a:mi></a:mrow></a:msup></a:mrow><a:mprescripts/><a:none/><a:mrow><a:mn>1</a:mn></a:mrow></a:mmultiscripts></a:mrow></a:math> autoionizing state, which decay by spontaneous emission of 40.74 eV photons in one out of 1600 cases. On average, a <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>4.1</c:mn><c:mo>±</c:mo><c:mn>1.6</c:mn><c:mo>%</c:mo></c:math> conversion of 63.67 eV photons was detected in the forward direction at <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mrow><e:mn>20</e:mn><e:mtext> </e:mtext><e:mtext> </e:mtext><e:mi mathvariant="normal">μ</e:mi><e:mi mathvariant="normal">J</e:mi></e:mrow></e:math> 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.