Using femtosecond transient absorption spectroscopy on excited-state helium atoms in a plasma created through optical field ionization, we measured the decay of $2{}^{3}S\mathrm{\text{−}}2{}^{3}P$ excitation with sub-ps temporal resolution. The population evolution shows that initial decay is significantly faster than the electron-atom collisions and three orders of magnitude faster than the single atom spontaneous decay rate. This indicates on superradiant coherent behavior of the atomic system inside the plasma.

• Received 27 February 2012

DOI:https://doi.org/10.1103/PhysRevLett.109.093604