Observation of two-electron transitions in dense non-Maxwellian laser-produced plasmas and their use as diagnostic reference lines

Two-electron satellite transitions near He_β, 1s2s3s-1s²2p and 1s2s3d-1s²2p, have been identified in dense laser-produced plasmas. Space resolved high-resolution spectra indicate that their existence is confined to the area of the laser spot thereby providing a localized emission source of the highest density plasma region. Detailed modeling with the MARIA-code verifies their advantageous use as diagnostic reference lines in dense optically thick plasmas. Close to the target surface, anomalous high emission is encountered for the Li-like two-electron transitions op = 1s2s²-1s²2p. The high resolution data taken with spherically bent mica crystals indicates that there is an intensity correlation with higher order intercombination satellite transitions 1s2131′-1s²21″ near He_α. An important finding is the very good agreement between the data and the simulations, when considering forbidden transitions only and when using two-electron transitions as reference lines. Based on these reference lines space resolved non-Maxwellian signatures could be determined.