Analysis of the M-shell spectra emitted by a short-pulse laser-created tantalum plasma

The spectrum of tantalum emitted by a subpicosecond laser-created plasma, was recorded in the regions of the $3d\ensuremath{-}5f,$ $3d\ensuremath{-}4f,$ and $3d\ensuremath{-}4p$ transitions. The main difference with a nanosecond laser-created plasma spectrum is a broad understructure appearing under the $3d\ensuremath{-}5f$ transitions. An interpretation of this feature as a density effect is proposed. The supertransition array model is used for interpreting the spectrum, assuming local thermodynamic equilibrium (LTE) at some effective temperature. An interpretation of the $3d\ensuremath{-}4f$ spectrum using the more detailed unresolved transition array formalism, which does not assume LTE, is also proposed. Fitted contributions of the different ionic species differ slightly from the LTE-predicted values.