Determination of the Li I 4d–4f Energy Separation Using Active Spectroscopy

An accurate knowledge of the Li i 4d–4f energy separation is essential for the determination of electric fields, as is pursued using several modern diagnostic techniques. However, there is a rather large spread in the values of this quantity in the available data sources. We have measured the Li i 4d–4f energy separation using a technique that combines laser-induced-fluorescence with the utilization of collisional excitations. The plasma used for these measurements is laser-produced, which allows for selection of an electron-density range for which line shifts due to the plasma microfields are sufficiently small. An observation of the forbidden 2p–4f line provides the information on the microfields that allows for accounting for the Stark-shifts and evaluating the 4d–4f energy separation in the field-free limit. A comparison of the measured value with a few previous measurements allows for resolving the uncertainties in this quantity.