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

Abstract

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.

Electric fields in plasmas under pulsed currents

“…The line shape simulation, similar to that described in Ref. [21], show the individual contribution of each of the three broadening mechanisms: the Stark effect, Doppler, and instrumental.…”

“…Therefore, the 4f-4d energy separation is crucial for an accurate modeling. We have measured its value independently [21] in order to resolve the ambiguity in light of the large spread in the values of this quantity in the available data sources.…”

“…A detailed discussion of the spectral calibration and the determination of the instrumental response of the system can be found in Ref. [21].…”

Term energies of 1s2nf high Rydberg states for the lithium atom

“…Radziemski et al [13] used low-current hollow cathode sources and Fourier-transform spectrometry to determine the 4d-4f energy splitting as (4.988 ± 0.003) cm −1 , without reflecting the accuracy in the field-free limit. Tsigutkin et al [14] measured the LiI 4d-4f energy separation using a technique that combines laser-induced-fluorescence with the utilization of collisional excitations; they evaluated the 4d-4f energy separation in the field-free limit as 5.1 ± 0.2 cm −1 . Safronova et al [19] presented a calculation of the 4d-4f energy separation in Li I with relativistic all-order perturbation theory values of E(SD) = 4.99 cm −1 , E(SDpT) = 4.92 cm −1 and a multiconfiguration Hartree-Fock value of E(MCHF) = 4.98 cm −1 .…”

“…The most obvious effect is the depression of energies for lower-l states, which arises from the polarization and penetration of the core by the Rydberg electron. For the Rydberg states of lithium, there are many experimental measurements reported in the literature [9][10][11][12][13][14][15], but there are few measurements on 1s 2 nf 2 F high Rydberg states. Baig et al [16] reported observations of the Rydberg states of atomic lithium excited by the two-colour three photon resonance ionization technique in conjunction with a space charge limited thermionic diode ion detector; the odd parity nf 2 F Rydberg series were detected for the first time for n = 13-48, only 50 cm −1 from the ionization limit.…”