Forbidden Transitions of LiI in Laser-Induced Fluorescence Spectroscopy for the Measurement of Low-Frequency Electric Fields in Plasmas

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Abstract
Three forbidden transition (42P-22P, 42D-32D and 22P-42F) due to the Stark effect in the LiI energy system were theoretically investigated for the measurement of low-frequency electric fields in plasmas. Numerical results concerning the intensities of the forbidden transitions and the polarization characteristics are presented in which the effect of the electron spin is taken into account. The excitation of the levels of n=4 is supposed to be possible using tunable lasers and thus, the effect of laser polarization is also considered. The first two transitions (42P-22P and 42D-32D) are suitable for measuring a rather strong electric field (1 kV/cm) and can also be used to give the direction of the electric field in plasmas. The third transition is applicable to a weak-field measurement down to about 0.1 kV/cm.