High-sensitive laser spectroscopic method using atomic interference to measure microscopic electric field in plasmas

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We have proposed a sensitive method to directly measure the electric field in plasmas by using a laser-induced fluorescence (LIF) of a helium atom. In this LIF technique the electric field can be measured by observing only the linear polarization of the allowed fluorescence (4^1D→2^1P) subsequent to the forbidden excitation of metastable He atoms (2^1S→4^1D) due to the Stark-induced electric dipole and the electric quadrupole (QDP) transitions. In the case of the magnetized plasma a new high sensitive LIF method will be possible. The crossed magnetic field to electric field can induce the interference between the Stark and QDP amplitudes. The forbidden excitation generates anisotropic population between magnetic sublevels with opposite sign of quantum number (orientation), e.g. m_l = +1 and -1. The anisotropy appears as the dichroism in circular polarization components of LIF.The electric field dependence of the degree of circular polarization induced by the interference was theoretica … More lly calculated based on the selection rules of the transitions involved in the LIF process. It showed that the circular dichroism has high sensitivity (the minimum detectable electric field : 10 V/cm) and wide dynamic range as compared with the linear one. A model-type experiment was performed with a cylindrical hollow cathode discharge with the magnetic field applied perpendiculany to the electric field in the cathode dark space. We observed simultaneously both the circularly polarized components of LIF from the plasma along the magnetic field direction. The observed LIF was strongly polarized in the sheath. The degree of the polarization agreed with a theoretically calculated value. It should be noted that in the plasma the interference effect was observed for the first time. From the facts described above we may conclude that the use of the circular that the use of the circular dichroism will make it possible to measure the microscopic electric field of the plasma separately to the macroscopic one. Less