POLARIZATION-SENSITIVE 4-PHOTON SPECTROSCOPY OF ATOMS AND IONS IN THEPLASMA OF OPTICAL-BREAKDOWN

An experimental technique of coherent ellipsometry of atomic and ionic resonances in the spectrum of three-color four-wave mixing in excited gas media is developed. We investigate the influence of the quasireso nant component of the nonlinear-optical susceptibility on frequency de pendences of the intensity and polarization parameters of the signal r esulting from four-wave mixing. It is demonstrated that the polarizati on technique in spectroscopy of four-photon scattering is an efficient tool for studying closely spaced and overlapping spectral lines of at oms and ions in plasmas. We develop an experimental technique for the two-dimensional mapping of the relative populations of excited states of atoms and ions in a low-temperature plasma of optical breakdown usi ng coherent four-wave mixing with hyper-Raman resonances. The proposed technique is employed for the investigation of the spatial distributi on of excited atoms and ions in a laser-produced plasma near a surface of a metal target. To increase the sensitivity of the developed techn ique, we implemented a polarization control of the shapes of resonance s in the spectrum of four-wave mixing.