Two-dimensional mapping of the relative populations of the excited states of atoms and ions in a laser-produced plasma by the method of coherent four-wave mixing

An experimental method was developed for two-dimensional mapping of the relative populations of excited states of atoms and ions in a low-temperature plasma formed by optical breakdown. The method was based on coherent four-wave mixing with a resonance of the hyper-Raman type. An investigation was made of the spatial distribution of excited atoms and ions in a plasma generated by laser radiation incident on the surface of a metal target. The sensitivity of the method was enhanced by polarisation control of the profile of resonances in the spectrum of four-wave mixing.