Panoramic two-dimensional visualisation of the spatial distribution of the atomic and ionic components of a laser plasma by the method of coherent four-photon spectroscopy

An experimental method is proposed for two-dimensional mapping of the relative populations of excited states of atoms and ions in a low-temperature optical-breakdown plasma. The method is based on a technique of coherent four-wave mixing combined with a hyper-Raman resonance. A study is reported of the possibility of obtaining information on the spatial distribution of the excited-state populations of atoms and ions from two-dimensional maps of the four-wave mixing signal taking account of the phase mismatch and of one-photon absorption. Conditions were found experimentally for the four-wave mixing in a plasma in a phase-matched regime without a significant influence of the one-photon absorption.