Abstract

A sensitive non-intrusive spatially, temporally as well as spectrally resolved and quantitative detection of oxygen atoms in hostile environments such as combustion processes, high pressure gases or discharges is of primordial importance for the study of a wide variety of chemical reactions. Most of the conditions mentioned imply an optical detection method which avoids direct contact with the sample. In order to avoid problems with noil-resonant background while at the same time maintaining high sensitivity and resolution a resonance enhanced nonlinear optical method such as degenerate four-wave mixing (DFWM) could be applied. In DFWM three laser beams from the same laser source are overlapped to generate a resonant, coherent superposition of electromagnetic fields, which scatters a fourth signal beam. The phase conjugate signal beam carries information about, the absorption properties of the irradiated medium via the third order nonlinear susceptibility tensor χ⁽³⁾ Here we apply a sophisticated variant of DFWM. namely two-photon DFWM to the 2p³P_J →3p³P′_J transitions in oxygen atoms, which are produced inside a microwave discharge. In contrast to one-photon DFWM only two of the four photons together are resonant with an absorption line. As a result, excited state population is insignifeant for the grating formation and thus the signal intensity does not suffer from collisional quenching and absorption processes.

© 1996 IEEE