Oxygen atom detection using third harmonic generation

Atomic oxygen has been detected using the four-wave mixing technique of third harmonic generation (THG). Ground state O((3)P(j)) atoms, generated using a microwave discharge, were monitored by focusing a 391-nm dye laser beam into the post-discharge region and observing radiation at 130 nm, near resonant with the 3s (3)S(0)(1)? 2p(4)(3)P(j) transition, produced along the laser propagation direction. The observed concentration dependent frequency shifts of the laser excitation spectrum verified the assignment of THG. Oxygen atoms were also detected in the same manner following photodissociation of NO(2). An important point for diagnostic purposes is that both the VUV intensity and peak frequency shift are dependent on O atom concentration. The relationship between these variables was explored by employing chemical titrations to establish absolute concentration. The demonstrated sensitivity is 5 x 10(13) cm(-3), but this could be significantly improved. Implications for three-photon resonant multiphoton ionization detection as well as in situ monitoring of atomic oxygen metastable states ((1)D(2) and (1)S(0)) are discussed. Third harmonic generation provides state selective and low aperture detection with sensitivity comparable to other optical methods and shows promise as a general detection technique.

Third harmonic generation, optical detection, four-wave mixing, oxygen, atomic detection, atomic spectroscopy, VUV radiation.