- 1998 by Z. Tang et al. The REMPI (Resonant Enhanced MultiPhoton Ionization) process in naturally occurring argon is a candidate for providing a long ionized channel through the atmosphere, which can be used for the control and guiding of electrical discharges. This paper will present a preliminary theoretical model and an experimental study on a very promising 3+1 ionization in neutral argon at fractional atmospheric pressure near 261.27 nm. Photo-ionization of argon is greatly enhanced by a three photon transition from the 3p6 1S0 ground state to the 3p53d [5/2] J=3 state of argon. The 3p53d [5/2] J=3 state has neither one photon nor two-photo allowed transitions to the ground state. A scheme of evaluating the generalized three-photon excitation cross section j(3) in neutral argon is outlined. For the laser line-width of 200 MHz, the estimated value of the upper bound of the three photon absorption cross section is j(3)=7.94l0-79 cm6s2. The one photon ionization cross section from the 3p53d [5/2] J=3 state at 261.27 nm is evaluated to be l.04l0-18 cm2. The output of a frequency tripled, injection seeded Ti: Sapphire laser was focused into an argon sample cell. The REMPI signal near 261.27 nm was observed and collected by a pair of biased-potential electrodes which serve as an ionization probe. The typical pulse excitation conditions were 25-nsec pulse duration, 200 MHz line-width, 3-5 mJ, 200 m spot diameter. The argon pressure was 60- 200 torr.