Modeling of the initiation and evolution of a laserhyphen;ionized column in the lower atmosphere: 314.5 nm wavelength resonant multiphoton ionization of naturally occurring argon

摘要

A model has been developed to examine the effects of a 314.5 nm wavelength laser pulse directed vertically through the atmosphere on the local ion and electron concentrations in the beam path. The 314.5 nm wavelength was selected to exploit a threehyphen;photon excitation resonance with the 3p54sthinsp;1P10excited state in argon. Absorption of a fourth photon of the same wavelength will ionize the excited atom. Using a rate equation formalism and a detailed collection of atmospheric chemistry reactions the model provides the concentrations of electrons and ions as functions of altitude, time, laser energy, relative humidity, focal characteristics of the pulse, and ambient atmospheric electrichyphen;field conditions. Both linear and nonlinear effects on the propagation of the laser pulse have been taken into account. The calculated charged particle concentrations are used to estimate the conductivity of the ionized column. Results presented indicate peak electron densities up to 108cmminus;3can be created using laser intensities on the order of 108W/cm2. Electron lifetimes in the column are typically 50ndash;200 ns, however, the lifetime of the conductive channel is long in comparison (100 mgr;s). Longhyphen;lived ions created by charge transfer and attachment reactions between laserhyphen;produced argon ions and electrons and other atmospheric species provide a persistent enhanced conductivity in the beam path. Model results demonstrate that laser pulses having peak powers attainable with current technology can perturb the conductivity of an ionized column by as much as seven orders of magnitude in comparison to ambient conditions. A distinct tradeoff between column length and the extent of ionization is evident in the results presented. The tradeoff is a result of the competing mechanisms of multiphoton ionization and stimulated Raman scattering and is discussed.