Multiphoton Ionization Via an Excited State; A Survey of its Effect on Laser Breakdown in the Atmosphere
NAVAL RESEARCH LAB WASHINGTON DC FUNDAMENTAL PLASMA PROCESSES
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The program on controlled atmospheric breakdown at a distance depends on both the compression and focusing of the laser beam, and also on the breakdown mechanism. For normal atmospheric constituents and available laser wavelengths, the ionization energy is much greater than the photon energy. If the laser pulse time is so short that avalanche breakdown cannot occur, multiphoton ionization becomes important. However, the more photons one needs, the more difficult it is to ionize. In this respect, ultraviolet photons are better than optical, which in turn are better than infrared. In this report, we examine multiphoton ionization via the excitation of an intermediate state. Most experimental data indicate that the ionization does occur through an intermediate state both for nanosecond ns and femtosecond fs pulses. This makes ionization easier to accomplish than it might otherwise have been and is thereby of potential importance to the ONR program. This report surveys some of the experimental data and also examines the theoretical basis for the frequent importance of multiphoton ionization via an intermediate state.
- Atmospheric Physics
- Atomic and Molecular Physics and Spectroscopy