Kinetic processes in a high pressure laser generated plasma
"A dense sodium vapor in a high-pressure noble buffer has been simultaneously excited by short (4 ns) laser pulses from two lasers: ii the first tuned to one of the 'D' line transitions and the second tuned to either the photoionization threshold of the 3p states near 406 nm or to a higher level 3p-nd resonance. The population densities of the excited levels of atomic sodium were measured with temporal resolution by absolute intensity measurements of excited state fluorescence. At early times (~100 ns), excited state populations are determined by energy transfer collisions between two laser-excited 3p atoms while the ion/electron density is controlled by super elastic heating of "seed electrons" in collision with 3p atoms followed by electron impact ionization of excited atoms. Excitation transfer rates into the 4d, 5d, 6d, and 6s levels are measured. At late times (~1 μs), excited state populations are controlled by collisional-radiative recombination. When the second laser is scanned through the various 3p-nd resonances, the degree of ionization obtained is a function of the oscillator strength of the transition at the laser wavelength. When such a scan is taken simultaneously in a cell with no buffer gas and one with a high pressure of buffer atoms, the collisional line shift due to the buffer atoms is measured. A comparison of shifts measured by this method with shifts of the same transitions at the same buffer pressure measured in the recombination fluorescence shows an additional shift in the recombination fluorescence emission due to the high density of electrons in the recombination regime"-- Abstract, p. ii
Missouri University of Science and Technology
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