The pressure effect and time resolution of helium molecular bands in a pulsed microwave plasma Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/br86b691x

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  • The behavior of all of the observed molecular band emission as well as the prominent atomic line emission from a pulsed microwave helium plasma is given. Extensive efforts to purify the helium include the use of a uranium getter, a quartz diffusion tube, and a cataphoretic discharge. The plasma is produced in a cylindrical pyrex tube contained in a square waveguide section using pulsed x-band radiation from a 725A magnetron. From the variations of the light intensity as a function of time, together with the pressure and power effects it is concluded that the molecular radiation follows from the same process both during and after the microwave excitation pulse. The dominant process is believed to be collisional recombination by the reaction He⁺₂ + 2e --- He[superscript *]₂ with subsequent radiation from the excited molecules. This radiation appears at about the same time as the atomic emission upon production of the plasma. The low molecular intensity during the microwave excitation pulse is probably due to quenching by the applied microwave field. While the initial degree of vibrational excitation of the helium molecular ion is not known, the observed molecular emission involved only the 0,0 vibrational levels both during the pulse and in the afterglow. Because of the extensive pressure variation, significant changes in the temporal light intensity of the atomic emission are also observed. These observations show that the excitation pulse period emission and the afterglow emission are significantly different. The dominant process during the excitation pulse is undoubtedly inelastic electron excitation of the helium atom. At higher pressures recombination at a reduced level is also present during the pulse and is dominant in the afterglow. At least part of the decrease in atomic light intensity after the plasma formation and during the pulse is associated in time with increased transmission of the incident microwaves. There is, however, no observed change in the reflected microwave pulse at the time of this intensity decrease.
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