Measurement of the temporal behavior of the H−-density in a pulsed hydrogen multipole discharge using the photodetachment technique

An increase of the H−-density in a pulsed low-pressure hydrogen multicusp discharge is observed after the current pulse, i.e. in the “post-discharge.” The H−-density is determined by laser induced photodetachment at a Langmuir probe. A semiempirical model is used to infer the total density of vibrationally excited molecules in the electronic ground state from a comparison with the measured shape of the transient H−-density in the post-discharge. The total density of vibrationally excited molecules is found to be independent of the discharge current up to 20 amps. The density evolution of vibrationally excited molecules during the discharge is obtained from measurements of the maximum H−-density in the post-discharge as a function of the duration of a single discharge pulse. After 200 μs the density is saturated. The lifetime of vibrationally excited molecules in the post-discharge is determined by varying the time interval between discharge pulses of short duration. The lifetime is found to be approximate...