This paper presents the results of an experimental investigation into collective effects in the transient plasma formed by multiphoton ionization of atomic deuterium with a pulsed laser. The laser wavelength is varied in a narrow range around 243 nm, so that the photoionization is resonant with the metastable ${2S}_{1/2}$ state. The ion yield, the ion time-of-flight spectra, and the yield of Lyman-α photons have been measured as a function of laser intensity (from 1 to 340 ${\mathrm{M}\mathrm{W}/\mathrm{c}\mathrm{m}}^2$) and laser detuning around the ${1S}_{1/2}{-2S}_{1/2}$ two-photon resonance. During and shortly after the laser pulse, collective effects resulting from the mutual interaction of the photoelectrons and the ions affect the spatial and temporal distribution of the ions. Because of the near-degeneracy of the ${2S}_{1/2},$ ${2P}_{1/2},$ and ${2P}_{3/2}$ states, the resonant multiphoton ionization is affected by the Stark mixing of these states in the collective field. As a result, the time-dependent yields of ions and of Lyman-α photons are modulated by the interplay of the multiphoton ionization of the atoms and the collective effects in the plasma. From the measurements it is deduced that collective effects are important above a critical charge density of $3\times {10}^8{\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}/\mathrm{c}\mathrm{m}}^3.$ An asymmetry is observed in the line profile of the total ion yield as a function of laser detuning. This asymmetry is interpreted to be due to the effect of the collective field upon the intermediate resonant ${2S}_{1/2}$ state of the photoionization process.

• Received 5 March 2002

DOI:https://doi.org/10.1103/PhysRevE.66.046411