Shortt, B.J and van der Burgt, P.J.M. and Giammanco, F. and Slevin, J.A. and Lanigan, W.
Wavelength dependent collective effects in the multiphoton ionization of atomic deuterium.
Physical Review E, 66 (4).
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 2S1/2 state. The ion
yield, the ion time-of-flight spectra, and the yield of Lyman-a photons have been measured as a function of
laser intensity ~from 1 to 340 MW/cm2! and laser detuning around the 1S1/2-2S1/2 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 2S1/2, 2P1/2 , and 2P3/2 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-a 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
33108 ions/cm3. 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 2S1/2 state of the photoionization process.
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