Observation of collective effects in the multiphoton ionization of atomic deuterium

The yields and time-of-flight spectra for ions produced in three-photon ionization of atomic deuterium have been investigated as a function of laser intensity from 0.4 to 200 MW/cm(2). A pulsed laser beam of 243 nm is used, sp that the photoionization is resonant with the metastable 2s state of deuterium. The production of resonantly excited 2s atoms is measured by detecting the quenched Lyman-alpha radiation at time intervals both during and after the laser pulse. The near degeneracy of the 2s-2p state manifold provides a particularly sensitive way to monitor the presence of any electric field in the interaction volume. As the laser intensity is increased, we observe the effects of a collective field arising from the electron-ion plasma produced by the laser pulse. We present the results of a theoretical model where the collective motion of the ions and the electrons is analyzed in terms of the fluid approximation. The essential features of the experimental observations including ion yields, ion time-of-flight spectra, and metastable production versus time, are well described by the theory. [S1050-2947(98)06508-1].