The three-photon ionization spectrum of Na has been investigated in the 5390–6000-Å region. The behavior of electrons and ions produced at the observed resonances has been analyzed as a function of the experimental parameters laser intensity, beam density, and focusing conditions with the aim of elucidating the role of long-range collective interactions between electrons and ions during the collection phase. The dependence of electron current on the externally applied electric field has been especially investigated as a determinant test of phenomena under study. Important deviations of the observed dependences from those usually expected for noninteracting particles are explained in terms of collective plasma interactions.

• Received 26 June 1989

DOI:https://doi.org/10.1103/PhysRevA.40.5160