Dynamics of Excess Electron Localization in Liquid Helium and Neon

In this paper, we address the relations between the structure, electronic level structure, energetics, and localization dynamics of an excess electron in a bubble in liquid 4He, 3He, and Ne. Our treatment of the dynamics of formation for the electron bubble rests on a quantum mechanical Wigner-Seitz description of the excess electron in conjunction with a hydrodynamic picture for the liquid. The dynamics of electron localization is described in terms of the initial formation of an incipient bubble of radius 3.3-3.5 Å followed by adiabatic bubble expansion in the ground electronic state. The hydrodynamic model for bubble expansion considers the expansion of a spherical cavity in an incompressible liquid with the energy dissipation being due to the emission of sound waves. This model predicts the bubble expansion time (τb ) in liquid 4He to be τb D ) 8.5 ps at P ) 0, exhibiting a marked pressure dependence (decreasing by a numerical factor of 4 at P ) 16 atm) and revealing a small isotope effect of τb (He)/τb (3He) ) 0.83 in liquid 3He, while for liquid Ne, τb D = 1 ps. The collapse times of the empty bubble formed by vertical photoionization of the electron bubble are τc ) 19.8 ps for 4He and τc ) 34 ps for 3He, with τc being considerably longer than τb , reflecting the effect of the kinetic electron energy on the fast electron bubble expansion. The interrogation of the electron bubble localization dynamics by femtosecond absorption spectroscopy is explored by the analysis of the temporal evolution of the electronic excitation energies.