Evidence of Metastable Atomic and Molecular Bubble States in Electron-Bombarded Superfluid Liquid Helium

Infrared absorption spectra of electron-bombarded superfluid liquid helium have established that (a) a³Σᵤ⁺ helium molecules and probably 2³S₁ atoms are present in the liquid in concentrations of the order of 10¹³/cm³ in an excitation volume {~}0.05 cm³, for a 1-{μ}A bombarding beam of 160-keV electrons; (b) these states are metastable with life-times of the order of a millisecond at 1.7^∘{}K; (c) the a³Σᵤ⁺ molecules are de-excited by a³Σᵤ⁺-a³Σᵤ⁺ collisions in the liquid; and (d) the local configuration of the surrounding liquid helium is different for different excited states of the helium atomic and molecular systems, and the configurational interaction with the host fluid has an appreciable effect upon the electronic energies associated with these excited states.