Photodynamics in superfluid helium: Femtosecond laser-induced ionization, charge recombination, and preparation of molecular Rydberg states

Femtosecond pulses (790 nm) are used for nonresonant laser excitation of superfluid liquid helium to prepare ionic and neutral excited states at energies above 18 eV. Measurements of laser-induced fluorescence and photocurrent enable a detailed description of the primary photoprocesses. A controllable excitation regime unique to femtosecond pulses is realized at laser intensities below the dielectric breakdown threshold, I<5×1013 W/cm2. A steady state of the long-lived triplet excimers He2*(3a) (lowest Rydberg state) is established; the concentration decays between laser pulses through diffusion-controlled bimolecular annihilation to ∼1012 cm−3 at a laser repetition rate of 500 Hz. The triplet population is amplified with each pulse in a sequence that involves: (1) ionization of the Rydberg electron of He2* via complete Coulomb barrier suppression; (2) cascade electron impact ionization of the ground-state He atoms by the ponderomotively accelerated quasifree electrons in liquid He; (3) localization and thermalization of the “hot” electrons and He+ cations to form electron “bubble” and He3+ “snowball” states; (4) recombination of these elementary charge carriers to form He2*. The amplification factor for the triplets M=2m characterizes the excitation sequence: m is the number of generations in the cascade (m=5 at I=4.5×1013 W/cm2), and m is proportional to the laser intensity and temporal pulse width. The laser-induced ionization cascade prepares an inhomogeneous initial distribution of spatially separated ions on three length scales: clumps of positive charges with an interionic separation determined by the cascade length of 60 Å; a cloud of electrons surrounding the clump at the electron thermalization length ∼103 Å; and interclump separation dictated by the concentration of the He2* precursors, ∼104 Å.