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 su perfluid liquid helium to prepare ionic and neutral excited states at energ ies above 18 eV. Measurements of laser-induced fluorescence and photocurren t enable a detailed description of the primary photoprocesses. A controllab le excitation regime unique to femtosecond pulses is realized at laser inte nsities below the dielectric breakdown threshold, I < 5 X 10(13) W/cm(2). A steady state of the long-lived triplet excimers He-2*((3)a) (lowest Rydber g state) is established; the concentration decays between laser pulses thro ugh diffusion-controlled bimolecular annihilation to similar to 10(12) cm(- 3) at a laser repetition rate of 500 Hz. The triplet population is amplifie d with each pulse in a sequence that involves: (1) ionization of the Rydber g electron of He-2* via complete Coulomb barrier suppression; (2) cascade e lectron impact ionization of the ground-state He atoms by the ponderomotive ly accelerated quasifree electrons in liquid He; (3) localization and therm alization of the "hot'' electrons and He+ cations to form electron "bubble' ' and He-3(+) "snowball'' states; (4) recombination of these elementary cha rge carriers to form He-2*. The amplification factor for the triplets M = 2 (m) characterizes the excitation sequence: m is the number of generations i n the cascade (m = 5 at I = 4.5 X 10(13) W/cm(2)), and m is proportional to the laser intensity and temporal pulse width. The laser-induced ionization cascade prepares an inhomogeneous initial distribution of spatially separa ted ions on three length scales: clumps of positive charges with an interio nic separation determined by the cascade length of 60 Angstrom; a cloud of electrons surrounding the clump at the electron thermalization length simil ar to 10(3) Angstrom; and interclump separation dictated by the concentrati on of the He-2* precursors, similar to 10(4) Angstrom. (C) 1999 American In stitute of Physics. [S0021-9606(99)01202-7].