We report the first measurements on the distribution of electron energies and associated recombination kinetics in tunnel-ionized plasmas of interest to recent x-ray laser proposals.Subpicosecond laser pulses, focused to an intensity of 1 X 10'6 W/cm, create a fully ionized helium plasma.Measurement of the resulting emission spectrum allows a determination of the time-dependent electron velocity distribution.The initial distribution is found to be strongly non-Maxwellian and subsequent recombination kinetics proceed more slowly than predicted for a Maxwellian distribution of electrons.PACS numbers: 52.50. Jm, 52.25.Nr, 52.40.Nk The creation of x-ray lasers pumped by recombination of electrons to highly ionized atoms has been an active area of research for several years [1]. An important criterion for gain in these systems is that the electron-ion recombination proceeds rapidly in comparison with the decay time of the lasing transition. Electron-ion recombination is expected to be rapid given a plasma with a sufficiently cold free electron bath [1] and recently it has been suggested that novel short-pulse (~1 ps) lasers may be used to create plasmas with controllable electron temperature [2]. In particular, Burnett and Corkum [2] have used a model based on tunneling ionization, the quasistatic model, to predict that nonequilibrium plasmas characterized by a high stage of ionization and a bath of cold free electrons should result from high-field ionization of a gas sample. Recent Thomson scattering measurements of tunnel-ionized plasmas confirm that the quasistatic model accurately predicts average electron energies [3]. Thomson scattering, however, does not reveal the detailed distribution of free electron energies. Since recombination kinetics are expected to depend sensitively on the distribution of electron energies [1], results fromThomson scattering measurements cannot necessarily be used to accurately calculate recombination times. Importantly, we are not aware of any experiments where recombination times were measured for an electron distribution produced by high-field ionization. In this work we report the first measurements on the distribution of free electron energies and resulting recombination time scales in tunnel-ionized plasmas produced at gas densities appropriate to recombination x-ray laser production. The spectrum of free-bound and associated boundbound fluorescence is used to record time-dependent recombinations kinetics within a plasma created by interaction of a high-power, subpicosecond laser pulse with a helium gas sample. We investigate three aspects of the time-resolved spectra. First, early time non-Maxwellian spectra are observed. Second, the time evolution of the electron energy distribution is used to estimate the plasma cooling rate and thermalization time scale. Third, the history of the spectral features is used to determine the recombination kinetics.Importantly, we find that the time scale for population cascade to the 2p state of Her? is observed to be on the order of 100 ti...