Dynamics of picosecond-laser-pulse plasmas determined from the spectral shifts of reflected probe pulses

The evolution of density profiles in plasmas produced by ${10}^{14}$-W ${\mathrm{cm}}^{\mathrm{-}2}$, 1-ps, 580-nm-wavelength laser pulses has been determined from spectral shifts imposed on a reflected picosecond probe pulse. The shifts are caused by two mechanisms: hydrodynamic expansion and ionization front motion. Both time-resolved expansion velocities and ionization rates are inferred from the spectral shifts and are compared to one-dimensional hydrodynamic simulations.