The ionization dynamics of free NaCl nanoparticles of 70 nm mean diameter is studied in a one-color pump-probe experiment using intense near-infrared femtosecond laser pulses. We find a significant delay dependence of the electron emission yield with a pronounced maximum for optimal delays in the picosecond range. The pump-probe dynamics is interpreted by model calculations in terms of fast pump-induced metallization of the target and resonant plasmon heating of the expanding nanoplasma by the probe pulse. The picture of collective resonance absorption is supported by the observed linear scaling of the electron yield at optimal delay with probe intensity. In contrast, for pump only excitation we find a strongly nonlinear intensity dependence of the electron yield, which is attributed to the intertwining of multi-photon and tunneling ionization with non-resonant heating. Our results provide compelling evidence for the general nature and robustness of transient plasmonic resonance heating in laser-excited nanoparticles in the size regime reaching up to 100 nm.
