  We present the results of a systematic study of photoelectron emission from
gasphase dielectric nanoparticles (SiO2) irradiated by intense 25 fs, 780 nm
linearly polarized laser pulses as a function of particle size (20 nm to 750 nm
in diameter) and laser intensity. We also introduce an experimental technique
to reduce the effects of focal volume averaging. The highest photoelectron
energies show a strong size dependence, increasing by a factor of six over the
range of particles sizes studied at a fixed intensity. For smaller particle
sizes (up to 200 nm), our findings agree well with earlier results obtained
with few-cycle, ~4 fs pulses. For large nanoparticles, which exhibit stronger
near-field localization due to field-propagation effects, we observe the
emission of much more energetic electrons, reaching energies up to ~200 times
the ponderomotive energy. This strong deviation in maximum photoelectron energy
is attributed to the increase in ionization and charge interaction for
many-cycle pulses at similar intensities.
