Small water droplets (20 micron in diameter) have been exposed to
intense (∼ 1019 W/cm2) laser pulses in order
to study ultrashort (∼ 35 fs) laser pulse driven ion acceleration. Ion
emission spectra registered simultaneously in forward and backward
direction in respect to the incident laser beam carry similar integral ion
energy but show different ion cutoff energies. With simple model
estimations on basis of the confined and spherical geometry of the
droplet-target, we inferred acceleration field strengths of about
(0.7–2) MV/μm. Up to 9% of the incident laser energy is
converted to kinetic energy of ions, which have been accelerated to
energies above 100 keV and up to 1.5 MeV. A laser pedestal at an intensity
of about 10−7 of the peak intensity at 1–2 ns in
front of the pulse peak still limits the achievable cutoff energies of
emitted protons from the droplet. The observed increase of cutoff energies
with an enhanced temporal contrast of the laser pulse is elucidated within
a simple acceleration model.