  X-ray free-electron lasers (XFELs) promise the diffractive imaging of single
molecules and nanoparticles with atomic spatial resolution. This relies on the
averaging of millions of diffraction patterns of identical particles, which
should ideally be isolated in the gas phase and preserved in their native
structure. Here, we demonstrated that polystyrene nanospheres and Cydia
pomonella granulovirus can be transferred into the gas phase, isolated, and
very quickly shockfrozen, i.e. cooled to 4~K within microseconds in a
helium-buffer-gas cell, much faster than state-of-the-art approaches.
Nanoparticle beams emerging from the cell were characterized using
particle-localization microscopy with light-sheet illumination, which allowed
for the full reconstruction of the particle beams, focused to
$<100\:\mu\text{m}$, as well as for the determination of particle flux and
number density. The experimental results were quantitatively reproduced and
rationalized through particle-trajectory simulations. We propose an optimized
setup with cooling rates for few-nanometers particles on nanoseconds
timescales. The produced beams of shockfrozen isolated nanoparticles provide a
breakthrough in sample delivery, e.g. for diffractive imaging and microscopy or
low-temperature nanoscience.
