Despite
the development of a myriad of mitigation methods, radiation
damage continues to be a major limiting factor in transmission electron
microscopy. Intriguing results have been reported using pulsed-laser
driven and chopped electron beams for modulated dose delivery, but
the underlying relationships and effects remain unclear. Indeed, delivering
precisely timed single-electron packets to the specimen has yet to
be systematically explored, and no direct comparisons to conventional
methods within a common parameter space have been made. Here, using
a model linear saturated hydrocarbon (n-hexatriacontane,
C36H74), we show that precisely timed delivery
of each electron to the specimen, with a well-defined and uniform
time between arrival, leads to a repeatable reduction in damage compared
to conventional ultralow-dose methods for the same dose rate and the
same accumulated dose. Using a femtosecond pulsed laser to confine
the probability of electron emission to a 300 fs temporal window,
we find damage to be sensitively dependent on the time between electron
arrival (controlled with the laser repetition rate) and on the number
of electrons per packet (controlled with the laser-pulse energy).
Relative arrival times of 5, 20, and 100 μs were tested for
electron packets comprised of, on average, 1, 5, and 20 electrons.
In general, damage increased with decreasing time between electrons
and, more substantially, with increasing electron number. Further,
we find that improvements relative to conventional methods vanish
once a threshold number of electrons per packet is reached. The results
indicate that precise electron-by-electron dose delivery leads to
a repeatable reduction in irreversible structural damage, and the
systematic studies indicate this arises from control of the time between
sequential electrons arriving within the same damage radius, all else
being equal.