  The vitrification of aqueous solutions through rapid cooling is a remarkable
achievement that launched the field of cryo-electron microscopy (cryo-EM) and
has enabled the cryopreservation of biological specimens. For judging the
feasibility of a vitrification experiment, the critical cooling rate of pure
water is a frequently cited reference quantity. However, an accurate
determination has remained elusive, with estimates varying by several orders of
magnitude. Here, we employ in situ and time-resolved electron microscopy to
obtain a precise measurement. We use shaped microsecond laser pulses to briefly
melt an amorphous ice sample before flash freezing it with a variable,
well-defined cooling rate. This allows us to directly measure the critical
cooling rate of pure water, which we determine to be $6.4\cdot10^{6}$ K/s. Our
experimental approach also expands the toolkit of microsecond time-resolved
cryo-EM, an emerging technique, in which a cryo sample is similarly flash
melted and revitrified with a laser pulse.
