  Spatial separation of water dimer from water monomer and larger
water-clusters through the electric deflector is presented. A beam of water
dimer with $93~\%$ purity and a rotational temperature of $1.5~$K was obtained.
Following strong-field ionization using a $35~$fs laser pulse with a wavelength
centered around $800~$nm and a peak intensity of $10^{14}~\text{W}/\text{cm}^2$
we observed proton transfer and $46~\%$ of the ionized water dimer broke apart
into a hydronium ion $\text{H}_3\text{O}^+$ and neutral OH.
