  Here we report the results of an experimental and theoretical study of the
gas-phase reactions between O($^1$D) and H$_2$O and O($^1$D) and D$_2$O at room
temperature and below. On the experimental side, the kinetics of these
reactions have been investigated over the 50-127 K range using a continuous
flow Laval nozzle apparatus, coupled with pulsed laser photolysis and pulsed
laser induced fluorescence for the production and detection of O($^1$D) atoms
respectively. Experiments were also performed at 296 K in the absence of a
Laval nozzle. On the theoretical side, the existing full-dimensional ground
X$^1$A potential energy surface for the H$_2$O$_2$ system involved in this
process has been reinvestigated and enhanced to provide a better description of
the barrierless H-atom abstraction pathway. Based on this enhanced potential
energy surface, quasiclassical trajectory calculations and ring polymer
molecular dynamics simulations have been performed to obtain low temperature
rate constants. The measured and calculated rate constants display similar
behaviour above 100 K, showing little or no variation as a function of
temperature. Below 100 K, the experimental rate constants increase
dramatically, in contrast to the essentially temperature independent
theoretical values. The possible origins of the divergence between experiment
and theory at low temperatures are discussed.
