  The reaction N(4S) + NO -> O(3P) + N2 plays a pivotal role in the conversion
of atomic to molecular nitrogen in dense interstellar clouds and in the
atmosphere. Here we report a joint experimental and computational investigation
of the N + NO reaction with the aim of providing improved constraints on its
low temperature reactivity. Thermal rates were measured over the 50 to 296 K
range in a continuous supersonic flow reactor coupled with pulsed laser
photolysis and laser induced fluorescence for the production and detection of
N(4S) atoms, respectively. With decreasing temperature, the experimentally
measured reaction rate was found to monotonously increase up to a value of (6.6
+- 1.3) x 10-11 cm3 s-1 at 50 K. To confirm this finding, quasi-classical
trajectory simulations were carried out on a previously validated,
full-dimensional potential energy surface (PES). However, around 50 K the
computed rates decreased which required re-evaluation of the reactive PES in
the long-range part due to a small spurious barrier with height 40 K in the
entrance channel. By exploring different correction schemes the measured
thermal rates can be adequately reproduced, displaying a clear negative
temperature dependence over the entire temperature range. The possible
astrochemical implications of an increased reaction rate at low temperature are
also discussed.
