Low temperature rate constants for the N(4S) + CH(X2{\Pi}r) reaction.
  Implications for N2 formation cycles in dense interstellar clouds
  Rate constants for the potentially important interstellar N(4S) +
CH(X2{\Pi}r) reaction have been measured in a continuous supersonic flow
reactor over the range 56 K < T < 296 K using the relative rate technique
employing both the N(4S) + OH(X2{\Pi}i) and N(4S) + CN(X2{\Sigma}+) reactions
as references. Excess concentrations of atomic nitrogen were produced by the
microwave discharge method upstream of the Laval nozzle and CH and OH radicals
were created by the in-situ pulsed laser photolysis of suitable precursor
molecules. In parallel, quantum dynamics calculations of the title reaction
have been performed based on accurate global potential energy surfaces for the
13A' and 13A" states of HCN and HNC, brought about through a hierarchical
construction scheme. Both adiabatic potential energy surfaces are barrierless,
each one having two deep potential wells suggesting that this reaction is
dominated by a complex-forming mechanism. The experimental and theoretical work
are inexcellent agreement, predicting a positive temperature dependence of the
rate constant, in contrast to earlier experimental work at low temperature. The
effects of the new low temperature rate constants on interstellar N2 formation
are tested using a dense cloud model, yielding N2 abundances 10-20 % lower than
previously predicted.
