Low temperature rate constants for the N(4S) + CH(X2Πr) reaction. Implications for N2 formation cycles in dense interstellar clouds

Abstract

Rate constants for the potentially important interstellar N(⁴S) + CH(X²Π_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(X²Π_i) and N(⁴S) + CN(X²Σ⁺) 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 in excellent 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.