A kinetic study of the reaction lithium + nitrous oxide: non-Arrhenius behavior over the temperature range 363-900 K

Abstract

A kinetic study is presented of the reaction between lithium atoms and nitrous oxide over the temperature range 363-900 K, using a new experimental system. Li atoms are produced in an excess of N/sub 2/O and He bath gas by pulsed photolysis of either LiI or LiCl vapor. The concentration of the metal atoms is then monitored in real time by the technique of laser-induced fluorescence of Li atoms at lambda = 670.7 nm using a pulsed nitrogen-pumped dye laser and box-car integration of the fluorescence signal. Second-order rate constants for the title reaction have been measured at T = 363, 411, 474, 545, 600, 657, 700, 800, and 900 K. When this data is plotted in the standard Arrhenius form, a strong departure from linearity above 600 K is evident with an activation energy increasing with temperature. The best analytical description of this temperature dependence over the temperature range studied is given by k/sub 1/(T) = (2.4 +- 0.5) x 10/sup -11/T/sup 1/2 exp(-(1468 +- 137)T) + (7.9 +- 4.7) x 10/sup -10/T/sup 1/2 exp(-(4157 +- 540)T). The first term of this expression describes a low-temperature Arrhenius behavior similar to that observed by other workers for the analogous reactionmore » of sodium atoms. The author interpret the form of the second term as describing the participation in the reaction at temperatures above 600 K of highly vibrationally excited N/sub 2/O, which enables the reaction to proceed by a close-range charge-transfer mechanism. This significantly enhances the reaction cross section. A simple two-channel model is discussed which accounts for the observed temperature dependence« less