The possible resurrection of the Chapman mechanism for atmospheric sodium chemiluminescence and ruminations on NaO reaction dynamics

Abstract

AbstractA critical reappraisal and kinetic modeling is presented of a sodium/N2O system. The analysis provides a compelling argument that previously reported kinetic decay rates may refer not to the reaction of O with NaO but rather to that of O with Na2O. The system shows pronounced propensity for a significant portion of the NaO to be quantitatively converted to Na2O on a time scale commensurate with that of the Na/N2O titration reaction. The O atom in the system does appear to originate from the photolysis of a residual level of NaO. The observed Na(2P) chemiluminescence, used to track the O atom decay rates, can be consistent with the O + NaO reaction as previously surmised. It is unlikely that the alternate O + Na2 and O + Na2O chemiluminescent channels can generate the observed intensity levels. This reanalysis, which provides for the observed first order dependences on N2O(Na2O) and O atom concentrations has significant implications for the Chapman atmospheric mechanism of the sodium airglow. Its conclusions resurrect the viability of the original scheme which requires efficient branching of the O + NaO reaction to Na(2P). Recent suggestions invoking the participation of NaO(A2Σ+) require the latter to have a metastable nature with respect to its radiative and collisional quenching (N2, O2) channels, for which there is no current evidence.An additional evaluation of the rate constant measurements for the fast reactions of NaO with NO or CO indicates that these most probably are kinetically complex and involve longlived transition states. Their rate constants are predicted to have small negative activation energies and pressure dependences. In the case of NaO with CO this may explain its low Na(2P) chemiluminescent efficiency. For the NaO + O reaction, a rate constant of about 4 ×10−11 cm3 molecule−1s−1 is predicted at room temperature. This is similar to that used in earlier atmospheric models. Its magnitude circumvents the consequences of the reaction's large entropy decrease which otherwise implies too large a cross‐section for the reverse reaction. A smaller value also is more likely to be consistent with a normal short‐lived collisional transition state, which will allow for a more significant Na(2P) quantum efficiency. © 1993 John Wiley & Sons, Inc.