Pressure, temperature, and isotope effects in the quenching reaction of metastable NH/ND(b1Σ+) with ammonia

Abstract

The quenching of metastable NH(b ¹ Σ ⁺ ) and ND(b ¹ Σ ⁺ ) radicals by NH ₃ and ND ₃ , respectively, has been studied in the presence of the diluent bath gases He, Ar, N ₂ and SF ₆ . Rate constants were determined by measuring the removal rates of the metastables directly. The complicated dependence of the rate constants on the pressure and nature of the bath gases, the temperature, and the isotope suggests complex formation for at least part of the reaction. Complex formation is rarely observed for electronically excited states of simple molecules. We therefore present a large set of experimental kinetic data which can be modelled by a reliable theory, as soon as sufficiently detailed information on the underlying potential-energy surface becomes available. Here we have fitted the data using a RRKM/master equation approach to show that a mechanism via complex formation appears to be adequate and in order to derive some gross features of the relevant potential-energy surface. A comparison is made with the reaction NH(a ¹ Δ)+NH ₃ .