C2 emission during the pyrolysis of acetone

Abstract

When dilute mixtures of acetone in argon are heated by shock waves to 4000°K, emission from the C₂ Swan band at 5165 Å rises to a peak immediately behind the front and then decays to a steady equilibrium level. Absolute intensity measurements of this radiation overshoot as a function of pressure, composition and temperature show that (i) the acetone is heated and substantially dissociated in the front, i.e., within 3 µsec; the enthalpy absorbed from the shock is 600 kcal mole^–1 acetone. (ii) A substantial fraction of the carbon appears as C₂ behind the front. This fraction is not the equilibrium value, is not strongly temperature dependent, and is independent at any one temperature of pressure of acetone or diluent. (iii) The concentration of C₂A³π is maintained in equilibrium with that of ground state C₂ by collisional steps C₂+M⇌C^*₂+M. The radiative overshoot from C₂ Swan bands is not due to chemiluminescent recombination of fragments. Spectral emissivity data for the C₂ 5165 Å Swan band are given.