Molecular-scale modeling of light emission by combustion: An ab initio study

Despite the advanced understanding of combustion, the mechanisms of subsequent light emission have not attracted much attention. In this work, we model the light emission as electronic excitation throughout the oxidation reaction. We examined the simple dynamics of the collision of an oxygen molecule (O₂) with a kinetic energy of 4, 6, or 10 eV with a stationary target molecule (Mg₂, SiH₄ or CH₄). Time-dependent density functional theory was used to monitor electronic excitation. For a collision between O₂ and Mg₂, the electronic excitation energy increased with the incident kinetic energy. In contrast, for a collision between O₂ and SiH₄ molecules, a substantial electronic excitation occurred only at an incident kinetic energy of 10 eV. The electronic excitation was qualitatively reproduced by analysis using complete active space self-consistent field method. On the other hand, collision between O₂ and CH₄ molecules shows reflection of these molecules indicating that small-mass molecules could show neither oxidation nor subsequent electronic excitation upon collision with an O₂ molecule. We believe that this work provides a first step toward understanding the light-emission process during combustion.