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

Yoshiyuki Miyamoto,Tokutaro Komatsu

doi:10.1038/s41598-019-49200-2

Yoshiyuki Miyamoto, Tokutaro Komatsu

Open Access

https://doi.org/10.1038/s41598-019-49200-2

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Abstract

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.

Highlights

The authors are grateful for the fruitful discussions of the current work in a computer-aided molecular and material design (CAMM) forum conducted by the Business Research Institute, Tokyo, Japan
We describe three cases for an O2 molecule colliding with a Mg2 molecule, a SiH4 molecule, and a CH4 molecule
For O2 → Mg2 collision, Mg–O bond formation was observed with an incident kinetic energy greater than 6 eV, and the electronic excitation energy increased with the incident kinetic energy

Summary

Computational Schemes

The TDDFT calculations were performed within the local density approximation using the Perdew-Zunger functional[16] fitted to the numerical result for the electron gas calculation[17]. In performing the TDDFT-Ehrenfest simulation, we used the spin-unpolarized approximation Within this approximation, the triplet ground state of the O2 molecule is mimicked by assigning half occupation for each of the doubly degenerate O2 π* MOs. The real-time propagation of the Kohn-Sham orbitals was computed by solving the time-dependent Kohn-Sham equation[13] as i ∂ψn(r, ∂t t). There should be several ways to monitor electronic excitation; The one is taking projections of the time-dependent Kohn-Sham orbitals obtained from Eq (2) to the occupied and empty static Kohn-Sham orbitals on the same atomic positions This method may approximately work when electronic charge density between the ground and excited states does not differ so much, and the Kohn-Sham Hamiltonian between the ground and excited states can be regarded as common. Small insets are snapshots of atomic coordinates for O atoms (red ball) and Mg atoms (gray balls), respectively. (b) Same as (a) but with incident kinetic energy of 10 eV

Results and Discussion

Summary

Author Contributions

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