Diagnostics and simulations of molecular formation in laser-induced plasmas

Highlights: • Molecular dynamic simulations on atomistic scales to obtain more detailed information of time-resolved LIBS experiments • Analysis of interactions of different species in LIBS plasmas • New method of molecular temperature estimation in the cooling phase of LIBS plasmas • Introducing the well-established ReaxFF method in plasma simulations • Distinction of different cement types by the emission of CaO molecular bands The formation of diatomic molecules or radicals in the cooling phase of laser-induced plasmas is mainly determined by thermodynamic parameters as the local plasma temperature, pressure and particle density. Better understanding of the molecular formation can be used for deeper material analysis and research. We adapted the well-established method reactive force field (ReaxFF) to simulate the formation of molecular bonds in time-resolved LIBS experiments. Instead of standard quantum mechanical or continuous simulation methods, ReaxFF uses a hybrid form based on the calculation of the bond order. The main advantage is the short computational time of molecular bonds compared to standard approaches. This allows the simulation of molecular formation at fixed temperatures in thermodynamic equilibria as well as temperature ramp simulations to get temperature dependent molecular concentration profiles. Molecular simulations enable the explanation of observed molecular band emission in LIBS experiments, the investigation of molecular interferences with other elements in the sample, and the signal optimization of molecular LIBS experiments. Furthermore, we show that the presented method can be used for a rough molecular temperature estimation of the plasma. As an application, we simulated the temperature behavior of the formation of calcium oxide (CaO) and compared the results to the law of mass action. Calcium and oxygen are the main constituents in cement, whose analysis is of high economic importance as it is part of concrete infrastructure buildings. LIBS measurements of CaO at different gate delays reveal the dynamic behavior of atomic and molecular emission. Furthermore, multivariate methods can be used for a cement separation on the basis of the molecular emission of CaO at 600.4 nm.