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Formation and detection of molecules in laser induced plasmas (LIP) is a hot topic. In analytical plasmas like LIBS, the detection of molecules is important for identification of geological and other materials, analysis of isotopes and difficult elements (Cl, F, etc.) via molecular emission. In chemical plasmas, like PECVD (plasma enhanced chemical vapor deposition) or PLD (pulsed laser deposition), molecules formed in the plasma determine a composition and a thickness of deposits. Similarly, molecules play an important role in microstructuring and oxidizing metal surfaces by laser ablation. It is unfortunate that different communities, which utilize plasma methods and seek for solutions of similar problems, do not strongly overlap, and do not fully use knowledge accumulated by each other.
In this presentation, mechanisms of formation of molecules will be analyzed on the example of LIPs used for chemical vapor deposition and metal microstructuring. Theoretical analysis includes equilibrium chemistry calculations combined with plasma hydrodynamics. First, LIP excited in a gas mixture of BCl3 or BF3 with H2 or CH4 will be analyzed; this chemical system is used for obtaining deposits of refractory solid boron and boron carbide. Second, a breakdown in the SiF4 + SiCl4 gas mixtures will be described; this method allows synthesis of fluorochlorosilanes SiFxCl4-x (x = 1, 2, 3), the good etching agents (Figure). Third, solid ablation of Mo in BF3+H2 and Ti in air will be considered aimed at obtaining deposits of high hardness MoxBy and films of TixOy on textured Ti surfaces, correspondingly.
In experiment, reaction gases before and after laser illumination, and solid deposits are analyzed by optical emission spectroscopy (OES), IR and mass spectrometry (MS), SEM, X-ray, and AFM. It will be shown that the hydrodynamic-chemical model adequately predicts the composition of LIPs, zones of molecular formation, dependence on reactant stoichiometry, plasma temperature and pressure.