Dynamics of atomic and molecular emission features from nanosecond, femtosecond laser and filament produced plasmas

Summary form only given. Recently there is a renewed interest in the detection of emission from molecular species in laser-produced plasma systems, particularly for isotopic detection.1 However, significant efforts have been made in the past decades to study the mechanisms leading to the formation of diatomic and larger molecular species (eg. fullerenes) and nanoparticles in LA plumes. The molecules in a laser-produced plasma system are generated through various formation channels: recombination between plasma species, combustion through interaction with atmospheric air, and fragmentation of large clusters. Even though extensive studies are available in the literature focusing on identifying molecular species in LA plumes and their emission features with respect to varying conditions, still very little is known about the evolutionary paths of molecular and nanoparticle formation. This is partly due to the complexity of numerous physical processes that co-exist in a transient laser-plasma system.It is well known that the emission features of ions, atoms, molecules and nanoparticles in a laser ablation plume strongly depend on the laser irradiation conditions.2 We report the temporal emission features of atomic and molecules in plasmas generated using a nanosecond laser, a femtosecond laser and filaments generated from a femtosecond laser. Our results show that, at a fixed laser energy, the persistence of atoms and molecules are found to be highest and lowest in ns laser and filament ablation plasmas respectively while molecular species are formed at early times for both ultrashort pulse (fs and filament) generated plasmas. For ns laser ablation plumes, the shockwaves formed in the plume front are found to be mediating the molecular formation. The persistence of atomic, and molecular emission features and its relation to fundamental properties (temperature and density) of ablation plumes generated using various irradiation methods (ns, fs, filaments) will be discussed in detail along with its implications for remote sensing applications.