Underlying physical processes for time dependent variations of He triplet and singlet intensities in laser-induced He plasma

A series of experiments are conducted for the study of time-dependent variations of major emission from the metastable excited He generated in various experimental conditions. Different Nd:YAG laser pulse widths and pulse energies, as well as purity of the He gas, are employed for studying quenching effects including the use of a Cu target in an experimental condition for spectrochemical analysis. It is found that in all cases investigated, the metastable excited He atom (He*) associated with the triplet He I 587.5 nm emission line has the unquestionable dominance over another triplet emission line of He I 388.8 nm and the singlet He I 667.8 nm emission. Further analysis of the present data combined with the results of previous studies suggested that the energy transfer between He atoms via the Penning-like collision-induced energy transfer process has so far remained less than appropriately addressed. It is strongly argued and demonstrated in this work that this underlying physical process is likely the moving force leading to the repeated He ionizations responsible for the self-propelled multiple amplification of the triplet He I 587.5 nm emission intensity particularly at the early stage of the shock wave formation. This study has convincingly demonstrated that the Penning-like collision-induced energy transfer process can also effectively take place between He* and the ablated atoms, implying its usefulness for spectrochemical laser-induced plasma spectroscopy analysis.