Abstract
A time-resolved spectroscopic study is performed on the enhancement signals of He gas plasma emission using nanosecond (ns) and picosecond (ps) lasers in an orthogonal configuration. The ns laser is used for the He gas plasma generation and the ps laser is employed for the ejection of fast electrons from a metal target, which serves to excite subsequently the He atoms in the plasma. The study is focused on the most dominant He I 587.6 nm and He I 667.8 nm emission lines suggested to be responsible for the He-assisted excitation (HAE) mechanism. The time-dependent intensity enhancements induced by the fast electrons generated with a series of delayed ps laser ablations are deduced from the intensity time profiles of both He emission lines. The results clearly lead to the conclusion that the metastable excited triplet He atoms are actually the species overwhelmingly produced during the recombination process in the ns laser-induced He gas plasma. These metastable He atoms are believed to serve as the major energy source for the delayed excitation of analyte atoms in ns laser-induced breakdown spectroscopy (LIBS) using He ambient gas.
Highlights
The ns laser is used for the He gas plasma generation and the ps laser is employed for the ejection of fast electrons from a metal target, which serves to excite subsequently the He atoms in the plasma
085105-2 Suyanto et al AIP Advances 6, 085105 (2016). He ambient gas have been reported previously.[9,10,11,12,13,14,15,16,17]. Those favorable effects of He ambient gas have been elucidated to be closely related to the crucial role of a He metastable excited state (He*) which serves as a temporary energy reservoir for the subsequent excitation of the ablated analyte atoms.[13,18]
In connection with the use of fast electrons ejected with the ps laser pulse for the excitation of He atoms in the He gas plasma, it is worthwhile to show that the process is physically reasonable
Summary
He ambient gas have been reported previously.[9,10,11,12,13,14,15,16,17] Those favorable effects of He ambient gas have been elucidated to be closely related to the crucial role of a He metastable excited state (He*) which serves as a temporary energy reservoir for the subsequent excitation of the ablated analyte atoms.[13,18] Further, the triplet He* was suggested to play the major role since the He I 587.6 nm emission associated with transition from the triplet excited state has much stronger intensity than that of He I 667.8 nm emission from the singlet excited state.[13,18] no report has so far been published on the explicit identification of the specific major He* species produced in the laser-induced plasma. The aim of this study is on the other hand devoted to reveal a more direct evidence for the dominant production of triplet He* over its singlet species in laser induced He plasma. For this purpose, the time-dependent emission intensities of He I 587.6 nm and He I 667.8 nm are measured using the orthogonal double pulse LIBS set-up similar to the one adopted previously,[13,18] but using the two lasers to serve different functions in the present experiments as explained
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