Hydrogen analysis in solid samples using laser-induced helium plasma at atmospheric pressure

Abstract

A special technique for the modification of laser-induced breakdown spectroscopy (LIBS) has been developed to improve the spectral quality of hydrogen emission from a solid sample in helium gas at atmospheric pressure. In this technique, the plasma was generated by focusing a fundamental Nd-YAG (yttrium aluminum garnet) laser into a surrounding helium gas. The helium atoms excited to their metastable states would then serve to excite the atoms of the solid material vaporized by using another Nd-YAG laser. When properly synchronized, the resulting hydrogen emission line of H I 656.2 nm shows a dramatic improvement of the emission intensity and the spectral quality over what was obtained by conventional LIBS technique. This study further reveals that this improvement is mainly due to the role of the metastable excited state in a helium atom, which allows the delayed detection to be performed at a favorable moment when the charged particles responsible for the strong Stark broadening effect in the plasma have mostly disappeared.

Investigation of helium addition for laser-induced plasma spectroscopy of pure gas phase systems: Analyte interactions and signal enhancement

“…The recent work by Kurniawan and co-workers has specifically addressed this issue for hydrogen analysis via LIBS [20][21][22][23][24]. Similar to the current results, they observed an increase in hydrogen emission intensities combined with a decrease in hydrogen line broadening with helium addition, in which the latter may be interpreted as a decrease in electron density although no direct electron density calculations were reported.…”

“…Helium was found to produce the optimal signal-to-noise ratio in comparison to air and argon, and resulted in reduced line broadening. Contemporary studies by Kurniawan and co-workers have specifically addressed the role of helium for enhancement of hydrogen analysis in solid samples, including zircaloy-4 alloys, via the LIBS technique [20][21][22][23][24]. In their studies, hydrogen emission intensities were enhanced while Stark broadening was reduced through the use of helium breakdown, using both single and double-laser LIBS methodologies.…”

Hydrogen analysis in solid samples by utilizing He metastable atoms induced by TEA CO2 laser plasma in He gas at 1 atm

“…In fact, even though there are improvement in the quality of target plasma to some extent reported in those other works, however, no any discussion on the possible role of the helium metastable-excited state was presented. It should be noticed that for the Nd-YAG laser case the excitation through He metastable-excited state can only be expected to occur in the case of using the double excitation scheme, such as that reported in our previous works [16,17].…”

“…However, the low-pressure plasma technique has limitations for practical application in nuclear power station, one of which is the inability for in-situ analysis. In improving this technique, we have proposed a double excitation method using He surrounding gas at 1 atmospheric pressure [16,17]. By this scheme, helium metastable-excited state which has long lifetime is produced in the He gas breakdown plasma, and the ablated hydrogen atoms from the sample can be excited well in the region of the He gas breakdown plasma where the helium metastable-excited states populated.…”

Direct evidence of mismatching effect on H emission in laser-induced atmospheric helium gas plasma

“…To overcome this problem, we have offered several a) Author to whom correspondence should be addressed. alternative techniques such as the double pulse laser irradiation 4,5 and introducing the use of helium surrounding gas allowing the metastable excited helium atoms to provide the excitation energy for the hydrogen atoms at atmospheric pressures [6][7][8][9][10][11][12][13] instead of relying on the conventional shock wave excitation mechanism. This technique was shown to yield the much desired strong and sharp hydrogen emission turning the so called "mismatching effect" to its advantage.…”