Ayaka Tamura

Spectroscopic Measurements of Solids Immersed in Water at High Pressure Using a Long-Duration Nanosecond Laser Pulse

The effects of pressure on the emission lines of a submerged metallic plate irradiated by a long 150 ns duration laser pulse have been investigated. While spectroscopic measurements using multiple-pulse irradiation of submerged targets are sensitive to pressure, the interactions of a long-pulse occur during a time window where the properties of the laser pulse dominate the transient pressures surrounding the ablated region. The emission lines are not sensitive to external pressures of up to 30 MPa, and the results demonstrate that long-pulse ablation is available as an analytical technique to study solids immersed at high pressure.

Simultaneous observation of nascent plasma and bubble induced by laser ablation in water with various pulse durations

We investigate the effects of pulse duration on the dynamics of the nascent plasma and bubble induced by laser ablation in water. To examine the relationship between the nascent plasma and the bubble without disturbed by shot-to-shot fluctuation, we observe the images of the plasma and the bubble simultaneously by using two intensified charge coupled device detectors. We successfully observe the images of the plasma and bubble during the pulsed-irradiation, when the bubble size is as small as 20 μm. The light-emitting region of the plasma during the laser irradiation seems to exceed the bubble boundary in the case of the short-pulse (30-ns pulse) irradiation, while the size of the plasma is significantly smaller than that of the bubble in the case of the long-pulse (100-ns pulse) irradiation. The results suggest that the extent of the plasma quenching in the initial stage significantly depends on the pulse duration. Also, we investigate how the plasma-bubble relationship in the very early stage affects the shape of the atomic spectral lines observed at the later delay time of 600 ns. The present work gives important information to obtain high quality spectra in the application of underwater laser-induced breakdown spectroscopy, as well as to clarify the mechanism of liquid-phase laser ablation.

Single-Pulse Underwater Laser-Induced Breakdown Spectroscopy with Nongated Detection Scheme

We investigated spatially resolved emission spectra of Al atoms in a very small (∼0.1 mm) laser ablation plasma produced by a single long-pulse (∼100 ns) irradiation of an Al target in water. The spectral feature varied considerably, depending on the position to be measured. The density of the plasma periphery was low enough to neglect the self-absorption effect, even when resonance lines were observed. By properly selecting the position, we successfully obtained well-resolved spectral lines even without time-gated detection. This suggests that time-gating is not necessary anymore in the practical applications of underwater laser-induced breakdown spectroscopy when employing spatially resolved detection system.

On-Site Quantitative Elemental Analysis of Metal Ions in Aqueous Solutions by Underwater Laser-Induced Breakdown Spectroscopy Combined with Electrodeposition under Controlled Potential

We propose a technique of on-site quantitative analysis of Zn(2+) in aqueous solution based on the combination of electrodeposition for preconcentration of Zn onto a Cu electrode and successive underwater laser-induced breakdown spectroscopy (underwater LIBS) of the electrode surface under electrochemically controlled potential. Zinc emission lines are observed with the present technique for a Zn(2+) concentration of 5 ppm. It is roughly estimated that the overall sensitivity over 10 000 times higher is achieved by the preconcentration. Although underwater LIBS suffers from the spectral deformation due to the dense plasma confined in water and also from serious shot-to-shot fluctuations, a linear calibration curve with a coefficient of determination R(2) of 0.974 is obtained in the range of 5-50 ppm.

Synergetic effects of double laser pulses for the formation of mild plasma in water: toward non-gated underwater laser-induced breakdown spectroscopy.

We experimentally study the dynamics of the plasma induced by the double-laser-pulse irradiation of solid target in water, and find that an appropriate choice of the pulse energies and pulse interval results in the production of an unprecedentedly mild (low-density) plasma, the emission spectra of which are very narrow even without the time-gated detection. The optimum pulse interval and pulse energies are 15-30 μs and about ~1 mJ, respectively, where the latter values are much smaller than those typically employed for this kind of study. In order to clarify the mechanism for the formation of mild plasma we examine the role of the first and second laser pulses, and find that the first pulse produces the cavitation bubble without emission (and hence plasma), and the second pulse induces the mild plasma in the cavitation bubble. These findings may present a new phase of underwater laser-induced breakdown spectroscopy.

Two-dimensional space-resolved emission spectroscopy of laser ablation plasma in water

We developed a method for two-dimensional space-resolved emission spectroscopy of laser-induced plasma in water to investigate the spatial distribution of atomic species involved in the plasma. Using this method, the laser ablation plasma produced on a Cu target in 5 mM NaCl aqueous solution was examined. The emission spectrum varied considerably depending on the detecting position. The temperature and the atomic density ratio NNa/NCu at various detecting positions were evaluated by fitting emission spectra to a theoretical model based on the Boltzmann distribution. We are successful in observing even a small difference between the distributions of the plasma parameters along the directions vertical and horizontal to the surface. The present approach gives direct information for sound understanding of the behavior of laser ablation plasma produced on a solid surface in water.

Effects of temporal laser profile on the emission spectra for underwater laser-induced breakdown spectroscopy: Study by short-interval double pulses with different pulse durations

We investigate the effects of temporal laser profile on the emission spectra of laser ablation plasma in water. We use short-interval (76 ns) double pulses with different pulse durations of the composing two pulses for the irradiation of underwater target. Narrow atomic spectral lines in emission spectra are obtained by the irradiation, where the two pulses are wide enough to be merged into a single-pulse-like temporal profile, while deformed spectra are obtained when the two pulses are fully separated. The behavior of the atomic spectral lines for the different pulse durations is consistent with that of the temporal profiles of the optical emission intensities of the plasma. All these results suggest that continuous excitation of the plasma during the laser irradiation for ∼100 ns is a key to obtain narrow emission spectral lines.

Laser-induced breakdown spectroscopy for in situ chemical analysis at sea

Spectroscopy is emerging as a technique that can expand the envelop of modern oceanographic sensors. The selectivity of spectroscopic techniques enables a single instrument to measure multiple components of the marine environment, and can form the bases of versatile tools to perform in situ geo-chemical analysis. This work investigates emission spectroscopy using laser-induced plasmas to perform multi-element chemical analysis of liquids and solids at sea. In situ measurements of both liquids and solids have been successfully performed at sea using the 3000m depth rated prototype I-SEA (In situ Seafloor Element Analyser). Techniques aimed at optimising the signals observed from plasmas generated at high pressure are described and their mechanisms discussed. I-SEA is just an example of a new generation of chemical cameras (ChemiCam) that can probe different aspects of the environment. It is hoped that through integration with platforms such as underwater vehicles, drilling systems and subsea observatories, this technology will contribute to more efficient scientific surveys, and serve as a tool to facilitate both spatially and temporally continuous study of the ocean.