Low-pressure Laser-induced Plasma Research Articles

Novel control of plasma expansion direction aimed at very low pressure laser-induced plasma spectroscopy.

A plasma confinement approach has been applied to enhance the signal intensity of laser-induced plasma in low pressure conditions down to 10(-2) torr. Detection of plasma emission spectrum is a daunting task at low pressure due to the low electron density and the short persistence time of plasma that undergoes a rapid expansion. Here we devised a spatial confinement setup that increases the electron density at various range of low pressures. A confining window is placed above the sample surface to control the direction of the expanding plasma aimed at optimizing the efficiency of the low pressure detection. More ions, atoms, and molecules can reach the detector by a direction-controlled confinement of an otherwise freely expanding plasma. The spectral intensities of neutral atoms increased up to 4 times with a single laser pulse by the proposed confining method at 1 torr. The signal of doubly ionized carbon atom which was detectable only at low pressure is also enhanced 4 times. The results of this study provide an important guideline for strengthening the otherwise weak signals at low pressure by controlling the plasma expansion direction.

Sensitive Measurement of Trace Mercury Using Low Pressure Laser-Induced Plasma

The emission of trace heavy metals, such as mercury (Hg), from power plants and other industries is a severe environmental problem concerning the public health. The laser-induced plasma technique was employed to measure Hg under various conditions, which reveals several merits of this method at low pressure. The main interferences of laser-induced breakdown spectroscopy (LIBS), which include the black-body-like emission from plasma itself and coexisting molecular and atomic emissions, decreased significantly using low pressure laser-induced plasma. Under low pressure conditions, Hg signal was rather clear without serious influence even if there is no delay time from the laser irradiation, which means the gated detection device is not necessary. This method featured the detection limit of 0.3 ppm at pressure 700 Pa. Additionally, the feasible of this method in real applications was demonstrated by measuring Hg in combustion gas which performed preferable results.

Comparative Studies on the Excitation Mechanism of Fe II Lines in Low-Pressure Laser-Induced Plasma of Argon and Neon

ABSTRACT Emission characteristics of Fe II lines excited from low-pressure laser-induced plasma were investigated when Ar or Ne was employed as a plasma gas. Emission intensity measured in time-resolved mode was strongly dependent both on the kind of plasma gas and on the excitation energy of Fe ionic lines. In the initial plasma, just after the breakdown, fast electrons and gas species were major energy donors for excitations of ablated Fe atoms. With an expansion of the plasma plume, particularly intense emission lines of Fe ion appeared in Ar and Ne plasmas. Especially, different emission lines of Fe ion were selectively excited when Ar or Ne were employed as the plasma gas: for example, the Fe II 242.415-nm and the Fe II 248.424-nm lines in Ar plasma, and the Fe II 250.388-nm and the Fe II 257.686-nm lines in Ne plasma. The excitation mechanism for these ionic lines are different in both plasmas: a charge-transfer collision with Ar ion principally caused their excitations in Ar plasma, while a Penning-type collision with the metastable state of Ne atom could populate the corresponding excited levels of Fe ion in Ne plasma.

Emission Characteristics of Copper Ionic Lines from the 3d95s-3d94p Transition in a Low-Pressure Laser-Induced Plasma

The Emission Characteristic of Copper Ionic Lines Requiring Large Excitation Energies Was Investigated in Low-Pressure Laser-Induced Plasma Spectroscopy (LP-LIPS), when Argon Was Employed as Plasma Gas. The Excited Mechanism of the Ironic Lines Whose Electron Transitions Were Assigned to the 3d95s-3d94p Configuration Was Understood from the Time-Resolved Spectra of Copper. The Emission Intensity of the Copper Emission Lines, Measured in a Time-Resolved Mode, Was Extremely Dependent on the Kind of Copper Lines and the Upper Energy. Generally, their Emission Intensities Dramatically Decreased with the Duration Time, along with the Recombination as Well as the De-Excitation of Copper Ions Requiring Larger Kinetics Energy which Mainly Were Produced in a Hot Breakdown Plasma. The Emission Behavior Excited from the 3d95s-3d94p Transition, such as the Cu II 254.4 nm and the Cu II 276.9 nm Lines, Was Generally Similar to that from 3d94p-3d94s Transition, although their Excitation Energies Were Different. This Effect Would Result from a Common and Dominant Ionization /excitation Mechanism, which Was Collision Energy Transfer from Energetic Particles such as Fast Electrons.

Emission Characteristics of the Dielectronic Recombination Line of Atomic Cu and Selectively Excited Ionic Cu Line by Resonance Charge-Transfer Collision in a Low-Pressure Laser-Induced Plasma

Emisson spectra and time-resolved two-dimensional (2D) emission images of the electron-ion dielectronic recombination (i.e. a reversal process of auto-ionization) line of neutral Cu atoms, the selectively excited Cu ionic line, and normal Cu atomic line were observed for understanding the excitation mechanisms of Cu neutral and ionic lines in a low-pressure laser-induced plasma (LP-LIP) of Ar. From the observations, the number of charged particles around the emitting species seems to increase with increasing Ar pressure. Different time-resolved 2D emission images were observed among the selectively excited Cu ionic line and other Cu emission lines resulting from the different excitation mechanisms of the respective emission lines. Collisions of the second kind and electron-ion recombinations were found to be one of the major excitation mechanisms of Cu in Ar LP-LIP.

Atomic Hydrogen Emission Induced by TEA CO2 Laser Bombardment on Solid Samples at Low Pressure and its Analytical Application

Hydrogen emission has been studied in laser plasmas by focusing a TEA CO(2) laser (10.6 microm, 500 mJ, 200 ns) on various types of samples, such as glass, quartz, black plastic sheet, and oil on copper plate sub-target. It was found that H(alpha) emission with a narrow spectral width occurs with high efficiency when the laser plasma is produced in the low-pressure region. On the contrary, the conventional well-known laser-induced breakdown spectroscopy (LIBS), which is usually carried out at atmospheric air pressure, cannot be applied to the analysis of hydrogen as an impurity. By combining low-pressure laser-induced plasma spectroscopy with laser surface cleaning, a preliminary quantitative analysis was made on zircaloy pipe samples intentionally doped with hydrogen. As a result, a good linear relationship was obtained between H(alpha) emission intensity and its concentration.

Effect of plasma gas for spectrometric analysis of tin and zinc using low-pressure laser-induced plasma

The emission characteristics of tramp elements such as Sn and Zn in low-pressure laser-induced plasma have been examined with reference to change of the surrounding gas (Ar, Ne and He). From the pressure dependence of the intensity of Sn I 326.23-nm, Sn II 335.22-nm, Zn I 213.86-nm and Zn II 210.00-nm emission lines, it was found that Sn and Zn atoms could be excited by the collision between surrounding gas species and ablated atoms with large kinetic energy by laser irradiation. Besides the collisional excitation, resonance charge-transfer collision between Zn atoms and Ne ions proved to be effective in the selective excitation of Zn II 206.42-nm and 210.00-nm emission lines, because the emission intensity of these lines was strongly enhanced in Ne atmosphere, and the sum of the excitation energy of these lines and the ionization potential of Zn is very close to the ground-state energy of Ne ions.

Emission Characteristics of a Low-Pressure Laser-Induced Plasma: Selective Excitation of Ionic Emission Lines of Copper

Under reduced pressures, the plasmas produced by irradiation of a Q-switched Nd:YAG laser have favorable features as an excitation source for solid samples in atomic emission spectrometry, such as relatively low fluctuation of the emission intensity and a high signal-to-background ratio. For the analytical application of low-pressure laser-induced plasma spectroscopy (LP-LIPS), the spectra should be examined regarding the type of emission lines and their relative intensities. Further, information on the excitation mechanism in the plasma could be useful in determining optimum operation conditions of the experimental parameters. Spectrum patterns of LP-LIP are principally determined by the type of the filled gas. Especially, particular ionic lines of Cu are selectively excited when Ar or Ne are employed as the filled gas. This feature seems to be due to the (quasi-) resonance charge transfer collisions between a ground state ion of the Ar or Ne gas and a ground state atom of Cu. The plasma plume expanding after the laser irradiation also becomes an excitation source for sample atoms introduced by laser ablation.

Analysis of Scrapped Materials by Low-pressure Laser-induced Plasma Spectroscopy.

Analysis of Scrapped Materials by Low-pressure Laser-induced Plasma Spectroscopy.

Line broadening mechanisms in the low pressure laser-induced plasma

The spectral profiles of Ca and Rb lines have been studied in a laser induced plasma as a function of pressure (1–10 torr) and delay time with respect to the plasma initiation (1–10 μs). Measurements were made in a plasma induced by the 1064-nm output of a Nd:YAG laser on a calcium carbonate matrix, doped with Rb. Spectral profiles were measured in absorption using a narrow-band cw Ti:Sapphire laser. It was shown that in the case of a trace element (Rb in a CaCO3 matrix), the broadening mechanism was Doppler-dominant, whereas for a major matrix component (Ca), resonance broadening was the main contributor to the line shape. The plasma was shown to be non-equilibrium provided by the difference between the kinetic (3000 K) and the excitation (8000 K) temperatures. The electron number density at delay times of 5–10 μs and pressures of 1–10 torr was estimated not to exceed 1015 cm−3. The number densities of Ca atoms in the ground and the excited (23652 cm−1) states were evaluated by measuring line width and peak absorption at 732.6 nm. They were found to be in the range of (1.5–2.2)×1017 cm−3 for the ground state and (1.5–33)×1011 cm−3 for the excited state.