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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.
© 2015 Optical Society of America
1. Introduction
Laser induced plasma spectroscopy is a component analysis technique using the spectra obtained from the plasma generated by the laser ablation, which allows real-time and stand-off detection of all phases of the sample without potential contamination of sample, and depth profiling and multi element detection are also possible.
Studies on laser induced plasma detection in low pressure condition have been performed for understanding the plasma characteristics that is sensitive to the ambient pressure [1–3]. It was reported that signal detection is effective at 1 to 10 torr due to a reduced plasma shielding effect and the existence of optimum mean free path at such pressure [3–6]. In a vacuum condition however, rapidly expanding and vanishing plasma causes its emission detection extremely difficult.
Several spatial confinement methods for enhancing the signal intensity of plasma emission have been reported in the literature. Shen et al. confined the plasma using two parallel walls and the cylindrical pipe of varying diameters [7], and showed that the cylindrical pipe is more effective than the parallel walls. The cylindrical pipe showed an enhancement in the signal due to the confined shock reflections that effectively reheated the plasma and strengthened the signal intensity. Guo et al. used a hemispherical cavity [8] for uniformly compressing the plasma within the cavity and studied the combined effects of spatial confinement and dual-pulse irradiation [9]. Popov et al. used a small cylindrical chamber with polished brass walls (4 mm in diameter and 4 mm in height) to confine the plasma [10], which resulted in the improvement in both signal intensity and the limit of detection (LOD). Ding et al. investigated the plasma confinement effect by the reflected shockwave using metal disks with 2 mm hole in its center [11]. The reflected shock wave allowed the secondary enhancement of the signal intensity from CN molecules. Tao et al. performed a numerical analysis of the confinement effect using the microhole in Ar atmosphere and suggested that the plasma temperature, pressure, and thrust are enhanced due to the pressure wave reflections by the presence of a sidewall [12]. Zeng and Mao studied the effect of cavity with various aspect ratios in fused silica samples [13,14]. The plasma temperature and electron number density were at maximum when using the largest aspect ratio. Hou et al. improved both pulse to pulse signal repeatability and signal intensity by combination of cylindrical confinement and spark discharge [15]. They reported on the enhancements of spectral line intensity and the reduction of shot-to-shot fluctuation using the plasma image. Corsi et al. drilled craters of different depths on a copper sample to create cavity for the signal enhancement [16]. Hao et al. used ring magnet to spatially and magnetically confine the plasma [17]. They reported the enhancement of temperature and electron density of V and Mn. Li et al. performed numerical analysis using molecular dynamic simulation to investigate the temperature increase by shock wave confinement [18]. The temperature, pressure, and number density increase were observed by the presence of shock reflections.
In all previous plasma confinement attempts, the key concept was to maximize the strength of the secondary pressure waves or the reflecting shock waves from confining the walls at an atmospheric pressure (760 torr). In doing so, a reheating of the plasma by shock reflection within the confinement was proven useful in the signal enhancement. With the lowered ambient pressure as in the present work, shock strength can no longer be sustained and thus an alternative approach is required for further enhancing the signal strength. In our previous paper [19], a confining window above the sample is placed for successful prevention of rapid plasma extinction. At 1 mm confining height at 1 torr, the signal intensity of Al III emission (452.89 nm) was enhanced up to 5.5 times the free expansion case. In the present study, confining walls are introduced for increasing the electron density at low pressure as the direction of plasma expansion is being controlled. Various neutral atoms and molecules are considered and five different confining windows of varying transmittance at 1064 nm are used in order to validate the concept of confinement. Also the reason for molecular signal enhancement by active recombination of excited atoms at 760 torr is explained by the increase in electron density under such plasma confinement. The present study provides an important guideline as to how to strengthen the weak signal intensity of the plasma at low pressure conditions by using a plasma direction controlled confinement scheme.
2. Experimental setup
A laser-induced plasma spectroscopy system (RT250-Ec, Applied Spectra Inc.) uses Q-switched Nd:YAG laser operating at 1064 nm with 5-7 ns pulse duration at pulse energy of 34.3 mJ at 0.5 Hz that is focused onto the surface of a sample placed inside of a vacuum chamber. The laser beam is perpendicular to a surface of the sample. A high resolution 6 channel-CCD spectrometer covers the spectrum ranging from 190 to 1040 nm. The spectral resolution of the spectrometer is less than 0.1 nm for UV to VIS and 0.12 nm for VIS to NIR range. The emission from the plasma was detected through the side-view window. To collect the plasma, uncoated quartz lens of 100 mm focal length was used. The gate delay is varied from 0.1 to 0.5 μs while gate width is set to 1.05 ms. Sample is mounted on a XYZ stage inside a chamber, de-pressurized from 760 to 0.1 torr where a rotary pump is used to evacuate the chamber to provide low pressure test conditions for the laser-induced plasma.
The confining window of 1 mm thickness and 12.7 mm diameter was placed at 2 mm above a sample by two confining walls that effectively guide the plasma to freely expand in a single direction (Fig. 1). When a shockwave strikes the confining material, part of the energy of the shockwave is transmitted to the material, and the remainder of the energy gives rise to forming the reflected shockwaves that travel back towards the sample. The impedance is the product of wave velocity and density, representing the measure of opposition. With a higher impedance mismatch between two contacting materials, reflection of shock wave is likely, as opposed to transmission. In our setup, this reflection of shockwave is no longer utilized as such a reflection would not occur at the near vacuum condition. Hence carefully selected confining material for this setup was acrylic which has low impedance. Potentially decreasing electron density at low pressure condition is effectively handled by the present confinement set up.
Various metal samples such as Al alloy, Brass, Ni, Ti, Sn and Graphite were considered for finding their elemental characteristics, and the USGS certified reference material was selected for the detection of Sulfur. The laser beam was focused on the sample surface and fired on 3 locations with 5 shots for each location. The confining window was replaced after irradiation of 5 pulses to prevent deposition of ablated particles on the surface of window. Acrylic window for its good price can be replaced for such repeated experiments. For the optimal window material selection, Acrylic, BK7, Sapphire, Fused silica, and Magnesium fluoride were considered with their inherent transmittance to a 1064 nm beam.
3. Results and discussion
3.1. Low pressure effects
Figure 2 shows the confinement effect of neutral atoms at 1 torr. Unconf and Conf represent the unconfinement (free expansion) and confinement cases, respectively. By confinement, signal intensity of all the elements improved at least 1.5 times compared to a free expansion using a single laser pulse. The plasma persistence time of carbon, nickel, and tin which have shorter persistence time than aluminum, copper, and zinc in our earlier results in Fig. 1 of [3] is also increased. The electron density of plasma rapidly decreases without confinement while the confinement allows the plasma to merge along a single direction of detection. The resulting plasma has high electron density and the extended persistence time. Significantly increased amount of ions, atoms, and molecules is collected along the controlled direction of the plasma.
Fig. 2 Effect of plasma confinement of the neutral atoms at 1 torr (a) Al I 396.152 nm, (b) Cu I 324.754 nm, (c) Zn I 472.2156 nm, (d) Ti I 334.9405 nm, (e) C I 247.8561 nm, (f) Ni I 341.4764 nm, (g) Sn I 326.2331 nm.
Table 1 compares signal intensities of neutral atoms with and without confinement using a single laser pulse. In our setup, energy loss occurs while the beam is transmitted through the acrylic window of 92% transmittance at 1064 nm. Thus signal intensity decreases at normal pressure where the confinement is not critical for generating a strong plasma for detection. The signal intensities of carbon and aluminum at 760 torr by confinement are greatly declined. These two elements can recombine with nitrogen and oxygen which are included in the air. When confinement is applied, the increase of electron density in limited space causes rapid recombination of excited atoms, and thus atomic signals have low intensity and short persistence time. The persistence time of neutral carbon is shorter than aluminum because carbon has a large electronegativity which is a measure of tendency for either atoms or functional group to attract the electrons towards themselves. Hence carbon has weak signal intensity and small persistence time due to rapid extinction by recombination in the presence of nitrogen.
Table 1. Confined/unconfined signal ratio of neutral atoms using single laser pulse
However confinement effect becomes clear at pressure below 100 torr where the signal intensity increases at least 1.5 times. The signal intensity can further increase by accumulation of the several laser pulses. Also, one notes that carbon and nickel are only detectable at 0.1 torr with confinement.
Table 2 shows the electron density of Al I (396 nm) at 2 µs delay time. Although the signal intensity of neutral aluminum by confinement is greatly decreased at 760 torr, the electron density is increased. The confining plasma expansion in a limited space using a confining window causes the increase in both electron density and persistence time. Furthermore, more ions, atoms, and molecules reach the detector along the controlled path in the proposed confinement set up. The plasma expansion occurs at the early times right after the plasma initiation, and thus the confinement effect is also started immediately.
Table 2. Electron density of Al I (396 nm) at 2 µs delay time
Figure 3 shows the change of molecular band signal from a graphite sample at atmospheric pressure. When confinement is used, the increase of electron density within the limited space causes rapid recombination of excited atoms. The rapid recombination causes decrease of atomic emission. Instead, strong molecular signal appears by the recombination of atoms as shown Fig. 3 CN molecule is generated by recombining the ablated carbon from the sample and the nitrogen of an air. Air-combined molecules have remarkably small persistence time by confinement because supplied air is insufficient for generating a strong molecular signal in the limited space.
Fig. 3 Effect of ablated mass confinement of the molecular bands at atmospheric pressure (a), (b) CN 388 nm, (c), (d) C2 516 nm.
In contrast to air-combined molecules, C2 molecular band that is formed from the product material of the sample showed strong intensity and longer persistence time with the confinement applied. The electronegativity [20] that defines the tendency of an atom to attract electrons towards itself, of nitrogen is higher than that of carbon, and thus the C-N bond is generated more easily than the C-C bond. This explains why the C2 band signal from the free expansion has a weak intensity. Whereas C2 band formation occurs rather strongly while the expansion of carbon atom towards an air is disrupted by the presence of an acrylic window in our system.
The signal increment by confinement of neutral carbon at low pressure in Table 1 supports the result of the molecular signals. The signal of neutral carbon with confinement decreases at high pressure conditions above 1 torr due to its recombination with nitrogen. Air becomes thinner as the pressure is lowered, thus the confinement allows for signal enhancement at pressure below 1 torr.
We have checked the molecular signals at such low pressures. Signals of CN and C2 molecular band decrease as pressure is decreased and disappear when below 10 torr. Therefore signal enhancement of neutral carbon by confinement starts to appear at 1 torr as in Table 1.
The highly ionized atom was also investigated. The doubly ionized carbon atom is not detected at 760 torr as shown Fig. 4 C III emission line appeared at 1 torr when no confinement is applied. The low pressure condition allowed for a reduction in the plasma shielding, resulting in lesser obstruction along the beam path. Besides, highly ionized atoms are also not readily combined with electrons. In the confinement case, the signal intensity (or signal to noise ratio) increase is quite evident. A rapid extinction of plasma is delayed as the plasma is effectively isolated within a 2 mm confinement spacing.
3.2. Hard-to-detect element – sulfur detection
The present confinement scheme may facilitate the detection of minor elements and those hard-do-detect elements such as sulfur, chlorine, and phosphorous. Strong emission lines of these elements are reported to occur either in the UV or near IR region, and the excited atoms readily react with atmospheric oxygen. Besides, these elements have high ionization energy, and thus detection of S, Cl, P has known quite challenging. In the past, researchers attempted to detect such elements by using the double pulses at high laser energy [21] or the buffer gases [22] and low pressure conditions [23] for potential detection. More recently, Labutin et al. detected S I 921 nm of 0.5-1% concentration using collinear double pulse approach [24]. Their double pulse allowed 5 times increase of signal intensity of sulfur at 2 µs interpulse delay using 55 mJ/pulse.
Here we provide a result of sulfur detection by using a single laser pulse at low laser energy combined with the present confinement method. The target sample is MASS-1 which is a USGS certified reference material that contains 27% of sulfur, and the accumulation of five spectra are used for generating the result. Figure 5 shows sulfur spectra with respect to pressure. Without confinement, neutral sulfur peaks appeared with a very low signal to noise ratio (SNR) at the atmospheric pressure, while the peak is quite non-distinguishable at the lowered pressures. Most of neutral atoms are easily detectable at atmospheric pressure with a CCD detector which has a long gate width as shown in Table 1. We find that the low pressure condition allows for an effective detection of certain element namely the sulfur which quickly recombines with the oxygen in air. Consequently SNR is enhanced. To estimate SNR of S I 921 nm, background subtraction was performed using the Aurora software (Applied Spectra Inc.). Two ratios of the peaks namely 921 nm and 915 nm were used for SNR. For 760 torr unconfined case, SNR was 3.34, and for 10 torr confined case, SNR was 20.15. Thus the present work is also effective in terms of utilizing the pressure conditions for detecting those hard-to-detect elements of a wide interest.
3.3. Effect of a confining window material
Up to now, we used acrylic window which has 92% of transmittance at 1064 nm wavelength due to high costs of replacing the windows. To compare the effect of window materials, five different materials were considered that include Acrylic, BK7, Sapphire, Fused silica, and Magnesium flouride. The transmittance of each material is summarized in Table 3.
Table 3. Transmittance of window material at 1064 nm
Figure 6 shows signal intensities ratio of confined and unconfined cases with respect to pressure for various materials. For acrylic window and sapphire window, the signal enhancement was noticeable only if the pressure is lowered below 100 torr due to energy losses. For all other window materials of higher transmittance, the signal enhancement was observed at all pressure range. Despite the low transmittance of only 83%, we examined sapphire as a window material as an extension of work reported [19] where we performed the signal enhancement at low pressure using sapphire. The confinement effect using sapphire was visible at pressure below 1 torr only. The results suggest the optimal window materials for the present confinement scheme aimed at low pressure detection.
4. Conclusion
An effective plasma confinement for enhancing the signal at low pressure is described. As the pressure approaches near vacuum, the utilization of the shock reflections known to improve the signal intensity by reheating the plasma at atmospheric pressure is no longer achievable, and thus an alternative scheme is desired. The present confinement configuration consists of two end walls capped with a transparent window that prevents rapid plasma dissipation while controlling the direction of the plasma expansion towards the detector. The enhancement factors of 1.5 to 4.2 in all elemental signals were obtained by the confinement at the low pressure conditions using a single laser pulse. The signal intensity and signal to noise ratio of highly ionized atoms which appear only at low pressure were also greatly improved by the present detection scheme. The direction control as opposed to shock-heating of the rapidly expanding plasma is proven quite effective when it comes to low pressure plasma detection. Further optimization of this technique would eventually guide the handling of those hard-to-detect elements of minor concentration.
Acknowledgments
Authors wish to acknowledge the financial support from the Korea National Research Foundation under National Space Laboratory Program 2014 (NRF-2014M1A3A3A02034903) through the IAAT at Seoul National University.
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