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Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 17, Iss. 11 — May. 25, 2009
  • pp: 8856–8870

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Enhanced temperature and emission from a standoff 266 nm laser initiated LIBS plasma using a simultaneous 10.6 μm CO2 laser pulse

Avishekh Pal, Robert D. Waterbury, Edwin L. Dottery, and Dennis K. Killinger  »View Author Affiliations


Optics Express, Vol. 17, Issue 11, pp. 8856-8870 (2009)
http://dx.doi.org/10.1364/OE.17.008856


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Abstract

A deep UV 266 nm laser induced LIBS plasma has been enhanced by using a simultaneous 10.6 μm CO2 laser pulse at standoff ranges up to 55 m for several targets including metals, ceramics and plastics. The LIBS plasma emission was produced, for the first time, by a 266 nm laser and was enhanced by several orders of magnitude using the CO2 laser pulse. The temperature of the enhanced LIBS plasma was measured, for the first time, and was observed to increase by about 3000K due to the addition of the CO2 laser pulse.

© 2009 Optical Society of America

1. Introduction and background

Laser-Induced-Breakdown Spectroscopy (LIBS) is a recognized laser detection technique for sensing the chemical composition of a wide range of materials including minerals, chemical substances, and trace species [1-6

A. Miziolek, V. Palleschi, and I. Schechter, eds., Laser Induced Breakdown Spectroscopy . (Cambridge University Press, 2006).

]. Recently, LIBS has been studied for the remote detection of a wide variety of substances such as surface contaminants and other trace materials [1-6

A. Miziolek, V. Palleschi, and I. Schechter, eds., Laser Induced Breakdown Spectroscopy . (Cambridge University Press, 2006).

]. There is a need to increase the detection range of such standoff LIBS systems, and, as a result, the need to increase the strength of the LIBS signal. It should be added that a wide range of LIBS systems are currently being studied including classical nanosecond laser pulse LIBS, dual-pulse LIBS, fs-LIBS, microwave enhanced LIBS, and Townsend Effect Plasma Spectroscopy (TEPS) CO2 laser enhanced nanosecond laser LIBS [7

J. Scaffidi, W. Pearman, J. C. Carter, and M. Angel, “Temporal dependence of the enhancement of material removal in femtosecond-nanosecond dual pulse laser-induced breakdown spectroscopy,” Appl. Opt. 43, 6492–6499 (2004). [PubMed]

]. Our work reported in this paper covers the latter technique, and is an outgrowth of our past LIBS/TEPS studies using a near-IR 1.064 μm laser for the LIBS plasma and enhancement using a simultaneous 10.6 μm laser IR CO2 laser pulse [8

D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, and E. L. Dottery, “Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse,” Opt. Express 15, 12905–12915 (2007). [PubMed]

].

Our previous TEPS studies were conducted using a classical single pulse nanosecond 1064 nm laser for the LIBS excitation followed by the addition of a nearly simultaneous CO2 laser pulse which resulted in signal enhancements on the order of 25 – 300 [8

D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, and E. L. Dottery, “Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse,” Opt. Express 15, 12905–12915 (2007). [PubMed]

]. The results presented in this paper build off our previous work and change the primary plasma formation laser wavelength from 1064 nm to 266 nm, for the first time to our knowledge. It is important to note that the use of 266 nm and 10.6 μm lasers is a technique that may have eye-safety advantages for stand-off applications since these two wavelengths have an increased minimum permissible exposure (MPE) limit for direct ocular viewing compared to 0.53 or 1.06 μm lasers [9

American National Standard for Safe use of Lasers Outdoors, Laser Institute of America, ANSI Z136.6 (2005).

].

The work presented in this paper is the first demonstration of remote LIBS using a 266 nm UV laser for the LIBS plasma generation and for identification of samples from a standoff distance out to 55 m using eye safe wavelengths. In addition, the use of a second laser at 10.6 μm CO2 enhanced the LIBS signal. It should be noted that the standoff distance for the CO2 laser was kept at a constant distance from the target of 5 m due to telescope and focusing limitations for the results presented herein; however, our recent field work has extended the CO2 laser range out to 20 m [10

R. D. Waterbury, P. M. Pellegrino, E. L. Dottery, A. R. Ford, and J. B. Rose, “Results if a UV TEPS/Raman system for standoff detection of energetic materials,” Chemical and Biological Defense Physical Science and Technology conference, New Orleans (2008).

]. Laser pulse energies on target were about 0.05 J/mm2 for both lasers, with LIBS emission depending upon both the UV and the CO2 laser pulse intensities. Enhancement in the LIBS emission on the order of 10x to 100x was observed for several targets, including metals, ceramics, and plastics. In addition, the LIBS plasma electron temperature was measured using the Boltzmann technique and an increase in the LIBS plasma temperature of about 3,000 K due to the addition of the CO2 laser pulse was measured, for the first time to our knowledge. The temporal overlap of the two lasers was found to be optimized for a 0.5 μs to 1.5 μs delay. It should be added that the research data presented in this paper was mostly obtained using a stand-off LIBS/TEPS and Raman system developed by Alakai to be used for field measurements and has demonstrated the first dual mode LIBS and Raman standoff detection of trace materials at these less eye-hazardous wavelengths [11

R. Waterbury, A. Pal, D. K. Killinger, J. Rose, E. Dottery, and G. Ontai, “Standoff LIBS measurements of energetic materials using a 266 nm excitation laser,” Proc. SPIE 6954, 409 (2008).

]. It may be added that organic targets have also been studied and will be reported in a future paper.

2. Deep UV LIBS and 10 μm laser TEPS experimental apparatus

Our TEPS enhanced 266 nm/10 μm LIBS system used a 4th harmonic nanosecond Nd:YAG laser to produce a LIBS plasma, and a simultaneous CO2 laser pulse to overlap spatially and temporally the LIBS spark. A schematic of the simultaneous dual-laser standoff LIBS system is shown in Fig. 1. As seen in Fig. 1, a Q-switched, 4th harmonic Nd:YAG laser (frequency quadrupled Quantel Brilliant B; 90 mJ/pulse, 10 Hz, 6 ns pulse length, M2 of 5) was focused onto the target with a custom designed beam expander and focusing optics using a 2.5 cm plano concave lens (focal length of 100 mm) and 10 cm double convex lens ( focal length of 760 mm). The focused spot size was measured by burn patterns to be about 1 mm in diameter at ranges close to 40 m. The plasma produced on the target emitted LIBS emission into a 2π steradian solid-angle cone, which was collected using a 35 cm diameter telescope (Meade LX200-14) and sent to a dichroc beam splitter and then focused onto two separate 600 micron core fiber optic fibers. The signal was transmitted to a 2-channel spectrometer (Ocean Optics: Model HR2000+; channel 1 for 300-570 nm, channel 2 for 640-850 nm) where it was detected by a linear CCD array with 2048 pixels and an optical resolution of 0.2 nm/ pixel. An optical order-sorting filter in each spectrometer eliminated higher order diffraction modes off the grating. The LIBS spectrum from the spectrometer was then transferred to a notebook computer using the Ocean Optics spectrometer software.

Fig. 1. Schematic of Deep UV 266 nm LIBS and 10.6μm CO2 laser enhancement LIBS standoff system.

A high-power, pulsed CO2 Transverse Electrode Atmospheric (TEA) laser (Lumonics Model 960; 1.4 J/pulse, 10 Hz) was used to produce 10.6 μm laser pulses that were routed using mirrors onto the same LIBS emission target area. Due to the moderately high mode pattern and beam divergence of the CO2 laser (0.01 radians), it was not made co-linear with the main Nd:YAG laser and its beam was focused by a separate lens (5 cm ZnSe lens, focal length of 25 cm) as shown in Fig. 1. The pulse length of the CO2 laser pulse had an initial TEA laser spike of about 100 ns length followed by a nitrogen-fed tail about 5 μs long. The CO2 laser output beam size was controlled using a 5 cm focal length ZnSe lens to have a diameter on target between 3 mm to 15 mm. For a 6.5 mm diameter beam, the energy density was about 40 mJ/mm2. The timing of the laser pulses was controlled with a digital time-delay generator (SRS Model DG535), and the laser pulses were detected using a fast Si photodiode (ThorLabs: Model 10A) for the Nd:YAG laser and a pyro-electric detector (Eltec:Model 420-0-1491) for the CO2 laser. The timing uncertainty (jitter) of the lasers was about 20 ns for the Q-switched Nd:YAG laser and about 500 ns for the TEA CO2 laser.

The angular separation of the Nd:YAG/LIBS collection beam and CO2 beam was about 20 degrees. The Nd:YAG laser beam was at near normal incidence to the sample. In this paper, the substrates used for the LIBS measurements were pure grade aluminum (99.99% Sigma-Aldrich), pure grade Copper (99.99% Sigma- Aldrich), iron (for temperature measurements), lead, and ceramic (alumina). One of the major components of remote sensing with LIBS is the ability to focus the laser onto a small spot to create a spark on a remote sample surface. For most solids the necessary irradiance to produce plasma is on the order of 107-108 W/cm2 [6

S. Palanco, C Lopez-Moreno, J. J. Laserna, and F. DeLucia Jr, “Design, construction and assessment of a field -deployable laser-induced breakdown spectrometer for remote elemental sensing,” Spectrochim. Acta, Part B 61, 88–95 (2006).

]. Since air breakdown in the proximity of the remote sample surface may result in false signals, care was taken in our remote LIBS experiment to make sure that the irradiance level was less than that required for breakdown of clean air (~1010 W/cm2)[6

S. Palanco, C Lopez-Moreno, J. J. Laserna, and F. DeLucia Jr, “Design, construction and assessment of a field -deployable laser-induced breakdown spectrometer for remote elemental sensing,” Spectrochim. Acta, Part B 61, 88–95 (2006).

].

The transmitted Nd:YAG laser beam was expanded and then focused down to a point at the remote target as shown in Fig. 1 using a negative and positive lens. The focus distance to the sample was adjusted by altering the distance between the diverging lens and the primary mirror with the help of a motorized moving platform (Thorlabs Model# TST 001/ZST25).

3. LIBS spectra using 266 nm Nd:YAG laser

LIBS spectra using only the Nd:YAG laser at 266 nm were obtained. As an example, Fig. 2 shows the measured spectra obtained for an aluminum target at a distance of about 25 m. As can be seen in Fig. 2, there are several atomic emission lines superimposed upon a broad continuum. For example the strong elemental line at 777.16 nm is due to O(I) and at 746.86 nm is due to N(I). Other lines in Fig. 2 are also identified. However, some of the lines are hidden in a broad continuum due to the black body radiation emitted by the plasma. The C2 Swan bands near 460 nm and 520 nm as reported by other groups can be clearly seen [4

C. López-Moreno, S. Palanco, J. J. Laserna, F. DeLucia Jr, A. W. Miziolek, J. Rose, R. A. Walters, and A. I. Whitehouse “Test of a stand-off laser-induced breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces,” J. Anal. At. Spectrom. 21, 55–60 (2006).

]. The absolute error in the spectrometer wavelength reading was about 1nm. The statistical variability in the LIBS signal was approximately 15% for 100 laser pulses. The emission from the LIBS plasma was observed to peak at 1~2 μs after the 266 nm laser pulse.

Fig. 2. LIBS emission signal from a pure Al target using a 266 nm Nd:YAG laser initiated plasma; standoff range of 25 m.

4. CO2 laser LIBS measurements

The influence of the CO2 laser by itself as a source of the LIBS emission was investigated. The energy density of the CO2 laser on the target substrate was varied by changing an iris in front of the CO2 laser beam and changing the position of the ZnSe focusing lens. Our results are given in Fig. 3 which shows the LIBS signal as a function of the CO2 laser density on a ceramic target. In this case no Nd:YAG laser was used and only the CO2 laser was used.

Fig. 3. LIBS signal from a pure ceramic (alumina) substrate using a pulsed CO2 laser initiated plasma as a function of CO2 energy density on the target.

As can be seen, the LIBS emission or plasma appears at a CO2 laser energy density of about 120 mJ/mm2. A lower density of about 100 mJ/mm2 produced a black body continuum while lower energy levels of 70 mJ/mm2 produced no LIBS signals. In subsequent sections of this paper, where the CO2 laser is used to enhance the 266 nm Nd:YAG laser produced LIBS signal, the energy density for the CO2 laser was about 30 mJ/mm2. As can be seen in Fig. 3, this energy density value does not cause breakdown. It may be added that at this CO2 energy density level of 30 mJ/mm2, negligible breakdown was observed for an Al target for the CO2 laser alone. It is interesting to note that the 100 mJ/mm2 energy density for a pulse length of 100 ns is equivalent to 108 W/cm2.

5. Enhanced 266 nm Nd:YAG LIBS emission using simultaneous 10.6 μm CO2 laser pulse

The 266 nm Nd:YAG laser was used to generate the plasma on an aluminum substrate at a distance 25 m away. The 10.6 μm CO2 laser was positioned about 5m away from the substrate for all of our experiments herein, due to limitations of the CO2 laser telescope that was available. The CO2 laser beam was focused at the LIBS plasma on the substrate and had a pulse energy of around 1.5 J, a spot size of around 7 mm that fully covered the LIBS plasma region, and an energy density of 30 mJ/mm2 on the substrate. This value ensured that the LIBS emission was not due to the CO2 laser alone.

The LIBS/TEPS spectrum from the remote aluminum target was measured and is shown in Fig. 4, where many of the lines have been tentatively identified and are given in a later section. As can be seen in Fig. 2 and Fig. 4, the LIBS signal has been greatly enhanced by the addition of the CO2 laser beam; note that the amplitude scale in Fig. 4 is about 100x greater than that shown in Fig. 2, where no CO2 laser was used. As can be seen, the signal strength in some cases is almost 100 times greater. This is consistent with our previous near–IR published work using a 1 μm Nd:YAG laser for plasma excitation and a CO2 laser for enhancement [8

D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, and E. L. Dottery, “Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse,” Opt. Express 15, 12905–12915 (2007). [PubMed]

]. In addition to the previously observed lines of oxygen and nitrogen in Fig. 2, more lines are seen in Fig. 4 indicating that a change in the plasma condition has occurred by addition of the CO2 laser. To better understand this change, additional studies involving the timing and plasma temperature were conducted, and are presented in the following sections.

Fig. 4. LIBS emission signal from a pure Al target using a 266 nm of Nd:YAG laser initiated plasma and 10.6 μm CO2 laser pulse; standoff range of 25 m.

6. Timing overlap of two lasers and its effect on LIBS plasma

The enhancement of the LIBS signal was found to be highly dependent on the timing overlap of the two lasers. The overlap was measured using the silicon photo detector which detected the 266 nm and the LIBS plasma emission, and the pyro-electric photo detector that measured the CO2 laser pulses. As an example, Fig. 5 shows a dual oscilloscope trace of the two laser signals when the beams were sampled. In this case the 266 nm beam was reduced in intensity so that no LIBS plasma emission occurred. As can be seen, the 6 ns 266 nm laser pulse and 0.1 μs CO2 laser pulse with 5 μs tail are evident.

Fig. 5. Oscilloscope trace of the CO2 laser and Nd:YAG laser pulses.

The delay generator was used to change the timing between the two lasers. Figure 6 shows the measured LIBS signal acquired at a range of 35 m as a function of the CO2 pulse delay with respect to the Nd:YAG laser pulse. Here, the optical intensity of the signal in the upper spectrometer channel was reduced to avoid saturation of the detector and resulted in a reduced relative intensity for the 640-850 nm lines. As can be seen, a delay of around 0.5 μs produced the most enhanced signal, however some elemental lines were more pronounced at 2.0 μs and 2.8 μs. Several of the lines were measured as a function of the CO2 laser pulse delay, and our results are shown in Fig. 7. As can be seen, the maximum enhancement was found to occur between 0.5 μs and 1.5 μs.

Fig. 6. LIBS spectrum from a pure Al target as a function of CO2 laser pulse delay compared to 266 nm Nd:YAG laser pulse; standoff range of 35 m.
Fig. 7. Various LIBS/TEPS emission lines as a function CO2 laser delay; stand-off range of 35 m.

7. LIBS/TEPS data on various substrates

The 266 nm Nd:YAG laser and 10.6 μm CO2 laser were used in our remote stand-off configuration to obtain enhanced LIBS/TEPS spectra for several different types of targets. Figures 8 to 12 show the measured LIBS signal on bare substrates of ceramic (alumina), copper, iron, lead, and plastic (polycarbonate) substrates at a standoff range of 20 m inside our laboratory building under ambient conditions. As can be seen, a number of molecular and atomic lines can be observed. It should be noted that many of these lines were considerably enhanced by use of the TEPS CO2 laser. For example, the LIBS signal from a plastic target was negligible (i.e., little emission), but was enhanced by 100x or more by the addition of the CO2 laser as can be seen in Fig. 12. Other lines for several targets were enhanced by several orders of magnitude. A detailed study of these enhancement ratios is planned for a later technical report.

Fig. 8. LIBS/TEPS signal on bare ceramic (alumina) substrate; standoff range of 20 m range.
Fig. 9. LIBS/TEPS signal on bare Copper substrate; standoff range of 20 m range.
Fig. 10. LIBS/TEPS signal on Iron substrate; standoff range of 20 m.
Fig. 11. LIBS/TEPS signal on Lead substrate; standoff range of 20 m.
Fig. 12. LIBS/TEPS signal on Plastic (polycarbonate) substrate; standoff range of 20 m.

Tentative identification of the lines using the NIST database was conducted by manual inspection of the spectra, and the results are given in Table 1; in some cases where multiple spectral peaks overlapped, the assignment was difficult and care was taken to exclude these lines from being used in subsequent analysis. As can be seen, ceramic (alumina) has Al, O, N and Fe lines, copper has Cu, Al, Fe, and N lines, iron has Pb, Fe, and Al lines and lead has Fe, Pb along with O, and N lines. There are several lines identified for the plastic sample, but several also overlap the C2-Swan bands near 500 nm. The emission near 656 nm may be H (I) emission related to ambient air [8

D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, and E. L. Dottery, “Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse,” Opt. Express 15, 12905–12915 (2007). [PubMed]

].

8. Electron temperature measurement of LIBS plasma temperature

In order to better understand the mechanism behind the CO2 laser plasma LIBS/TEPS enhancement, we studied the LIBS plasma temperature using the Boltzmann method on a iron (Fe) target. Previous experiments by other groups have shown that the emission lines from iron have sufficient strength and upper electron energy level spacing to be useful for plasma temperature measurements [12-14

A. A. Khalil, M. Richardson, C. Barnett, and L. Johnson, “Double Pulse UV laser induced breakdown spectroscopy of stainless steel,” Appl. Spectrosc. 73, 735–742 (2006).

].

The Boltzmann plot method is a simple and widely used technique for spectroscopic measurement, especially for measuring the electron temperature of plasma from using the relative intensity of two or more line spectra emitted from energy levels having a relatively large energy difference. For the case of lines emitted between two energy levels, lower level Ei and upper level Ej, and having densities of Ni and Nj, respectively, under thermal equilibrium conditions, the relation between Ni and Nj can be written as [15-16

G. V. Marr, Plasma Spectroscopy , (Amsterdam: Elsevier, 1968)

]

N j N i= g j g ie x p [− ( E j− E i k T e)],
(1)

where gi and gj are the statistical weights of the respective states, k is the Boltzmann constant (1.38 × 10-23 J/K), and Te is the temperature in K. The emission intensity from the upper energy level, Ej to the lower energy level, Ei, can be given as [15-16

G. V. Marr, Plasma Spectroscopy , (Amsterdam: Elsevier, 1968)

]

ln ( ε ji λ ji A ji g j)= E j k T+C,
(2)

where λji is the wavelength of the emitted light, h is Planck’s constant (6.626 × 10-34 J∙s), and Aji is the transition probability, which is the probability per second that an atom in state j spontaneously emits radiation, C is a constant and, εji is the intensity of the emission line (also called the emission coefficient of the spectral line).

Table 1.  Identified species and LIBS/TEPS emission lines from various substrates.
oe-17-11-8856-i001

We conducted LIBS/TEPS temperature experiments using an iron substrate as our target. As an example, Fig. 13 and Fig. 14 show LIBS and LIBS/TEPS measurements for an iron target at a standoff range of 25 m. In this case, the collection optics bypassed the telescope so that the input optical fibers were placed directly next to the substrate in order to increase the signal strength. Figure 13 shows the spectrum from the iron substrate due to the 266 nm Nd:YAG only laser emission and Fig. 14 shows the emission spectrum on the iron substrate from the plasma generated by the 266 nm Nd:YAG and enhanced by the 10.6 μm CO2 laser pulse. As can be seen in Fig. 14, the LIBS intensity increases with the addition of the CO2 laser. Note that the y-axis scale for Fig. 14 is about 30 times greater than the scale in Fig. 13. It should be noted that the enhancement for all the lines was not uniform, possibly due to the relative presence of different species in the plasma formed by the first laser pulse; further studies are planned to elucidate this mechanism. For comparison purposes, similar lines are identified and labeled in Figs 13 and 14. The assignments of these lines as the levels belonging to the lower and upper state configurations are well known and tabulated in the NIST database [17

Y. Ralchenko, A. E. Kramida, and J. Reader NIST ASD Team, “NIST Atomic Spectra Database,” National Institute of Standards and Technology, Gaithersburg, MD (2008). http://physics.nist.gov/asd3.

]. There are several lines between 380 nm and 450 nm belonging to the neutral iron Fe(I), and assignments of the observed lines are tabulated in Table 2, along with their transition probabilities, A, and uncertainty in the A value, ΔA. Assignments were conducted using the wavelengths recorded in the spectral data file from the spectrometer, and were more accurate than that implied by the arrows designating the lines in Fig. 13, and Fig. 14.

Fig. 13. LIBS emission spectrum from Iron substrate when only the 266 nm Nd:YAG laser was used to generate the plasma; standoff range of 25 m.
Fig. 14. LIBS/TEPS emission spectrum from Iron substrate when both 266 nm Nd:YAG and 10.6 μm CO2 lasers were used to generate the plasma; standoff range of 25 m.
Table 2.  Spectroscopic constants of the neutral Fe (I) lines used in Boltzmann plot temperature determination; from NIST database [15

G. V. Marr, Plasma Spectroscopy , (Amsterdam: Elsevier, 1968)

]
λ(nm)ConfigurationsA(s-1)ΔA(%)gEj(eV)
404.583p63d7(4F)4s - 3p63d7(4F)7.50 × 107 2594.55
411.853p63d7(2H)4s - 3p63d7(2H)5.80 × 107 25136.58
426.053p63d6(5D)4s4p(3P0) - 3p63d6(5D)3.70 × 107 50115.31
430.793p63d7(4F)4s - 3p63d7(4F) 3.50 × 107 2594.44
432.583p63d7(4F)4s - 3p63d7(4F)5.10 × 107 2574.47
438.353p63d7(4F)4s - 3p63d7(4F)4.60 × 107 25114.31
440.483p63d7(4F)4s - 3p63d7(4F)2.50 × 107 2594.37

The electron temperature was calculated using the ratio of the relative line to background intensities of about seven spectral lines using Eq. 2 and the data in Table 2. In order to increase the accuracy of this technique, lines were chosen which had a large range of upper energy levels, over the range of 4.37 eV to 6.58 eV.

Figure 15 is the resultant Boltzmann plot of the Iron (I) lines measured from the 266 nm laser LIBS and that with the addition of the CO2 laser; here there was a 0.5 μs delay between the two laser pulses. The slope of the linear regression yields the Boltzmann temperature. As can be seen, the temperature measured for the LIBS plasma was about 8700 K, and for the LIBS/TEPS CO2 enhanced plasma was about 12,000 K. This suggests that the LIBS/TEPS plasma is hotter due to absorption of the CO2 laser radiation by the thermal electrons in the LIBS plasma. This indicates that the simultaneous CO2 laser pulse super-heats or enhances the electron temperature of the original 266 nm Nd:YAG laser produced plasma. Of importance is that the correlation value, R2, is high with a value of 0.77 to 0.87, and tends to indicate that Local Thermodynamic Equilibrium (LTE) conditions were present within the plasma. It may be added that the line near 404.58 nm in Fig. 14, is somewhat wider and may be due to a combination of lines. However, this data point at 4.55 eV in Fig. 15 does not significantly change the derived temperature, since much larger Ej values are also used.

Fig. 15. Boltzmann plot of Fe (I) lines of the laser-induced plasma for 266 nm Nd:YAG LIBS, and enhanced LIBS/TEPS plasma using both Nd:YAG and CO2 laser pulses.
Fig. 16. Measured electron plasma temperature as a function of interpulse delays between the 266 nm Nd:YAG laser pulse and CO2 laser pulse.

The electron temperature of the LIBS/TEPS plasma was also measured as a function of the CO2 laser pulse delay. Figure 16 shows the measured Boltzmann temperature as the delay between the 266 nm Nd:YAG laser pulse and 10.6 μm CO2 laser pulse was varied. As can be seen, the temperature reached a peak on the order of 25,000 K at a delay of about 1.5 μs. In addition, we also measured the intensity of several prominent lines as a function of the pulse delay. This data is shown in Fig. 17 along with the temperature data from Fig. 16. As can be seen, both the emission and temperature seems to peak around a pulse delay of about 0.5 μs to 3 μs. However, it is interesting to note that the peak N (746.83 nm) and O (777.42 nm) emission seems to occur at a delay of 0.5 μs, while the peak temperature and peak Fe emissions (438.35 nm and 430.79 nm) is at around 1.5 μs. This may be due to differences in the second pulse absorption and re-heating process dynamics for the different species.

Fig. 17. LIBS emission intensity for selected lines from Fe target, and measured plasma temperature, as a function of LIBS/TEPS CO2 laser pulse delay/overlap.

9. Initial standoff LIBS/TEPS measurements as a function of range

In order to study the range dependence of the LIBS/TEPS signal, an initial series of LIBS/TEPS experiments were conducted at varying standoff ranges from the target. In this case, however, due to CO2 laser transmitter limitations, the CO2 laser was kept at a constant distance of 5 m away from the target. Figure 18 shows a plot of the LIBS/TEPS spectrum measured as a function of range for a target of pure Al. As can be seen, the LIBS signal strength decreased as the range became progressively longer. This is consistent with the effects of atmosphere attenuation and the 1/R2 factor as found in the LIBS lidar equation due to the reduced fraction of LIBS emission captured by the telescope area as the range is increased. Further studies are being conducted to better quantify this range dependence, including absorption of various parts of the LIBS emission spectrum by molecules in the atmosphere [18

D. Plutov and D. K. Killinger, “Atmospheric transmission and Lidar modeling of LIBS and Raman remote sensing of distant compounds,” paper JMA 24, OSA:LACSEA conference, St. Petersburg, Fl (2008).

].

In addition, we also measured the volume of the crater produced by the LIBS plasma on the target as a function of standoff range. Using a precision profilometer (Dektek 3030), the volume of the ablated matter was determined as a function of standoff range for an Al target, and our results are shown in Fig. 19. As can be seen in Fig. 19, the volume of the ablated material decreased as the range increased, but seemed to reach a minimum at around 25 m; similar results were found for the volume of the crater formed. This trend is consistent with the interplay of the minimum beam diameter reached at the longer ranges and the increased confocal beam parameter, zo, at these longer ranges; the confocal beam parameter, zo, for the Nd:YAG laser was calculated to be about 6 m at a range of 25 m [19

A. Yariv, Quantum Electronics (Wiley & Sons, New York, 1989).

]. Our results agree with the results obtained by Palanco who found a similar trend in the LIBS signal as a function of range; they measured an inverse third power [6

S. Palanco, C Lopez-Moreno, J. J. Laserna, and F. DeLucia Jr, “Design, construction and assessment of a field -deployable laser-induced breakdown spectrometer for remote elemental sensing,” Spectrochim. Acta, Part B 61, 88–95 (2006).

].

Fig. 18. LIBS/TEPS signal as a function of standoff range for an Al target.
Fig. 19. Volume of LIBS/TEPS ablated material for an Al target as a function of range.

10. Conclusions

A dual laser (266 nm and 10.6 μm) LIBS/TEPS system has demonstrated enhanced LIBS emission for the remote sensing of standoff targets. The LIBS plasma temperature was measured and found to increase by several thousands of degrees due to the use of the CO2 TEPS laser. Our results are the first 266 nm stand-off LIBS measurements with TEPS enhancement and the first quantifiable temperature enhancement of the LIBS signal due to the CO2 laser pulse. The 100x enhancement for LIBS/TEPS we observed is greater than that normally seen for two-color multiphoton ionization (enhancement of 4 to 6) or that for Dual-pulse LIBS (enhancement of 10-20x), but the actual mechanism may incorporate these latter effects also [8

D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, and E. L. Dottery, “Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse,” Opt. Express 15, 12905–12915 (2007). [PubMed]

,9

American National Standard for Safe use of Lasers Outdoors, Laser Institute of America, ANSI Z136.6 (2005).

,20

S. Yamada, “Ultraviolet and infrared laser excited two-color multiphoton ionization for determination of molecules in solution,” Anal. Chem. 61, 612–615 (1989).

,21

J. A. Torresano and M. Santos, “One and two-color infrared mutiphoton dissociation of C3F6 ,” J. Phys. Chem. A. 101, 2221–2228 (1997).

]. Our results help quantify the LIBS/TEPS enhancement and provide initial indication of the mechanism behind the enhancement, which involves super heating of the initial LIBS plasma.

Acknowledgments

“The research reported in this document was performed in part by contract W911QX-07-C-0044 with the U.S. Army Research Laboratory under Prime Contract to Alakai Consulting & Engineering, Inc., and a Florida Matching Grant FHT 08-11. The views and conclusions contained in this document/presentation are those of the authors and should not be interpreted as presenting the official policies or position, either expressed or implied, of the U.S. Army Research Laboratory or the U.S. Government unless so designated by other authorized documents. Citation of manufacturer’s or trade names does not constitute an official endorsement or approval of the use thereof. The U.S. Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon.”

References and links

1.

A. Miziolek, V. Palleschi, and I. Schechter, eds., Laser Induced Breakdown Spectroscopy . (Cambridge University Press, 2006).

2.

A. Cremers and L. J. Radzeimki, Handbook of Laser induced breakdown Spectroscopy . (Wiley, 2006).

3.

C. Lopez-Moreno, S. Palanco, J. J. Laserna, F. DeLucia Jr., A. W. Miziolek, J. Rose, R. A. Walters, and A. I. Whitehouse, “Test of a stand-off laser-induced-breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces,” J. Anal. At. Spectrom. 21, 55 (2006).

4.

C. López-Moreno, S. Palanco, J. J. Laserna, F. DeLucia Jr, A. W. Miziolek, J. Rose, R. A. Walters, and A. I. Whitehouse “Test of a stand-off laser-induced breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces,” J. Anal. At. Spectrom. 21, 55–60 (2006).

5.

L. M. Cabalin and J. J. Laserna., “Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q-switched laser operation,” Spectrochim. Acta, Part B 53, 723–730 (1998).

6.

S. Palanco, C Lopez-Moreno, J. J. Laserna, and F. DeLucia Jr, “Design, construction and assessment of a field -deployable laser-induced breakdown spectrometer for remote elemental sensing,” Spectrochim. Acta, Part B 61, 88–95 (2006).

7.

J. Scaffidi, W. Pearman, J. C. Carter, and M. Angel, “Temporal dependence of the enhancement of material removal in femtosecond-nanosecond dual pulse laser-induced breakdown spectroscopy,” Appl. Opt. 43, 6492–6499 (2004). [PubMed]

8.

D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, and E. L. Dottery, “Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse,” Opt. Express 15, 12905–12915 (2007). [PubMed]

9.

American National Standard for Safe use of Lasers Outdoors, Laser Institute of America, ANSI Z136.6 (2005).

10.

R. D. Waterbury, P. M. Pellegrino, E. L. Dottery, A. R. Ford, and J. B. Rose, “Results if a UV TEPS/Raman system for standoff detection of energetic materials,” Chemical and Biological Defense Physical Science and Technology conference, New Orleans (2008).

11.

R. Waterbury, A. Pal, D. K. Killinger, J. Rose, E. Dottery, and G. Ontai, “Standoff LIBS measurements of energetic materials using a 266 nm excitation laser,” Proc. SPIE 6954, 409 (2008).

12.

A. A. Khalil, M. Richardson, C. Barnett, and L. Johnson, “Double Pulse UV laser induced breakdown spectroscopy of stainless steel,” Appl. Spectrosc. 73, 735–742 (2006).

13.

K. J. Grant and G. L. Paul, “Electron temperature and density profiles of Excimer laser-induced plasmas,” Appl. Spectrosc. 44, 1349–1354 (1990).

14.

D. Lacroix and G. Jeandel, “Spectroscopic characterization of laser-induced plasma created during welding with a pulsed Nd:YAG laser,” J. Appl. Phys. 81, 6599–6606 (1997).

15.

G. V. Marr, Plasma Spectroscopy , (Amsterdam: Elsevier, 1968)

16.

H. R. Griem, Plasma Spectroscopy , (McGraw Hill, New York, 1964)

17.

Y. Ralchenko, A. E. Kramida, and J. Reader NIST ASD Team, “NIST Atomic Spectra Database,” National Institute of Standards and Technology, Gaithersburg, MD (2008). http://physics.nist.gov/asd3.

18.

D. Plutov and D. K. Killinger, “Atmospheric transmission and Lidar modeling of LIBS and Raman remote sensing of distant compounds,” paper JMA 24, OSA:LACSEA conference, St. Petersburg, Fl (2008).

19.

A. Yariv, Quantum Electronics (Wiley & Sons, New York, 1989).

20.

S. Yamada, “Ultraviolet and infrared laser excited two-color multiphoton ionization for determination of molecules in solution,” Anal. Chem. 61, 612–615 (1989).

21.

J. A. Torresano and M. Santos, “One and two-color infrared mutiphoton dissociation of C3F6 ,” J. Phys. Chem. A. 101, 2221–2228 (1997).

OCIS Codes
(140.3440) Lasers and laser optics : Laser-induced breakdown
(300.6365) Spectroscopy : Spectroscopy, laser induced breakdown
(280.6780) Remote sensing and sensors : Temperature

ToC Category:
Spectroscopy

History
Original Manuscript: April 6, 2009
Revised Manuscript: May 1, 2009
Manuscript Accepted: May 4, 2009
Published: May 12, 2009

Citation
Avishekh Pal, Robert D. Waterbury, Edwin L. Dottery, and Dennis K. Killinger, "Enhanced temperature and emission from a standoff 266 nm laser initiated LIBS plasma using a simultaneous 10.6 μm CO2 laser pulse," Opt. Express 17, 8856-8870 (2009)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-11-8856


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References

  1. A. Miziolek, V. Palleschi, and I. Schechter, eds., Laser Induced Breakdown Spectroscopy. (Cambridge University Press, 2006).
  2. A. Cremers and L. J. Radzeimki, Handbook of Laser induced breakdown Spectroscopy. (Wiley, 2006).
  3. C. Lopez-Moreno, S. Palanco, J. J. Laserna, F. DeLuciaJr., A. W. Miziolek, J. Rose, R. A. Walters, and A. I. Whitehouse, "Test of a stand-off laser-induced-breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces," J. Anal. At. Spectrom. 21, 55 (2006).
  4. C. López-Moreno, S. Palanco, J. J. Laserna, F. DeLuciaJr, A. W. Miziolek, J. Rose, R. A. Walters and A. I. Whitehouse "Test of a stand-off laser-induced breakdown spectroscopy sensor for the detection of explosive residues on solid surfaces," J. Anal. At. Spectrom. 21, 55-60 (2006).
  5. L. M. Cabalin and J. J. Laserna., "Experimental determination of laser induced breakdown thresholds of metals under nanosecond Q-switched laser operation," Spectrochim. Acta, Part B 53, 723-730 (1998).
  6. S. Palanco, C Lopez-Moreno, J. J. Laserna, and F. DeLucia Jr, "Design, construction and assessment of a field -deployable laser-induced breakdown spectrometer for remote elemental sensing," Spectrochim. Acta, Part B 61, 88-95 (2006).
  7. J. Scaffidi, W. Pearman, J. C. Carter, and M. Angel, "Temporal dependence of the enhancement of material removal in femtosecond-nanosecond dual pulse laser-induced breakdown spectroscopy," Appl. Opt. 43, 6492-6499 (2004). [PubMed]
  8. D. K. Killinger, S. D. Allen, R. D. Waterbury, C. Stefano, and E. L. Dottery, "Enhancement of Nd:YAG LIBS emission of a remote target using a simultaneous CO2 laser pulse," Opt. Express 15, 12905-12915 (2007). [PubMed]
  9. American National Standard for Safe use of Lasers Outdoors, Laser Institute of America, ANSI Z136.6 (2005).
  10. R. D. Waterbury, P. M. Pellegrino, E. L. Dottery, A. R. Ford, and J. B. Rose, " Results if a UV TEPS/Raman system for standoff detection of energetic materials," Chemical and Biological Defense Physical Science and Technology conference, New Orleans (2008).
  11. R. Waterbury, A. Pal, D. K. Killinger, J. Rose, E. Dottery, and G. Ontai, "Standoff LIBS measurements of energetic materials using a 266 nm excitation laser," Proc. SPIE 6954, 409 (2008).
  12. A. A. Khalil, M. Richardson, C. Barnett, and L. Johnson, "Double Pulse UV laser induced breakdown spectroscopy of stainless steel," Appl. Spectrosc. 73, 735-742 (2006).
  13. K. J. Grant and G. L. Paul, "Electron temperature and density profiles of Excimer laser-induced plasmas,"Appl. Spectrosc. 44, 1349-1354 (1990).
  14. D. Lacroix and G. Jeandel, "Spectroscopic characterization of laser-induced plasma created during welding with a pulsed Nd:YAG laser," J. Appl. Phys. 81, 6599-6606 (1997).
  15. G. V. Marr, Plasma Spectroscopy, (Amsterdam: Elsevier, 1968)Q2
  16. H. R. Griem, Plasma Spectroscopy, (McGraw Hill, New York, 1964)
  17. Y. Ralchenko, A. E. Kramida, J. Reader, and NIST ASD Team, "NIST Atomic Spectra Database," National Institute of Standards and Technology, Gaithersburg, MD (2008). http://physics.nist.gov/asd3.
  18. D. Plutov, D. K. Killinger, "Atmospheric transmission and Lidar modeling of LIBS and Raman remote sensing of distant compounds," paper JMA 24, OSA:LACSEA conference, St. Petersburg, Fl (2008).
  19. A. Yariv, Quantum Electronics (Wiley & Sons, New York, 1989).
  20. S. Yamada, "Ultraviolet and infrared laser excited two-color multiphoton ionization for determination of molecules in solution," Anal. Chem. 61, 612-615 (1989).
  21. J. A. Torresano and M. Santos, "One and two-color infrared mutiphoton dissociation of C3F6," J. Phys. Chem. A. 101, 2221-2228 (1997).

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