Laser-induced breakdown spectroscopy of Zr in short ultraviolet wavelength range

Laser-induced breakdown spectroscopy of Zr in short ultraviolet wavelength range

Spectrochimica Acta Part B 85 (2013) 93–99 Contents lists available at SciVerse ScienceDirect Spectrochimica Acta Part B journal homepage: www.elsev...

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Spectrochimica Acta Part B 85 (2013) 93–99

Contents lists available at SciVerse ScienceDirect

Spectrochimica Acta Part B journal homepage: www.elsevier.com/locate/sab

Laser-induced breakdown spectroscopy of Zr in short ultraviolet wavelength range M. Gaft a,⁎, L. Nagli a, I. Gornushkin b a b

Laser Distance Spectrometry, 11 Granit Street, Petach Tikva 49514, Israel BAM Federal Institute for Materials Research and Testing, Richard-Willstätter-Strasse 11, 12489 Berlin, Germany

a r t i c l e

i n f o

Article history: Received 5 June 2012 Accepted 8 April 2013 Available online 30 April 2013 Keywords: Laser-induced breakdown spectroscopy Zirconium Triple ionization Double ionization Unknown lines

a b s t r a c t Emission and absorption spectra of a laser-induced zirconium plasma are studied at early times after plasma formation in the short UV spectral range from 190 to 240 nm. Many lines from highly ionized Zr ions, such as Zr IV at 216.4 and 228.7 nm and Zr III at 194.1, 194.7, 193.1, 196.3, 196.6, 197.5, 199.0, 200.3, 200.8, 202.8, 203.6, 205.7, 206.1, 207.1, 208.6, 210.2, 211.4, 217.6, and 219.1 nm have been found in the plasma under ambient and vacuum conditions. These lines could be detected in both single-pulse and double-pulse ablation modes. Several of the detected lines, namely those observed at 195.0, 198.2, 199.7, 229.4, 231.6, 232.4 and 233.0 nm, could not be attributed to Zr after inspecting known spectral data bases. From the temporal persistence behavior of these lines, it is concluded that they most likely belong to Zr I and Zr II emission lines that have not previously been reported. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Zirconium is a very important metal in the mining industry; the most important Zr-bearing industrial minerals are baddeleyite (ZrO2) and zircon (ZrSiO4). The task of online quality control of baddeleyite and zircon ores and concentrates may be solved by on-line analyzers using laser-induced breakdown spectroscopy (LIBS). For such task, the optimal Zr analytical lines need to be determined, particularly in the case of Zr–Fe ores, where multiple Fe emission lines may interfere. The literature data on LIBS of Zr are rather limited. Several lines in the 327.3–472.2 nm range were characterized analytically in terms of detection limits achievable using Atomic Absorption Standard (ASS) solutions deposited on a filter [1]. To the best of our knowledge, Zr analysis by LIBS in a short wavelength UV range of 190–240 nm has not been performed before. Our interest for this specific spectral range was further motivated by the fact that multiple Zr III and Zr IV lines are reported in the NIST spectral data base [2], while Zr I and Zr II lines are absent. Recently, we found that doubly-ionized species, such as B III and Fe III, can exist during the first 150–200 ns of plasmas produced in air with typical laser irradiances of 109–1011 W/cm2. The emission from these ions was detected using both double-pulse and single-pulse operation modes. It was concluded that, in order to observe emission from multiple ionized ions, the sum of the second ionization energy and the energy of corresponding excited states should be less than approximately 30 eV [3]. Zirconium appears to be a promising target because several Zr III and even Zr IV lines have very high transition probabilities combined with suitable sums of the ionization and excitation energies. ⁎ Corresponding author. E-mail address: [email protected] (M. Gaft). 0584-8547/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sab.2013.04.006

The aim of the present work is to report on the experimental findings obtained under the conditions described above using laser-induced breakdown spectroscopy of Zr in air. 2. Experimental setup The experimental setup is a conventional confocal single-pulse (SP) and double-pulse (DP) plasma configuration previously described in details [4]. The system consists of a Quantel Nd:YAG twin laser system containing two Big Sky Ultra 50 lasers, characterized by a maximum energy of 50 mJ/pulse at 1064 nm, with a pulse width (FWHM) of 5.8 ns. In our experiments, we used 30 mJ of maximum energy for the second laser pulse and 3–30 mJ for the first laser pulse. A delay generator controls the timing of the laser pulses, separated typically by 250 ns between each other. The laser beams are focused with a 25 cm focal length quartz lens located at about 20 cm above the sample. The laser spot diameter, determined by the knife-edge method, was about 300 μm on the sample surface: in this way, the energy of 10 mJ/pulse resulted in the fluence of 21 J/cm2. The emitted plasma radiation is collected by a fiber (0.22 NA) and guided to a spectrometer (Shamrock SR 303i–A) equipped with 1200 and 2400 lines/mm diffraction gratings. The wavelength and spectral resolution of the spectrometer were calibrated using a low pressure Hg lamp by measuring both the spectral positions of the lines and their spectral profiles. Using a spectrometer geometrical slit width of 50 μm, the spectral resolution found for the 2400 lines/mm grating and for the 1200 lines/mm grating was 0.1 nm and 0.16 nm, respectively. The detector is an intensified chargecoupled device camera (Andor DH-720 25F-03). The time resolution of the ICCD camera was determined by analyzing the temporal profile of the laser pulse used in our experiment. To this purpose, the second harmonic of the Nd-YAG was utilized. Using the kinetic series technique

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Fig. 1. Single-pulse (a, b) and double-pulse (c, d) breakdown spectra of metallic zirconium with different delays and gates in spectral range 192.5–220 nm in air.

with a gate width and step each 1 ns wide, the measured HWFM of the laser pulse was 6 ns, which corresponds to its passport value. The lines intensity on the figures is presented in arbitrary units (a.u.), which are comparable inside each figure, but not comparable between different figures.

For the experiments carried out under vacuum, the sample was place inside a vacuum chamber equipped with MgF2 windows and evacuated down to ~ 0.06 Torr. The persistence data were measured by a kinetic series approach, which is particularly well suited for recording the temporal evolution

Fig. 2. Very narrow gate (G = 1 ns) single-pulse (a, b) and double-pulse (c, d) emission LIBS spectra with small delay of 30 ns and bigger delay of 150 ns in spectral range of 205–235 nm.

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of a process. The overall duration of the acquisition was 500 ns: during this time, 100 sets have been measured, with 5 ns steps from the initial delay between the 2nd laser pulse and the data acquisition. The sample was metallic Zr (99.95% purity), positioned on a moving holder to insure that each laser shot is delivered to a fresh spot on the sample surface.

3. Experimental results Fig. 1 presents the SP and DP LIBS spectra of metallic Zr measured in the UV spectral range 192–220 nm in ambient air. In the SP spectrum, setting the ICCD at a zero delay and a relatively narrow gate of 100 ns, many emission lines are detected. The strongest peaks are observed at nominal wavelength readings at 194.1, 195.6, 199.1, 200.8, 203.7, 207.2, 208.7 and 216.4 nm (Fig. 1a). When the delay time is increased to 400 ns and for a relatively broad gate of 1 μs (Fig. 1b), the lines at 203.7, 207.2 and 216.4 nm disappear, while other lines persist and several new lines become visible, the strongest intensities occurring at 193.1, 195.0, 196.7, 197.6, 201.7, 203.2, 206.1, 209.6 and 211.0 nm. The most significant change occurs with the line observed at 199.7 nm, which becomes very strong (compare Figs. 1a and b). The DP excitation results in lower background emission, narrower line widths and higher lines intensity. Nevertheless, the lines' enhancements are different. For example, in the DP spectrum obtained at zero delay and relatively narrow gate of 100 ns (Fig. 1c), the line at 194.1 nm becomes 8 times stronger compared with the SP spectrum, while the line at 216.4 nm increases 17 times. Besides, new lines become detectable, the strongest intensity occurring at 194.7, 196.3, 197.5, 202.8, 205.7, 206.1, 209.2 and 210.3 nm. When the delay time increases to 400 ns and with a relatively broad gate of 1 μs (Fig. 1d), the lines at 209.2 and 216.4 disappear, the lines at 196.3, 199.0, and 200.3 nm become relatively weaker, while the line at 199.7 nm becomes nearly the strongest one.

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The SP and DP LIBS spectra similar to those obtained in Fig. 1 are shown in Fig. 2, except that in this case the UV spectral interval ranges from 205 to 235 nm, a region in which emission from Zr IV and Zr III is most likely to occur [2]. The spectrum was measured using the shortest possible gate of 1 ns, which is most suitable for detection of multiply ionized ions. The delay time was varied from 0 to 500 ns. Three main emission lines are detected at the very beginning of the SP plasma lifetime: these lines peak at 208.7, 216.4 and 228.6 nm (Fig. 2a). At an increased delay time of 100–150 ns, the two lines at 216.4 nm and 228.6 nm almost disappear, the former line showing a much slower decrease in intensity compared to the latter. At this delay time, several lines, whose peak intensities occur at 206.0, 207.1, 209.5, 229.4, 231.6, 232.5 and 233.0 nm, dominate the spectrum (Fig. 2b). The persistence of the line at 216.4 nm is approximately 35 ns, while that of the line at 208.7 nm is 65 ns, nearly twice as long. Under our DP experimental conditions, the lines at 216.4 and 228.6 nm become narrower and dominate the spectrum at the very beginning of the plasma (Fig. 2c). As compared to the SP spectra, more lines become visible in the DP spectra at the delay time of 150 ns: these lines occur at 206.0, 207.1, 209.5, 210.2, 211.4, 217.6, 219.1, 220.6, 228.6, 229.4, 230.7, 232.4 and 233.0 nm (Fig. 2d). The persistence of the line at 216.4 nm is approximately 72 ns, while that of the line at 208.7 nm is 185 ns; both the values are substantially longer than those observed in the SP plasma. 4. Discussion 4.1. Zr IV According to Refs. [4,5], the strongest lines of Zr IV should appear at 209.15, 209.23, 216.37 and 228.67 nm with relative intensities of 70,000; 80,000; 10,000,000 and 4,000,000 units [2,4,5]. Our experiments enabled to find very strong emission lines at 216.4 and 228.7 nm. They exist only during a very short time, i.e., less than 150 ns after the onset

Fig. 3. Double-pulse breakdown spectra of metallic Zr in air with zero delay and narrow gate of 200 ns (a) and long delay of 200 ns and broad gate of 1 μs (b) in the spectral range 228.0–233.5 nm. Also shown are the spectra obtained with a delay of 150 ns and very narrow gate of 2 ns (c) and a long delay of 1 μs and broad gate of 100 μs (d) in the spectral range 228.0–247.0 nm.

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Table 1 Spectroscopic data for model calculations. Element

λ, nm

Elow

Eup

f

glow

gup

ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIII ZrIV ZrIII ZrIII ZrIII ZrIII ZrIV ZrIII ZrIII

192.192 193.249 193.643 193.664 193.721 194.024 194.105 194.605 194.657 196.192 196.620 197.491 198.979 200.191 200.681 200.817 202.670 202.987 203.535 203.686 205.601 205.866 206.082 207.036 207.406 207.788 208.092 208.678 208.945 209.695 210.228 210.309 211.400 211.403 211.405 211.408 216.368 217.569 219.103 225.773 227.649 228.667 230.146 230.811

8327 56,077 55,616 8064 8327 11,051 8840 56,438 8327 57,349 8840 57,684 48,507 53,649 5743 60,358 55,558 56,077 56,077 55,558 57,684 56,077 8840 57,684 56,438 8327 62,591 5743 57,349 56,438 8064 57,349 8327 57,349 56,438 59,699 38,258 60,358 59,699 60,358 62,591 38,258 62,117 18,805

60,358 107,824 107,257 59,699 59,948 62,591 60,358 10,7824 59,699 108,319 59,699 108,319 98,763 103,585 55,558 110,139 104,883 105,326 105,193 104,637 106,306 104,637 57,349 105,970 104,637 56,438 110,631 53,649 105,193 104,111 55,616 104,883 55,616 104,637 103,725 106,986 84,461 106,306 105,326 104,637 106,504 81,977 105,555 62,117

0.0797 0.2730 1.0300 0.1650 0.0771 0.1020 0.1840 0.7910 0.0568 0.7570 0.0424 0.2470 1.9900 0.7990 0.1380 0.2960 0.5750 0.0861 0.9590 0.4570 0.1600 0.0590 0.0436 1.0200 0.2010 0.0538 1.1900 0.2190 0.1310 0.3840 0.0808 0.2400 0.0079 0.1420 0.0479 0.1630 1.0000 0.5090 0.5790 0.0916 0.0955 1.0000 0.3150 0.0270

3 7 3 1 3 9 5 5 3 7 5 9 1 5 5 5 5 7 7 5 9 7 5 9 5 3 7 5 7 5 1 7 3 7 5 3 2 5 3 5 7 2 3 5

5 7 5 3 1 7 5 7 3 9 3 9 3 7 5 5 7 5 9 7 7 7 7 11 7 5 9 5 9 5 3 7 3 7 3 3 4 7 5 7 5 2 3 3

of the laser plasma in the SP-mode and 400 ns after the 2nd laser shot in the DP-mode: this behavior is characteristic for multiply ionized ions. It may therefore be concluded that the emission attributed to Zr IV is found in the laser-induced plasma in air at the beginning of the plasma lifetime. To the best of our knowledge, it is the first time that such highly ionized ion emission is detected in conventional laser-induced plasma experiments using moderate, ~50 mJ, laser pulse energy. A possible reason for their detection is their very high relative intensities combined with the low energies of the corresponding excited states (~10 eV) in addition to the relatively low third ionization energy (23 eV) [2,4,5]. A very weak line at 209.2 nm characterized by a very short persistence time was also detected. It may be assumed that this line also belongs to Zr IV; its much lower intensity results from a combination of substantially lower transition probability with higher excitation energy (25.7 eV). For comparison, the excitation energies of the Zr IV lines at 216.4 and 228.7 nm are 10.5 eV and 10.2 eV, respectively. Zr IV lines in the plasma can also be seen in absorption. They may be detected only in the DP plasma within a very short time interval between the two laser pulses, i.e., 200–250 ns. We note that this may be attributed to a Fraunhofer-type absorption in which de-excited ions remaining from the first pulse absorb broadband plasma emission resulting from the second pulse [6]. If this explanation is correct, it is the first time when triply ionized ions are detected by such the technique.

This means that the very high temperature of the plasma is populating highly excited levels from which absorption takes place. In our case, the lower energy level of the 216.4 and 228.7 nm transitions is 4.75 eV, indicating that temperatures can be of the order of 55,000 K.

4.2. Zr III The intense emission lines detected in the SP and especially DP operation modes at the beginning of the plasma lifetime appeared at 194.1, 194.7, 193.1, 196.3, 196.6, 197.5, 199.0, 200.3, 200.8, 202.8, 203.6, 205.7, 206.1, 207.1, 208.6, 210.2, 211.4, 217.6, and 219.1 nm: these lines can be attributed to Zr III [2,4,5]. The same holds for the lines observed at 230.1 and 230.8 nm in the vacuum plasma. Zr II is characterized by a low second ionization potential of 13.13 eV. However, the detected Zr III lines are only those that result from the combination of high emission probabilities with low energies of corresponding emitting levels. This is the case, for example, for levels with energies of 7.76, 6.89 and 6.65 eV [2,4,5]. The strongest of the above mentioned lines are also observable in the Fraunhofer-type absorption spectra. This is easily understandable, taking into account that the absorption originates from relatively low energy levels in the range of 0.7–1.37 eV.

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Fig. 4. Double-pulse breakdown spectra with delay of 100 ns and broad gate of 1 ms determined by the gated CCD spectrometer of baddeleyite ZrO2 (a) and magnetite (Fe2+Fe3+2O4) (b) in ambient conditions.

4.3. Unidentified lines Besides known lines of Zr III and Zr IV emissions, several other lines were observed, which could not be identified using available data bases. These lines can be seen in the spectra at the following wavelengths: 195.0, 198.2, 199.7, 229.4, 231.6, 232.4 and 233.0 nm. Since these lines are also observed in spectra obtained under vacuum, they cannot be attributed to N II and O II species. Moreover, it is also unlikely that the lines belong to trace impurities in metallic Zr, since the total residual trace metal content is less than 300 ppm (Certificate of Analysis, Sigma-Aldrich). This conclusion is supported by the following considerations. The highest impurity level (190 ppm) belongs to Fe, followed by Hf (29 ppm), Cr (8.4 ppm), Si (9.5 ppm), Al (7.1 ppm), Ti (5.3 ppm) and Ni (4.1 ppm). LIBS spectra were obtained from pure Fe, Ti, Al and Ni metals, which were at our disposal. The experimental conditions, e.g., excitation/detection geometry, MCP gain, number of accumulations, delay time and gate width were identical to those used for Zr. In particular, the spectral resolution was the same. From the substantial spectral differences observed, it was concluded that the above unidentified lines could not result from the presence of these tested elements. Furthermore, the line emission intensities for all elements are of the same order of magnitude as those observed with Zr, making therefore very unlikely that these intensities correspond to elements present at the trace impurity level, i.e., about two orders of magnitude lower. The logical conclusion is that the unidentified lines are due to the metallic Zr matrix. Fig. 3 shows the spectrum observed in the window 228–233.5 nm. It is worth noting again that the spectra were obtained with the best resolution achievable with our apparatus, i.e., 0.1 nm. Although the lines listed in the available wavelength tables are reported with 7 significant digits, in our case, we can only report 4 significant digits. In the spectra

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obtained at zero delay and with a narrow gate of 200 ns, the line at 228.7 nm corresponds well to the known line of Zr IV listed at 288.66 nm [2,4,5]. The same conclusion can be reached for the lines at 230.2 and 230.8 nm, which correspond to known lines of Zr III at 230.146 and 230.811 nm [2; see also Table 1]. Moreover, several other lines are found at 229.1, 229.4, 231.7, 232.2, 232.5 and 233.0 nm (Fig. 3a). After the delay of 200 ns and with the gate width of 1 μs, the Zr IV line totally disappears, the Zr III lines remain, and the lines at 229.1, 229.4, 231.7, 232.5 and 233.0 nm dominate the spectrum (Fig. 3b). According to known databases [2], and as also reported in our previous paper [6], there are no Zr lines listed between 228.66 and 230.14 nm. Therefore, the lines at 229.1 and 229.4 nm, determined with an accuracy of ±0.1 nm, belong to a spectral range where Zr emission lines are absent. The same holds for the lines at 231.7 and 232.4 nm, which are also observed inside the spectral range 230.8 to 231.27 nm, where no Zr lines are listed [2,4]. The line at 233.0 nm is attributed to Zr III, but its relative intensity is 2000 times lower compared with the lines at 230.2 and 230.8 nm [2,4]. Our suggested conclusion is that all those lines may be considered as unknown, which should belong to Zr I or Zr II species, in view of the fact that their emission persists longer than the emission due to Zr IV and Zr III. We studied another UV spectral window, from 234 to 247 nm, where the emission from well identified Zr I and Zr II lines could be seen, in order to compare the behavior of these lines with the unknown lines. Fig. 3c presents the corresponding data for DP LIBS, obtained in air with a very narrow gate width of 1 ns. It may be seen that the lines at 240.5, 242.0, 244.4 and 244.8 nm are detected, which correspond well to the strongest Zr III lines at 240.58, 242.07, 244.5 and 244.9 nm in this spectral window [2]. Besides that, additional lines appear mainly peaking at 235.2 and 235.6 nm. At the long delay of 1 μs and with the broad gate of 10 μs (Fig. 3d), the lines at 237.3, 238.7 and 238.9 nm are detected: these lines correspond well to known lines of Zr I emission at 237.4, 238.80 and 238.90 nm [2]. The lines at 245.0 and 245.8 nm correspond to known lines of Zr II at 245.01 and 245.82 nm [2]. On the other hand, the lines appearing at 235.2, 235.6, 242.7 and 243.5 nm may not be identified in known data bases, where Zr lines are absent in the 234.7–236 nm and 242.2–243.8 nm spectral ranges [2,6]. We are therefore led to conclude once again that those lines may be considered as unknowns, which should be attributed to Zr I or Zr II states because they have longer persistence time compared to Zr IV and Zr III. The line at 229.4 nm has an analytical merit. Iron is a main impurity in baddeleyite (ZrO2) industrial concentrate samples and represents a strong interfering factor for online quality control of this product by LIBS. Fig. 4 shows the DP LIBS spectra of baddeleyite and magnetite (Fe 2+Fe 3+2O4) industrial concentrates obtained using the gated CCD AvaSpec spectrometer operated with a delay time of 100 ns and a gate width of 1 ms. It can be clearly seen that the nearest emission line of Fe I is situated at 229.9 nm and it is relatively weak (Fig. 4b). Thus the line at 229.4 nm is clean from the Fe emission influence and may be used for analytical purpose. 4.4. Computer simulations of absorption and emission in Zr plasma Emission and absorption spectra of Zr for both single (SP) and double pulse (DP) modes were simulated on a computer and compared to the observed plasma behavior. The collisional-dominated plasma model employed the Navier–Stokes equations coupled to the state and radiative transfer equations. The one dimensional Kurganov–Tadmor highresolution schemes [7,8] have been extended to Navier–Stokes equations with axial symmetry in space. The algorithm is a third-order semidiscrete central scheme for conservation laws and convectiondiffusion equations. It is a higher order version of the original Godunov shock-wave capturing method. The output of plasma dynamic simulations was used to calculate the plasma emissivity by the radiative transfer code as in Refs. [9,10].

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Fig. 5. Top row, left and right panels: spatial distributions of plasma density and temperature for SP- and DP-LIBS, respectively, obtained from the model at 5 ns gate delay and 1 ns gate width. Inter-plasma separation in the DP mode is 1 μs. Note the position of the primary shock in the DP regime and rapid decrease of density behind the shock front. Bottom row: SP and DP LIBS spectra.

DP plasma was simulated by introducing a smaller size secondary plasma in center of the primary plasma at a prescribed time after the creation of the first plasma. The secondary plasma was allowed to have its own initial parameters (the size, temperature, and density), while the outer parameters (the ambient pressure, temperature, and density) were determined by the expanding primary plasma. The

outputs of the model were the plasma temperature, number densities, and emission spectra as functions of spatial coordinates and time. The theoretical plasma was composed of 100% Zr. Argon was taken as an ambient gas as this made the calculations easier compared to the expansion in air. The use of argon, not the air, did not significantly affect the early plasma dynamics because at this evolution stage

Fig. 6. Same profiles and spectra as in Fig. 5 calculated for 100 ns gate delay and 1 ns gate width. Note the formation of two shocks by two laser pulses in the DP mode.

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(1's to 100's of ns) the plasma expansion is nearly adiabatic and unaffected by the surrounding gas. A total of 44 strong spectral lines of Zr III and Zr IV were chosen in the spectral window of 192–232 nm and are reported in Table 1, with wavelengths given to a 7-digit accuracy (Table 1). Spectroscopic parameters required for spectra calculations were taken from the following sources: transition probabilities and level characteristics from the Kurucz database [11]; partition functions from the Drawing and Fellenbock tables [12], Stark broadening parameters from Ref. [13]. Wherever Stark parameters were not available, they were substituted by fake values taken to be close to the known values for similar lines. The Zr plasma was “created” at time t = 0 within the spherical volume 4/3πr3, r = 0.2 mm. The initial gas density and temperature were 1.6 1019 cm−3 (0.6 the atmospheric density) and 80,000 K, correspondingly. For DP calculations, the primary plasma was allowed to propagate for 1 μs when second spherical plasma with r = 0.2 mm was created in the center of the first plasma. The density of second plasma was higher, 3.1∙1019 cm−3 (1.2 the ambient density), reflecting the higher power of the second pulse and better ablation conditions (no plasma shielding). The initial plasma temperature was the same 80,000 K. Figs. 5 and 6 present the calculated Zr spectra and spatial profiles of plasma density and temperature for SP and DP conditions. In Fig. 5, which depicts first 5 ns of the SP plasma propagation, the relatively simple density and temperature profiles with the shock front just started forming (left top panel) result in the quasi continuous plasma spectrum (left bottom panel) with small bumps due to Zr III and Zr IV lines. As in experiment, no absorption lines are formed. On the contrary, for the DP 5 ns-old plasma, the complex density and temperature distributions with a strong front shock (right top panel) produce a spectrum with prominent absorption lines from Zr III and especially Zr IV at 216.4 nm and 228.7 nm (right bottom panel). The high temperature at the shock favors a formation of the high population of Zr III and Zr IV ions, while lower ionization states and neutrals are suppressed (see the corresponding profiles in Fig. 5, right top panel). The Bremsstrahlung and photo recombination continuum radiation emitted by the plasma core is absorbed at strong transitions of doubly and triply ionized Zr ions. This qualitatively explains the Zr III and Zr IV absorption lines seen in the experimental DP spectrum. As the plasma cools down due to expansion and radiative energy loss, the amount of continuum radiation from the plasma core decreases that transforms the absorption Zr III and Zr IV spectra into emission ones just after only 100 ns of the DP plasma evolution (see Fig. 6 right bottom panel). The differences between SP and DP plasma spectra taken at this delay time become insignificant with only change in the line widths (compare left and right bottom panels in Fig. 6). The latter occurs because the second plasma in the DP mode expands into the rarefied atmosphere created by the first pulse and by the time of 100 ns its density drops more significantly than that in the SP plasma. The simulations provide a picture qualitatively close to that observed in the experiment. Note that the model employs quite a high initial plasma temperature of 80,000 K which may well correspond to true experimental conditions (see, e.g.,

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section A). To measure such high temperatures in early plasma by traditional methods like the Boltzmann plot or line-to-continuum ratio methods would be difficult if only single ionization occurs and ion lines are outshined by strong continuum radiation. In the case of Zr plasma, where Zr III and Zr IV lines can be detected, the range of applicability of these traditional diagnostic methods can be expanded toward earlier times on a 1's–10's of ns scale. 5. Conclusions Experimental data and theoretical model calculations show that in typical single and double pulse laser-induced plasmas excited with irradiances of 109 to 1011 W/cm 2 in air or vacuum, triply and doubly ionized Zr species are present. The persistence of these species is very short; emission and absorption from these ions can only be detected during the earliest, 150–200 ns, stage of the plasma lifetime. Several unidentified lines were detected and most likely attributed to Zr I and Zr II emission. Acknowledgements Financial support from the DFG-NSF grant GO 1848/1-1 and NI 185/38-1 (USA, Germany) is gratefully acknowledged. The authors also thank Professor Shabanov for his valuable contribution to plasma modeling. References [1] D. Cremers, L. Radziemski, Handbook of Laser Induced Breakdown Spectroscopy, Wiley, 2006. [2] NIST Atomic Spectra Database Lines, http://physics.nist.gov/PhysRefData/ASD/ lines_form.html. [3] M. Gaft, L. Nagli, Y. Gornushkin, Y. Groisman, Doubly-ionized ion emission in laser-induced breakdown spectroscopy in air, Anal. Bioanal. Chem. 400 (2011) 3229–3237. [4] N. Acquista, J. Reader, Spectrum and energy levels of triply ionized zirconium (Zr IV), J. Opt. Soc. Am. 70 (1980) 789–792. [5] S. Afzal, S. Khatoon, K. Rahimulla, A Review of experimentally observed zirconium spectra IAEA, Int. Nucl. Data Comm., INDC (IND)-0047, November 2005. [6] L. Nagli, M. Gaft, I. Gornushkin, Fraunhofer type absorption lines in double pulse laser induced plasma, Appl. Opt. 51 (2012) 201–212. [7] A. Kurganov, D. Levy, A third-order semidiscrete central scheme for conservation laws and convection-diffusion equations, SIAM J. Sci. Comput. 22 (2000) 1461–1488. [8] A. Kurganov, E. Tadmor, New high-resolution central schemes for nonlinear conservation laws and convection–diffusion equations, J. Comput. Phys. 160 (2000) 241–282. [9] I.B. Gornushkin, A.Y. Kazakov, N. Omenetto, B.W. Smith, J.D. Winefordner, Radiation dynamics of post-breakdown laser induced plasma, Spectrochim. Acta Part B 59 (2004) 401–418. [10] I. Gornushkin, S.V. Shabanov, N. Omenetto, J.D. Winefordner, Theoretical modeling of a non-isothermal asymmetric expansion of laser-induced plasma in vacuum, J. Appl. Phys. 100 (2006) 073304. [11] P. Smith, C. Heise, J. Esmond, R. Kurucz, “Atomic spectral line database built from atomic data files” from R.L. Kurucz' CD-ROM 23, http://www.pmp.uni-hannover. de/cgi-bin/ssi/test/kurucz/sekur.html. [12] H.W. Drawin, P. Felenbok, Data for Plasmas in Local Thermodynamic Equiibrium, Gauthier-Villars, Paris, 1965. [13] L.Ĉ. Popović, N. Milovanović, M.S. Dimitrijević, The electron-impact broadening effect in hot star atmospheres: the case of singly- and doubly-ionized zirconium, Astron, Astrophys. 365 (2001) 656–659.