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In this paper, the effect of laser pulse energy on orthogonal double femtosecond pulse laser induced breakdown spectroscopy (LIBS) in air is studied. In the experiment, the energy of the probe pulse is changeable, while the pump pulse energy is held constant. At the same time, a systematic study of the laser induced breakdown spectroscopy signal dependence on the inter-pulse delay between the two pulses is performed. It is noted that the double pulse orthogonal configuration yields 2–32 times signal enhancement for the ionic and atomic lines as compared to the single pulse LIBS spectra when an optimum temporal separation between the two pulses is used, while there is no significant signal enhancement for the molecular lines in the studied range of the delay. It is also noted that the dependence of the enhancement factor for ionic and atomic lines on the inter-pulse delay can be fitted by Gaussian function. Furthermore, the electron temperature obtained by the relative line-to-continuum intensity ratio method was used to explain the LIBS signal enhancement.
© 2013 Optical Society of America
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is a spectroscopic technique that uses a focused laser pulse to form a spark in air as well as in or on liquid and solid surfaces [1–8]. Major advantages of the technique include its speed, no sample preparation, and the ability to perform the analysis from a distance. Exploiting these advantages, LIBS has been successfully applied in many application areas, such as environmental monitoring [9–12], material analysis [13], biological identification [14], remote sensing [15], lightning control [16, 17], and so on. Though the applications of LIBS are vast, LIBS suffers a main limitation in sensitivity when compared to other spectroscopy techniques. Especially in the case of femtosecond LIBS, due to the absence of laser-plasma interaction [18], the intensity of the overall signal is much lower than that in the case of nanosecond (ns) LIBS. In the effort to overcome this drawback, several groups have explored the use of double-pulse (DP) configuration [19, 20]. It was demonstrated that DP-LIBS represents a very effective approach for the improvement of analytical capabilities of the LIBS technique, and the use of DP-LIBS can provide significant increases in signal-to-noise and/or spectral intensity compared to single-pulse (SP) LIBS. Although the DP technique has been well established for twenty years, the underlying mechanisms which lead to signal enhancement are not yet well understood [21, 22], and some proposed mechanisms are even contradictory [23]. The study of the mechanisms would lead a further improvement to the applications of DP-LIBS. The DP-LIBS technique can be carried out in two different pulse configurations called orthogonal and collinear [24–26], referring to the relative directions of the two laser pulses. In a DP configuration, two laser pulses with an inter-pulse delay are used for generating plasma. Furthermore, these two pulses can be generated from either a single laser or two different lasers. Although several works dealing with the effect of pressure on the LIBS signal using nanosecond pulses can be found in the existing literature [27], until now rather few works focused on studying the effects of laser energy on the LIBS signal using femtosecond laser pulses, and to the authors knowledge, correlational studies about the inter-pulse delay on the order of femtoseconds have not been reported yet. In this paper, we investigated DP femtosecond LIBS measurements in ambient air, using IR (810 nm) femtosecond (33 fs) laser pulses in an orthogonal, pump-probe type of configuration based on an optical delay line. The signal enhancement was observed in the delay time range from several femtoseconds to several hundreds of femtoseconds. Meanwhile, the orthogonal DP-LIBS scheme was used along with manipulation of the energy of the probe laser beam to investigate the potential for the further improvements in sensitivity.
2. Experimental setup
The orthogonal DP femtosecond LIBS experiments were carried out with a Ti:Sapphire chirped-pulse amplification laser system emitting 33 fs pulses with energy of 2.0 mJ at 1 kHz repetition rate, and a central wavelength of 810 nm. A schematic of the experimental setup is shown in Fig. 1. The seed laser beam was split into two parts by a 3:2 beam splitter (BS1). A half wave plate and a Glan laser polarizer were put in the probe beam (beam 1) path in order to change the incident pulse energy. The energy of the probe pulse was set to 1.0, 0.8, 0.6, 0.4 and 0.2 mJ, while the energy of the pump beam (beam 2) was fixed at 0.8 mJ. Then, the pump beam was focused by a 100 mm focal length lens (L1) to produce an air breakdown and subsequent laser-induced plasma, the probe beam was aligned perpendicularly to the pump beam and focused by a 200 mm focal length lens (L2). The inter-pulse delay was realized by using the path length difference of the two pulses after the 3:2 BS. A computer-controlled translation stage (Model: TSA-50; resolution: 78.125 nm) provided the change of the path length. In the orthogonal configurations, the zero point of the delay time in the femtosecond range was difficult to be found with respect to the collinear case. Thus a third-harmonic generation frequency-resolved optical-gating (THG FROG) technique was introduced in our experiment [see Fig. 1 (b)]. From Ref. [28], it is possible to define the zero point of the delay time as the intensity of the third-harmonic is the highest. After the optical path difference introduced by the L1, L2 and BS2 is eliminated from the paths, the sparks induced by two laser beams in the orthogonal configuration can be assumed spatiotemporally intersected. The LIBS emission from the spark was collected through an optical quartz fiber (diameter=400 μm), placed perpendicularly to the two pulses propagation at a 3 mm distance from the air spark. The spectrum was measured by a no-gated spectrometer (Ocean Optics, USB4000). The spectra range of the spectrometer is 200–900 nm with a resolution of 1.33 nm. The dark current background of the detector was subtracted from the measured spectroscopic data for each measurement.
Fig. 1 Schematic diagram of the experimental setup, BS, beam-splitter; M1–8, 810 nm high-reflection mirrors; M9, 270 nm high-reflection mirrors; WP, 1/2 wave plate; GP, Glan polarizer; L1–4, focusing lenses; BBO, Beta-Bariume Borate Crystal.
In order to reduce the spectral fluctuations, each measurement was performed over a series of 100 pairs of laser pulses and duplicated 100 times. Hence the final spectral data was averaged over ten thousand consecutive shot-pairs for each inter-pulse delay.
3. Results and discussion
3.1. The LIBS spectra
For getting a better understanding of the effect of probe pulse energy on the enhancement in DP-LIBS spectra as compared to SP-LIBS spectra, we performed the experiment fixing the pump pulse energy E1 at 0.8 mJ and reducing the probe pulse energy (E2) from 1.0 mJ down to 0.2 mJ, corresponding, respectively, to 5/4 and 1/4 of E1, while the pulse energy was set to equal to E1 (0.8 mJ) in the SP-LIBS experiment. In Fig. 2 LIBS spectra from the air spark in double- and single-pulse configurations are displayed and the inter-pulse delay in all of the DP-LIBS experiment was adjusted to be 0 fs. Generally, in the case of SP-LIBS, the spectra consist of continuous spectra [Bremsstrahlung emission (free-free transitions) and radiative recombination (free-bound transitions)] and a series of nitrogen and oxygen lines, e.g., N I at 746.8, 744.1, 742.3 nm, N2 second positive band (C3Π → B3Π) with peaks at 357.7, 337.7 and 315.0 nm, first negative system band ( ) with a peak at 391.4 nm [29, 30], N II at 500.5 and 399.5 nm, O I at 844.6, 822.7, 777.2 nm, H I at 656.2, etc. From Fig. 2, it is easy to find that the continuous spectra and all the line spectra appearing in the SP-LIBS experiment are also present in the DP-LIBS spectra along with some additional lines, e.g., N II at 567.9 and 463.0 nm.
In order to study the effect of inter-pulse delay on the DP-LIBS spectra, we manipulated the delay from −500 fs into 500 fs (maximum inter-pulse delay studied here). The positive delay refers to the probe pulse following the pump pulse in time. In general, the spectra in Fig. 3 are similar to those in Fig. 2, and nearly all the lines and the continuous spectra appearing in Fig. 2 are also present in Fig. 3.
Fig. 3 The DP-LIBS spectra from air spark with different inter-pulse delays at the fixed probe pulse energy E2=1.0 mJ.
Stark broadening of spectral lines in plasma results from the collision with charged species, and is considered as the primary mechanism influencing the emission spectra. The Lorentz function can be used to fit these spectra and is expressed as:
where w is the full width half maximum (FWHM), xc is the center wavelength, y0 is the background emission, and A is the integrated area of the emission line. These parameters were obtained by fitting the experiment spectral data (as shown in Fig. 4) and were used to deduce the signal enhancement factor and electron temperature.Fig. 4 Lorentzian fitting of the stark broadened profile for N II 500.5 nm at inter-pulse delay 0 fs in the DP-LIBS scheme. The ratio of the integrated spectral line intensity (A) and continuum intensity (y0) at the center wavelength were used for the calculation of electron temperature.
3.2. The signal enhancement
Figure 5 shows variations of the enhancement factor (IDP/ISP) of the selected emission lines as the inter-pulse delay, which is varied from −500 fs to 500 fs for all cases of probe pulse energy. Incidentally, IDP/ISP is defined as the ratio of DP- and SP-LIBS intensity, while the ISP is the algebraic sum of the signal intensity obtained separately with pump pulse and probe pulse. Obviously, the Gaussian function can be also used to fit all the obtained results. The full widths at half maximum (FWHM) for the selected lines is listed in Table 1. The FWHM indicates the sensitivity of the signal enhancement factor of one line to the inter-pulse delay, and the smaller of the FWHM, the higher sensitivity. Additionally, it is easy to find that nearly all of the maximum enhancements are at 0 fs inter-pulse delay. The comparison between the SP- and DP-spectra at inter-pulse delay 0 fs reveals more than two times enhancement both for nitrogen and oxygen lines for the lowest probe pulse energy, while the maximum value of enhancement factor of the 500.5 nm line is about 32 for the highest probe pulse energy. According to this plot, the optimum delay was found for ionic and neutral lines, all the lines, independent of ionization state show the same qualitative behavior with the inter-pulse delay and reach a maximum emission when the delay=0 fs.
Fig. 5 Influence of the inter-pulse delay, on the signal enhancement factor for several emission lines in the DP-LIBS measurements with all cases of probe pulse energy. (a), (b), (c) and (d) indicate the results of the spectra of 777.2, 746.8, 656.2 and 500.5 nm, respectively.
Table 1. The FWHM for various emission lines observed in the LIBS spectra corresponding to different probe pulse energies.
Particularly, one interesting thing found in the present work is that no matter how the inter-pulse delay and the probe pulse energy change, there is no significant signal enhancement found for the N2 molecular lines (391.4, 357.7, 337.7, 315.0 nm). Incidentally, the values of enhancement factor of these selected molecular lines are relatively equivalent and around IDP/ISP = 1.05 ± 0.03.
In order to understand the details of probe pulse energy effect on the intensity of the LIBS emission signal, Fig. 6 depicts how the enhancement factor, IDP/ISP, corresponding to different emission lines from ion, atom and molecule, varies as a function of laser pulse energies ratio (E2/E1) used in this study. In general, the enhancement factor, IDP/ISP, for the emission lines of both neutral and ionic, shows a steady increase with enhancing the probe pulse energy, while the enhancement factors of N2 molecular lines are relatively constant and the value maintains at about 1.05 ± 0.03 in all cases of the energy ratio. It is also worth noting the tendency that the emission line from ion (N II 500.5 nm) exhibit a higher degree of enhancement at higher values of E2/E1 is observed. Such an effect was also noticed in ns/ns DP-LIBS experiments and attributed to an increase in the plasma temperature brought about by the second laser pulse [31, 32].
Fig. 6 The variation of signal enhancement factor (IDP/ISP) as a function of laser pulses energies ratio (E2/E1). With the inter-pulse delay between the two pulses is 0 fs in all the cases.
From these obtained results, we can find that the DP-LIBS can provide not only significant increases in spectral intensity, but also an effective way to control the ratio among different emission lines. It can also ensure the discrimination of molecular lines and atomic lines from ionic lines in spectra, which may be particularly important to identify elements in the unknown samples.
To understand the signal increases obtained in the DP-LIBS approach, the influence of the plasma temperature was studied. Under the assumption of local thermodynamic equilibrium (LTE), the population densities of atomic or ionic electronic states are described by Boltzmann distributions and the electron temperature Te can be assumed equal to the excitation temperature Texc, namely Te = Texc = T. Therefore the electron temperature (T) (in K) can be estimated by the relative line-to-continuum intensity ratio, and the equation for the method is given by [33–35] as:
where Cr = 2 × 10−5 (sK); εl and εc = y0/Δλ are the line emission coefficient and the continuum coefficient, respectively, the εl/εc ratio can be calculated from the integrated line intensity (A) and continuum intensity (y0) for a certain bandwidth (Δλ); Akigk is the Einstein transition probability weighted by the upper energy level degeneracy; Ui is the partition function for ion, which is a weak function of temperature; λc and λl are continuum wavelength and center wavelength of the spectral line in nm units, respectively, and it is assumed that λc very nearly equals λl; Ei and Ek are the ionization potential and the upper level energy, respectively; ΔEi is the lowering of the ionization potential of atoms in the presence of a field of ions and is small enough to be insignificant; ξ and G are the free-bound continuum correction factor and the free-free Gaunt factor, respectively, for nitrogen we have used ξ = 1.8 and G=1.1 obtained for argon in Ref. [33]; the constants h, c and k are the Plancks constant, the speed of the light and the Boltzmanns constant, respectively.In this study, the N II 500.5 nm line to continuum ratio was used to deduce the electron temperature. Since only the time-integrated emission was measured with the no-gated detection system, the time-averaged electron temperature was obtained. Figure 7 shows the evolution of the time-averaged electron temperature as a function of the inter-pulse delay between the two laser pulses in the DP-LIBS measurements with the probe pulse energy E2=1.0 mJ. One can see that the electron temperature changes in the range of 8538 k to 10422 k and reaches its maximum value at the inter-pulse delay 0 fs. The experimental errors on temperature calculation (the errors coming from the uncertainty of transition probability Aki, Ei and Ek are neglected here) are of the order of 8%, coming mainly from the uncertainties of the intensities and the fitting of the line profile. It should be point out that the increase of time-averaged electron temperature in our DP experiments with respect to the SP-LIBS case (Te ∼ 8469 k) is relatively great (of the order of 23%) at its maximum. Incidentally, the insert of Fig. 7 shows the relation between the signal enhancement factor IDP/ISP of N II 500.5 nm line and the electron temperature (T) within the inter-pulse delay range of −200 fs to 0 fs. It is easily found that the enhancement depends on the increased temperature and is well described by an exponential function as y = A * exp(−B * (1 x − C)) (A, B and C are constant parameters). Consequently, the plasma temperature is a key parameter to understand the increases in intensity in the DP-LIBS scheme.
Fig. 7 The time-averaged electron temperature as a function of the inter-pulse delay between the two laser pulses in the DP-LIBS measurements with the probe pulse energy E2 = 1.0 mJ. The insert shows the relation between the signal enhancement factor of N II 500.5 nm line and the electron temperature within the inter-pulse delay range of −200 fs to 0 fs.
The increase of the electron temperature can fully explain the differences of the enhancement factor of different lines (the neutral and ionic emission lines) illustrated in the Figs. 5 and 6. This reasoning becomes clear when the Boltzmann equation is used to express the emission enhancement. The expression Eq. (3), first introduced by Gautier et al.[31], relates the emission intensity of the lines in the SP- and DP-LIBS cases with the electron temperature.
where the increase of the species density NDP/NSP does not depend on the spectral line properties and affects all the lines in the same way; for neutral lines and for ionic lines. Thus, for a given value of , the signal enhancement depends on the temperature and can be well described by an exponential function for both neutral and ionic species. The insert of Fig. 7 is quite uniform across this description. On the other hand, for a given value of temperature, lines with higher will show a greater enhancement; and this is the main reason why the ionic lines show a greater enhancement with respect to the atomic lines (see Fig. 6).As studied in Ref. [22], for the inter-pulse delay between the two laser pulses tending to zero, the pulse-plasma coupling effects and the plasma re-heating effects can be neglected. From these results, the temperature can be assumed to be responsible for the evolution of the emission intensities in the orthogonal DP-LIBS experiment.
4. Conclusion
Preliminary studies on the feasibility of the use of femtosecond DP-LIBS in an orthogonal geometry have been carried out experimentally. Enhancements of 2–32 folds, in comparison with the single pulse case, have been found with different probe pulse energy when an optimum inter-pulse delay is used. Moreover, several additional emission lines of ionic are also observed in the configuration of the DP-LIBS comparing with the single pulse configuration. And, more remarkably, in the case of higher value of E2/E1, the emission lines arising from ions exhibit a higher degree of enhancement, while the enhancement factors of N2 molecular lines are relatively constant and the value maintains at about 1.0 ± 0.03 in all cases of the energy ratio. Studies of the LIBS intensity as a function of the inter-pulse delay reveal two main regimes of interaction. Firstly, for the atomic and ionic emission lines, the signal enhancement shows a significant variation in the inter-pulse delay range studied here, and the dependence of the enhancement factor on the inter-pulse delay can be fitted by Gaussian function. Secondly, the enhancement factors of molecular lines remain relatively constant in the studied range of inter-pulse delay.
Furthermore, the electron temperature was obtained by the relative line-to-continuum intensity ratio method, and the behavior of the evolution of the time-averaged electron temperature as a function of the inter-pulse delay is in good agreement with that of signal enhancement. The observed great increase in temperature can be assumed to explain the LIBS signal enhancement. These findings, besides giving a further insight on the mechanisms underlying femtosecond laser interaction with gas in DP configuration, can also be useful for optimization of actual LIBS experiments in a number of practical applications.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grant No. 11135002, 11075069, 91026021, 11075068 and 10975065).
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