Femtosecond and Nanosecond Dual-Laser Optical Emission Spectroscopy of Gas Mixtures Cheng-Hsiang Lin,a Zhi Liang,a Jun Zhou,b Hai-Lung Tsaia,* a b
Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409 USA Department of Mechanical Engineering, Pennsylvania State University Erie, The Behrend College, Erie, PA 16563 USA
A method employing an integrated femtosecond (fs) and nanosecond (ns) dual-laser system was developed to generate plasma with desired radical species from gas mixtures via a fs laser pulse and then to excite selected radical species to higher electronic states using a wavelength-tunable ns laser pulse. An optical spectrometer was used to measure the emission spectra and identify the transition from the excited electronic state to the ground state. The proposed technique has been demonstrated for an N2–CO2 mixture with various time delays between the two fs and ns pulses. The results have indicated that the population of selected radical species at the excited electronic state can be increased using the subsequent ns laser pulse, which also enhances the intensity of emission spectra allowing better identifications of the radical species. This technique holds a promise of detection and identification of signature plasma species, particularly for trace elements and long-distance standoff detections. Index Headings: Laser-induced breakdown spectroscopy; Dualbeam spectroscopy; Plasma generation; Species manipulation.
Since ultrashort lasers have become available, the capability and accuracy of various scientific tools such as the optical nonlinear imaging and time-resolved measurements have improved.1–3 From a material processing standpoint, fabrication of functional micro/ nanoscale three-dimensional devices4,5 and micromachining of dielectrics6–8 were realized by employing the femtosecond (fs) laser multi-photon absorption effect, which otherwise would be difficult to achieve using conventional technologies. Although the fs laser itself is a powerful tool already, a multi-laser system including an fs laser was employed to improve the system performance. For example, a dual-laser system consisting of fs and nanosecond (ns) lasers was proposed to improve the overall machining efficiency for dielectrics.7 Optical emission spectroscopy is a useful method for species identification in plasma and is important in the material coating process9 or other applications.10,11 In the past decades, the technology of laser-induced breakdown spectroscopy (LIBS) has been developed for real-time surface analysis and depth profile analysis of various materials in all phases.12,13 In standard LIBS technology, a laser is used to locally generate radical species and a spectrometer is used to detect their optical emission spectra. Since optical emissions occur when there are transitions from higher energy states to lower energy states, the signal strength could be enhanced if the generated free radicals could be re-excited back to the higher states. It was reported that a double-pulse Received 25 July 2013; accepted 3 October 2013. * Author to whom correspondence should be sent. E-mail:
[email protected]. DOI: 10.1366/13-07237
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(from one or two lasers) LIBS can enhance the signal by thermal reheating and increasing the material ablation rate.14 Furthermore, due to fs laser filamentation, longdistance (90 m) remote LIBS can be achieved.15 To the best knowledge of the authors, there is no report on femtosecond–nanosecond (fs–ns) dual-laser LIBS or fs– ns dual-laser spectroscopy. In this work, an fs–ns duallaser spectroscopy system was proposed to generate and manipulate the populations of radical species at the ground and excited electronic states. The dual-laser optical emission spectroscopy system consists of a Ti:sapphire fs laser (Legend-F, Coherent), a diode-pumped solid-state ns laser (AVIA-X, Coherent), a spectrometer (Andor) equipped with a time-gated intensified charge-coupled device (CCD; iStar Gen II, Andor), a digital delay generator (DG645, Stanford Research), and a gas chamber with fused silica windows for holding gas mixtures. The central wavelength, pulse duration, and maximum pulse energy for the fs laser are, respectively, 800 nm, 120 fs, and 1 mJ, and are 355 nm, 30 ns, and 0.23 mJ, respectively, for the ns laser. As shown in the schematic diagram Fig. 1, each of the fs laser pulses (FLPs) and ns laser pulses (NLPs) first passes through the combination of a half-wave plate and a linear polarizer to adjust the pulse energy of both FLP and NLP to 0.2 mJ. Then, both laser beams are combined into the same optical path via a long-pass dichroic mirror. Next, both the FLPs and NLPs are focused into a gas chamber via an off-axis replicated parabolic mirror. In order to precisely control the time delay between the FLPs and NLPs, a digital delay generator is employed. A negative time delay is defined when the NLP is shot ahead of the FLP, and a positive time delay implies the NLP is shot behind the FLP. The emission spectra from the gas chamber are collected with a lens and then projected onto an optical fiber connected to a spectrometer, and a time-gated intensified CCD camera is used to collect and amplify the spectra. The gate width of the intensifier is set to 5 ls, which is long enough to collect all optical emission signals because the time constant of electronic transition is in the range of tens to hundreds of nanoseconds.11 Each spectrum is accumulated in 2 s, which includes 2000 intensified signals because the repetition rate of the duallaser system is 1 kHz. The gas chamber is continuously flushed and vented with a gas mixture of 50% nitrogen (N2) and 50% carbon dioxide (CO2). The reason to select N2 and CO2 as experimental gases is that both of them are relatively stable, and only limited kinds of radical species would be generated, which facilitates the analysis. The optical emission spectrum of N2–CO2 excited using a FLP is shown in Fig. 2. The spectral lines of
0003-7028/14/6802-0222/0 Q 2014 Society for Applied Spectroscopy
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FIG. 1. System setup of the dual-laser optical emission spectroscopy. HWP and LP are, respectively, half-wave plate and linear polarizer.
CN, N, and O were observed, which are consistent with the published results.9,10 Note there might be some other radical species whose spectral lines are out of the measuring range or the signal is too low to be detected. The results show that radical species can be generated using FLPs via multi-photon absorption to dissociate N2 and CO2 molecules, and then some of these free radicals combined to become another species such as CN. In the following, we assume CN is the desired radical species, and the ns pulse will be used to increase the population of CN in the excited electronic state. To study the effect of laser intensity (LI) on the population of generated species, the spectra excited using various LIs were recorded and analyzed. The fs LI and signal intensity (SI) of each spectral line are normalized respectively using the maximum LI and its corresponding SI for each spectral line. The spectra in the range between 383 and 390 nm include energy bands of (0, 0), (1, 1), (2, 2), (3, 3), and (4, 4) of the CN free radical at Dv = 0, where in the expression of (A, B), A represents the vibrational state in the ground state X2Rþ and B represents the vibrational state in the second excited electronic state B2Rþ of the CN radical, and the difference Dv is defined as A minus B (Dv = A B). Note the emission spectra caused using the transition between the first excited electronic state A2PI (and its vibrational states) and the ground state X2Rþ (and its vibrational states) are very weak and, hence, this study focused only on the transition between the ground state and the second excited electronic state. Since the 355 nm NLPs can be reflected using the gas chamber surface, which
FIG. 2. Optical emission spectra of N2–CO2 mixture.
FIG. 3. (a) Spectra for CN at different normalized intensities of fs laser. Trace (1) 0.25; trace (2) 0.41; trace (3) 0.59; trace (4) 0.75; trace (5) 0.88; trace (6) 1. (b) Signal intensity vs. laser intensity of CN violet band at Dv = 0.
would interfere with the measurement of CN band at Dv = þ1 (Fig. 2), the following discussion focused on the CN violet band of Dv = 0 and Dv = 1 to reveal the effects of NLPs on radical species populations. Figure 3a shows the SIs of CN violet bands at Dv = 0 excited using different fs LIs of the N2–CO2 mixture. The spectral lines and the intensity ratio of each line are similar to the published results.9 When the normalized SIs (NSIs) for each spectral line are plotted with regard to the normalized LIs (NLIs), the results are shown in Fig. 3b. The measured data in Fig. 3b can be well fit using the equation NSI = C 3 NLI2, where C is a constant. This is consistent with the theoretical study that the population ratio between electronic states and vibrational states for the same radical or between any two different radicals can be represented as a function of the transition dipole moment of each radical at its electronic states and vibrational states.16 In other words, the population ratio between electronic states and their associated vibrational states or between different radical species is independent of LI but depends on the intrinsic molecular property, initial state of the molecule, and photon energy. Figure 4 illustrates the results of N2–CO2 gas mixture excited using fs–ns dual-laser pulses. When the N2–CO2
FIG. 4. Dual-laser spectra of CN bands at Dv = 0. (a) Fs laser only, (b) dual-laser at time delay 30 ns, (c) 0 ns, (d) þ30 ns, (e) þ60 ns, (f) þ330 ns, and (g) normalized intensity for different time delays.
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gas mixture is excited using a NLP alone, no specific signal can be identified in the spectrum (results not shown) because the laser photon energy is insufficient to dissociate molecules, i.e., the ns laser energy cannot be absorbed using N2 or CO2 molecules. In contrast, Fig. 4a shows the spectrum of CN radical at Dv = 0 excited using a FLP alone, which would be used as a reference for normalization. The spectra excited using fs–ns pulses with time delay from 30 ns to þ330 ns are shown, respectively, in Fig. 4b through Fig. 4f. For convenience of comparison, the maximum intensity of (4, 4) and (0, 0) lines in Fig. 4a are marked with dashed lines in each subfigure of Fig. 4 with identical scale. In the case of time delay 30 ns, Fig. 4b, no apparent enhancement is observed due to only a very small portion of the ns laser energy being absorbed using the generated CN. In this case, most of the ns energy was available before the CN species were generated using the FLP, which leads to little or no contribution to the spectral lines. In Fig. 4c, excited using dual-laser with 0 ns time delay, all spectral lines, except (0, 0) transition, of the spectrum are enhanced at different levels. Note the energy transfer from NLPs to radical species can be achieved through a direct resonant absorption and/or indirectly using molecular collisions. Apparently, the CN species have absorbed the ns laser energy and were excited to the higher vibrational states in the B2Rþ electronic state and the population ratios at different vibrational states have been changed. For this 0 ns time-delay case, only 50% of the ns laser pulse energy is available for the generated CN species. For the cases of þ30 ns delay, Fig. 4d, and þ60 ns delay, Fig. 4e, their SIs are stronger than the corresponding energy bands for the case of 0 ns delay, Fig. 4c, because more of the NLP energy was absorbed. However, for the case of þ330 ns time delay, Fig. 4f, the SIs are very close to those in the case of FLP alone. The NLP mainly increases the SIs of (4, 4) and (3, 3) lines, and the SIs of other lines, (0, 0), (1, 1), and (2, 2), have minor increments. This is probably because the corresponding transition dipole moments for (4, 4) and (3, 3) lines are bigger than those for other lines, and the excitation rate is proportional to the square of transition dipole moment.16 Only a small portion of excited CN radicals at the 3rd and 4th vibrational states will relax to the lower vibrational states through collisions. Hence, the enhancements in SIs of (0, 0), (1, 1), and (2, 2) are small. Our observations are similar to those in the detection of trace phosphorus in steel using LIBS.17 The result in Fig. 4 also means the population of CN radicals in the B2Rþ electronic state truly increases from the X2Rþ ground state rather than the population transfer between the vibrational levels of the B2Rþ electronic state. The SI of each spectral line at different time delays normalized using the SI of FLP alone is shown in Fig. 4g. The maximum SI ratio, 1.86, occurs for (3, 3) line at the þ60 ns time delay. In contrast, the maximum SI ratio of (0, 0) line is only 1.12. Similar phenomena can be observed for CN violet bands at Dv = 1, Fig. 5, and the maximum SI ratio 1.98 occurs for (3, 4) line at þ90 ns time delay. It is seen that the SIs of Dv = 1 band are smaller than those in Dv = 0 band, but the enhancement using NLPs can still be clearly seen. From Figs. 4 and 5, it is concluded that the
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FIG. 5. Dual-laser spectra of CN bands at Dv = 1. (a) Fs laser only, (b) dual-laser at time delay 30 ns, (c) 0 ns, (d) þ30 ns, (e) þ60 ns, (f) þ330 ns, and (g) normalized intensity for different time delays.
NLP with a wavelength of 355 nm can significantly increase the population of CN radical in the 3rd and 4th excited vibrational states for the B2Rþ electronic state. However, the enhancement did not occur for N radical, Fig. 6, or for O radical (results not shown). For these two radicals, the SI of spectral lines is time-delay independent and are almost identical to the case of fs laser alone. Hence, by using the 355 nm NLP, one can excite the selected radical species that have emission spectra near the laser wavelength. In other words, the technique can be used to identify the existence of selected radical species, particularly for very little radical species and/or low signals that may be caused by, for example, trace elements or long-distance standoff detections. Note that if the ns laser was tunable and had a wavelength of about 740 nm, the SI for N could have been enhanced, and the existence of N could have been identified. From the above results, we have demonstrated that for an N2–CO2 gas mixture, we can generate radical species CN via a FLP and then increase the population of the second excited electronic state using a NLP. By choosing different gas mixtures, one can generate almost any desired radical species via laser dissociation using an FLP, and the population of the desired radical species can be manipulated using the subsequent NLP. As each radical species has its own absorption band, if the wavelength of the ns laser is tunable, it would be possible to manipulate the population of any desired radical species. For mixtures of complex molecules, a multi-laser system may be needed to achieve multiradical manipulations. The potential application of the
FIG. 6 Dual-laser spectra of N. (a) Spectra excited (1) using fs laser alone and dual-laser with (2) time delay 30 ns, (3) 0 ns, (4) þ30 ns, (5) þ60 ns, (6) and þ330 ns. (b) The normalized intensity for different time delays.
proposed technique is, for example, to detect some signature species for trace toxic chemicals, such as the Cl species in the decomposition of perchloroethylene (Cl2C = CCl2).18 In summary, a dual-laser optical emission spectroscopy was carried out for generating and measuring radical species. The tunable NLP provides another energy source to increase the population of the desired excited electronic state, which in turn enhances the emission signal strength. This technology offers an opportunity to identify the existence of some radical species, particularly for the cases with trace radical species, weak signals, and/or long-distance detections.
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