Journal of Analytical Atomic Spectrometry

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www.rsc.org/jaas Volume 27 | Number 12 | December 2012 | Pages 1995–2140

XXXVIII Colloquium Spectroscopicum JAAS

Internationale June 16 – 21, 2013, Tromsø, Norway Organized by the Norwegian Chemical Society

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Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E ,ǇƉŚĞŶĂƚĞĚƚĞĐŚŶŝƋƵĞƐͬ>ĂƐĞƌƐƉĞĐƚƌŽƐĐŽƉǇͬ/ŵĂŐŝŶŐƚĞĐŚŶŝƋƵĞƐ Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54. EƵĐůĞĂƌƚĞĐŚŶŝƋƵĞƐ;DƂƐƐďĂƵĞƌƐƉĞĐƚƌŽƐĐŽƉǇ͕'ĂŵŵĂƐƉĞĐƚƌŽƐĐŽƉǇ͕EͿ DĞƚŚŽĚƐŽĨƐƵƌĨĂĐĞĂŶĂůǇƐŝƐĂŶĚĚĞƉƚŚƉƌŽĨŝůŝŶŐ

Application of spectroscopy in DĂƚĞƌŝĂůƐĐŝĞŶĐĞƐ &ŽŽĚĂŶĂůǇƐŝƐ ;ŶĂŶŽͬŵŝĐƌŽ͕ƐƵƌĨĂĐĞĂŶĚŝŶƚĞƌĨĂĐĞĂŶĂůǇƐŝƐͿ ŝŽůŽŐŝĐĂůĂƉƉůŝĐĂƚŝŽŶƐ ŶǀŝƌŽŶŵĞŶƚĂůĂŶĚŐĞŽĐŚĞŵŝĐĂůĂŶĂůǇƐŝƐ &ƵĞůƐĂŶĚďŝŽĨƵĞůƐ ƌĐŚĂĞŽŵĞƚƌǇĂŶĚĐƵůƚƵƌĂůŚĞƌŝƚĂŐĞ ^ƉĞĐŝĂƚŝŽŶĂŶĂůǇƐŝƐͬDĞƚĂůůŽŵŝĐƐ ůŝŶŝĐĂůĂŶĚƉŚĂƌŵĂĐĞƵƚŝĐĂůĂŶĂůǇƐŝƐ DŝŶŝĂƚƵƌŝƐĂƚŝŽŶĂŶĚŶĂŶŽƚĞĐŚŶŽůŽŐǇ DĂƐƐƐƉĞĐƚƌŽŵĞƚƌǇŝŶƉŽƐƚͲŐĞŶŽŵŝĐƐĂŶĚƉƌŽƚĞŽŵŝĐƐ ^ƉĞĐŝĂůĞŵƉŚĂƐŝƐǁŝůůďĞŐŝǀĞŶƚŽƚŚĞƚŽƉŝĐƐ͗ ůŝŵĂƚĞĐŚĂŶŐĞ͕ŶĞǁŵĂƚĞƌŝĂůƐ͕ŚƵŵĂŶŚĞĂůƚŚĂŶĚƚŚĞĞŶǀŝƌŽŶŵĞŶƚ Pages 1995–2140 ISSN 0267-9477

PAPER Richard E. Russo et al. Time-resolved LIBS of atomic and molecular carbon from coal in air, argon and helium 0267-9477(2012)27:12;1-9 View Article Online

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Cite this: J. Anal. At. Spectrom., 2012, 27, 2066 www.rsc.org/jaas PAPER Time-resolved LIBS of atomic and molecular carbon from coal in air, argon and helium

Meirong Dong,ab Xianglei Mao,b Jhanis J. Gonzalez,b Jidong Lua and Richard E. Russo*b

Received 30th July 2012, Accepted 1st October 2012 DOI: 10.1039/c2ja30222e

Laser ablation chemical analysis of a coal sample was studied by LIBS (laser-induced breakdown spectroscopy). Ablation was performed using a 266 nm Nd:YAG laser in different gas environments (air, argon and helium) at atmospheric pressure. We present characteristics of spectra measured from

coal with special attention to atomic and molecular carbon including CI, C2 and CN. The inﬂuence of the ambient gas on the laser-induced coal plasma was studied by using time-resolved analysis. Atomic iron emission lines were employed to construct Boltzmann plots for the plasma excitation temperature.

Computer simulations of C2 spectra were used to deduce the molecular rotational temperature. Electron density and total atomic and molecular number density are reported to describe emission differences of atomic and molecular carbon in the different gas environments. These data demonstrate that the plasma excitation temperature is the primary factor contributing to differences among the atomic carbon emission in the gas environments. Reactions between the plasma species and ambient gas, and the total molecular number are main factors inﬂuencing molecular carbon emission. Finally, the inﬂuence of laser energy on the rotational temperature was studied in the air environment to

demonstrate that the rotational temperature derived from C2 band emission can be utilized to correct plasma fluctuations. Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54. 1. Introduction Sr and Ba) and predict slag propensity for five coal blends.17 Feng et al. utilized LIBS combined with PLS to analyze the carbon Laser-induced breakdown spectroscopy (LIBS) is a powerful tool 18 1–6 content in coal. Lu et al. performed a series of studies for for chemical analysis. The high-temperature ionized plasma elemental detection19,20 and also to evaluate primate analysis formed by a focused laser pulse can be used to determine the 21,22 7–9 (volatile matter and ash) by using multivariate analysis. Coal elements that constitute the samples. The direct solid-state analysis is limited by the calibration for some elements and the detection of coal in real time is an important practical problem. application of statistical analysis to improve precision of detection The determination of chemical composition of coal prior to is often implemented. Only a few studies have addressed the combustion is vitally important for a power plant to obtain 10,11 plasma properties on coal ablation; a systematic investigation of optimal boiler control. Recently, Chadwick et al. have inves- temporal variations of plasma characteristics from coal has not tigated lignite samples and obtained detection limits of Ca, Al, Na, been reported to the best of our knowledge. Optical emission Fe, Mg and Si, and the measurement accuracies for inorganic depends on the gas environment and plasma properties such as 12–14 components (e.g. Al, Si, and Mg) were typically within 10%. plasma temperature and electron number density. These proper- Blevins et al. utilized LIBS to detect Na, K, and Ca in the flue gas 15 ties in turn depend on the laser parameters (pulse duration, of a power generation boiler. Zhang et al. measured the organic wavelength, and fluence) and physical–chemical properties of the oxygen content in pulverized anthracite coal under atmospheric sample. The interaction between the plasma plume and ambient conditions with LIBS, with an average relative error in quanti- 16 gas can significantly influence optical emission and the spectral tative measurement of 19.39%. Ctvrtnickova et al. utilized LIBS information available for chemical analysis. Several studies have and Thermo-Mechanical Analysis (TMA) to determine the coal reported the ambient gas influence on the temperature in the elemental composition (C, H, Si, Al, Fe, Ti, Ca, Mg, Na, K, Mn, plasma for various ambient gases.23–25 Additionally the ambient gas can also play a role in the chemistry of the plasma, e.g. through aSchool of Electric Power, South China University of Technology, the formation of oxides as shown during LIBS of mercury and Guangzhou, Guangdong, 510640, China. E-mail: [email protected]; Fax: titanium.26,27 The use of noble gases for the ambient atmosphere +86-20-87110613; Tel: +86-20-87114081 can prevent such chemical reactions.28 bLawrence Berkeley National Laboratory, University of California, Coal is a heterogeneous material with a complex chemical and Berkeley, CA 94720, USA. E-mail: [email protected]; Fax: +1-510-486- 7303; Tel: +1-510-486-4258 physical structure, containing many of the elements in the

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onto the entrance of an optical fiber coupled to a Czerny-Turner spectrometer (Horiba JY 1250M) with an Intensified Charge- Coupled Device (ICCD) (Princeton Instruments, PI MAX 1024 Gen II). The detection system provides a spectral window of 13 nm and a resolution of typically 0.04 nm. Bituminous coal powder samples were prepared with a grain size of 100 mm and pressed by a 25 ton press into 31 mm diameter and 5 mm thick pellets. For time-resolved analysis, each coal sample location was ablated using 50 laser shots and one spectrum per location was obtained by accumulating all consecutive laser shots. This procedure was repeated for the 4 locations 29 Fig. 1 Wiser coal chemical structure model. analyzed on the sample in order to have statistics of the measurements. In order to obtain the strong intensity for periodic table. There exist many macromolecular organic species, different spectral windows at the same time, the gate width was whose chemical structure model is shown in Fig. 1.29 Carbon is varied for each gate delay but the ratio of gate width to gate delay the major element in coal and it is widely distributed in different was fixed at 0.75. The integrated intensity should be divided by samples. Carbon emission has been used in diagnosis applica- the gate width to get the actual intensity as a function of delay. tions such as detection,30 thin film preparation,31 and material The sample chamber was filled with the desired gas and the flow 32 identification. Molecular bands from CN and C2 can be easily of gas was controlled by a valve located before the chamber. measured in the LIBS spectra of samples containing carbon.33 Measurements were performed in air, argon and helium gas The molecular emission is related to the interaction between the environments. Before experiments were performed in argon and plasma species and ambient gas or from direct excitation of helium, a pure graphite sample was ablated in the chamber; no

ablated molecular species such as C2. A number of groups have observation of CN molecular emission from the graphite plasma reported time-resolved measurement of molecular emission, was used to identify whether ambient air was completely involving laser ablation of graphite,34–37 organic compounds,38 or excluded from the chamber. Experiments showed that a flow of 1 polymers.39 Several groups presented a kinetic model to describe 2.0 L min (argon and helium) was sufficient to exclude ambient the formation of molecular emission.40,41 Information on the air from the chamber. plasma excitation temperature was not available in these experiments because of the lack of emission lines suitable for a reliable Boltzmann analysis. Although comparison of the measured 3. Results and discussion rotational–vibrational properties of molecular emission with 3.1 Emission spectra of atomic and molecular carbon simulated spectra could yield rotational and vibrational Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54. temperatures,42,43 the excitation temperature and molecular Emission spectra were measured from the laser-induced coal temperature are not necessarily related if the plasma is not in plasma in the ultraviolet and visible regions. For coal samples LTE. The excitation temperature is a measure of the distribution containing aromatic cycles (five- and six-membered rings) and of atomic or ionic population within the source and is often used the diatomic structures C–C, C]C, C–N as intermolecular as a quantitative means of comparing different atomic emission bonds (shown as Fig. 1), molecular C2 emission from the 44 3P / 3P sources. swanP system (dP g a m) and CN from the violet system 2 + 2 + In this research, we present characteristics of time-resolved (B / X ) can be easily measured, with atomic carbon 33 nanosecond pulsed 266 nm LIBS for coal analysis, with an emission as described in previous work. Fig. 2 shows emission emphasis on atomic and molecular emission. A bituminous coal spectra of atomic and molecular carbon from coal plasma sample was analyzed in different gas environments (air, argon formed in air, argon and helium. The spectral characteristics of and helium). A kinetic study of the plasma properties, namely atomic and molecular carbon emission are dependent on the temperature, electron density and total atomic and molecular gas environment due to differences in physical and chemical number, was used to describe the atomic and molecular carbon properties of the plasma. emission from coal samples. The availability of elemental atomic Fig. 3 shows time-evolution for the atomic emission of C I at and molecular emission allows us to demonstrate the correlation 247.8 nm (Fig. 3(a)) and the molecular C2 (0–0) emission between excitation and rotational temperatures. (Fig. 3(b)). Data in Fig. 3(a) show the area of the emission line (C I 247.8 nm) after background subtraction. Fig. 3(b) shows the 2. Experimental intensity of the C2 (0–0) band from the accumulation of the line intensity of each pixel in the spectral range of 513.5–516.6 nm, A Q-switch Nd:YAG laser operated at 266 nm with a 4 ns pulse- after background correction. The horizontal section of the

duration was used as the ablation source. Pulse energy was spectrum adjacent to the C2 (0–0) band head was chosen as the adjustable from 1.5 mJ to 20 mJ. For the time-resolved analysis, background of the C2 band. The temporal behavior for atomic the laser energy was 14 mJ and the laser beam was focused using carbon is signiﬁcantly different from the C2 band. For atomic a quartz lens with a spot diameter of 250 mm. The sample was carbon, the intensity decreased with increased gate delay, as is placed in a chamber provided with optical windows for laser commonly measured. However, the intensity of molecular band irradiation and spectroscopic observation of the plasma. A single emission increased ﬁrst and then decreased as the gate delay quartz lens was used to collect laser-induced plasma emission increased. Overall, Ar provided a higher intensity over a longer

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Fig. 2 Emission spectra of (a) atomic carbon and (b) molecular CN and

C2 observed during coal ablation in air, argon and helium. The gate delay Fig. 3 Time-evolution of the (a) atomic carbon 247.8 nm and (b) Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54. and width were 400 ns and 300 ns, respectively. molecular C2 band. The integrated intensity of C I 247.8 nm and C2 (0–0) band are presented as a function of gate delay (nanoseconds).

time for both atomic and molecular carbon emission, whereas air and He display similar behavior. It was expected that the total density N(T) of a neutral atom or ion of this element by molecular emission would last longer than that of atomic emis- Boltzmann’s law:45 sion since it takes time for the plasma to cool and for molecular hc NðTÞ E species to form. The difference between carbon emission in air I ¼ g A exp m (1) mn 4pl UðTÞ m mn kT and He can be attributed to the spectral linewidth; carbon mn

emission is stronger and its linewidth (FWHW) is narrowest in where lmn, Amn and gm are, respectively, the wavelength, the He atmosphere (Fig. 2(a)); carbon emission in air is weaker but transition probability, and the statistical weight for the upper

the linewidth is wider. The differences in linewidth will be dis- level; Em is the excited level energy; T is the temperature; and k cussed in Section 3.2, and related to the variations of electron and h are Boltzmann and Planck constants, respectively. U(T)is density with time delay. From Fig. 3(a), atomic carbon emission the partition function. There are two main factors inﬂuencing the dropped faster as the gate delay increased in air than that in emission line intensity. The ﬁrst is the number density of the helium, which is opposite to molecular carbon emission atoms and the second is the temperature of the plasma. The (Fig. 3(b)) time behavior. The quenching in air due to collisions plasma temperature can be obtained by means of Boltzmann of oxygen and nitrogen with carbon41 will enhance the reduction plots which are described in detail elsewhere.46,47 For this of atomic carbon emission and increase the formation of research, non-resonant atomic Fe lines were chosen to estimate molecular CN, C2, CO and other molecular species. the plasma excitation temperature. The parameters for these Fe I lines are taken from (ref. 48) and given in Table 1. A typical coal plasma spectrum in different gas environments is shown in Fig. 4 3.2 Excitation temperature and electron density and a plot for the estimation of plasma temperature is shown

The atomic emission spectral line intensity Imn is a measure of the in Fig. 5. population of the corresponding energy level of the element in The only two groups of energy levels of Fe lines for the the plasma. If the plasma is in local thermodynamic equilibrium Boltzmann plots are limited by the narrow detection window of (LTE), the population of an excited level can be related to the the spectrometer. They are very close and not blended in the

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2 Table 1 The parameters used for plasma temperature calculation T DA1 DA2 DI1 DI2 (atomic Fe lines). Gaps correspond to data not available in the literature DT ¼ þ þ þ (2) 0:625ðE1 E2Þ A1 A2 I1 I2 Wavelength/nm Log gfa A [108 s1] Uncertaintyb E (cm1) m The main sources of errors are caused by the transition 370.5566 1.334 0.03214 27394.688 probabilities and energy difference. The uncertainty of transition 370.9246 0.646 0.1564 10% 34328.749 probabilities is 10%–20% (seen in Table 1). The detection 372.2563 1.287 0.04968 27559.581 condition with the delay and width of 300 ns and 225 ns in the air 372.7619 0.631 0.2244 10% 34547.206 373.7131 0.574 0.1414 25% 27166.819 environment was chosen as an example here to show the accu- 374.8262 1.016 0.09146 27559.581 racy and precision of the calculated excitation temperature. The 375.8233 0.027 0.6336 34328.749 low energy level (373.7131 nm) and high energy level (376.7192 376.3789 0.238 0.5441 10% 34547.206 nm) were chosen to obtain the precision according to eqn (2). 376.7192 0.389 0.06393 10% 34692.144 With T ¼ 7102 K, the accuracy was an error of 225 K (seen in a b Ref. 48. Ref. 50. g is the statistical weight and f is the oscillator Fig. 5) and the precision was an error of 5056 K. Although strength. the precision of the calculated temperature was not so high, the accuracy was enough to present the relative variation of the excitation temperature.

The FWHM of Stark broadened lines Dl1/2 is related to the electron number density by the expression:51 " # ! 1=4 ne ne 3 1=3 Dl = ¼ 2W 1 þ 1:75A 1 N 1 2 1016 1016 4 D (3)

3 where Dl1/2 is in nm and electron number density ne is in cm . The coefﬁcient W is the electron impact parameter which is related to temperature, A is the ion broadening parameter, and

ND is the number of particles in the Debye sphere. The ﬁrst term on the right side of eqn (3) represents the broadening due to electron contribution and the second term is the ion correction factor. For non-hydrogenic ions, Stark broadening is dominated by electron impact. Since the perturbations caused by ions are negligible compared to electrons, the ion correction factor can be Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E

Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54. Fig. 4 Atomic iron emission in the range of 367–378 nm from coal safely neglected. Moreover, in the most acquisition condition, plasmas in different gas environments (air, argon and helium). The gate FWHW of CI 247.8 nm is more than instrumental broadening. delay and width were 300 ns and 225 ns, respectively. The number density presented here was used to show the relative temporal variation in different gas environments and it is enough to consider the electron impact. Therefore eqn (2) reduces to spectrum, making corrections for the spectral response of the system unnecessary. On the other hand, the evaluation eqn (2) of ne 49 Dl = ¼ 2W (4) the line pair method could be used here to obtain the precision. 1 2 1016

The value of W for C 247.8 nm can be found from (ref. 52) according to the plasma excitation temperature obtained above (Table 2). A stark broadened line proﬁle was approximated by a Lorentzian function. The FWHM of atomic carbon C (I) at 247.8 nm was used and according to the plasma temperature obtained above, the electron number density was calculated to estimate the effect on atomic carbon emission. The temporal variations of the temperature and electron number density are shown in Fig. 6 and 7, respectively. The temperature decreased as the gate delay increased in different gas environments. The temperature was higher in argon than in air and helium. Conversely, only small temperature changes could be measured between air and helium. As can be seen in Fig. 4,

Table 2 The impact width W of C (I) 247.8 nm

Temperature (K) 5000 10 000 20 000 40 000 Fig. 5 Example of Boltzmann plot obtained in air, argon and helium. The gate delay and the gate width were 300 ns and 225 ns, respectively. Width (nm) 0.317 103 0.361 103 0.417 103 0.480 103 The slope yields the temperature.

This journal is ª The Royal Society of Chemistry 2012 J. Anal. At. Spectrom., 2012, 27, 2066–2075 | 2069 View Article Online

lower atomic mass, the vapor plume expands faster for ablation in helium since the background gas provides less momentum drag. Also, with the higher ionization potential, the background gas is less ionized behind the shockwave for ablation in helium. This higher electron-number-density vapor plume absorbs a signiﬁcant portion of the incoming laser energy, thus increasing the plasma temperature and increasing the rates of electron- impact processes during the laser irradiation in the argon environment. On the other hand, argon which has a great propensity for formation of electrons would contribute to the high electron concentration leading to the enhancement of plasma emission intensity in an argon atmosphere.24,56,57 Therefore, the intensity, temperature, and the electron number density were higher in argon than that in air and helium. The lower limit for the electron density for which the plasma 58 Fig. 6 Time-resolved plasma excitation temperature derived from the will be within 10% of LTE is given by: Boltzmann analysis by using atomic iron lines for different gas envi- 12$ 1/2$ D 3 ronments (air, argon and helium). ne > 1.61018 10 Te ( E) (5)

3 where ne is the electron density (in cm ), Te is the plasma temperature (in K), and DE is the higher energy difference (in eV) of the levels whose populations are given by LTE conditions. We can obtain the DE from the carbon emission 247.8 nm to be 5 eV. The range of the electron density from 3.5 1018 cm3 to 2.5 1019 cm3 is obtained for the three different gas environments. We deduce that LTE is satisﬁed in the plasmas generated in this work. The total atomic number N(T) can also be used to estimate the inﬂuence of the plasma parameters on the atomic emission line intensity. According to eqn (1), the qualitative formula of N(T) can be expressed by:

Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E Em Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54. NðTÞfI UðTÞ exp (6) mn kT where the partition function of C(I) can be found from (ref. 59) Fig. 7 Temporal behavior of the electron density for atomic carbon at 247.8 nm. according to the temperature. This calculation only holds when LTE exists and the temperature is homogeneous in the plasma. The calculated total number is a non-dimensional value which iron emission was much lower in helium than in air and argon. could be used to qualitatively compare the time-resolved The iron content in this coal sample was 0.861%. The iron emission dropped faster in helium and there was not enough iron emission intensity for plasma temperature calculations after 600 ns. The lower emission for iron in the helium environment, to some degree, would increase the error for the temperature calculations. Also, the electron number density was much lower in helium than in air and argon. However, the difference in electron density was smaller than the temperature difference between air and argon. These data demonstrate that the temperature plays a dominant role on the atomic emission in different gas environments especially for air and argon. Plasma expansion is known to vary for ablation in different gas environments.53 In helium, the vapor plume expands faster which is strongly related to the atomic mass and the ionization potential. Wen et al. discussed these phenomena from both theoretical and experimental perspectives.54,55 The major differences in air, argon and helium are the atomic mass and the ionization potential. Helium has a much smaller atomic mass and a much higher ionization potential compared to that of nitrogen (80% in air) Fig. 8 Temporal variation of total atomic number Nc for atomic carbon and argon (24.587 eV versus 14.5341 eV, 15.759 eV). With the CI 247.8 nm.

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h i P 2 behavior in air, argon and helium. The data are shown in Fig. 8. N exp synthð ; D ; Þ i Ii Ii Trot l C The total atomic carbon number decreased with gate delay in 2 c ðTrot; Dl; CÞ¼ (9) air, argon and helium. A significant difference can be observed NðN 1Þ at earlier times (before 400 ns). Again, these data support exp synth where Ii and Ii are the measured and calculated intensities that temperature is the dominant factor influencing atomic th of the i pixel of an N point spectrum, respectively. The Trot emission in different gas environments, except at the earlier value corresponding to the smallest c2 was taken as the appro- times. priate calculated temperature value. The calculation procedure can be found elsewhere.63,64 Each rotational transition was approximated by Lorentzian 3.3 Simulation of molecular C2 (0–0) band spectra curve fitting and its integral equal to the intensity at the wave- Comparison of an experimental emission spectrum60,61 with its length corresponding to the maximum. The Lorentzian curves corresponding synthetic spectrum allows determination of the for each branch, organized in table form, are compiled and then rotational and vibrational temperatures of molecular bands. The added. The spectrum shape is obtained by superposition of all calculation procedure provides both Trot and Tvib for all diatomic rotational transitions with J, up to the fixed maximum rotational molecules in various states (singlet, doublet or triplet). According quantum number. At the beginning of all iterations, the to the Born Oppenheimer approximation,62 which assumes the normalization to the maximum must be performed first, so as to existence of thermal equilibrium among the rotational and compare the spectra obtained with different computational vibrational states, the emission intensity of a molecular band parameters. The width for the Lorentzian function could directly (In0,n00,v0,v00,J0,J00) is obtained by the formula: use the apparatus width (Dl) which is measured by the standard

0 00 0 00 light. It could be an ‘‘a priori’’ known parameter in the spectrum qv ;v SJ ;J 4 F 0hc=kT G0hc=kT 0 00 0 00 0 00 ¼ ð 0 00 Þ rot vib In ;n ;v ;v ;J ;J Ce 0 vJ ;J e e (7) simulation. However, in order to obtain the more accurate fitting Qrotv results in our fitting program, the width was also settled as the where h is Planck’s constant, c is the velocity of light in vacuum, k fitting parameter and determined as an unknown parameter. is Boltzmann’s constant and Ce is the emission constant. v and J From the final fitting results, it could be found that the fitting are the vibrational and rotational quantum numbers, respec- width is very close to the apparatus width and seldom changes as 0 00 tively. (v ,v ) is the vibrational transition, qv0,v00 is the Franck– the gate delay. 0 00 0 00 Condon coefficient for (v ,v ) and SJ ,J is the rotational line The C2 emission swan system is an electronic transition strengths (also known as Honl–London€ factors if two states are between two P states and its rotational bands include P, Q and R 62 0 0 in Hund’s limiting case). F and G are the rotational and branches. Fig. 9 presents experimental spectra and computer

vibrational energy terms, respectively. Qrot is the rotational simulations for the (0, 0) band of C2 which were measured in partition function. A detail description of diatomic molecules’ different gas environments (air, argon and helium); experimental

Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E spectral terms may be found in (ref. 62). In this case, the well-

Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54. spectra and synthetic spectra are in excellent agreement. resolved molecular C2 (0–0) band was calculated to deduce the The temporal variation of the rotational temperatures is rotational temperature. Therefore eqn (6) can be simpliﬁed as shown in Fig. 10. The rotational temperature dropped as the gate follows: delay increased, similar to the behavior of the excitation temperature shown earlier. But the trend is different in different 4 F0hc/kT 0 00 0 00 0 00 ¼ 0 00 0 00 rot In ,n ,v ,v ,J ,J Cem(vJ ,J ) SJ ,J e (8) gas environments. For the rotational temperature, it was lowest in helium but only small differences were measured between air The simulation was performed by three parameters: the and argon. By comparing the rotational (Fig. 10) and excitation emission constant (C ), the rotational temperature (T ), and em rot temperature (Fig. 6), these two temperatures are close in argon the apparatus width (Dl). The molecular constants used for these and helium; in air the rotational temperature is much higher than calculations are given in Table 3. The rotational temperature Trot was obtained by minimizing:

Table 3 Molecular constants according to Ballick and Ramsay.65,66 Gaps correspond to data not available in the literature. Te is the electronic energy level, ceue and geue are anharmonic oscillators. Be and ae are vibrational constants, De is the rotational constant, and re is the internuclear distance

3 3 Constants d Pg a Pm

1 Te (cm ) 20022.50 714.24 1 ue (cm ) 1788.22 1641.35 1 ceue (cm ) 16.440 11.67 1 geue (cm ) 0.5067 1 Be (cm ) 1.7527 1.63246 1 6 De (cm ) 6.44 10 1 Fig. 9 Experimental (black line) and synthetic spectra (red line) for ae (cm ) 0.01608 0.01661 different gas environments (air, argon and helium). The gate delay and re (A) 1.2661 1.31190 the gate width were 1 ms and 0.75 ms, respectively.

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Fig. 12 Time-evolution of the molecular CN band. The intensity band Fig. 10 Time-resolved rotational temperature derived from the C2 (0–0) head (0–0) at 388.3 nm was measured as a function of gate delay band for different gas environments (air, argon and helium). (nanosecond).

the excitation temperature. The temperature from statistical helium. The reaction of carbon and nitrogen is the main mechanics on the molecular level is the result of the motion of the contribution to the total molecular number in air. In helium, the species that exist in plasma. The motion of particles would carry rotational temperature and the total molecular number are both kinetic energy. Temperature increases as the motion and kinetic lower than in the air environment. These data showed that the energy increase. The rotational temperature is directly related to total molecular number is the dominant factor inﬂuencing the motion of particles in plasma. The strong reaction of carbon molecular carbon emission in different gas environments, espe- and nitrogen in air would increase the motion of molecular cially for air and argon. particles so that the rotational temperature would be higher than Fig. 12 shows time-evolution of the molecular CN (0–0) band the excitation temperature in air and close to the rotational head at 388.3 nm. The intensity of CN is strong in air, and it can temperature in argon. be detected in argon and helium because of the existing CN In order to further explain this phenomenon, the total number radicals in coal; the nitrogen content is approximately 1%. The of molecular C2 species was qualitatively calculated according to nitrogen atomic emission line was not measured from ablation of the following relationship:62 this coal sample. Although the CN intensity in argon and helium Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54. XN is not very strong and only the CN band head at 388.3 nm could 4 F 0ð jÞhc=kT ð Þf ð Þ 0 ð 0 00 Þ rot be measured, these data are important for the indirect N T IC2ð0-- 0ÞQ Trot SJ ;J00 vJ ;J e (10) j¼0 measurement of N in coal. CN detected in argon and helium is associated with the nitrogen in coal. Nonmetallic elements exist where the partition function of C can be found from (ref. 59) 2 in coal with various forms.29 Nitrogen is the only element in coal according to the rotational temperature. The temporal variation with a single organic form connected with carbon. As mentioned of the total molecular number is shown in Fig. 11. The total previously, the pure graphite sample was used to conﬁrm that air molecular number is highest in argon, followed by air and was completely excluded from the chamber.

3.4 Inﬂuence of laser ﬂuence on C2 rotational temperature Comparing the calculation results for excitation (Fig. 5) and

rotational temperatures (Fig. 9), the error in the C2 rotational temperature is smaller than that for the excitation temperature. The molecular temperature is derived from ﬁtting experimental to synthetic spectra. The good agreement shows that the molecular temperature in the plasma could better indicate ﬂuc-

tuations in plasma properties. The emission of molecular C2 is much stronger than iron atomic emission and the strong C2 emission intensity could reduce the calculation error. In order to better understand the role of molecular information for coal

chemical analysis by LIBS, the influence of laser fluence on C2 rotational temperature was studied in the air environment. To study the influence of laser fluence, the gate delay and gate width were 1 ms and 40 ms, respectively. The ablation was

Fig. 11 Time-resolved analysis of total molecular number Nc2 for the C2 repeated at 5 sample locations for each laser energy condition, (0–0) band in different gas environments (air, argon and helium). and one spectrum per location was obtained by accumulating

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Fig. 13 Variation of rotational temperature derived from the C2 molecule versus laser ﬂuence. Published on 01 October 2012 http://pubs.rsc.org | doi:10.1039/C2JA30222E Downloaded by Lawrence Berkeley National Laboratory on 05/04/2013 16:43:54.

Fig. 15 Plasma rotational temperature correction for laser fluence: (a) I I I correction factor for the Mg/ C2 ratio and (b) corrected results for the Mg/ IC ratio. I I versus 2 Fig. 14 Variation of intensity ratio Mg/ C2 laser fluence. 4. Conclusion 50 laser shots. The variation of rotational temperature with laser We studied time-resolved analysis of laser-induced coal plasmas fluence is shown in Fig. 13. The rotational temperature increased in different gas environments (air, argon and helium). Coal with the increase of fluence due to larger number of molecules ablation allowed us to simultaneously measure elemental and excited by the higher laser energy. The emission intensity is not molecular emission for understanding temporal changes in only related to the plasma temperature but also related to the plasma properties (electron density, number density and number density. The intensity ratio can be used to decrease the temperature). Ambient gas had a significant influence on atomic effect of number density variations which is a common calibra- and molecular emission. Plasma excitation temperature was a tion method used in LIBS 67,68 The atomic emission line for Mg I primary factor driving the atomic carbon emission in each gas

518.36 nm can be measured with the molecular C2 band environment. However, for molecular carbon emission, the (Fig. 2(b)) in the spectral window from 513–519 nm. Fig. 14 atomic number from recombination processes contributed to a I I shows the relationship between intensity ratios of Mg/ C2 with significant difference between excitation and rotational temper- I I laser fluence. The intensity ratio ( Mg/ C2) significantly changed as atures. The CN molecular band could be measured not only in the laser fluence increased. The relationship between the intensity air but also in Ar and He; CN emission in Ar and He plasmas was ratio and rotational temperature is shown in Fig. 15(a); the from the nitrogen in the coal sample. Due to the single form of intensity ratio increased as the rotational temperature increased. nitrogen in coal, CN emission intensity can be used to represent A correction factor was acquired by polynomial fitting these the quantity of nitrogen in coal. The influence of laser energy on

data. By applying this correction factor, the ﬂuctuation of the the C2 rotational temperature was measured and the correlation intensity ratio was reduced and the relative standard deviation of rotational plasma temperature derived from molecular band

(RSD) improved from 43.74% to 20.27%, as seen in Fig. 14 C2 emission was found to be a stable parameter for improving and 15(b). LIBS chemical analysis.

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