The Diagnostics of Laser-Induced Fluorescence (LIF) Spectra of PAHs

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The Diagnostics of Laser Induced Fluorescence (LIF) Spectra of PAHs in Flame with TD-DFT : Special Focus on 5-Membered Ring Peng Liu, Zhenwu He, Gao-Lei Hou, Bin Guan, He Lin, and Zhen Huang J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b10114 • Publication Date (Web): 08 Dec 2015 Downloaded from http://pubs.acs.org on December 10, 2015

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The Journal of Physical Chemistry

The Diagnostics of Laser Induced Fluorescence (LIF) Spectra of PAHs in Flame with TD-DFT : Special Focus on 5-Membered Ring Peng Liu1, Zhenwu He1, Gao-Lei Hou2, Bin Guan1, He Lin*1, Zhen Huang1 1

Key Laboratory for Power Machinery and Engineering of Ministry of Education, School of Mechanical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 2

Physical Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA

Abstract: The electronic emission characteristics of 13 gas-phase PAHs, ranging from phenlylacetylene to rubicene, were investigated to diagnose laser induced fluorescence (LIF) spectra of PAHs in flame by DFT, TD-DFT and premixed flame modeling methods. It was found that the maximum emission wavelengths of the PAHs with 5-membered ring are located in visible region and insensitive to the number of C atom. However, the fluorescence wavelengths of the PAHs without 5-membered ring increase with the number of C atom due to the reduced HOMO-LUMO gap. In addition, the fluorescence wavelength of the PAHs without 5-membered ring with linear arrangement is longer than that of PAHs with non-linear arrangement. According to the Franck-Condon principle, the vibrationally-resolved electronic fluorescence spectra were obtained. The results show that fluorescence bandwidth of the PAHs with 5-membered ring is much broader than that of the PAHs without 5-menbered ring. The concentration of PAHs was calculated using the premixed flat-flame model with KM2 mechanism. Based on the fluorescence bandwidth and the concentration of the PAHs, the potentially fluorescence distribution of PAHs in flame was mapped. One can distinguish the specific PAHs according to the mapped fluorescence distribution of PAHs in this study. It was found that naphthalene should be responsible for the fluorescence located in 312-340 nm region in flame. 1-ethynylnaphthalene is the most possible candidate to emit the fluorescence located in 360-380 nm region. The fluorescence signals with the wavelength longer than 500 nm are likely emitted by the PAHs with 5-membered ring. This study contributes to enhance the selectivity of PAHs in LIF technology, especially the visible region.

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1. Introduction Polycyclic aromatic hydrocarbons (PAHs) formed in combustion of fossil fuels are the precursors of soot particle, and have received great attention in recent years for the mutagenicity and carcinogenicity.1-8 The mechanisms of PAHs formation and evolution in flame have been studied for decades both theoretically2,7 and experimentally.9 In 1980s, laser induced fluorescence (LIF) technique was introduced into combustion diagnosis.10,11 This method can obtain the non-intrusive, temporally and spatially resolved information of PAHs in flame.12 In LIF measurement, 266 nm from Nd:YAG laser is widely used as excitation wavelength, by which most of the PAHs can be excited. Two emission bands can be detected upon excitation at 266 nm in fuel-rich flame: a UV band between 280 and 380 nm and a second one in the visible range of 400-650 nm.13,14 The size of the PAHs can be distinguished according to the fluorescence spectra.15,16 Generally, the fluorescence shifts toward longer wavelengths when the molecular size of the PAHs increases.13,15,17-19 The signal in UV region is regarded as the fluorescence being emitted by the PAHs with 2-3 rings.17,20,21 However, the attribution of the visible fluorescence signal is still ambiguous. For example, it seems that only extremely large PAHs could emit the visible fluorescence beyond 550 nm.14,22,23 However, such PAHs are unlikely to emit detectable fluorescence signal due to their low concentration in flame. The dimer of large PAHs (larger than pyrene) can potentially emit visible signal and survive at flame temperature.9,24 But their concentration is even lower than that of large PAHs. It is well known that the 5-membered ring plays a key role in PAH evolution, and can be formed in PAHs growth and 6-membered ring oxidation.8 The PAHs with 5-membered ring are expected to have relatively high concentration and may contribute to the first solid nuclei in flame.25 In addition, this kind of PAHs is also able to emit visible signal in flame. For example, the maximum fluorescence wavelength of fluoranthene and acenaphthylene is 450 and 546 nm, respectively.26,27 In diagnosis of PAHs with LIF technique, it is important to know the absorption spectra and corresponding fluorescence spectra of the selected PAHs. However, the spectroscopic data of gas-phase PAHs in flame is very scarce,12,14 and most fluorescence spectra of PAHs were measured in liquid phase or solid phase.14,15,28-31 Nevertheless, the database of liquid or solid phase PAHs fluorescence spectra is far from complete. The quantum chemical time-dependent density functional theory (TD-DFT)32 is a promising method to obtain the gas-phase spectra of PAHs, and has been successfully applied to predict the spectra of various organic and dye molecules.33-37 The accuracy of TD-DFT is satisfying since extensive studies showed that the predicted vertical emission energy matches well with corresponding experimental data, and the deviation is within 0.3 eV at 6-31+G(d,p) level.34, 38-41 2


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In this study, our focus is to diagnose the attribution of the visible fluorescence signal of the PAHs with 5-membered ring. The PAHs with fluorescence in UV regions were also investigated to diagnose the LIF spectra of PAHs in flame. Therefore, the electronic emission characteristics of 13 gas-phase PAHs including vertical emission energy of S1-S0 transition, oscillator strength and the molecular orbital (MO) contribution were calculated by using the combination of DFT and TD-DFT methods. Subsequently, the vibrationally resolved electronic fluorescence spectra were investigated based on the Franck-Condon principle. We also interpreted the LIF spectra detected in premixed CH4 and C2H4 flame tentatively. The overlap of fluorescence spectra was further diagnosed by calculating the concentration distribution of PAHs with premixed flat-flame model. 2. Calculation Details The combination of DFT and TD-DFT methods was employed to compute the fluorescence spectra of PAHs, as well as to investigate the nature of electronic transition of PAHs. In this study, the ground state geometry optimization of the PAHs as listed in Fig. 1 was carried out using the DFT B3LYP hybrid functional42,43 with the 6-31+G(d,p)44 basis set.2,45,46 Previous studies have shown that the transition energies are not sensitive to basis sets for this type calculations.34,47 Frequency analyses were performed to verify that the optimized structures were the real local minima. Based on the linear response theory, the emission properties were obtained by optimizing the first excited state with TD-DFT/6-31+G(d,p) method. It is notable that some PAHs have a plane of symmetry in the ground state but the symmetry will be broken in the excited state,34 therefore, the ground state geometry was perturbed slightly to break symmetry at the beginning of the structures optimization of excited state. The frequency analyses were performed again to confirm that the optimized excited state geometry is a minimum on the excited state potential energy surface. The frequencies of gas-phase PAHs were used in the Franck-Condon factors (FCFs) calculations to obtain the vibrationally resolved electronic spectra.48-50 The energy of the 0-0 transition and half-width at half-maximum of the spectral bands are the key input parameters in FCFs calculation. The former is the energy difference between the vibrational ground states of the two electronic states, which are computed at ground state and excited state minimum geometry respectively. The latter was set as 135 cm-1. In the Franck–Condon principle, the probability of transitions is determined by the overlap of the wavefunctions at the initial and final energy levels. The maximum overtone and the maximum number of integrals were set as 13 and 100 respectively. All quantum chemistry calculations were performed by using the Gaussian 0951 program package.

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Phenylacetylene

Naphthalene

1-ethynylnaphthalene

Acenaphthylene

Phenanthrene

Anthracene

Naphthacene

Fluoranthene

Pyrene

Cyclopenta(cd)pyrene

Indeno(1,2,3-cd)pyrene

Coronene

Rubicene Figure 1. The structures of the investigated PAHs The premixed flat-flame is effectively one-dimensional and can be made very steady, facilitating detailed experimental measurements of temperature and species profiles. The fluorescence signal intensity of PAHs detected by LIF technique in premixed flat-flame is proportional to the concentration of PAHs.12 To further diagnose the LIF spectra, the maximum mole fraction of the PAHs in premixed C2H4 flame were computed using the premixed flat-flame model in Chemkin Pro software with the KM2 mechanism,52 which includes 202 species and 1351 reactions. The total flow in the simulation is 8 L/min (C2H4/O2/Ar = 1.455/1.745/4.8), and the pressure is 1 atm. 3. Results and Discussion 3.1 The electronic emission characteristic of PAHs According to the Kasha's rule, the molecule at high electronic states will be rapidly redistributed to the first excited state (S1), and then the excited molecule undergoes S1-S0 (ground state) transition and emits fluorescence. To interpret the LIF spectra of PAHs in flame, the electronic emission characteristics of S1-S0 transition of PAHs were calculated using DFT combined with time-dependent extension TD-DFT methods. In these calculations, vertical emission wavelength (or vertical emission energy), oscillator strength and molecular orbital (MO) contribution were inspected. The investigated PAHs in this study were divided into 4


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two groups according to the structure. One group consists of phenylacetylene, naphthalene, 1-ethynylnaphthalene, anthracene, phenanthrene, pyrene, coronene, and naphthacene, whose structures do not contain 5-membered ring. The other group is the PAHs with 5-membered ring, including acenaphthylene, fluoranthene, indeno(1,2,3-cd)pyrene, cyclopenta(cd)pyrene, and rubicene. 3.1.1 PAHs without 5-membered ring The calculated electronic emission characteristics of the gas-phase PAHs without 5-membered ring were presented in Table 1. The existing experimental data was also presented to test against the calculated results. As can be seen from Table 1, the predicted maximum emission energies are in agreement with the corresponding experimental results, and the maximum deviation is 0.28 eV (anthracene). The deviation may be caused by the solvent effect as most of the experimental data was obtained in the solid or liquid phase53, or by the accuracy of the TD-DFT method.54-57 The rank of the calculated emission wavelengths is in the following order: phenylacetylene < naphthalene < phenanthrene < 1-ethynylnaphthalene < pyrene < coronene < anthracene < naphthacene, and this order is generally in accord with the size (the number of C atom).

Table 1. The main calculated electronic transition properties of the PAHs. The transition wavelengths, oscillator strengths and MO contributions are given. H (L) denotes the HOMO (LUMO). Species

Maximum

Oscillator

S1-S0

Experimental

Solvents used in

emission

strength

Transition

results

experiment

nature

(nm, eV)

H-1→L(48 %)

None

None

328 (3.78)58

Nitrogen58,59

wavelength (nm, eV) Phenylacetylene

266 (4.66)

0.0006

H→L+1(51 %) Naphthalene

324 (3.83)

H→L (98 %)

0.0774

324 (3.83)59 1-ethynylnaphthalene

356 (3.48)

0.1891

H→L (99 %)

None

None

Anthracene

443 (2.80)

0.0605

H→L (99 %)

403 (3.08)53

Vinylic and Olefinic Polymers53

Phenanthrene

336 (3.69)

0.0031

H-1→L (36 %) H→L+1 (63 %) 5


354 (3.50)60

N-hexane60


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Pyrene

367 (3.38)

0.3429

H-1→L+1

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372 (3.33)30

Gas state30

408 (3.04)61

Perfluorohexane61

563 (2.21)62

Anthracene

(5 %) H→L (95 %) Coronene

404 (3.07)

0

H-1→L (49 %) H→L+1 (49 %)

Naphthacene

585 (2.12)

H→L (100 %)

0.0571

Crystal62

For naphthalene, 1-ethynylnaphthalene, anthracene, pyrene, and naphthacene, the H→L transition dominantly (≥ 95 %) contributes to S1-S0 transition, as shown in Table 1. Therefore, the energy gaps (Δ EH-L) between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) were calculated to explain the rank of maximum emission wavelengths.63 Generally, the molecule with a narrower energy gap is more likely to emit the fluorescence with longer wavelength. The frontier orbital energies of all investigated PAHs andΔEH-L are listed in Table S1 in the supporting information. When the number of C atom of PAHs increases, the HOMO energy generally moves up and LUMO energy moves down, resulting in the energy gap narrowing down except that of anthracene, as illustrated in Fig.2. TheΔEH-L is ranked in the following order: naphthalene (4.7328) > 1-ethynylnaphthalene (4.2805) > pyrene (3.7925) > anthracene (3.5382) > naphthacene (1.7087), as shown in Fig. 2. The ΔEH-L rank agrees well with the rank of maximum emission wavelengths.

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Figure 2. Energy gaps, and contour plots for HOMO and LUMO of PAHs.

In addition, the possibility of S1-S0 transition of the investigated PAH molecules was judged by the oscillator strength. As shown in Table 1, the oscillator strength of coronene in S1-S0 transition is zero, implying that this transition is forbidden, which has been confirmed by experiment.61 The S1-S0 transitions of the other PAHs in this study are all allowed as their oscillator strengths are not zero.

3.1.2 PAHs with 5-membered ring The electronic emission characteristics of the PAHs with 5-membered ring, including acenaphthylene, fluoranthene, indeno(1,2,3-cd)pyrene, cyclopenta(cd)pyrene, and rubicene are presented in Table 2. It was found that the maximum emission wavelengths of these PAHs are in the range of 492-703 nm, and there is no forbidden transition. The predicted maximum emission wavelengths are also in agreement with the corresponding experimental results. To our knowledge, the fluorescence information of rubicene has not been reported yet. The deviation between the calculated maximum emission wavelength and the available experimental data of acenaphthylene, fluoranthene, indeno(1,2,3-cd)pyrene and cyclopenta(cd)pyrene is 0.25, 0.24, 0.09 and 0.01 eV, respectively.

Table 2. The main calculated electronic transitions corresponding to the experimental emission properties of 7



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the PAHs. The transition wavelengths, oscillator strengths, and MO contributions are given. Species

maximum

oscillator

S1-S0

Experimental

Solvents used

emission

strengths

Transition nature

results (nm, eV)

in experiment

0.0109

H-2→L (2 %)

542 (2.29)27

Cyclohexane27

wavelength (nm, eV) Acenaphthylene

609 (2.04)

H→L (98 %) Fluoranthene

492 (2.52)

0.0077

H→L (98 %)

450 (2.76)64

Nitrogen64

Indeno(1,2,3-cd)pyrene

556 (2.23)

0.0521

H-1→L (16 %)

535 (2.32)65

Cyclohexane65

H→L (83 %) Cyclopenta(cd)pyrene

704 (1.76)

0.0112

H→L (98 %)

700 (1.77)66

Heptane66

Rubicene

651 (1.90)

0.1398

H-1→L (13 %)

None

None

H→L (86 %) The correlation between the maximum wavelengths and the number of C atom of PAHs was summarized and shown in Fig. 3. It is obvious that the 5-membered ring has great influence on the emission wavelength. For the PAHs with 5-membered ring (the species with blue symbol in Fig. 3), the maximum wavelengths are all located in the visible region and insensitive to the number of C atom. The maximum wavelength of the PAHs without 5-membered ring (the species with black and red symbols) generally increases with the number of C atom. In addition, the maximum wavelength is dependent on the molecular structure. The fluorescence wavelength of linear PAHs (the species with red symbol) is longer than that of non-linear PAHs. For example, the maximum emission wavelength of anthracene is longer than that of its isomer phenanthrene. In this study, the PAHs with or without 5-membered ring are non-alternant or alternant hydrocarbons, respectively. It means that the molecular orbits (MOs) of former ones are asymmetric, and latter ones are symmetrical. This may be responsible for their difference in fluorescence characteristics.

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Figure 3. The calculated maximum emission wavelength of the PAHs studied in this work.

3.2 Vibrationally-resolved electronic fluorescence spectrum In previous vertical emission energy calculations, a sole fluorescence peak was obtained. In practice, continuous spectra will be measured due to the vibration of PAH molecules.67 Therefore, the vibrationally-resolved electronic spectra at 0 K was investigated to obtain the bandwidth of fluorescence spectra, given that the location of fluorescence spectral line almost remains unchanged, regardless of the temperature changes.58,59,64,68 First of all, we checked our calculation by comparing the simulated fluorescence spectrum of pyrene with the experimental one.30 As shown in Fig. 4, the main characteristic peaks measured in experiment are 366.1, 371.8, 378.1, 383.2, 387.7, 392.8, 405.2, 411.7, and 419.7 nm. The corresponding computed peaks are 363.7, 369.6, 375.5, 382.8, 388.8, 395.2, 403.3, 410.3, and 417.2 nm. The maximum deviation of the location of spectral line is within 3 nm. The deviation may result from the experimental measurement error, as well as the inherent frequency calculation error of B3LYP/6-31+G(d,p) method, which may predict the frequencies to be 1.04 times larger than the measured values.69 As shown in Fig. 4, there is also deviation between the computed intensities and the experimental ones. This may be due to the temperature effect49,57 since that the experimental spectrum was measured at room temperature. Besides, the deviation may also be due to the basis set effect.33 Anyway, the effective and useful information extracted from the vibrationally-resolved electronic spectrum calculation is the bandwidth of fluorescence spectrum.

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Figure 4. The simulated and experimental fluorescence spectra30 of pyrene. The vibrationally-resolved electronic fluorescence spectra of the 13 gas-phase PAHs were computed. The fluorescence bandwidths of phenylacetylene (267.1-292.3 nm), naphthalene (312.9-363.9 nm), 1-ethynylnaphthalene (343.7-404.8 nm), anthracene (397.0-482.6 nm), phenanthrene (346.8-384.5 nm), pyrene (363.7-417.2 nm), coronene (407.7-459.5 nm), naphthacene (560.5-611.6 nm), acenaphthylene (500.0-690.8

nm),

fluoranthene

(437.7-540.4

nm),

cyclopenta(cd)pyrene

(579.0-763.3

nm),

indeno(1,2,3-cd)pyrene (498.4-766.1 nm), and rubicene (614.8-743.1 nm) were shown in Fig. 5 (a)-(l), respectively. The fluorescence bandwidths of the PAHs without 5-membered ring (Fig. 5 (a)-(g)) are within 50 nm except anthracene, and are narrower than those of PAHs with 5-membered ring (Fig. 5 (h)-(l)). The fluorescence

bandwidths

of

the

latter ones vary

from

102.7

(fluoranthene) to

(indeno(1,2,3-cd)pyrene).

(a)

(b)

(c)

(d)

(e)

(f)

10


194.3

nm

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(g)

(h)

(i)

(j)

(k)

(l)

Figure 5. The simulated vibrationally-resolved electronic spectra of PAHs. 3.3 Diagnostics of LIF Spectra of PAHs in premixed flame As shown in Fig. 5, fluorescence spectra overlapping is inevitable for most investigated PAHs. For this reason, the contribution of the specific PAHs in the overlap area was further diagnosed by calculating the PAHs concentration. The maximum mole fraction of the PAHs were computed using the premixed flat-flame model in Chemkin Pro software with the KM2 mechanism,52 which includes 202 species and 1351 reactions. The KM2 mechanism is able to predict the concentration of PAHs up to coronene and has been tested against the premixed C2H4 flames.52 In this study, the maximum mole fraction of phenylacetylene, naphthalene, 1-ethynylnaphthalene, acenaphthylene, phenanthrene, pyrene, cyclopenta(cd)pyrene, and coronene were calculated. As illustrated in Fig. 6, the maximum mole fraction of acenaphthylene is the highest one among these PAHs, and reaches 1.5×10-4. The maximum mole fraction of phenlylacetylene closely follows that of acenaphthylene. The maximum mole fractions of naphthalene, 1-ethynylnaphthalene, cyclopenta(cd)pyrene, and coronene are close to each other (around 1.0×10-5) and the fluctuation is within 3 times. The computed values of phenanthrene and pyrene are only 1.9×10-6 and 1.6×10-6, respectively, which are almost smaller than that of acenaphthylene by two order of magnitudes. It should be noticed that fluoranthene, rubicene, indeno(1,2,3-cd)pyrene, anthracene, and naphthacene are not involved in KM2 mechanism, hence, some assumptions have to be made to evaluate their concentration. The concentration of anthracene and naphthacene were assumed to be much lower than that of pyrene based 11



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on the fact that the formation enthalpies of this kind of PAHs are much higher than that of pyrene.70 Therefore, the fluorescence contributions of anthracene and naphthacene in flame are neglected in this study. As shown in Fig.6, the concentrations of phenylacetylene, naphthalene, 1-ethynylnaphthalene, phenanthrene, and pyrene decrease with the number of C atom. Thereby, the concentrations of 5-membered ring PAHs with visible fluorescence are assumed as the following mole-fraction order: acenaphthylene > fluoranthene > cyclopenta(cd)pyrene > indeno(1,2,3-cd)pyrene > rubicene, based on the order of their C atom number. Here the concentrations of acenaphthylene and cyclopenta(cd)pyrene can be calculated with KM2 mechanism.

Figure 6. The maximum mole fraction of main PAHs in the premixed C2H4/O2/Ar (17.4/22.6/60) flame at 1 atm (Φ = 2.3) The evolution mechanism of PAHs in fuel-rich flame is similar, regardless of the kind of fuels.71 Generally, large PAHs form at the cost of consuming small PAHs, thereby the fluorescence of small PAHs occurs prior to that of large PAHs. In other words, the fluorescence of small PAHs can be detected at low height above the burner (HAB), while the fluorescence of large ones occurs at high HAB at the cost of weakening the fluorescence intensity of small PAHs. In light of the fluorescence bandwidth and the concentration ranking of PAHs, the potential fluorescence distribution of PAHs in flame was mapped in Fig. 7. Two LIF spectra measured in C2H4 premixed flame13 and CH4 premixed flame14 respectively were presented in Fig. 7 to illustrate the evolution of PAHs. The former was measured at HAB = 2 mm, and the later at HAB = 13 mm. With the help of fluorescence distribution of PAHs, we can diagnose the specific PAHs and investigate the evolution detail of PAHs in flame according to the corresponding fluorescence wavelength. For example, naphthalene should be responsible for the fluorescence located in 312-340 nm region in flame because the fluorescence in this region cannot be emitted by other PAHs. Pyrene, phenanthrene, and 1-ethynylnaphthalene are all able to emit the fluorescence located in 360-380 nm region, but 1-ethynylnaphthalene is the most possible candidate because its concentration is almost 10 times of pyrene 12


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and phenanthrene. In addition, acenaphthylene, cyclopenta(cd)pyrene, rubicene, and indeno(1,2,3-cd)pyrene were recommended to be the dominant contributors for fluorescence wavelength beyond 500 nm in flame. Two emission bands were detected at HAB = 2 mm in C2H4 flame13: a UV band located between 280 and 380 nm and a second one within the visible range of 400-650 nm. The results suggest that the smallest PAHs (phenylacetylene, naphthalene, 1-ethynylnaphthalene, and phenanthrene) are abundant, and the concentration of larger PAHs is low. Only the visible band was detected at HAB = 13 mm in CH4 flame14, and red-shift of fluorescence peak in visible region was observed. It is likely that the smallest PAHs (phenylacetylene, naphthalene, 1-ethynylnaphthalene and phenanthrene) have been consumed to generate larger PAHs (like coronene and 5-membered PAHs) at HAB = 13 mm.

Figure 7. The potential fluorescence distribution of PAHs in flame. The red dash line is the LIF spectra at HAB = 2 mm from reference13, and the blue dash line is the LIF spectra at HAB = 13 mm from reference14.

Conclusion The LIF spectra of the PAHs in flame were diagnosed by using the combination of DFT and TD-DFT methods. The following conclusions can be remarked. The electronic emission characteristics of the 13 gas-phase PAHs ranging from phenlylacetylene to rubicene were investigated. It was found that the transition of S1-S0 for the investigated PAHs is allowed except coronene, and the maximum emission wavelength is significantly sensitive to the 5-membered ring. The maximum emission wavelengths of the PAHs with 5-membered ring are within visible region and insensitive to the number of C atom, while the maximum emission wavelengths of the PAHs without 5-membered were found to be shorter than 450 nm and sensitive the structure, and generally increase with the number of C atom due to the reduced HOMO-LUMO gaps. The vibrationally-resolved electronic fluorescence spectra were calculated based on Franck-Condon principle. It was found that the fluorescence bandwidths of the PAHs without 5-menbered ring are generally 13



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within 50 nm, and narrower than those of the PAHs with 5-menbered ring. The fluorescence bandwidths of the latter are beyond 102.7 nm. To diagnose the overlap of fluorescence spectra, the mole fraction of PAHs was calculated using the premixed flat-flame model with KM2 mechanism. It was found that the PAHs with 5-membered ring like acenaphthylene and cyclopenta(cd)pyrene have relative high mole fraction. Two assumptions were made to get the mole fraction rank of fluoranthene, rubicene, indeno(1,2,3-cd)pyrene, anthracene, and naphthacene according to the number of C atom rank and formation enthalpy respectively. The potential fluorescence distribution of PAHs in flame was mapped based on the fluorescence bandwidths and the maximum mole fraction of PAHs, with the help of which, the measured fluorescence spectra in C2H4 and CH4 premixed flame are well interpreted. The PAHs with 5-membered ring, including acenaphthylene, cyclopenta(cd)pyrene, rubicene, and indeno(1,2,3-cd)pyrene were suggested to be the contributors for fluorescence wavelength beyond 500 nm in flame.

Acknowledgment This work was supported by National Natural Science Foundation of China (91441129, 51210010, 51176118) and the National Basic Research Program of China (973 Program) (2013CB228502).

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The Diagnostics of Laser-Induced Fluorescence (LIF) Spectra of PAHs

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