R2PI Spectroscopy of Aromatic Molecules ... - ACS Publications

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R2PI Spectroscopy of Aromatic Molecules Produced in an EthyleneRich Flame Yvain Carpentier,†,‡ Thomas Pino,† and Philippe Bréchignac*,† †

Institut des Sciences Moléculaires d’Orsay, CNRS UMR 8214, Université Paris-Sud, Bât. 210, F-91405 Orsay, France Laboratoire de Physique des Lasers, Atomes et Molécules (PhLAM), UMR CNRS 8523, Centre d’Études et de Recherches Lasers et Applications, Université de Lille 1, F-59655 Villeneuve d’Ascq Cedex, France

‡

ABSTRACT: Laser spectroscopy, combined with mass spectrometry, was applied to study the spectra of aromatic molecules produced in a premixed ethylene-rich flat flame. These studies produce new gas-phase electronic spectra of polyaromatic compounds, which ultimately will guide the understanding of the chemical processes that lead to polycyclic aromatic hydrocarbon (PAH) growth or PAH formation locking. Resonant two-photon ionization (R2PI) spectra of all species detectable in a specific fuel-rich flame were recorded simultaneously during a single scan of the laser wavelength, within the 220−330 nm range. Comparison with spectra available in the literature allowed us to identify 16 aromatic species. In the PAH forming region of this flame, we found that the main PAHs are accompanied by a great diversity of other species, including in particular various side-chains on aromatic networks. We also show that this technique allows, at least in some cases, to distinguish between different isomers associated with the same mass peak, although the extracted PAHs are only cooled down to room temperature. nanoparticles produced by a flat premixed flame at low pressure. To identify such large molecules as PAHs in the flame, in situ laser techniques such as cavity ring-down spectroscopy (CRDS) or laser-induced fluorescence (LIF) are not suited because the spectra are not specific enough at high temperature. So, the online extraction of a molecular beam (MB) in which the species are cooled was considered to be a good solution. Different techniques can then be used such as LIF14 or mass spectrometry15 (MS). In the latter case, to firmly identify species, especially isomers, resonance-enhanced multiphoton ionization (REMPI) is more suitable than electron impact ionization, which has a limited energy resolution. However, up to now, spectroscopy of flame molecules using a combined resonance-enhanced multiphoton ionization molecular-beam mass-spectrometry (REMPI-MBMS) has been scarcely used for aromatic compounds.16−18 The main difficulty is caused by the spectral congestion exhibited by large and hot molecules after extraction from the flame, as well as the technical difficulty of ensuring that all experimental parts are working together. Another way to identify the species in the mass spectra is to use their specific ionization potential thresholds.19,20 We report in this article an extended application

1. INTRODUCTION Polycyclic aromatic hydrocarbons (PAHs) constitute a rich class of molecules that has received a growing attention over the past three decades, in particular in both the combustion1 and astrophysics2 communities. PAHs are actually a major product in incomplete combustion processes, with considerable implications for health and environmental issues.3 They are also thought to be involved in the formation of soot.4,5 Physico-chemical routes leading to PAHs and to soot are therefore an active research field. A precise characterization of the PAH content of flames, including knowledge of their structure and their abundance, is needed to refine formation models with laboratory data. In space, the existence of such polycyclic aromatic compounds are invoked to account for the observation of the aromatic infrared bands (AIBs) seen in emission in numerous astrophysical objects.2 Energetic considerations in emission models constrain the size of the molecules within a range of 30−200 carbon atoms. The PAHs were first proposed in 19846,7 as a tentative explanation for numerous astrophysical signatures including the diffuse interstellar bands (DIBs)8,9 and the UV bump at 217.5 nm10−13 in the interstellar extinction curve. For these reasons gas-phase spectroscopy of cold PAHs in the UV−visible wavelength range is needed. These issues led us to the construction of a dedicated experiment called “nanograins” at the Institut des Sciences Moléculaires d’Orsay. With this apparatus we perform spectroscopic gas-phase studies of PAHs and carbonaceous © 2013 American Chemical Society

Special Issue: Oka Festschrift: Celebrating 45 Years of Astrochemistry Received: January 27, 2013 Revised: July 18, 2013 Published: July 18, 2013 10092

dx.doi.org/10.1021/jp400913n | J. Phys. Chem. A 2013, 117, 10092−10104

The Journal of Physical Chemistry A

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Figure 1. Sketch of the experimental setup.

total fresh-gas inlet flow was set to 3 L min−1. The species present in the flame are extracted at 15 mm from the burner by means of a quartz cone with a 500 μm diameter circular aperture, cooled by water circulation at its base (Figure 1). These conditions can be kept constant for hours and are reproducible from one day to the next. Note that the burner itself is mounted on a translation (onaxis) stage that can be externally adjusted while the flame is running. The present distance (15 mm) was chosen so that the sampling was done just before the beginning of the sooting region. The temperature of the species at the sampled position within the flame has been estimated to be roughly 2000 K. This value has been derived from the measurement of the emission of the soot particles in the flame and comparison with a blackbody radiation source. The species are cooled to room temperature by collisions with a carrier rare gas (He) in a dedicated thermalization chamber. A molecular beam is formed after skimming an effusive jet. The extracted species can be deposited then on a substrate, potentially further processed, and analyzed ex situ, as reported in previous papers,21,22,28 or directly characterized online in the gas phase by time-of-flight mass spectrometry (TOF-MS) and REMPI. TOF-Mass Spectrometry and UV Spectroscopy. The mass distribution of the species extracted from the flame was probed by means of a TOF-MS. A home-designed and homemade instrument was used. The species are extracted and accelerated perpendicularly to the molecular beam up to a total kinetic energy of 5 kV. In this configuration the mass resolving power was of the order of 600 at 128 amu (mass of naphthalene). PAHs are known to exhibit a decrease of their ionization energy with increasing size. For small PAHs, it ranges between 7 and 8 eV with a maximum value for naphthalene (8.14 eV). For benzene and its derivatives the ionization energy lies higher in the 8−9 eV energy range with a maximum value for benzene (9.24 eV). Consequently, in a one-color R2PI scheme, access to the ionization energy requires a laser wavelength shorter than 305 and 269 nm for naphthalene and benzene, respectively. Conversely, electronic transitions lying below respectively 4.07

of these techniques to a specific flame for the identification of aromatic molecules. In section 2, experimental details are given. The main results are presented in section 3 and discussed in the context of PAHs spectroscopy in section 4.

2. EXPERIMENTAL SECTION The experimental setup, for the spectroscopic study of carbonbearing molecules to carbonaceous nanoparticles, is equipped with a time-of-flight mass spectrometer and surrounded by various tunable laser sources allowing us to perform online laser spectroscopy of the flame products. The flame byproducts can also be analyzed off-line.21,22 The Flame. The source of carbonaceous species is a premixed flat flame burning at low pressure and stabilized on a McKenna burner. This kind of reactor is commonly used in the combustion community as a model reactor to study chemistry and kinetic processes in flames. Because it offers a onedimensional flow, with a high degree of reproducibility, these flat laminar flames can be more easily compared with models. In such a premixed flame, the reaction time of the combustion processes scales with the distance to the burner. From the surface of the burner where a mixture of fuels and oxygen is injected, a rich and complex chemistry develops, leading to the formation of more complex molecular structures ranging from small carbonaceous molecules or radicals to soot particles as a function of the distance to the burner. At low pressure (a few tenths of millibar) the flame is spatially extended, which renders an accurate sampling easier and thus facilitates the analysis by spectroscopic methods. In particular, this burner is well suited for the formation of a molecular beam using a sampling cone and further analysis by gas-phase spectrometry. Ethylene flames are able to produce high amounts of large PAHs23−26 but in this article we focus our attention on small PAHs, up to the mass of tetraphene (C18H12, 228 amu). The burning conditions have been tuned to improve the PAH signals during a previous study.27 For all measurements whose results are presented here, the flame conditions have been kept constant: the fuel precursor, i.e., ethylene (C2H4), is mixed with oxygen and controlled to maintain the C/O ratio fixed at 1.1, which corresponds to an equivalent ratio of 3.3. The pressure was kept at 50 mbar using a controlled butterfly valve, and the 10093



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results in fluctuations of the mass peak intensities that are difficult to correct by any simple method, such as normalization by the laser power. To overcome this difficulty, we have developed and applied a point-to-point correction method to the raw data for the scans with λ ≤ 287 nm. The variation of 15 prominent mass peaks was considered. From one wavelength point to the next, their intensity ratio was calculated. The mass gates for which the deviation from the mean value was maximum were sequentially rejected until five were left. The associated species were supposed not to undergo a strong transition at this wavelength and they were used to scale the new point. The different scans were scaled by a multiplication factor so that the overlapping points were superimposed.

eV/4.62 eV are out of reach. To overcome this limitation, a resonant two-color two-photon ionization (R2C2PI) scheme has been adopted for scanning over the 300−330 nm wavelength range, by using the frequency-quadrupled output of a Nd:YAG laser at 266 nm. To identify species by MB-MS combined with REMPI in a flame, two strategies can be followed. If one searches for a specific molecule, one can scan an adequate small region of interest (usually associated with the S1 ← S0 transition) at high resolution. On the contrary, if one wants to detect all the possible isomers, one has to scan a wide wavelength region to monitor several electronic transitions for a given species/ isomer. We focused our effort on the second option. For this reason and from measurement time considerations, the spectra were recorded with a wavelength step of 0.25 or 0.5 nm depending on the wavelength range, whereas the laser resolution was smaller than 0.01 nm. This choice of the scanning step value does not affect the practical resolution because PAH absorption features in the wavelength range associated with Sn ← S0 transitions (with n ≥ 2) generally exhibit broad bands due to the relatively short lifetimes of the involved excited states.29,30 Different series of scans were performed in the 220−330 nm wavelength range. Most of the scans involved intervals wider than 20 nm, and up to 45 nm. Only a selection of spectra is presented in this article. Three different pulsed (10 Hz) laser systems were separately used to access the whole wavelength region: an OPO system (Spectra Physics MOPO-730) pumped by the third harmonics at 355 nm of a Nd:YAG laser (Spectra Physics Quanta-Ray Pro 250) and its associated frequencydoubling stage (Spectra Physics FDO-900), a dye laser (Lambda Physik LPD-3000) pumped by an excimer laser at 308 nm (Coherent COMPexPro 201), and another dye laser (Quantel TDL-50) pumped by another Spectra Physics YAG laser. Both dye laser beams were frequency-doubled using BBO crystals housed in the dedicated stage of the TDL 50. In the case of R2C2PI, the ionization laser beam was the fourth harmonics of a Nd:YAG laser (Continuum Surelite I) at 266 nm. In all cases the laser energy was kept below 500 μJ/pulse at the entrance of the ionization zone of the TOF-MS to avoid fragmentation (revealed by the presence of mass peaks with m < 78 amu). Signal Acquisition and Processing. The signal from the microchannel plates (MCP) was directly digitalized by a dedicated card (Acquiris DC252 10 bits) without using any preamplifier, and the mass spectra were monitored and processed by a LabVIEW interface. In brief, after time-tomass calibration of the TOF, a series of time gates are set at the onset and fall-down times of each ion pulse, so that the total signal corresponding to an ion of a given mass is obtained by integration of the signal through the corresponding time gate. This results in a series of mass gates, in which the ion signals are stored at every wavelength step. The UV spectra of all mass gates were thus generated all at once. The REMPI spectrum of a mass channel corresponds to the variation of the content of its mass gate versus the laser wavelength. For the scans with λ ≥ 286 nm, the integration was done directly by the LabVIEW software, whereas for the scans with λ ≤ 287 nm, the mass spectra averaged over 1000 laser shots were saved individually and the R2PI spectra were reconstructed afterward. Over such wide scanning intervals, the profile of the laser beams was not uniform, which induced changes of the photon flux density in the ionization zone. This

3. RESULTS Mass Spectrometry. Mass spectra reveal that a rich chemistry develops in the flame. Depending on the excitation wavelength, the dominating peaks differ, as seen in Figure 2.

Figure 2. Mass spectra resulting from R2PI of the neutral species produced in the flame (the burning conditions are defined in the text) for selected laser wavelengths. See text for explanation of the “break” between 17 and 18 carbon atoms.

Therefore, none of the REMPI mass spectra represent the true distribution of species in the flame. Nevertheless, the global mass spectrum is poorly wavelength specific and a lot of mass peaks appear in several data sets at different wavelengths, which reflects partly the fact that the species are not very cold in the molecular beam, and partly the fact that the vibronic transitions in this wavelength region have some degree of spectral diffuseness. The first two spectra in Figure 2 display only a few mass peaks below 78 amu (benzene) and the mass peak of the fuel precursor can be recognized at 28 amu. The laser intensity was kept low enough to avoid the appearance of peaks in this range corresponding to fragments. Above this mass, the spectrum is organized in groups of peaks associated with defined numbers of carbon atoms. In each group, one or two species dominate and are accompanied by minor peaks that correspond to species with a different number of hydrogen 10094



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Figure 3. R2PI spectra of the species appearing at the mass-gates 78, 92, 94, 102, 104, 106, 108, and 126 amu in the mass spectra of the species extracted from our fuel-rich flame. The red points correspond to the data at all the wavelength steps, they are linked together by straight lines for readability. These spectra could be associated with benzene and its derivatives (see text for the assignments). The skeletal formula of the identified molecules are indicated.

atoms or with a different 13C content. The flame conditions have been tuned to increase the signals of mass peaks at 78, 128, and 178 amu, where contributions from purely aromatic molecules are expected, in the central part of the scanned spectral range. A special voltage sequence was used for recording the high mass part of this spectrum: beyond the time-of-flight corresponding to the mass of 210 amu, the voltage applied to the microchannel plate detector (MCP) was pulsed at a higher value to increase the sensitivity for the cations of larger masses. This emphasizes that the amount of PAHs of mass beyond 202 amu (pyrene) drops drastically, as was already demonstrated by Keller et al.31 For this reason, only spectra of mass peak 228 amu could be recorded, and spectroscopy of species having

masses larger than 228 amu could not be achieved under the present burning and extraction conditions. Further dedicated experiments will be needed to record their spectra. REMPI Spectroscopy. The goal of this work being primarily the identification of individual PAHs produced in the flame, one should ideally require a benchmark of reference gas-phase absorption spectra (at room temperature) to which direct comparison could be made. Unfortunately, such reference spectra do not exist, except for the low-mass species having a large enough vapor pressure, which gives a practical limit at the mass of naphthalene. A few heavier PAHs or derivatives have been studied in free jet expansions or molecular beams, often using laser induced fluorescence, but the low values of the reached temperature in such cases may 10095



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thermalization chamber and the expansion jet to cool the species extracted from the flame to about room temperature. The spectra at mass-gates 92, 94, and 102 amu are similar (although red-shifted) to the benzene spectrum and could be unambiguously assigned to benzene derivatives. The mass channels 92 and 94 can be assigned to toluene (methylbenzene, C6H5CH3) and phenol (C6H5OH), respectively, by comparison with vapor-phase spectra.38 The vibronic features in the signal at 102 amu correspond to the spectrum of phenylacetylene (ethynylbenzene, C6H5C2H).43,44 For this species, we could also measure the strongest band of the S2 ← S0 transition at 238.5 nm. The spectrum at mass 104 amu presents broad bands with narrow features superimposed on them. It could be associated with the spectrum of styrene (ethenylbenzene, C6H5CHCH2). The signal at λ ≤ 265 nm drastically decreases and the intensity of the band at 252 nm, belonging to the second electronic transition is weaker than in the absorption spectrum, when compared with the band intensities in the first transition, as expected when the fluorescence quantum yield decreases. However the secure assignment to styrene is based on the long wavelength part of the spectrum. For mass 106 amu (C8H10) several isomers must be considered to explain the spectrum. Probable contributors are the ethylbenzene molecule (C6H5CH2CH3) and the three isomers of xylene (i.e., o-, m- and p-dimethylbenzene, C6H4(CH3)2). The presence of p-xylene and m-xylene can be confirmed from the observation of major bands at 272.5 and 270.75 nm, respectively. Moreover, ethylbenzene must be a major contributor to the band at 266.75 nm besides the two above-mentioned isomers, which present features in the same region. The definite presence of ethylbenzene is confirmed by the observation of the prominent band at 260 nm. The presence of o-xylene is more difficult to infer because the vaporphase spectrum of this molecule is not very specific. The relative abundances of these isomers will be discussed in the next section. The spectrum at mass 108 is associated with the cresol (methylphenol, C6H4CH3OH) isomers. A vibronic progression characteristic of the p-cresol molecule38 can be recognized with bands at 283.25, 279.75, 276.75, 273.75, 270.75, and 267.75 nm. However, the observation of an underlying plateau and of some additional bands suggests the presence of the two other isomers, i.e., o- and m-cresol. The R2PI spectra of the three molecules have been recorded by Tembreull.45 It should be noted that the signal at this mass channel is the lowest intensity for which an identification of the absorbing molecule was possible. The spectrum at the 126 amu gate (C10H6) does not present the same pattern as the first electronic transition of benzenerelated species. Naphthyne, diethynylbenzene, and phenyldiacetylene could contribute to this spectrum. However, the spectra of the two naphthyne isomers46 do not show any absorption feature at the position seen in our spectrum (261 nm). In the hypothesis of diethynylbenzene, the spectrum can be complicated by the presence of three position isomers as in the case of xylene at 106 amu. Only the S1 ← S0 transitions of these isomers have been measured in a jet using R2C2PI.47 The scan in the corresponding region cannot reveal any vibronic feature because the ionization potentials of these molecules lie too high in energy for R2PI measurements.48 The results of the calculations of the energies of the S2 ← S0 transition for the three position isomers reported in the same article do not match our data. However, these calculations do not even

considerably change the spectral pattern. For the molecules bearing more than about ten carbon atoms, three kinds of condensed-phase data are available: spectral atlas of species in solution (absorption data),32 similar data for species isolated in glassy or Shpolskii33 matrices, and articles reporting absorption or fluorescence excitation spectra of molecules isolated in low temperature rare gas matrices.34−36 Unfortunately, all the latter techniques share a common drawback for gas-phase identification: all the spectra exhibit a transition-specific and molecule-specific red shift, which depends on the nature of the solvent or of the solid medium. By its principle, the REMPI technique takes advantage of (1) the resonant absorption of a first photon which carries the spectral signature when its wavelength is scanned and (2) the sensitivity of charged particle detection and the ability of their mass discrimination when a second photon ionizes the molecules in the previously excited state. Consequently, it provides an essential help to the characterization in limiting the complex superimposition of too many individual spectra. It should be noted, however, that a REMPI spectrum, depending on the excited state lifetime and photoionization efficiency, is not equivalent to an absorption spectrum of the same species. This point will be further discussed below, when necessary. Tuning the wavelength of the laser and recording the intensities of the peaks in the mass spectra, referred to as mass gates below, allowed us to obtain the REMPI spectra of all species seeded in the molecular beam at the same time. The species identified in the flame by this method cover a wide range of sizes, from a single aromatic ring (benzene) up to four aromatic rings (in different steric arrangements). The R2PI spectra of the mass gates at 78, 92, 94, 102, 104, 106, 108, and 126 amu are displayed in Figure 3. To achieve the identification of these small species, the spectra were mainly compared with vapor absorption spectra of monocyclic aromatic hydrocarbons at room temperature.37,38 The spectrum of mass 78 could be assigned to the first electronic transition (S1 ← S0) of benzene. The true origin band of this transition at 262.56 nm (38 086 cm−1)39,40 is symmetry forbidden and cannot be seen in the spectrum. The vibrational progression displayed by the three major bands at 259, 253, and 247.25 nm is ascribed to a combination of the modes ν6 and ν1.40 A secondary vibrational progression implying the combination of the mode ν16 with ν6 is also distinguishable. The absence of vibronic bands observed at lower wavelengths (λ < 245 nm) in direct absorption spectra40 is the result of a faster nonradiative process occurring in the intermediate S1 state above 3000 cm−1 of excess vibrational energy, such as the electronic relaxation toward a triplet state or to a conical intersection connecting to the ground state, leading in any case to the falloff of the fluorescence quantum yield.41,42 Similar phenomena may also affect some of the present spectra appearing in different mass gates, such as the one at mass 104. The low resolution of these spectra does not allow an accurate integrated intensity of individual bands to be derived, which thus prevented us from determining the exact temperature of the species in the molecular beam. However, the similarity of the band intensity ratios with those of reference absorption spectra suggests that the molecules are close to room temperature. As an example, the spectrum at mass 104 amu can be compared to the absorption spectrum of styrene reported by Etzkorn et al.38 This emphasizes the ability of the 10096



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reproduce the measured positions of the S1 ← S0 transition. Finally, a spectrum of the photoproducts of benzene and diacetylene has been obtained by Robinson et al.49 and assigned to phenyldiacetylene by comparison with a vapor-phase spectrum at room temperature. This spectrum actually presents a strong band around 261 nm, but it is accompanied by two other bands at 247 and 275 nm, which are not seen in the present data. Therefore, a contribution from the phenyldiacetylene molecule can be ruled out. Thus the diethynylbenzene isomers remain the most probable choice for assignment in agreement with the identification of 1,4-diethynylbenzene in flames reported by Yang et al.19 by using simultaneously mass and ionization potential diagnostics. To confirm the presence of styrene at mass 104 amu and to identify the species at the origin of the spectrum at mass 116 amu, spectra were recorded with a finer scan step in the 286− 295 nm wavelength range. The spectra of mass-gates 104 and 116 amu are displayed in Figure 4. The wavelength calibration

values are characteristic of PAHs with respectively 2, 3, and 4 aromatic rings. The R2PI spectral features of these mass gates are plotted in Figure 5. The upper-left panel confirms the presence of naphthalene at the mass-gate 128 amu. The vibronic structure of the S2 ← S0 transition displays three prominent features at 268.5, 275, and 279 nm. It can be directly compared with the absorption and fluorescence excitation spectra reported by Suto et al.53 Note that these positions are also in agreement with Ne-matrix spectra of naphthalene within 0.4 nm.34 The smallness of this spectral shift, a consequence of the low polarizability of the neon atom, justifies making use of Ne-matrix data for comparison. The origin of the S1 ← S0 transition at 310 nm could not be measured due the weakness of this transition. R2PI scans down to 220 nm allowed the determination of the position of the origin of the third band system at 221.5 nm, appearing in the work by Suto et al.,53 and also in agreement with the value in Ne-matrix (assigned to S3 ← S0 by Salama and Allamandola34). The signal in the mass-gate 178 is shown in the upper right panel of Figure 5. It exhibits a complex spectrum with two major bands at 233.5 and 241.5 nm and additional ones at 251, 273, and 283.75 nm. These features can be correlated with those appearing in the Ne-matrix spectrum of the phenanthrene molecule.35 They have been assigned to vibronic absorption bands of the transitions from the electronic ground state to the excited states S2 to S5 by Salama and Allamandola.35 It should be noted that this three-ring PAH has a well-known isomer of the acenes series: anthracene. In the 220−287 nm wavelength range, the anthracene isomer presents a single very strong and broad absorption system around 42 300 cm−1 (≈236 nm).54,55 It should then appear as superimposed over the complex and only partially resolved structure due to phenanthrene, presenting a dip at this wavelength. Consequently, although it is difficult to definitely exclude the presence of anthracene, it can be deduced that its abundance in the flame is minor relative to phenanthrene, because the photoabsorption cross section of anthracene is at least 1 order of magnitude larger than that of phenanthrene at this wavelength.33 Measurements at longer wavelengths in the region of their S1 ← S0 transitions, whose origins in free jet spectra lie at 341 nm for phenanthrene56,57 and 361 nm for anthracene,55,58 should allow us to discriminate the two isomers and to give a quantitative value of their relative abundance. The R2PI spectrum of the mass-gate 202 (lower left panel in Figure 5) displays three major broad bands at 264.25, 253, and 232 nm. The bands at 264.25 and 232 nm correspond respectively to the origins of S3 ← S0 and S4 ← S0 transitions of pyrene, as confirmed by comparison with its fluorescence excitation spectrum59 and its absorption spectrum in Ar36 and Ne60 matrices. The 253 nm band involves vibration in the S3 state. This assignment to pyrene is further confirmed by the observation of two additional bands in other R2C2PI spectra recorded in the flame at longer wavelengths, occurring at 322.5 and 318.5 nm. These two vibronic bands belong to the second electronic transition of pyrene.61 The pyrene molecule also has a well-known isomer: the fluoranthene. Although this species has a prominent S4 ← S0 transition around 280 nm,62 it could not be detected. For the spectrum recorded in mass-gate 228 (C18H12), shown in the lower right panel of Figure 5, the presence of five isomers can be explored. The related structures are fourmembered rings PAHs, i.e., chrysene, tetracene, tetrahelicene, tetraphene, and triphenylene. Coherent data sets of spectra for

Figure 4. Identification of styrene and indene in the flame thanks to their R2PI spectra. The arrows indicate the positions of the origin bands for the S1 ← S0 transitions.

was done by introducing some aniline vapor in the thermalization chamber and using its band positions given by Brand et al.50 as reference. In this wavelength region, we identify the first electronic transition of styrene and indene respectively by comparison with vapor-phase spectra.51,52 The position of the origin for the S1 ← S0 transition are indicated by an arrow in the figure. In both spectra, the features appearing to the red of the origin band arise from the population of vibrational states in the ground electronic state (hot bands). The identification of the largest species in the mass spectra remains difficult due to the paucity of gas-phase data in the considered wavelength range. For this reason, the spectra have been mainly compared to solution or matrix spectra. This makes secure assignments more complex because of the matrixinduced spectral shift and broadening due to the interaction with the host molecules of the solution or matrix. Beyond the mass values of the previously discussed benzene derivatives, the mass spectra are dominated by mass peaks at 128, 178, and 202 amu when λ ≃ 260−270 nm. These mass 10097



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Figure 5. R2PI spectra of small PAHs identified in the C2H4 flame.

these species can be found in cyclohexane solution63,64 and in Shpolskii matrices.33 A comparison of these spectra with the latter spectrum requires some caution because of the solutionor matrix-induced red shifts. For that reason a preliminary comparison of gas-phase and Shpolskii spectra has been done for four well-measured PAHs, namely, naphthalene, phenanthrene, benzo[def ]fluorene, and pyrene. From the positions of the twelve main bands within the relevant spectral window and for both conditions of the samples, it appears that the Shpolskii red shift of a band ranges from 10 to 12 nm. This was used to specify a “confidence” wavelength interval for the gas-phase associated with every band of the five candidate molecules, as shown in Table 1. As a result, the REMPI bands at 270 and 282.5 nm can be assigned to tetraphene (2,3-benzophenanthrene), and the band at 276 nm to tetrahelicene (3,4benzophenanthrene), whereas chrysene, tetracene, and triphenylene do not find any counterpart in the REMPI spectrum. This, however, leaves an unassigned band near 235 nm in the REMPI spectrum at mass 228. Indeed, vinylpyrene (C16H9− C2H3) offers also a set of position isomers at mass 228 amu. To our knowledge no UV spectrum of these species is available in the literature. Nonetheless, similarly to what appears for 1ethynylpyrene, the spectrum of which was reported by Rouillé

Table 1. Wavelengths (nm) of the Absorption Bands in Shpolskii Matrices33 Compared to Those Observed in the REMPI Spectrum at Mass 228 amua species

λShpolskii

Δλgas

“gas phase”

triphenylene

260 251 243 270.5 260.5 243 278 284.5 274 291 280 269.5

248−250 239−241 232−234 259−261 249−251 232−234 266−268 274−276 263−265 280−282 269−271 258−260

253.6

chrysene

tetracene tetrahelicene tetraphene

obs bands

235 ? 260.5

267.2 274.3 280.5

276 282.5 270

The most intense bands of each species are in boldface. Δλgas is the confidence interval for the gas-phase values derived from matrix data (see text). The “gas-phase” values in the fourth column correspond to the positions of the Clar’s32 β bands derived from measurements in solution, after correction for solvent shift (see Schmidt66); their energies are listed in eV in ref 66. a

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Table 2. List of Aromatic Molecules Identified in the C2H4 Flamea

a

mass, amu

formula

species

78 92 94 102 104 106 106 106 106 108 116 126 128 140 142 152 154 178 190 192 202 216 228 228 228

C6H6 C6H5−CH3 C6H5−OH C6H5−C2H C6H5−C2H3 C6H5−C2H5 C6H4−(CH3)2 C6H4−(CH3)2 C6H4−(CH3)2 OH−C6H4−CH3 C9H8 C6H4−(C2H)2 C10H8 C11H8 C10H7−CH3 C10H7−C2H C10H7−C2H3 C14H10 C15H10 C14H9−CH3 C16H10 C16H9−CH3 C18H12 C18H12 C16H9−C2H3

benzene toluene phenol phenylacetylene styrene ethylbenzene o-xylene m-xylene p-xylene o-, m-, p-cresol indene p-diethynylbenzene naphthalene ethynylindene 2-methylnaphthalene 2-ethynylnaphthalene vinylnaphthalene phenanthrene benzo[def ]fluorene methylphenanthrene pyrene methylpyrene tetraphene tetrahelicene vinylpyrene ?

main bands, nm 259 267 275.25 275 287.7

253 263.75 268.25 268

269 270.8 272.8 283.25 288 279

279.75

275 274

276.5 283.75 278.75 322.5 325.2 282.5

273 268.75 269 264.25 321.1 270 276

247.25 260.5 261.5 261.5

238.5

266

263

267.2 267 277

263.8 264 273.75

261 268.5 225.5 234 234 241.5 244.5 244.5 253

220.5 224 220.5 233.5

232

235

Names in bold are firm identifications, and names in italic are proposed structures.

et al.65 in the Ne matrix, a red shift by a few nanometers relative to the pyrene β′ band at 232 nm sounds reasonable, and a contribution from one position isomer of vinylpyrene is suggested. The spectra of mass-gates 142, 152, 190, and 192 amu are plotted in the middle panels of Figure 5. The spectrum from mass-gate 152 amu shows two prominent bands at 224 and 234 nm accompanied by a low-intensity band system in the 260− 280 wavelength region. For this mass value, the possible presence of four isomers, i.e., biphenylene, acenaphthylene, and 1- and 2-ethynylnaphthalene, has been examined. The spectrum of biphenylene in an n-hexane Shpolskii matrix is dominated by a band at 252 nm.33 Considering the above value of possible matrix shift, no corresponding band is seen in the flame spectrum, which allows us to rule out this species. For the three other isomers, the spectrum has been compared with solution spectra in an acetonitrile/water mixture.67,68 Taking into account a solution red shift value ranging from 8 to 10 nm,32,63 only 2-ethynylnaphthalene can explain the presence of the band at 234 nm and the group of bands at longer wavelengths. A contribution of 1-ethynylnaphthalene and acenaphthylene at shorter wavelengths cannot be excluded. The spectra recorded in mass-gates 142, 190, and 192 amu can be tentatively assigned to naphthalene- and phenanthrenerelated species considering the similarities of the general spectral pattern with the spectra of the parent molecules. We then propose 2-methylnaphthalene (the 1-position isomer can be excluded thanks to the gas-phase results by Suto et al.53), benzo[def ]fluorene, and isomers of methylphenanthrene as main carriers for the spectra in mass-gates 142, 190, and 192 amu, respectively. Concerning this last mass value, the spectra of the four position-isomers in Shpolskii matrices are available.33 The wavelength of the most intense band changes

from 253 nm (carbon 4) to 258 nm (carbon 1). Consistently with the above-mentioned range of spectral shifts from 10 to 12 nm, the substitution on carbon atoms 2 and 3 sounds more plausible, and if we consider the overall pattern of the spectrum, the 2-methylphenanthrene would fit better. Nonetheless, coexistence of several isomers cannot be excluded. Note finally that the assignment of benzo[def ]fluorene (4,5-methylenephenanthrene) at mass 190 amu is very securely asserted because every detail present in the REMPI spectrum of the flame is consistent in position and relative magnitude with its counterpart in the n-hexane Shpolskii matrix spectrum33 when shifted by 10 or 11 nm.

4. DISCUSSION Distribution of the Identified Species. The results of the above-discussed identifications are summarized in Table 2. This table is organized in lines, each of which corresponds to a massgate channel value, listed in increasing values of the mass (first column). The 16 species (including 2 isomers of xylene) that have been securely identified as present in the flame are listed in bold characters in the third column. In addition, 10 proposed assignments, considered as very plausible, are listed in italics, which will be discussed below. Only one species is doubtful (question mark in the last line). The last few columns of Table 2 give the wavelength values of the main spectral bands that were used for identification. This study has revealed the presence at the sampled position in the flame, besides the purely aromatic hydrocarbons, of a large diversity of species containing side groups. The identified functional groups are the methyl (CH3), ethynyl (C2H), ethenyl (C2H3), ethyl (C2H5), and hydroxyl (OH) groups. Aromatic species containing two side groups such as xylene, 10099



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molecules, as a result of symmetry breaking.32 Assuming this trend to be the same for larger PAHs, and due to the low concentration of oxygenated species in the considered mass range, it seems that the role of oxygen is mainly in the thermal input through the combustion reaction heat rather than playing a significant role in the chemical network.79 A more quantitative evaluation of the relative abundances should require the knowledge of the absorption cross sections. It has been done in the case of the benzene derivatives for which the UV absorption cross sections are known in the vapor phase at room temperature.38 A question arises at this point about the behavior of the ionization cross sections. It can be noted that, in the worst case (largest IP values) of toluene, phenylacetylene, and ethylbenzene, the excess energy above the IP threshold ranges from 0.2 to 1.5 eV when the wavelength of the first laser is scanned from 290 to 220 nm. This range rises to 0.5−1.8 eV in the case phenol and xylenes, and to 0.9 −2.2 eV for indene and naphthalene. It is even higher for the other compounds. We thus expect a relatively slow and smooth rise of these cross sections at shorter wavelengths. Making the hypothesis that the species have similar excited state lifetimes and ionization cross sections, the relative abundance of two species a and b is given by the ratio Na/Nb = Iaσb/Ibσa, where I, N, and σ represent the ion signal intensity of a given species (i.e., a or b), its number density, and its UV absorption cross section. In the case of a single mass gate, which may contain signals from different isomers, it is valuable to determine their relative abundances. Figure 6 shows an example in the case of mass-gate 106 amu where the procedure can be applied. The assignment of the spectrum to the ethylbenzene molecule and the xylene isomers has already been discussed. A root-mean-square fitting routine has been applied to these data using the calibrated reference spectra of Etzkorn available in the MPI-Mainz-UV−vis Spectral Atlas of Gaseous Molecules.80 The reference spectra are plotted in the upper part of the figure. The overall shape of the four absorption spectra look similar, except if one considers the most structured parts showing up on the long wavelength side, which appear as more isomer-specific. For this reason, the routine has been applied to the signal spectral derivatives and not directly to its intensity. Its output provides a good reproduction of the spectral features and their relative intensities over the complete wavelength range, as it can be seen in the bottom panel of Figure 6. This choice of the spectral derivatives is, in principle, not very sensitive to smooth spectral signals like the one of oxylene, whose abundance is predicted to be between 0 and 10%. We consider then the identification of this isomer to not be securely established (Table 2). The limits of relative abundance of the other isomers present in this flame conditions have been deduced: from 5 to 10%, 20 to 30%, and 50 to 70% for p- and m-xylene and ethylbenzene, respectively. Figure 6 also shows that the simulation, based upon reference absorption spectra, largely overestimates the REMPI signal at short wavelengths. This fact reflects the reduction of the excited state lifetimes when the vibronic energy increases, resulting from the increase of the relaxation rates by intramolecular radiationless transitions. The overall result, in terms of the ratio of the measured R2PI spectrum at mass 106 amu to the linear combination spectrum (bottom panel), is in excellent agreement with the experimental fluorescence quantum yields of xylene isomers reported by Suto et al.53 Note finally that a direct fit using the fluorescence excitation

cresol, and diethynylbenzene were also shown to be present (even if we must remain cautious for the last one). These identifications of substituted species were easily done for the benzene derivatives. But for polyaromatic structures, the assignments were more difficult. It can be noted that the lack of good gas-phase reference spectra is a limitation. However, the present data are also limited by an intrinsic lack of selectivity: in the considered wavelength range, the observed bands for polyaromatic molecules imply excited electronic transitions (Sn ← S0) with n ≥ 2. In most cases these bands are actually broader as a result of short lifetimes, limited by nonradiative transitions.29,30 Spectral scans above 300 nm concentrating on the first electronic transitions should allow us to overcome the difficulty, even if the oscillator strengths are usually smaller, because the S1 states are characterized by longer lifetimes. An effort to further decrease the temperature of the species will also be profitable in this respect to reduce the spectral congestion and access the true intrinsic profiles of these highly excited electronic states.30,69 Fuel rich ethylene flames have been widely studied mainly in the context of PAH and soot nucleation,70 but the present burning conditions were not studied yet to our knowledge. Though many investigations have focused on off-line analysis of condensable species,26,71,72 only a few performed online analysis.25,73−75 In the former case, mainly the nonvolatile PAHs could be retrieved, i.e., those containing more than 16 carbon atoms, whereas the latter approach allowed detection of the smaller aromatic molecules. Among the techniques, gaschromatographic mass-spectrometry (GCMS) was used and provided a clear distribution of species.71,73 MS provided most of the available data, among which many were obtained via single photon ionization using synchrotron radiation.19,76,77 Nearly all species detected in the present work had already been observed in ethylene and acetylene flames or even incineration reactors where xylene isomers and phenanthrene (as a major component relative to anthracene) were detected.78 The main differences in terms of identifications in the present results, as compared with previous studies, are about the details obtained by R2PI for masses that could not be identified in GCMS,71,73 in particular concerning vinyl, ethyl substitution, and the analysis of the mass 228 amu signal. However, although REMPI may provide much detailed information about the flame as apparent here from the rich substitution pattern, it requires a complete knowledge of the electronic spectra over a broad range of UV wavelength, our goal as a matter of fact. Therefore, a detailed analysis of the longitudinal profile of the different species is out of the scope of the present work. Relative Abundances. The spectra plotted in Figures 3 and 5 correspond to the same data set. The numbers appearing on the vertical scales (intensity) allow an approximate estimation of the relative abundances of the species. From the results presented in the third column of Table 2, it appears that the methyl (CH3) and ethynyl (C2H) aliphatic entities are the major side groups for benzene. On the contrary, oxygen bearing aromatic molecules are scarce, and the oxygen atoms are only found within the hydroxyl side groups (phenol, cresol) rather than involved in the carbon skeleton. This is compatible with the fuel-rich dioxygen-poor conditions of the flame that we used. The concentration of the aromatic molecules containing an OH side group must be very low in the flame because the absorption cross sections of the corresponding S1 ← S0 transitions in these species are usually about 1 order of magnitude larger than that of pure aromatic hydrocarbon 10100



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techniques are ideally suited to cooled samples in a molecular beam, as our work here shows. Yang et al.19 and Zang et al.20 used the evolution of the photoionization threshold as a function of the chemical structure and could identify the isomers of most species present in their flame using tunable synchrotron radiation, by measurements of the photoionization efficiency (PIE) spectra. The difficulty in their case lies in distinguishing isomers that have similar and/or poorly determined ionization potentials. Moreover, it may happen that the ionization curve of the species with the lowest ionization potential hides the presence of other isomers. Gittins et al.18 and Desgroux et al.14,76 used a sampling method coupled to a supersonic expansion jet to perform realtime ex situ concentration measurements using respectively REMPI-TOFMS or LIF techniques. They were able to characterize the distribution of some PAHs in methane flames, up to the mass of pyrene, although they could detect a few larger sizes. The most efficient technique to detect and quantify the concentrations of a large number of species seems to be the molecular beam extracted from a laminar flame and PIE measurements at a synchrotron radiation facility. Yang et al.19 reported an impressive list of small radicals and PAHs containing up to 16 carbon atoms, obtained in this way from a benzene flame. Most of the PAHs and derivatives identified in the present work and listed in Table 2 are also present in the list by Yang et al.,19 with the exception of vinylnaphthalene, benzo[def ]fluorene, and the heaviest at masses 216 and 228 amu. Both flames are laminar and premixed but use a different fuel gas and equivalent ratio. The PAHs substituted by side groups are surprisingly the same. This finding points toward a similar chemical network, which could be of interest for the combusion science. The ethynyl-group-bearing molecules can reasonably be considered as a key step in the growth of the aromatic compounds, as it is generally accepted for ethylene flames. Consequently there is a clear need of spectral references to probe their presence. During this work they have been found to have a very high concentration, comparable to the concentration of pure polyaromatic hydrocarbons. The detection of ethynyl-substituted aromatic species (phenylacetylene, diethynylbenzene, ethynylindene, and ethynylnaphthalene) in abundance in the flame supports the idea of a growth via a hydrogen abstraction acetylene addition (HACA) mechanism.81,82 Another very significant result is the discrimination observed in this work between the two three-ring PAHs: phenanthrene was found much more abundant than anthracene, and this information certainly has consequences for the involved chemical routes. This may explain why the build-up of the compact four-ring system pyrene is efficient, thanks to the “inner” hydrogen reaction sites of the “bay” region. According to Clar’s sextet rule83 the acenes having a “zigzag” periphery are less stable and more reactive than their isomers with non aligned rings. The identification of the nonplanar tetrahelicene with a “cove” region strengthens this observation. Of course, other chemical routes79,84 should also be considered, and extensive chemical modeling is certainly needed to progress in the understanding of such a complex problem.

Figure 6. R2PI spectrum of mass 106 (lower panel) compared to a linear combination of the isomers’ spectra determined by lowering the mean differences of the spectral derivatives (see text). The gas-phase spectra38 at room temperature of the individual isomers are plotted in the upper panel.

spectra reported by Suto et al.,53 instead of the Etzkorn80 absorption spectra, also gives a very good adjustment with similar relative abundances. Production of Isomer Selective Spectra for Combustion Science. Determining the isomer content of the flame is a key step to identify the main routes leading to the PAH growth. The present study shows that it is possible, thanks to the 2dimensional information provided by the spectrally resolved mass spectrometry using laser R2P2CI, to experimentally determine in real time the distribution of the combustion byproducts thanks to online extraction at a sampling position within a laminar flame. Despite the large number of isomeric structures that can appear on each mass channel, their spectral discrimination was found to be possible in most cases up to the mass of pyrene, by using of a broad spectral coverage to measure several electronic transitions. Various techniques are commonly applied for determining the molecular content of flames. The most common ones are the chromatographic methods,23 which imply the extraction of a sample with a probe, followed by an ex situ analysis. In that case, the amount of material required is quite important and the most reactive species cannot be preserved. In situ optical measurements such as cavity ring-down spectroscopy (CRDS) or laser-induced fluorescence (LIF) in the flame constitute alternative ways, but they suffer from a poor selectivity in the absence of mass information, and this is even more critical as the conditions of temperature and pressure in the flame imply severe spectral congestion. To overcome this difficulty, optical

5. CONCLUSION A combination of mass spectrometry and REMPI spectroscopy was used to characterize the aromatic production of a fuel-rich C2H4 flame. The main goal was to identify online the aromatic 10101


The Journal of Physical Chemistry A species present at a given position (the sampling zone) in the flame, which remains a very difficult task with other techniques. Although the actual analysis was done ex situ (in the molecular beam generated out of the thermalization chamber), its online character and the built-in capability to probe the axial distribution through external adjustment of the distance from the burner without any change of the burning conditions makes it attractive. The spectra of low-mass species (

R2PI Spectroscopy of Aromatic Molecules ... - ACS Publications

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