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Electron impact ionization cross-sections of toluene
Chemical Physics Letters 434 (2007) 188–193 www.elsevier.com/locate/cplett
Electron impact ionization cross-sections of toluene J.R. Vacher *, F. Jorand, N. Blin-Simiand, S. Pasquiers Laboratoire de Physique des Gaz et des Plasmas, UMR CNRS 8578, Baˆt. 210, Universite´ Paris XI, 91405 Orsay Cedex, France Received 28 June 2006; in final form 29 November 2006 Available online 8 December 2006
Abstract Electron impact ionization of toluene is studied using mass spectrometry. Cross-sections for the formation of molecular ions and ionic þ fragments are measured between 10 and 78 eV with a total cross-section of 1:5 1015 cm2 towards 60 eV. C7 Hþ 8 and C7 H7 contribute to þ þ 75% of the total cross-section at 78 eV. The molecular ion is the most abundant below 25 eV. Four ionic fragments: C5 H5 ; C4 Hþ 3 ; C5 H3 þ and C3 Hþ , are detected above 20 eV. Enthalpy considerations can lead to think that C H is issued directly from the molecular ion, 5 5 3 whereas the three other species result from two step pathways. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction The toluene is available for commercial use but it is also one of many volatile organic compounds (VOCs) released in the atmosphere. The removal of pollutants in emitted effluents using pulsed electric discharges is the object of a growing interest but much works remain to be done in order to understand all the processes converting VOCs to less harmful molecules, in particular for aromatic hydrocarbons [1–3]. A complete understanding of the physical and chemical mechanisms involved in toluene removal, requires a detailed knowledge of the plasma kinetic. Data are needed concerning the electron collision processes on the hydrocarbon, i.e. the values of the cross-sections and the types of ionic and neutral species formed via the dissociative excitation and ionization processes. Previous studies of the fragmentation of cations of toluene and its isomers have mainly been carried out by electron impact ionization for energy of 70 eV [4]. Even though this information is useful for the identification of molecules, the data are inadequate to achieve a more complete understanding of ion and radical productions in nonthermal plasmas created by electrical discharges, for which
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0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.12.014
the electron energy distribution function covers a broad energy range. In this Letter, mass spectrometry measurements of the electron impact ionization of toluene are reported, and cross-sections for the formation of fragment ions are measured. Partition processes leading to observed ions are suggested. Such data will be useful for kinetic plasma modelling such those developed for detailed investigations on VOC conversion processes using phototriggered discharges [5,6]. 2. Experimental and theoretical method The experimental apparatus and procedure have been previously described in detail elsewhere [7–9]. Some improvements have been made by modifying the ionization process and the vacuum system. Toluene (Aldrich, 98%) is mixed with argon (Air Liquide, 99.995%) at 0.5 Torr to prevent the condensation of the hydrocarbon. The gas mixture leaked, through a 50 lm diameter hole, into the analysis chamber. Ions are formed in the ionization chamber (at a constant pressure of 5 106 Torr) by the impact of a focalised electron beam over the energy range 10–78 eV. Based on a comparison with rare gas ionization thresholds, the accuracy of the measured electron energy is estimated to be ±0.5 eV. The ions are then accelerated, focused and
J.R. Vacher et al. / Chemical Physics Letters 434 (2007) 188–193
analysed in a quadrupole mass spectrometer with a resolution (M/DM) better than 400. The various ionic species are detected by means of a channel-electron multiplier followed by a Faraday cup. The intensity ratios of the ionic fragments of toluene to Ar+ ions give the cross-sections for the formation of the fragments relative to that of argon ionization [10], the partial pressures of argon and toluene being known. The choice of cross-sections given by Wetzel et al. [10] is due to the availability of more detailed values near threshold of 15.76 eV [11] than other cross-sections [12–14] given by the literature. For the intensity ratio measurement, the transmission factor caused by mass segregation into the analyser is taken into account [8,9]. The total single ionization cross-section can be calculated using the Binary-Encounter-Bethe (BEB) model developed by Kim et al. for atoms [15] and molecules [16]. The cross-section for ejecting an electron from an orbital by electron impact, rBEB ðT Þ with incident kinetic energy T, needs the knowledge of binding and kinetic energies of this orbital. The total single ionization cross section is given by the sum over all occupied molecular orbitals: X rðT Þ ¼ rBEB ðT Þ Eighteen of these orbitals contribute to the BEB cross section below 100 eV. Orbital energies are computed from ab initio theory at the medium rhf/6-31G(d) level of theory and using GAUSSIAN 03 [17]. To ensure that the cross-section start at the ionization threshold, the calculated binding energy of the highest occupied molecular orbital (HOMO) are replaced by 8.828 eV, the experimental values of the ionization energies [11]. 3. Results and discussion The cross-sections for the formation of various Cn Hþ m ions, the sum of which contributes to more than 80% of the total ionization, are shown in Fig. 1. We estimate [8,9] that the uncertainty of the given values is 20% above 20 eV and 30% below 20 eV. The total ionization cross-section plotted in Fig. 2 is the sum of all the cross-sections
Fig. 1. Cross-sections for the formation of ions issued from toluene.
189
Fig. 2. Total ionization cross-section for the formation of ions from toluene above 20 eV and the cross-section calculated with BEB theory.
listed in Table 1. As the electron impact energy increases, the total cross-section for the ion formation from toluene shows a threshold level at 9–10 eV, rising up to 25 eV and reaching a maximum value of 1:5 1015 cm2 at around 60 eV. The BEB cross-section using energies computed from the ab initio previously defined theory are also shown in Fig. 2. Measured total cross-section agrees well with the cross-section given by the theoretical model above 20 eV, taking into account the experimental uncertainties. Below 20 eV, our results are higher than those given by the BEB model. From the ionization threshold of argon (15.76 eV) to 20 eV, the measured intensity is very small so that the uncertainty is much higher than 30% for the lowest energy value studied. Below 20 eV, the cross-sections plotted in Fig. 1 for the ions of 91 and 92 amu are corrected in order to normalize the sum of these two cross-sections to the value given by the BEB model. About forty different masses are observed but only six of them have been selected: those whose ionization crosssection is greater than 5 1017 cm2 at 78 eV, the maximum usable voltage (Tables 1 and 2). All the relative cross-sections are larger than 4% of the total cross-sections at different voltages. Two principal fragment ions (91 and 92 amu) contribute to more than 75% of the total crosssection at this maximum voltage. The molecular ion C7 Hþ 8 of M ¼ 92 amu is the most abundant below 25 eV. þ þ þ Four other fragment ions, C5 Hþ 5 ; C4 H3 ; C5 H3 and C3 H3 , are detected between 20 eV and 78 eV. We have compared (Table 1) our results to the NIST intensity values of the mass spectrum of toluene at 70 eV [11] normalised to C7 Hþ 7 . For the molecular ion and minor þ fragments C5 Hþ and C 3 H3 , our results are consistent with 5 those given by the NIST within ±20%. For the two other þ minor fragments (C4 Hþ 3 and C5 H3 ), measured relative intensities are larger of 35–45% than those of the NIST. These two ions contribute to less than 12% of the total ionization (6% for the NIST). We limited our discussion to the six more intense ions. The other ions detected at 78 eV were very minor and
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Table 1 Cross-sections r (1016 cm2) for the formation of the main ions from toluene at 78 eV (maximum voltage used), 50 eV (near maximum total cross-section) and 15 eV Mass (amu)
Ions C7 Hþ 7 C7 Hþ 8 C5 Hþ 5 C4 Hþ 3 C5 Hþ 3 C3 Hþ 3
91 92 65 51 63 39
NISTa (70 eV)
78 eV
Intensity (%)
r
Intensity (%)
r
Intensity (%)
r
Intensity (%)
100 77.6 12.1 6.4 7.4 10.7
7.2 4.7 1.1 0.87 0.82 0.68
100 65.3 15.3 12.1 11.4 9.4
5.7 3.9 1.6 1.3 1.1 1.0
100 68.4 28.1 22.8 19.3 17.5
0.13 2.4 – – – –
5.4 100 – – – –
50 eV
15 eV
Intensities (%) are normalised to the main ion. Ions are listed in order of decreasing cross-section for 78 eV. a From [11].
Table 2 Formulas, heat of formation and corresponding ionisation energies (eV) for the species of Fig. 2 Mass
Ion
92 92 91 91 65 65 51 63 39 39 77
TAC7 Hþ 8 CAC7 Hþ 8 BAC7 Hþ 7 CAC7 Hþ 7 VAC5 Hþ 5 CAC5 Hþ 5 C4 Hþ 3 C5 Hþ 3 CAC3 Hþ 3 PAC3 Hþ 3 C6 Hþ 5
a b c *
Df H (ion)
Name of the ion Toluene Cycloheptatriene Benzyl Tropylium Vinylcyclopropenyl Cyclopentadienyl Protonated diacetylene Ethynylcyclopropenyl* Cyclopropenyl Propargyl Phenyl
a
9.35 10.23a 9.39a 8.93a 10.49b 10.90b 12.49b 11.66c 11.19a 12.18a 11.83a
I.E. (neutral) 8.83a 8.29a 7.24a 6.28a 8.41a 8.31a 6.60a 8.67a 8.32a
From [11]. From [21]. From [29]. Structure defined in Ref. [28].
disappeared below 60 eV, so they did not present interesting profile of cross-sections. Among these minor ions, we can mention, in order of decreasing intensity, the ions of 50, 93, 62, 89, 38 and 52 amu which have a cross-section larger than 1017 cm2 at 78 eV. The formation of an appreciable number of ions from toluene results from primary processes: kinetic energy resulting from the electron ionising collision is converted into internal energy that can lead to the dissociation of the ion into a smaller one and a neutral fragment. The molecular ion C7 Hþ 8 , with one unpaired electron and an even mass number, noted OEþ as usual, can decompose into a lighter ion with an even number of electrons and an odd mass number noted EE+, and an odd mass number radical noted R : OEþ ! EEþ þ R (Table 3).
It has been shown from many years that unimolecular isomerization involving scrambling of carbon and hydrogen atoms can occur between toluene and cycloheptatriene cations [18,19]. Various reasonable mechanisms can be written for this isomerization. Table 2 shows that the enthalpy of formation for the toluene cation ðTAC7 Hþ 8Þ is lower by 0.9 eV than the one for the cycloheptatriene cation ðCAC7 Hþ 8 Þ. A transition state with energy of 1.4 eV above the one of the toluene cation was calculated [20]. Thus, near the ionization threshold, C7 Hþ 8 can be described as the toluene cation, but for higher energies, the two cations can be present. These two species have been be taken into account to determine possible fragmentation pathways in the following, as described in Table 3. þ 3.2. The hydrogen atom loss C 7 H þ 8 ! C7 H 7 þ H
3.1. The molecular ion The cross-section for the formation of C7 Hþ 8 is practically equal to 4 1016 cm2 between 78 eV and 20 eV and then decreases slowly to the ionization threshold (Fig. 1). This ion becomes the majority ion below 25 eV. As a molecule of formula C7H8 can be described by many isomers such toluene or cycloheptatriene, C7 Hþ 8 can be described by corresponding cations.
The cross-section for the formation of ion of 91 amu, in Fig. 1, is above the one for the molecular ion at energy values higher than 25 eV; the ratio r(91)/r(92) is 1:53 0:05 and constant between 78 eV and 40 eV. It is interesting to note that this result is identical to the ratio of the intensities measured in the dissociative photoionization of toluene by Field et al. [22]. Between 40 eV and 25 eV, the cross-section decreases at first slowly and later quickly: the ratio r(91)/
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Table 3 Dissociation reactions of the molecular ions (OE+) leading to the detected ions OE+ (1) (2) (3) (4) (5) (6) (7) (8) (9) (11) (10) (12) (13) (14) (15) (16) (17)
EE+ TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 þ TAC7 H8 CAC7 Hþ 8
ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ
! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !
BAC7 Hþ 7 ð91Þ BAC7 Hþ 7 ð91Þ CAC7 Hþ 7 ð91Þ CAC7 Hþ 7 ð91Þ VAC5 Hþ 5 ð65Þ VAC5 Hþ 5 ð65Þ CAC5 Hþ 5 ð65Þ CAC5 Hþ 5 ð65Þ VAC5 Hþ 5 ð65Þ VAC5 Hþ 5 ð65Þ CAC5 Hþ 5 ð65Þ CAC5 Hþ 5 ð65Þ C4 Hþ 3 ð51Þ C4 Hþ 3 ð51Þ C6 Hþ 5 ð77Þ C5 Hþ 3 ð63Þ C5 Hþ 3 ð63Þ
R (+M)
DrH° (eV)
DE (eV)
H (1) H (1) H (1) H (1) C2H3 (27) C2H3 (27) C2H3 (27) C2H3 (27) H ð1Þ þ C2 H2 H ð1Þ þ C2 H2 H ð1Þ þ C2 H2 H ð1Þ þ C2 H2 C3H5 (41) C3H5 (41) CH3 (15) C2H5 (29) C2H5 (29)
+2.30 +1.42 +1.84 +0.96 +4.24 +3.35 +4.65 +3.76 +5.75 +4.86 +6.16 +5.27 +4.91 +4.02 +4.00 +8.14 +7.25
+11.13 +11.13 +10.67 +10.67 +13.07 +13.07 +13.48 +13.48 +14.58 +14.58 +14.99 +14.99 +13.74 +13.74 +12.83 +16.97 +16.97
(26) (26) (26) (26)
Masses (between brackets) are given in amu.
r(92) is 0.5 at 20 eV. As the molecular ion can be described like toluene or cycloheptadiene cation, C7 Hþ 7 can be described like benzyl cation ðBAC7 Hþ Þ or tropylium cation 7 þ ðCAC7 Hþ Þ. A third form of C H , the tolyl ion, can exist 7 7 7 with three isomers. Their heats of formation being grater than those of benzyl and tropylium cation [21], tolyl forms are neglected in this study. As for the molecular ion, isomerization can occur: a rearrangement of benzyl cation can leads to the tropylium ion [20] via a transition state; the tropylium ion being the most stable [19,20]. In Table 3, DrH° values show that reactions (3) and (4) leading to the tropylium cation are less endothermic by 0.46 eV than reactions (1) and (2) leading to the benzyl cation. In the right column of Table 3, values of DE give the threshold energy to obtain the ion from the neutral toluene via the considered process. Activation energy is not taken into account in the present results. As show the two first values of DE in the right column, the formation of the tropylium ion is favoured at low ionization energy. C7 Hþ 7 is detected for 12 0:5 eV and the appearance energy (A.E.) was estimated to be at 11:3 0:2 eV [22]. It seems that the dissociation of the molecular ion is fast and occurs before leaving the ionization source: an excess of energy (kinetic shift) allows a correct detection of this ion in our apparatus. As the cross-sections for the formaþ tion of C7 Hþ 8 and C7 H7 are normalised with the BEB model below 20 eV, we do not take into account the role of the minor ions because they are not detected below 20 eV. If these ions are present, the cross-sections of the two major ions given by the BEB model are over-estimated. It should be noticed that, for 20 eV, the ration between the cross-section of the majority ion (92 amu) and C5 Hþ 5 is 250. Thus, the cross-sections of the two major ions are far from affected by the presence or not of minority ions.
3.3. The formation of minority ions The appearance energy of C5 Hþ 5 from toluene with electron impact was found to 16:4 0:2 eV [23]. The experiments of Bombach et al. [24] gave an onset for C5 Hþ 5 formation from toluene at 16 eV with photo ionization. The fact that C5 Hþ 5 is not observed below 20 eV (Fig. 1) suggests that the kinetic shift is not sufficient to create this ion in the source. In this case and just above 20 eV, the cross-section for C5 Hþ 5 may be underestimated. The experiments of Field et al. [22] gave þ A.E. of 17 eV for C4 Hþ 3 , 17.61 eV for C3 H3 and 16.5 eV for þ C5 H3 . These ions are not observed below 23 eV. Thus, a conclusion similar to the preceding one may be emitted on our measured cross-sections at low energy. 3.3.1. C 5 H þ 5 The ion of 65 amu is the most abundant of the four minor fragment ions from 20 eV up to 78 eV (Fig. 1). The cross-section for the formation of C5 Hþ 5 accounts for 11% of the total cross-section at 50 eV. We have considered in Table 3, two isomers having the lower enthalpy of formations: vinylcyclopropene ðVAC5 Hþ 5 Þ and cyclopentadiene ðCAC5 Hþ Þ cations. 5 Reactions (5)–(8) lead to the formation of a vinyl radical and the two selected isomers. We observe that the reactions (5) and (6) with the formation of VAC5 Hþ 5 are less endothermic of 0.41 eV than reaction (7) and (8). Reactions (9)–(12) lead to the formation of an H atom with acetylene molecule and the two isomers. As previously, the formation of VAC5 Hþ 5 in reactions (9) and (10) are less endothermic than reactions (11) and (12), but these four enthalpies are 1.51 eV higher than the corresponding four preceding enthalpies. It seems that, at low ionization energy, the formation of the ion of 65 amu can be described by the reactions (5) and (6) leading to the formation of vinylcyclopropene cation as show the values of DE.
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One other pathway leading to the formation of the ion of 65 amu must be considered. It results of two steps: at first, the molecular ion loses an H atom (reactions (1)– (4)); then the resultant ion loses acetylene. The reaction þ C7 Hþ 7 ! C5 H5 þ C2 H2 is observed in the photodissociation of toluene [24,25]. Of course, the values of DE, for this pathway, are the same than those of reactions (9)– (12), the products (H and C2H2) being the same. Yet, the speed of formation of ions C5 Hþ 5 must be slower than that given by the direct reactions (9)–(12). The fact that the cross-section for the formation of the ion of 65 amu, between 78 eV and 30 eV, is about 25% of the one of the ions of 91 amu, resulting from the direct reactions (1)–(4), suggests that C5 Hþ 5 results from such two steps pathway. It must be pointed out that experimental heath of forþ mation of VAC5 Hþ 5 and CAC5 H5 (Table 2) are lower than theoretical results given by Glukhovtsev et al. [26]: 11.20 eV and 11.30 eV. The first isomer is always the most stable. Using these results, DE values are 0.72 eV and 0.40 eV higher, respectively. That does not change the general trend in the Table 3 nor the preceding conclusions. 3.3.2. C 4 H þ 3 The cross-section for the formation of the ion of 51 amu reaches almost 9% of the total cross-section at 50 eV (Fig. 1). The lowest calculated energy isomer of C4 Hþ 3 is found for the linear C1-protonated diacetylene [27]. Two other isomers of which one with a cyclic structure can be considered but we assume the more stable linear structure in the following considerations. Reactions (13) and (14) in Table 3 lead to the formation of this ion and allyl radical from the two molecular ions taken as precursors. The DE value is found between the ones of reactions (5)–(8) and of reactions (9)–(12) whereas the cross-section is weaker than that of C5 Hþ 5 and the estimated threshold is higher. One other pathway leading to the formation of C4 Hþ 3 must be considered. It results of two steps: at first, the toluene ion loses CH3 by a simple bond split (reactions (15)); then the resultant benzyl ion loses acetylene. The reaction þ C6 Hþ 5 ! C4 H3 þ C2 H2 ðDr H ¼ þ3:01 eVÞ is well known and is expected to be a major channel in the photodissociation of toluene [24]. The fact that the benzyl ion (77 amu) is not observed in our study suggests that this reaction is very fast and then the formation of C4 Hþ 3 is controlled by the speed of the reaction (15). The value of DE for the formation of C4 Hþ 3 via these two steeps is 15.84 eV, higher than the threshold for the formation of C5 Hþ 5 . The shape of the curves, for the ions of 65 and 51 amu, close to the thresholds, can indicate that C4 Hþ 3 results from such two steps pathway. 3.3.3. C 5 H þ 3 The cross-section for the formation of the ion of 63 amu is almost identical to those of the three other minor
fragments above 50 eV, but becomes smallest below 45 eV. There are discrepancies between theoretical [28] and experimental enthalpies of formation of C5 Hþ 3 and also between experimental results themselves. From experimental results and generic equations, Schwell et al. [29] propose the average value of Df H ¼ 11:66 eV. Reactions (16) and (17) in Table 3 lead to the formation of C5 Hþ 3 and ethyl radical, from the two molecular ions taken as precursors. This heat of formation gives DE ¼ 12:37 eV. One other pathway must be considered. It results of two steps: at first, the molecular ion loses an H atom (reactions (1)–(4)); then the resultant ion loses ethylene. The reactions þ þ C7 Hþ 7 ! C5 H3 þ C2 H4 with Dr H ¼ 2:81 eV ðBAC7 H7 Þ þ and 3.27 eV ðCAC7 H7 Þ lead, via these two steps, to the value of DE ¼ 13:95 eV. From Fig. 1, the threshold for the formation of the ion of 63 amu can be considered higher than that of the ion of 51 amu (15.84 eV). Assuming the heat of formation of 11.66 eV as reliable, our results may reflect experimental errors near the threshold or the existence of other pathway to C5 Hþ 3 formation. Another two steps process can be considered: at first the formation of C5 Hþ 5 via the reactions (9)–(12), then the ion loses a H2 molecule by C5 Hþ 5 ! C5 Hþ þ H . This pathway leads to DE ¼ 15:75 eV which 2 3 is always lower but in better agreement with the threshold energy of C4 Hþ 3. 3.3.4. C 3 H þ 3 The ion of 39 amu is the least abundant of the selected ions from 50 eV to 78 eV. C3 Hþ 3 can be described by two isomers: the cyclopropenyl cation, which is the most stable, and the propargyl cation. A possible pathway may be considered with the formation of C5 Hþ 5 via the reactions (5)–(8) and þ then the loss of acetylene: C5 Hþ 5 ! C3 H3 þ C2 H2 . This two steps pathway leads to DE ¼ 16:07 eV for the formation of the cyclic isomer which is consistent with the DE values þ for C5 Hþ 5 (13–15 eV) and C4 H3 (15.84 eV). 4. Conclusion The electron impact ionization of toluene produces molecular ions and fragment ions with a total cross-section of 1:5 1015 cm2 towards 60 eV. Crosssections for the formation of the major species are measured between 13 and 78 eV. The molecular ion (92 amu) and the ion of 91 amu contribute to more than 75% of the total cross-section at 78 eV. The molecular ion C7 Hþ 8 is the most abundant below 25 eV. Four fragment ions þ þ þ (C5 Hþ 5 ; C4 H3 ; C5 H3 and C3 H3 Þ are detected above 20 eV. þ C5 H5 (65 amu) is the most abundant of these four fragment ions from 20 eV to 78 eV. This ion may be issued directly from the dissociation of the molecular ion, whereas the other species may result from two steps pathways. Heat of formation of various isomers describing these ions and measured cross-sections near the thresholds make possible to consider such processes.
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Electron impact ionization cross-sections of toluene J.R. Vacher *, F. Jorand, N. Blin-Simiand, S. Pasquiers Laboratoire de Physique des Gaz et des Plasmas, UMR CNRS 8578, Baˆt. 210, Universite´ Paris XI, 91405 Orsay Cedex, France Received 28 June 2006; in final form 29 November 2006 Available online 8 December 2006
Abstract Electron impact ionization of toluene is studied using mass spectrometry. Cross-sections for the formation of molecular ions and ionic þ fragments are measured between 10 and 78 eV with a total cross-section of 1:5 1015 cm2 towards 60 eV. C7 Hþ 8 and C7 H7 contribute to þ þ 75% of the total cross-section at 78 eV. The molecular ion is the most abundant below 25 eV. Four ionic fragments: C5 H5 ; C4 Hþ 3 ; C5 H3 þ and C3 Hþ , are detected above 20 eV. Enthalpy considerations can lead to think that C H is issued directly from the molecular ion, 5 5 3 whereas the three other species result from two step pathways. Ó 2006 Elsevier B.V. All rights reserved.
1. Introduction The toluene is available for commercial use but it is also one of many volatile organic compounds (VOCs) released in the atmosphere. The removal of pollutants in emitted effluents using pulsed electric discharges is the object of a growing interest but much works remain to be done in order to understand all the processes converting VOCs to less harmful molecules, in particular for aromatic hydrocarbons [1–3]. A complete understanding of the physical and chemical mechanisms involved in toluene removal, requires a detailed knowledge of the plasma kinetic. Data are needed concerning the electron collision processes on the hydrocarbon, i.e. the values of the cross-sections and the types of ionic and neutral species formed via the dissociative excitation and ionization processes. Previous studies of the fragmentation of cations of toluene and its isomers have mainly been carried out by electron impact ionization for energy of 70 eV [4]. Even though this information is useful for the identification of molecules, the data are inadequate to achieve a more complete understanding of ion and radical productions in nonthermal plasmas created by electrical discharges, for which
*
Corresponding author. Fax: +33 169 157 844. E-mail address: [email protected] (J.R. Vacher).
0009-2614/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.12.014
the electron energy distribution function covers a broad energy range. In this Letter, mass spectrometry measurements of the electron impact ionization of toluene are reported, and cross-sections for the formation of fragment ions are measured. Partition processes leading to observed ions are suggested. Such data will be useful for kinetic plasma modelling such those developed for detailed investigations on VOC conversion processes using phototriggered discharges [5,6]. 2. Experimental and theoretical method The experimental apparatus and procedure have been previously described in detail elsewhere [7–9]. Some improvements have been made by modifying the ionization process and the vacuum system. Toluene (Aldrich, 98%) is mixed with argon (Air Liquide, 99.995%) at 0.5 Torr to prevent the condensation of the hydrocarbon. The gas mixture leaked, through a 50 lm diameter hole, into the analysis chamber. Ions are formed in the ionization chamber (at a constant pressure of 5 106 Torr) by the impact of a focalised electron beam over the energy range 10–78 eV. Based on a comparison with rare gas ionization thresholds, the accuracy of the measured electron energy is estimated to be ±0.5 eV. The ions are then accelerated, focused and
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analysed in a quadrupole mass spectrometer with a resolution (M/DM) better than 400. The various ionic species are detected by means of a channel-electron multiplier followed by a Faraday cup. The intensity ratios of the ionic fragments of toluene to Ar+ ions give the cross-sections for the formation of the fragments relative to that of argon ionization [10], the partial pressures of argon and toluene being known. The choice of cross-sections given by Wetzel et al. [10] is due to the availability of more detailed values near threshold of 15.76 eV [11] than other cross-sections [12–14] given by the literature. For the intensity ratio measurement, the transmission factor caused by mass segregation into the analyser is taken into account [8,9]. The total single ionization cross-section can be calculated using the Binary-Encounter-Bethe (BEB) model developed by Kim et al. for atoms [15] and molecules [16]. The cross-section for ejecting an electron from an orbital by electron impact, rBEB ðT Þ with incident kinetic energy T, needs the knowledge of binding and kinetic energies of this orbital. The total single ionization cross section is given by the sum over all occupied molecular orbitals: X rðT Þ ¼ rBEB ðT Þ Eighteen of these orbitals contribute to the BEB cross section below 100 eV. Orbital energies are computed from ab initio theory at the medium rhf/6-31G(d) level of theory and using GAUSSIAN 03 [17]. To ensure that the cross-section start at the ionization threshold, the calculated binding energy of the highest occupied molecular orbital (HOMO) are replaced by 8.828 eV, the experimental values of the ionization energies [11]. 3. Results and discussion The cross-sections for the formation of various Cn Hþ m ions, the sum of which contributes to more than 80% of the total ionization, are shown in Fig. 1. We estimate [8,9] that the uncertainty of the given values is 20% above 20 eV and 30% below 20 eV. The total ionization cross-section plotted in Fig. 2 is the sum of all the cross-sections
Fig. 1. Cross-sections for the formation of ions issued from toluene.
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Fig. 2. Total ionization cross-section for the formation of ions from toluene above 20 eV and the cross-section calculated with BEB theory.
listed in Table 1. As the electron impact energy increases, the total cross-section for the ion formation from toluene shows a threshold level at 9–10 eV, rising up to 25 eV and reaching a maximum value of 1:5 1015 cm2 at around 60 eV. The BEB cross-section using energies computed from the ab initio previously defined theory are also shown in Fig. 2. Measured total cross-section agrees well with the cross-section given by the theoretical model above 20 eV, taking into account the experimental uncertainties. Below 20 eV, our results are higher than those given by the BEB model. From the ionization threshold of argon (15.76 eV) to 20 eV, the measured intensity is very small so that the uncertainty is much higher than 30% for the lowest energy value studied. Below 20 eV, the cross-sections plotted in Fig. 1 for the ions of 91 and 92 amu are corrected in order to normalize the sum of these two cross-sections to the value given by the BEB model. About forty different masses are observed but only six of them have been selected: those whose ionization crosssection is greater than 5 1017 cm2 at 78 eV, the maximum usable voltage (Tables 1 and 2). All the relative cross-sections are larger than 4% of the total cross-sections at different voltages. Two principal fragment ions (91 and 92 amu) contribute to more than 75% of the total crosssection at this maximum voltage. The molecular ion C7 Hþ 8 of M ¼ 92 amu is the most abundant below 25 eV. þ þ þ Four other fragment ions, C5 Hþ 5 ; C4 H3 ; C5 H3 and C3 H3 , are detected between 20 eV and 78 eV. We have compared (Table 1) our results to the NIST intensity values of the mass spectrum of toluene at 70 eV [11] normalised to C7 Hþ 7 . For the molecular ion and minor þ fragments C5 Hþ and C 3 H3 , our results are consistent with 5 those given by the NIST within ±20%. For the two other þ minor fragments (C4 Hþ 3 and C5 H3 ), measured relative intensities are larger of 35–45% than those of the NIST. These two ions contribute to less than 12% of the total ionization (6% for the NIST). We limited our discussion to the six more intense ions. The other ions detected at 78 eV were very minor and
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Table 1 Cross-sections r (1016 cm2) for the formation of the main ions from toluene at 78 eV (maximum voltage used), 50 eV (near maximum total cross-section) and 15 eV Mass (amu)
Ions C7 Hþ 7 C7 Hþ 8 C5 Hþ 5 C4 Hþ 3 C5 Hþ 3 C3 Hþ 3
91 92 65 51 63 39
NISTa (70 eV)
78 eV
Intensity (%)
r
Intensity (%)
r
Intensity (%)
r
Intensity (%)
100 77.6 12.1 6.4 7.4 10.7
7.2 4.7 1.1 0.87 0.82 0.68
100 65.3 15.3 12.1 11.4 9.4
5.7 3.9 1.6 1.3 1.1 1.0
100 68.4 28.1 22.8 19.3 17.5
0.13 2.4 – – – –
5.4 100 – – – –
50 eV
15 eV
Intensities (%) are normalised to the main ion. Ions are listed in order of decreasing cross-section for 78 eV. a From [11].
Table 2 Formulas, heat of formation and corresponding ionisation energies (eV) for the species of Fig. 2 Mass
Ion
92 92 91 91 65 65 51 63 39 39 77
TAC7 Hþ 8 CAC7 Hþ 8 BAC7 Hþ 7 CAC7 Hþ 7 VAC5 Hþ 5 CAC5 Hþ 5 C4 Hþ 3 C5 Hþ 3 CAC3 Hþ 3 PAC3 Hþ 3 C6 Hþ 5
a b c *
Df H (ion)
Name of the ion Toluene Cycloheptatriene Benzyl Tropylium Vinylcyclopropenyl Cyclopentadienyl Protonated diacetylene Ethynylcyclopropenyl* Cyclopropenyl Propargyl Phenyl
a
9.35 10.23a 9.39a 8.93a 10.49b 10.90b 12.49b 11.66c 11.19a 12.18a 11.83a
I.E. (neutral) 8.83a 8.29a 7.24a 6.28a 8.41a 8.31a 6.60a 8.67a 8.32a
From [11]. From [21]. From [29]. Structure defined in Ref. [28].
disappeared below 60 eV, so they did not present interesting profile of cross-sections. Among these minor ions, we can mention, in order of decreasing intensity, the ions of 50, 93, 62, 89, 38 and 52 amu which have a cross-section larger than 1017 cm2 at 78 eV. The formation of an appreciable number of ions from toluene results from primary processes: kinetic energy resulting from the electron ionising collision is converted into internal energy that can lead to the dissociation of the ion into a smaller one and a neutral fragment. The molecular ion C7 Hþ 8 , with one unpaired electron and an even mass number, noted OEþ as usual, can decompose into a lighter ion with an even number of electrons and an odd mass number noted EE+, and an odd mass number radical noted R : OEþ ! EEþ þ R (Table 3).
It has been shown from many years that unimolecular isomerization involving scrambling of carbon and hydrogen atoms can occur between toluene and cycloheptatriene cations [18,19]. Various reasonable mechanisms can be written for this isomerization. Table 2 shows that the enthalpy of formation for the toluene cation ðTAC7 Hþ 8Þ is lower by 0.9 eV than the one for the cycloheptatriene cation ðCAC7 Hþ 8 Þ. A transition state with energy of 1.4 eV above the one of the toluene cation was calculated [20]. Thus, near the ionization threshold, C7 Hþ 8 can be described as the toluene cation, but for higher energies, the two cations can be present. These two species have been be taken into account to determine possible fragmentation pathways in the following, as described in Table 3. þ 3.2. The hydrogen atom loss C 7 H þ 8 ! C7 H 7 þ H
3.1. The molecular ion The cross-section for the formation of C7 Hþ 8 is practically equal to 4 1016 cm2 between 78 eV and 20 eV and then decreases slowly to the ionization threshold (Fig. 1). This ion becomes the majority ion below 25 eV. As a molecule of formula C7H8 can be described by many isomers such toluene or cycloheptatriene, C7 Hþ 8 can be described by corresponding cations.
The cross-section for the formation of ion of 91 amu, in Fig. 1, is above the one for the molecular ion at energy values higher than 25 eV; the ratio r(91)/r(92) is 1:53 0:05 and constant between 78 eV and 40 eV. It is interesting to note that this result is identical to the ratio of the intensities measured in the dissociative photoionization of toluene by Field et al. [22]. Between 40 eV and 25 eV, the cross-section decreases at first slowly and later quickly: the ratio r(91)/
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Table 3 Dissociation reactions of the molecular ions (OE+) leading to the detected ions OE+ (1) (2) (3) (4) (5) (6) (7) (8) (9) (11) (10) (12) (13) (14) (15) (16) (17)
EE+ TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 CAC7 Hþ 8 TAC7 Hþ 8 þ TAC7 H8 CAC7 Hþ 8
ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ ð92Þ
! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! !
BAC7 Hþ 7 ð91Þ BAC7 Hþ 7 ð91Þ CAC7 Hþ 7 ð91Þ CAC7 Hþ 7 ð91Þ VAC5 Hþ 5 ð65Þ VAC5 Hþ 5 ð65Þ CAC5 Hþ 5 ð65Þ CAC5 Hþ 5 ð65Þ VAC5 Hþ 5 ð65Þ VAC5 Hþ 5 ð65Þ CAC5 Hþ 5 ð65Þ CAC5 Hþ 5 ð65Þ C4 Hþ 3 ð51Þ C4 Hþ 3 ð51Þ C6 Hþ 5 ð77Þ C5 Hþ 3 ð63Þ C5 Hþ 3 ð63Þ
R (+M)
DrH° (eV)
DE (eV)
H (1) H (1) H (1) H (1) C2H3 (27) C2H3 (27) C2H3 (27) C2H3 (27) H ð1Þ þ C2 H2 H ð1Þ þ C2 H2 H ð1Þ þ C2 H2 H ð1Þ þ C2 H2 C3H5 (41) C3H5 (41) CH3 (15) C2H5 (29) C2H5 (29)
+2.30 +1.42 +1.84 +0.96 +4.24 +3.35 +4.65 +3.76 +5.75 +4.86 +6.16 +5.27 +4.91 +4.02 +4.00 +8.14 +7.25
+11.13 +11.13 +10.67 +10.67 +13.07 +13.07 +13.48 +13.48 +14.58 +14.58 +14.99 +14.99 +13.74 +13.74 +12.83 +16.97 +16.97
(26) (26) (26) (26)
Masses (between brackets) are given in amu.
r(92) is 0.5 at 20 eV. As the molecular ion can be described like toluene or cycloheptadiene cation, C7 Hþ 7 can be described like benzyl cation ðBAC7 Hþ Þ or tropylium cation 7 þ ðCAC7 Hþ Þ. A third form of C H , the tolyl ion, can exist 7 7 7 with three isomers. Their heats of formation being grater than those of benzyl and tropylium cation [21], tolyl forms are neglected in this study. As for the molecular ion, isomerization can occur: a rearrangement of benzyl cation can leads to the tropylium ion [20] via a transition state; the tropylium ion being the most stable [19,20]. In Table 3, DrH° values show that reactions (3) and (4) leading to the tropylium cation are less endothermic by 0.46 eV than reactions (1) and (2) leading to the benzyl cation. In the right column of Table 3, values of DE give the threshold energy to obtain the ion from the neutral toluene via the considered process. Activation energy is not taken into account in the present results. As show the two first values of DE in the right column, the formation of the tropylium ion is favoured at low ionization energy. C7 Hþ 7 is detected for 12 0:5 eV and the appearance energy (A.E.) was estimated to be at 11:3 0:2 eV [22]. It seems that the dissociation of the molecular ion is fast and occurs before leaving the ionization source: an excess of energy (kinetic shift) allows a correct detection of this ion in our apparatus. As the cross-sections for the formaþ tion of C7 Hþ 8 and C7 H7 are normalised with the BEB model below 20 eV, we do not take into account the role of the minor ions because they are not detected below 20 eV. If these ions are present, the cross-sections of the two major ions given by the BEB model are over-estimated. It should be noticed that, for 20 eV, the ration between the cross-section of the majority ion (92 amu) and C5 Hþ 5 is 250. Thus, the cross-sections of the two major ions are far from affected by the presence or not of minority ions.
3.3. The formation of minority ions The appearance energy of C5 Hþ 5 from toluene with electron impact was found to 16:4 0:2 eV [23]. The experiments of Bombach et al. [24] gave an onset for C5 Hþ 5 formation from toluene at 16 eV with photo ionization. The fact that C5 Hþ 5 is not observed below 20 eV (Fig. 1) suggests that the kinetic shift is not sufficient to create this ion in the source. In this case and just above 20 eV, the cross-section for C5 Hþ 5 may be underestimated. The experiments of Field et al. [22] gave þ A.E. of 17 eV for C4 Hþ 3 , 17.61 eV for C3 H3 and 16.5 eV for þ C5 H3 . These ions are not observed below 23 eV. Thus, a conclusion similar to the preceding one may be emitted on our measured cross-sections at low energy. 3.3.1. C 5 H þ 5 The ion of 65 amu is the most abundant of the four minor fragment ions from 20 eV up to 78 eV (Fig. 1). The cross-section for the formation of C5 Hþ 5 accounts for 11% of the total cross-section at 50 eV. We have considered in Table 3, two isomers having the lower enthalpy of formations: vinylcyclopropene ðVAC5 Hþ 5 Þ and cyclopentadiene ðCAC5 Hþ Þ cations. 5 Reactions (5)–(8) lead to the formation of a vinyl radical and the two selected isomers. We observe that the reactions (5) and (6) with the formation of VAC5 Hþ 5 are less endothermic of 0.41 eV than reaction (7) and (8). Reactions (9)–(12) lead to the formation of an H atom with acetylene molecule and the two isomers. As previously, the formation of VAC5 Hþ 5 in reactions (9) and (10) are less endothermic than reactions (11) and (12), but these four enthalpies are 1.51 eV higher than the corresponding four preceding enthalpies. It seems that, at low ionization energy, the formation of the ion of 65 amu can be described by the reactions (5) and (6) leading to the formation of vinylcyclopropene cation as show the values of DE.
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One other pathway leading to the formation of the ion of 65 amu must be considered. It results of two steps: at first, the molecular ion loses an H atom (reactions (1)– (4)); then the resultant ion loses acetylene. The reaction þ C7 Hþ 7 ! C5 H5 þ C2 H2 is observed in the photodissociation of toluene [24,25]. Of course, the values of DE, for this pathway, are the same than those of reactions (9)– (12), the products (H and C2H2) being the same. Yet, the speed of formation of ions C5 Hþ 5 must be slower than that given by the direct reactions (9)–(12). The fact that the cross-section for the formation of the ion of 65 amu, between 78 eV and 30 eV, is about 25% of the one of the ions of 91 amu, resulting from the direct reactions (1)–(4), suggests that C5 Hþ 5 results from such two steps pathway. It must be pointed out that experimental heath of forþ mation of VAC5 Hþ 5 and CAC5 H5 (Table 2) are lower than theoretical results given by Glukhovtsev et al. [26]: 11.20 eV and 11.30 eV. The first isomer is always the most stable. Using these results, DE values are 0.72 eV and 0.40 eV higher, respectively. That does not change the general trend in the Table 3 nor the preceding conclusions. 3.3.2. C 4 H þ 3 The cross-section for the formation of the ion of 51 amu reaches almost 9% of the total cross-section at 50 eV (Fig. 1). The lowest calculated energy isomer of C4 Hþ 3 is found for the linear C1-protonated diacetylene [27]. Two other isomers of which one with a cyclic structure can be considered but we assume the more stable linear structure in the following considerations. Reactions (13) and (14) in Table 3 lead to the formation of this ion and allyl radical from the two molecular ions taken as precursors. The DE value is found between the ones of reactions (5)–(8) and of reactions (9)–(12) whereas the cross-section is weaker than that of C5 Hþ 5 and the estimated threshold is higher. One other pathway leading to the formation of C4 Hþ 3 must be considered. It results of two steps: at first, the toluene ion loses CH3 by a simple bond split (reactions (15)); then the resultant benzyl ion loses acetylene. The reaction þ C6 Hþ 5 ! C4 H3 þ C2 H2 ðDr H ¼ þ3:01 eVÞ is well known and is expected to be a major channel in the photodissociation of toluene [24]. The fact that the benzyl ion (77 amu) is not observed in our study suggests that this reaction is very fast and then the formation of C4 Hþ 3 is controlled by the speed of the reaction (15). The value of DE for the formation of C4 Hþ 3 via these two steeps is 15.84 eV, higher than the threshold for the formation of C5 Hþ 5 . The shape of the curves, for the ions of 65 and 51 amu, close to the thresholds, can indicate that C4 Hþ 3 results from such two steps pathway. 3.3.3. C 5 H þ 3 The cross-section for the formation of the ion of 63 amu is almost identical to those of the three other minor
fragments above 50 eV, but becomes smallest below 45 eV. There are discrepancies between theoretical [28] and experimental enthalpies of formation of C5 Hþ 3 and also between experimental results themselves. From experimental results and generic equations, Schwell et al. [29] propose the average value of Df H ¼ 11:66 eV. Reactions (16) and (17) in Table 3 lead to the formation of C5 Hþ 3 and ethyl radical, from the two molecular ions taken as precursors. This heat of formation gives DE ¼ 12:37 eV. One other pathway must be considered. It results of two steps: at first, the molecular ion loses an H atom (reactions (1)–(4)); then the resultant ion loses ethylene. The reactions þ þ C7 Hþ 7 ! C5 H3 þ C2 H4 with Dr H ¼ 2:81 eV ðBAC7 H7 Þ þ and 3.27 eV ðCAC7 H7 Þ lead, via these two steps, to the value of DE ¼ 13:95 eV. From Fig. 1, the threshold for the formation of the ion of 63 amu can be considered higher than that of the ion of 51 amu (15.84 eV). Assuming the heat of formation of 11.66 eV as reliable, our results may reflect experimental errors near the threshold or the existence of other pathway to C5 Hþ 3 formation. Another two steps process can be considered: at first the formation of C5 Hþ 5 via the reactions (9)–(12), then the ion loses a H2 molecule by C5 Hþ 5 ! C5 Hþ þ H . This pathway leads to DE ¼ 15:75 eV which 2 3 is always lower but in better agreement with the threshold energy of C4 Hþ 3. 3.3.4. C 3 H þ 3 The ion of 39 amu is the least abundant of the selected ions from 50 eV to 78 eV. C3 Hþ 3 can be described by two isomers: the cyclopropenyl cation, which is the most stable, and the propargyl cation. A possible pathway may be considered with the formation of C5 Hþ 5 via the reactions (5)–(8) and þ then the loss of acetylene: C5 Hþ 5 ! C3 H3 þ C2 H2 . This two steps pathway leads to DE ¼ 16:07 eV for the formation of the cyclic isomer which is consistent with the DE values þ for C5 Hþ 5 (13–15 eV) and C4 H3 (15.84 eV). 4. Conclusion The electron impact ionization of toluene produces molecular ions and fragment ions with a total cross-section of 1:5 1015 cm2 towards 60 eV. Crosssections for the formation of the major species are measured between 13 and 78 eV. The molecular ion (92 amu) and the ion of 91 amu contribute to more than 75% of the total cross-section at 78 eV. The molecular ion C7 Hþ 8 is the most abundant below 25 eV. Four fragment ions þ þ þ (C5 Hþ 5 ; C4 H3 ; C5 H3 and C3 H3 Þ are detected above 20 eV. þ C5 H5 (65 amu) is the most abundant of these four fragment ions from 20 eV to 78 eV. This ion may be issued directly from the dissociation of the molecular ion, whereas the other species may result from two steps pathways. Heat of formation of various isomers describing these ions and measured cross-sections near the thresholds make possible to consider such processes.
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