- Email: [email protected]
Dissociative electron impact ionization of methyl tert-butyl ether: total ionization cross-section and kinetic energy distributions
Chemical Physics Letters 400 (2004) 191–195 www.elsevier.com/locate/cplett
Dissociative electron impact ionization of methyl tert-butyl ether: total ionization cross-section and kinetic energy distributions T.M. Di Palma
a,*
, B. Apicella b, M. Armenante c, R. Velotta
d,e
, X. Wang e, N. Spinelli
d,f
a
Istituto Motori – CNR, Via Marconi 8, 80125 Napoli, Italy Istituto di Ricerche sulla Combustione – CNR, P.le V.Tecchio 80, 80125 Napoli, Italy c INFN, Via Cintia 26, 80126 – Napoli (Italy) Dip. Scienze Fisiche, Universita` di Napoli Federico II, Via Cintia 26, 80126 – Napoli (Italy) e Coherentia-INFM, Via Cintia 26, 80126 – Napoli (Italy) f INFM – Unita` di Napoli, Via Cintia 26, 80126 – Napoli (Italy) b
d
Received 21 July 2004; in final form 22 October 2004 Available online 11 November 2004
Abstract Kinetic energy distributions and yields of the ions produced in the electron impact ionization of methyl tert-butyl ether (MTBE) have been measured by TOF mass spectrometry. The detection efficiency as a function of the initial ion kinetic energy has been carefully evaluated by means of a Montecarlo simulation of the experimental conditions. The resulting kinetic energy spectra show that almost all the heaviest ions are produced with quasi-thermal energy distribution, while the smaller fragment ions H+ and CHþ 3 exhibit in addition substantial non-thermal components. As a final result, the total ionization cross-section of MTBE in the range 20–150 eV of the ionizing-electron energy has been derived and calibrated against the argon, chosen as a reference gas. 2004 Elsevier B.V. All rights reserved.
1. Introduction The methyl tert-butyl ether (MTBE, C5H12O) is an additive of the gasoline used at concentration up to 15% by volume, in order to increase the oxygen content, to enhance the efficiency in internal combustion engines and to reduce the emissions of carbon oxide, benzene and other organic compounds from motor vehicles. MTBE is an ether wherein the oxygen atom is bonded to two alkyl groups and has an unbounded electron pair. The particular geometry around the oxygen atom and the nature of the CAO bond make MTBE a polar molecule [1]. The polarity leads to a high water solubility that, combined to MTBEÕs high vapour pressure and its widespread use in the fuels, causes additional pollu-
*
Corresponding author. Fax: +39 081 2396097. E-mail address: [email protected] (T.M. Di Palma).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.106
tion of the urban storm water, the groundwater and the surface water [2]. Accordingly, most of the recent literature on MBTE deals with its potential impact on the environment [2] and the human health [3]. Previous studies published in the eighties and nineties analysed MTBE oxidation and pyrolysis in different operative conditions (e.g. [4]), whereas only few gas phase experiments and ab initio computational studies are reported on such contaminant [1,5–8]. On the other hand, experiments based on electron-impact ionization of such molecule are of relevance, since they can shed light on the role played by MTBE in complex processes, e.g. the chemical reactions leading to detonation (ÔknockÕ) in spark-ignition engines [9]. Thus, additional data on its interactions with electrons can help understanding the role of the gasoline components in the ÔknockÕ phenomenon. The electron impact (EI) ionization cross-sections for atoms and simple molecules have been measured and
192
T.M. Di Palma et al. / Chemical Physics Letters 400 (2004) 191–195
calculated since the 1930Õs [10]. More recently, experiments have demonstrated that accurate ionization cross-section measurements require to take carefully into account the non-thermal (ÔfastÕ) ions, which are produced in the EI dissociative ionization of molecules and may constitute a non-negligible part of the total ions [11,12]. This is particularly true when using conventional time of flight (TOF) or Quadrupole mass spectrometers, whose collection efficiencies are strongly dependent on the ion kinetic energy. A procedure to correlate the ion kinetic energy to the shape of the peaks detected in a time of flight mass spectrometer was proposed in [13] and has been successfully applied to the determination of the kinetic energy distributions (KEDs) of the ions produced in the electron impact of some molecules [14]. Moreover, since the kinetic energy of ions in the dissociative ionization is related to the fragmentation pathways, the KED analysis turns out to be a powerful tool for the identification of the fragmentation channels, particularly if combined with measurements of the appearance energies and of quantum chemistry calculations [15]. In this work, we report on experiments on the EI dissociative ionization of MTBE, carried out using an effusive beam, of MTBE in Ar, a pulsed ionizing electron gun, and a linear TOF mass spectrometer, all described in Section 2. In Section 3 we discuss the analysis of experimental data to obtain the kinetic energy distributions of the fragment ions, based on a Montecarlo simulation of ion trajectories in the spectrometer, evidencing the considerable amounts of fast nonthermal ions (protons and CHþ 3 ) produced by electron impact on MTBE. Finally, we discuss the resulting KEDs in Section 4 and provide the value of the total ionization cross-section of MTBE as a function of the energy in the range 20–150 eV.
2. Experimental apparatus The experimental apparatus, described in detail previously [16], consists of an ionization chamber, pumped down to 105 Pa and equipped with an effusive beam, crossed at 90 by an electron gun. The chamber is coupled to a Wiley-McLaren TOF mass spectrometer [17], with the axis normal to both the effusive beam and the electron gun. The effusive beam is produced by a 100 lm stainless steel needle injector. During the measurements, the local pressure in the interaction region is about 101 Pa, as evaluated after the calibration of the gas inlet system. The liquid sample (tert-butyl methyl ether 99.8% HPLC grade, Aldrich) is kept in a reservoir at a temperature of 28 C in argon atmosphere. The pressure in the reservoir (1.6 · 105 Pa) is stable within ±102 Pa, as measured by a Lesker fast response diaphragm manometer (range 1.3 · 102 to 2 · 105 Pa).
Since the temperature-induced fluctuations of the partial pressure of MTBE in Ar seriously affect the crosssection measurements, the temperature of the liquid is continuously measured with a copper-constantan thermocouple and is kept constant within 1 C during all the experiment. At 28 C the vapour pressure of the methyl tert-butyl ether is 3.6 · 104 Pa, as deduced from the Antoine coefficients [18], corresponding to 28% of the argon pressure in the reservoir. A 100 ns, 1–10 lA pulsed electron beam, operating at a repetition rate of 10 kHz, ionizes the effusive beam carrying the gas target; the resulting ions are extracted from the source region by a positive pulse applied to the repeller plate. Such a pulse, with amplitude and duration of 29 V and 2 ls, respectively, is switched on at the end of the electron pulse. In the ionization region the value of the extraction field is 32 V/cm, being the repeller and the extraction grid 0.9 cm apart. After the extraction, the ions travel through the acceleration region under a constant electric field of 300 V/cm, then enter a 95-cm long field free drift tube, and finally are detected by a 40-mm wide Chevron MCP. In order to retrieve the kinetic energy distributions of all the fragments, the extraction and accelerating fields have been chosen in order to obtain the first-order space-focusing condition. The MCP signal is firstly amplified (SRS amplifier Mod. SR445), then discriminated (300 MHz discriminator Phillips Scientific Mod. 6904) and finally sent to a multichannel scaler (EG&G Mod. 914P), operating over 10 000 channels with a time resolution of 5ns/channel. The mass resolution in these conditions, checked for Ar ions, is m/Dm 210. All the mass spectra are accumulated over 5 · 105 electron shots. The absolute electron energy has been calibrated against the Ar ionization threshold with an accuracy of ±0.5 eV, according to previous calibrations [16]. The control of the sample temperature within 1 C keeps the variations of MTBE vapour pressure at 28 C below 5%; in these conditions repeated TOF measurements resulted in fluctuations within 10% of the ratio between the areas under the most abundant peaks and the Ar+ peak, corrected as reported in Section 4.
3. Analysis of TOF peak shapes and retrieval of KEDs In space-focusing conditions, the TOF distributions of single ions (peaks shapes) are closely related to their initial kinetic energy distributions, each distribution being symmetrical when the initial energy distribution is symmetrical. Then, the KED of an ion can be evaluated from the analysis of only one of the two TOF half-peak shapes. However, a preliminary treatment of the experimental data is needed, which consists in the correction of the measured ion yields by the collection efficiency of the spectrometer. The ion trajectories have
T.M. Di Palma et al. / Chemical Physics Letters 400 (2004) 191–195
been worked out using the Monte Carlo method and the ion optics simulation software 3D SIM.ION [19], assuming isotropic angular distributions for the produced fragments and considering one million trajectories for each species. The resulting collection efficiency curves are independent on the ion mass and are reported in Fig. 1a for the ÔforwardÕ ions (i.e., those travelling towards the detector, solid line) and ÔbackwardÕ ions (travelling towards the repeller plate, dashed line). As expected, the two curves overlap up to a maximum kinetic energy (8 eV in our experimental conditions), where a sudden drop occurs in the efficiency of ÔbackwardÕ ions, as a consequence of the hit taking place on the repeller plate. In the KED analysis, we considered only the shorter-time side of the TOF distributions, corresponding to ÔforwardÕ ions, due to the above mentioned drawback of the ÔbackwardÕ case. The relation between ion initial kinetic energy and TOF has been deduced by means of a numerical inversion of the flight kinematic equation [20]. The KEDs are obtained by weighing the shorter-time side of the TOF distributions with the efficiency curve of the spectrometer. As a preliminary check, we have tested the correction procedure with Ar ions, as they are produced with a purely thermal energy distribution. The results are shown in Fig. 1b, where the Ar+ KED is reported by dots, for an incident electron energy of 150 eV. The
Fig. 1. (a) Collection efficiency curves of the time of flight spectrometer for the ions directed towards the detector (solid line) or the repeller (dashed line); (b) kinetic energy distribution of the Ar+ at 150 eV electron energy corrected for the collection efficiency curve (dots) and thermal KED calculated at 26 meV (line).
193
maximum value of the energy distribution E0 allowed us readily to evaluate the beam temperature from the relationship KBT = 2E0 [21]. The resulting value of KBT = 26 meV is in very good agreement with the room temperature, thus confirming the reliability of our correction procedure.
4. Results and discussion In Fig. 2a a typical TOF mass spectrum of MTBE is shown. We find out that, at 70 eV incident electron energy, the parent ion yield is about 0.3% of t-butyloxonium ion (CH3)2C+–OCH3 (m/z = 73), the most intense peak, in agreement with previous results [6–8]. Other intense peaks are found at m/z = 57, 43, 41, 29, 27, 15; moreover, a weak peak at m/z = 1 is also detected. As pointed out above, the collection efficiency of the TOF spectrometer is strongly dependent on the initial kinetic energy of the produced fragments. So, the experimental TOF distributions have been analysed and corrected very carefully, in order to unveil the possible presence of fast ions. The results are reported in Figs. 2b and c for m/z = 1 and m/z = 15, respectively, each at electron energies of 70 and 150 eV. Complex TOF structures appear around the H+ and CHþ 3 peaks. We definitely exclude that the side bands in these TOF distributions structures belong to double ions or masses other than H+ and CHþ 3 ; rather, they are signatures of the production of such ions with non-thermal kinetic energy. Corresponding KEDs of protons, methyl and t-butyloxonium ions are reported in Fig. 3a–c. We observe that the t-butyl oxonium ion is produced with a quasi-thermal distribution, while H+ and CHþ 3 have both quasithermal and non-thermal components of kinetic energy. Other fragments, not reported here, show mostly
Fig. 2. Mass spectrum of MTBE at 70 eV (a); zoom of the spectrum in correspondence of m/z = 1 (b), and m/z = 15 (c).
194
T.M. Di Palma et al. / Chemical Physics Letters 400 (2004) 191–195
Fig. 3. Kinetic energy distributions of protons (a), methyl ions (b) and t-butyl oxonium ions (c) at 70, 100 and 150 electron energies.
quasi-thermal energy distributions; the maximum E0 of these distributions is related to the average thermal energy through the relation Eth = 3E0 [21]. Table 1 reports the ion average thermal energies for several masses, as measured at 150 eV electron energy. These energies are related to the kinetic energy release in a given fragmentation path, which in turn provides information about the transition state involved in the dissociation reaction [22]. The dissociation mechanism of methyl tert-butyl ether into a methyl radical and the t-butyl oxonium ion is well established by isotope labeling experiments [6,8]. For this reaction, by using the momentum end energy conservation laws [21], the kinetic energy released to the center of mass is derived and turns out to be about 0.7 eV. Such value is consistent with the hypothesis of a potential energy barrier for the dissociation of methyl tert-butyl ether into a methyl radical and the tbutyl oxonium ion. This is in agreement with ab initio calculations [23], which predict a barrier of about
0.44 eV, corresponding to the formation of an ion– molecule complex in the transition state. The CHþ 3 KEDs, reported in Fig. 3b for 70, 100 and 150 eV electron energy, have a broad thermal component (Eth = 0.45 eV) and an additional, well separated non-thermal component at 2.7 eV. The lowest electron energy at which we observed the high energy components is 30 eV. These findings are consistent with similar results in other EI experiments on many organic compounds, wherein CHþ 3 fast production has been ascribed to the Coulomb explosion of doubly charged fragments [24,25]. In addition, electronic mechanisms may lead as well to fast ions production [15]. Further insights on the KEDs of methyl ion could hopefully be gained from measurements at higher electron-energy resolution, particularly around the methyl appearance energy, and from quantum chemistry computations. However, such tasks appear beyond the scope of this Letter, mainly focused onto the measurement of the total EI crosssection, conventionally carried out with a step of 5 eV, through the correct derivation of the MTBE fragmentsÕ KEDs. The hydrogen ion signals are much weaker than those of heavier ions, involving an even more difficult analysis. The KED of H+ has complex and broad high-energy components superimposed to the thermal one, thus impeding the evaluation of each contribution. We notice that, at high electron energy, the contribution of the fast components in the H+ KED becomes dominant and two rather broad maxima can be distinguished in Fig. 3a at about 4 and 12 eV, respectively; however, at lower electron impact energies (about 35 eV), approaching the H+ appearance energy, the KED does not show well-defined features, which instead could help elucidating the formation mechanism of the fast protons. In our data we find out that, for incident electron energy of 70 eV, the fraction of total ions with kinetic energy above 2 eV is about 10% and increases up to 15% at 150 eV; this finding highlights the importance of careful data correction in the evaluation of KEDs, ion yields and EI ionization cross-sections. In fact, the experimental limitations in the collection of energetic ions may lead to quite different results [12,26]. We have carried out an analysis of our data, not reported here, on ion yield of the fragments, considered without and with the application of the data correction procedure. We find out that the experimental fragmentation mass spectra are quite different from the corrected spectra even at 70 eV electron energy, a typical value in many experi-
Table 1 Values of the thermal energy of the fragment ions, measured at 150 eV electron energy Mass Eth
88 0.039
73 0.15
57 0.081
56 0.12
55 0.057
45 0.141
43 0.45
41 0.201
39 0.45
31 0.201
29 0.165
27 0.045
15 0.45
T.M. Di Palma et al. / Chemical Physics Letters 400 (2004) 191–195
195
Acknowledgement This work was partially supported by Centro Regionale di Competenze ÔAMRAÕ.
References
Fig. 4. Total electron-impact ionization cross-section of MTBE in the electron energy range 20–150 eV.
ments and in some reference databases (e.g. NIST). For example, at 70 eV electron energy, from the experimental mass spectrum, the relative abundance of the CHþ 3 with respect to the t-butyl oxonium ions turns out to be about 10%, but it increases to 40% if the data are corrected for the collection efficiency. These differences become more pronounced at higher electron energy, where a larger amount of non-thermal ions is produced. Finally, we use the absolute ion yield at different electron energies in order to evaluate the ionization crosssection of MTBE. Once the partial pressures of MTBE and Ar are known, the ion yield ratio of a MTBE fragment to that of Ar gives the fragmentÕs (partial) ionization cross-section, relative to that well-known of Ar [27]. The total cross-section is the sum of the partial crosssections of all the fragments and is reported in Fig. 4 over the energy range 20–150 eV, wherein the estimated overall relative errors are within 15%.
5. Conclusions We have performed an experiment of electron impact ionization of the MTBE in the range 20–150 eV. After taking into account the collection efficiency of the spectrometer by means of a Montecarlo simulation of ion trajectories, we have retrieved the kinetic energy distributions of the fragments, among which we have found detectable amounts of fast, non-thermal H+ and CHþ 3. Finally, we have measured the ion yields and the total ionization cross-section of MTBE, previously available only at 70 eV [28]. As of our knowledge, this is the first measurement of the total cross-section of MTBE in the range 20–150 eV.
[1] R.D. Suenram, F.J. Lovas, W. Pereyra, G.T. Fraser, A.R. Hight Walker, J. Mol. Spectrosc. 181 (1997) 67. [2] O.C. Braids, Environ. Forensics 2 (2001) 189. [3] W. Dekant, U. Bernawer, E. Rosner, A. Amberg, Toxicol. Lett. 124 (2001) 37. [4] A. Ciajolo, A. DÕAnna, M. Kurz, Combust. Sci. Technol. 123 (1997) 49. [5] L.N. Gregerson, J.S. Siegel, K.K. Baldridge, J. Phys. Chem. A 104 (2000) 11106. [6] S.D. Chambreau, J. Zhang, J.C. Traeger, T.H. Morton, Int. J. Mass Spectrom. 199 (2000) 17. [7] M. Bu¨chner, H.F. Gru¨tzmacher, Int. J. Mass Spectrom. 199 (2000) 141. [8] M.C. Bisonette, M. George, J.L. Holmes, Int. J. Mass Spectrom. Ion Proc. 101 (1990) 309. [9] J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill Book Company, New York, 1988. [10] T.D. Ma¨rk, G.H. Dunn (Eds.), Electron Impact Ionization, Springer, Wien, 1985. [11] C. Tian, C.R. Vidal, J. Phys B: At. Mol. Opt. Phys. 31 (1998) 895. [12] C. Tian, C.R. Vidal, J. Chem Phys. 108 (3) (1998) 927. [13] K. Scha¨fer, W.Y. Baek, K. Fo¨ster, D. Gassen, W. Neuwirth, Z. Phys. D 21 (1991) 137. [14] I. Torres, R. Martı´nez, M.N. Sa´nchez Rayo, F. Castan˜o, Chem. Phys. Lett. 328 (2000) 135. [15] I. Torres, R. Martı´nez, M.N. Sa´nchez Rayo, F. Castan˜o, J. Phys. B: At. Mol. Opt. Phys. 33 (2000) 3615. [16] R. Velotta, P. Di Girolamo, V. Berardi, N. Spinelli, M. Armenante, J. Phys. B: At. Mol. Opt. Phys. 27 (1994) 2051. [17] W.C. Wiley, I.H. McLaren, Rev. Sci. Instrum. 26 (1955) 1150. [18] A. Arce, J. Martinez-Ageitos, A. Soto, J. Chem. Eng. Data 41 (1996) 718. [19] SIMION -3D Version 6.0, Idaho National Engineering Laboratory, EG&G Idaho Inc., Idaho Falls, ID 83415, 1995. [20] R.J. Cotter, Time of Flight Mass Spectrometry: Instrumentation and Applications in America Chemical Society, America Chemical Society, Washington DC, 1997. [21] H.U. Poll, V. Grill, S. Matt, N. Abramzon, K. Becker, P. Scheier, T.D. Mark, Int. J. Mass Spectrom. 177 (1998) 143. [22] J. Laskin, C. Lifshitz, J. Mass Spectrom. 36 (2001) 459. [23] W. Beveridge, J.A. Hunter, C.A.F. Jonson, J.E. Parker, Org. Mass Spectrom. 27 (1992) 543. [24] J. Olmsted, III, K. Street Jr., A.S. Newton, J. Chem. Phys. 40 (6) (1964) 2114. [25] K. Gluch, J. Fedor, S. Matt-Leubner, O. Echt, A. Stamatovic, M. Probst, P. Sheier, T.D. Ma¨rk, J. Chem. Phys. 118 (7) (2003) 3090. [26] H. Deutsh, K. Becker, S. Matt, T.D. Mark, Int. J. Mass Spectrom. 197 (2000) 37. [27] R. Rejoub, B.J. Lindsay, R.F. Stebbings, Phys. Rev. A 65 (2002) 42713. [28] A.G. Herrison, E.G. Jones, S.K. Gupta, G.P. Nagy, Can. J. Chem. 44 (1966) 1966.
Dissociative electron impact ionization of methyl tert-butyl ether: total ionization cross-section and kinetic energy distributions T.M. Di Palma
a,*
, B. Apicella b, M. Armenante c, R. Velotta
d,e
, X. Wang e, N. Spinelli
d,f
a
Istituto Motori – CNR, Via Marconi 8, 80125 Napoli, Italy Istituto di Ricerche sulla Combustione – CNR, P.le V.Tecchio 80, 80125 Napoli, Italy c INFN, Via Cintia 26, 80126 – Napoli (Italy) Dip. Scienze Fisiche, Universita` di Napoli Federico II, Via Cintia 26, 80126 – Napoli (Italy) e Coherentia-INFM, Via Cintia 26, 80126 – Napoli (Italy) f INFM – Unita` di Napoli, Via Cintia 26, 80126 – Napoli (Italy) b
d
Received 21 July 2004; in final form 22 October 2004 Available online 11 November 2004
Abstract Kinetic energy distributions and yields of the ions produced in the electron impact ionization of methyl tert-butyl ether (MTBE) have been measured by TOF mass spectrometry. The detection efficiency as a function of the initial ion kinetic energy has been carefully evaluated by means of a Montecarlo simulation of the experimental conditions. The resulting kinetic energy spectra show that almost all the heaviest ions are produced with quasi-thermal energy distribution, while the smaller fragment ions H+ and CHþ 3 exhibit in addition substantial non-thermal components. As a final result, the total ionization cross-section of MTBE in the range 20–150 eV of the ionizing-electron energy has been derived and calibrated against the argon, chosen as a reference gas. 2004 Elsevier B.V. All rights reserved.
1. Introduction The methyl tert-butyl ether (MTBE, C5H12O) is an additive of the gasoline used at concentration up to 15% by volume, in order to increase the oxygen content, to enhance the efficiency in internal combustion engines and to reduce the emissions of carbon oxide, benzene and other organic compounds from motor vehicles. MTBE is an ether wherein the oxygen atom is bonded to two alkyl groups and has an unbounded electron pair. The particular geometry around the oxygen atom and the nature of the CAO bond make MTBE a polar molecule [1]. The polarity leads to a high water solubility that, combined to MTBEÕs high vapour pressure and its widespread use in the fuels, causes additional pollu-
*
Corresponding author. Fax: +39 081 2396097. E-mail address: [email protected] (T.M. Di Palma).
0009-2614/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.10.106
tion of the urban storm water, the groundwater and the surface water [2]. Accordingly, most of the recent literature on MBTE deals with its potential impact on the environment [2] and the human health [3]. Previous studies published in the eighties and nineties analysed MTBE oxidation and pyrolysis in different operative conditions (e.g. [4]), whereas only few gas phase experiments and ab initio computational studies are reported on such contaminant [1,5–8]. On the other hand, experiments based on electron-impact ionization of such molecule are of relevance, since they can shed light on the role played by MTBE in complex processes, e.g. the chemical reactions leading to detonation (ÔknockÕ) in spark-ignition engines [9]. Thus, additional data on its interactions with electrons can help understanding the role of the gasoline components in the ÔknockÕ phenomenon. The electron impact (EI) ionization cross-sections for atoms and simple molecules have been measured and
192
T.M. Di Palma et al. / Chemical Physics Letters 400 (2004) 191–195
calculated since the 1930Õs [10]. More recently, experiments have demonstrated that accurate ionization cross-section measurements require to take carefully into account the non-thermal (ÔfastÕ) ions, which are produced in the EI dissociative ionization of molecules and may constitute a non-negligible part of the total ions [11,12]. This is particularly true when using conventional time of flight (TOF) or Quadrupole mass spectrometers, whose collection efficiencies are strongly dependent on the ion kinetic energy. A procedure to correlate the ion kinetic energy to the shape of the peaks detected in a time of flight mass spectrometer was proposed in [13] and has been successfully applied to the determination of the kinetic energy distributions (KEDs) of the ions produced in the electron impact of some molecules [14]. Moreover, since the kinetic energy of ions in the dissociative ionization is related to the fragmentation pathways, the KED analysis turns out to be a powerful tool for the identification of the fragmentation channels, particularly if combined with measurements of the appearance energies and of quantum chemistry calculations [15]. In this work, we report on experiments on the EI dissociative ionization of MTBE, carried out using an effusive beam, of MTBE in Ar, a pulsed ionizing electron gun, and a linear TOF mass spectrometer, all described in Section 2. In Section 3 we discuss the analysis of experimental data to obtain the kinetic energy distributions of the fragment ions, based on a Montecarlo simulation of ion trajectories in the spectrometer, evidencing the considerable amounts of fast nonthermal ions (protons and CHþ 3 ) produced by electron impact on MTBE. Finally, we discuss the resulting KEDs in Section 4 and provide the value of the total ionization cross-section of MTBE as a function of the energy in the range 20–150 eV.
2. Experimental apparatus The experimental apparatus, described in detail previously [16], consists of an ionization chamber, pumped down to 105 Pa and equipped with an effusive beam, crossed at 90 by an electron gun. The chamber is coupled to a Wiley-McLaren TOF mass spectrometer [17], with the axis normal to both the effusive beam and the electron gun. The effusive beam is produced by a 100 lm stainless steel needle injector. During the measurements, the local pressure in the interaction region is about 101 Pa, as evaluated after the calibration of the gas inlet system. The liquid sample (tert-butyl methyl ether 99.8% HPLC grade, Aldrich) is kept in a reservoir at a temperature of 28 C in argon atmosphere. The pressure in the reservoir (1.6 · 105 Pa) is stable within ±102 Pa, as measured by a Lesker fast response diaphragm manometer (range 1.3 · 102 to 2 · 105 Pa).
Since the temperature-induced fluctuations of the partial pressure of MTBE in Ar seriously affect the crosssection measurements, the temperature of the liquid is continuously measured with a copper-constantan thermocouple and is kept constant within 1 C during all the experiment. At 28 C the vapour pressure of the methyl tert-butyl ether is 3.6 · 104 Pa, as deduced from the Antoine coefficients [18], corresponding to 28% of the argon pressure in the reservoir. A 100 ns, 1–10 lA pulsed electron beam, operating at a repetition rate of 10 kHz, ionizes the effusive beam carrying the gas target; the resulting ions are extracted from the source region by a positive pulse applied to the repeller plate. Such a pulse, with amplitude and duration of 29 V and 2 ls, respectively, is switched on at the end of the electron pulse. In the ionization region the value of the extraction field is 32 V/cm, being the repeller and the extraction grid 0.9 cm apart. After the extraction, the ions travel through the acceleration region under a constant electric field of 300 V/cm, then enter a 95-cm long field free drift tube, and finally are detected by a 40-mm wide Chevron MCP. In order to retrieve the kinetic energy distributions of all the fragments, the extraction and accelerating fields have been chosen in order to obtain the first-order space-focusing condition. The MCP signal is firstly amplified (SRS amplifier Mod. SR445), then discriminated (300 MHz discriminator Phillips Scientific Mod. 6904) and finally sent to a multichannel scaler (EG&G Mod. 914P), operating over 10 000 channels with a time resolution of 5ns/channel. The mass resolution in these conditions, checked for Ar ions, is m/Dm 210. All the mass spectra are accumulated over 5 · 105 electron shots. The absolute electron energy has been calibrated against the Ar ionization threshold with an accuracy of ±0.5 eV, according to previous calibrations [16]. The control of the sample temperature within 1 C keeps the variations of MTBE vapour pressure at 28 C below 5%; in these conditions repeated TOF measurements resulted in fluctuations within 10% of the ratio between the areas under the most abundant peaks and the Ar+ peak, corrected as reported in Section 4.
3. Analysis of TOF peak shapes and retrieval of KEDs In space-focusing conditions, the TOF distributions of single ions (peaks shapes) are closely related to their initial kinetic energy distributions, each distribution being symmetrical when the initial energy distribution is symmetrical. Then, the KED of an ion can be evaluated from the analysis of only one of the two TOF half-peak shapes. However, a preliminary treatment of the experimental data is needed, which consists in the correction of the measured ion yields by the collection efficiency of the spectrometer. The ion trajectories have
T.M. Di Palma et al. / Chemical Physics Letters 400 (2004) 191–195
been worked out using the Monte Carlo method and the ion optics simulation software 3D SIM.ION [19], assuming isotropic angular distributions for the produced fragments and considering one million trajectories for each species. The resulting collection efficiency curves are independent on the ion mass and are reported in Fig. 1a for the ÔforwardÕ ions (i.e., those travelling towards the detector, solid line) and ÔbackwardÕ ions (travelling towards the repeller plate, dashed line). As expected, the two curves overlap up to a maximum kinetic energy (8 eV in our experimental conditions), where a sudden drop occurs in the efficiency of ÔbackwardÕ ions, as a consequence of the hit taking place on the repeller plate. In the KED analysis, we considered only the shorter-time side of the TOF distributions, corresponding to ÔforwardÕ ions, due to the above mentioned drawback of the ÔbackwardÕ case. The relation between ion initial kinetic energy and TOF has been deduced by means of a numerical inversion of the flight kinematic equation [20]. The KEDs are obtained by weighing the shorter-time side of the TOF distributions with the efficiency curve of the spectrometer. As a preliminary check, we have tested the correction procedure with Ar ions, as they are produced with a purely thermal energy distribution. The results are shown in Fig. 1b, where the Ar+ KED is reported by dots, for an incident electron energy of 150 eV. The
Fig. 1. (a) Collection efficiency curves of the time of flight spectrometer for the ions directed towards the detector (solid line) or the repeller (dashed line); (b) kinetic energy distribution of the Ar+ at 150 eV electron energy corrected for the collection efficiency curve (dots) and thermal KED calculated at 26 meV (line).
193
maximum value of the energy distribution E0 allowed us readily to evaluate the beam temperature from the relationship KBT = 2E0 [21]. The resulting value of KBT = 26 meV is in very good agreement with the room temperature, thus confirming the reliability of our correction procedure.
4. Results and discussion In Fig. 2a a typical TOF mass spectrum of MTBE is shown. We find out that, at 70 eV incident electron energy, the parent ion yield is about 0.3% of t-butyloxonium ion (CH3)2C+–OCH3 (m/z = 73), the most intense peak, in agreement with previous results [6–8]. Other intense peaks are found at m/z = 57, 43, 41, 29, 27, 15; moreover, a weak peak at m/z = 1 is also detected. As pointed out above, the collection efficiency of the TOF spectrometer is strongly dependent on the initial kinetic energy of the produced fragments. So, the experimental TOF distributions have been analysed and corrected very carefully, in order to unveil the possible presence of fast ions. The results are reported in Figs. 2b and c for m/z = 1 and m/z = 15, respectively, each at electron energies of 70 and 150 eV. Complex TOF structures appear around the H+ and CHþ 3 peaks. We definitely exclude that the side bands in these TOF distributions structures belong to double ions or masses other than H+ and CHþ 3 ; rather, they are signatures of the production of such ions with non-thermal kinetic energy. Corresponding KEDs of protons, methyl and t-butyloxonium ions are reported in Fig. 3a–c. We observe that the t-butyl oxonium ion is produced with a quasi-thermal distribution, while H+ and CHþ 3 have both quasithermal and non-thermal components of kinetic energy. Other fragments, not reported here, show mostly
Fig. 2. Mass spectrum of MTBE at 70 eV (a); zoom of the spectrum in correspondence of m/z = 1 (b), and m/z = 15 (c).
194
T.M. Di Palma et al. / Chemical Physics Letters 400 (2004) 191–195
Fig. 3. Kinetic energy distributions of protons (a), methyl ions (b) and t-butyl oxonium ions (c) at 70, 100 and 150 electron energies.
quasi-thermal energy distributions; the maximum E0 of these distributions is related to the average thermal energy through the relation Eth = 3E0 [21]. Table 1 reports the ion average thermal energies for several masses, as measured at 150 eV electron energy. These energies are related to the kinetic energy release in a given fragmentation path, which in turn provides information about the transition state involved in the dissociation reaction [22]. The dissociation mechanism of methyl tert-butyl ether into a methyl radical and the t-butyl oxonium ion is well established by isotope labeling experiments [6,8]. For this reaction, by using the momentum end energy conservation laws [21], the kinetic energy released to the center of mass is derived and turns out to be about 0.7 eV. Such value is consistent with the hypothesis of a potential energy barrier for the dissociation of methyl tert-butyl ether into a methyl radical and the tbutyl oxonium ion. This is in agreement with ab initio calculations [23], which predict a barrier of about
0.44 eV, corresponding to the formation of an ion– molecule complex in the transition state. The CHþ 3 KEDs, reported in Fig. 3b for 70, 100 and 150 eV electron energy, have a broad thermal component (Eth = 0.45 eV) and an additional, well separated non-thermal component at 2.7 eV. The lowest electron energy at which we observed the high energy components is 30 eV. These findings are consistent with similar results in other EI experiments on many organic compounds, wherein CHþ 3 fast production has been ascribed to the Coulomb explosion of doubly charged fragments [24,25]. In addition, electronic mechanisms may lead as well to fast ions production [15]. Further insights on the KEDs of methyl ion could hopefully be gained from measurements at higher electron-energy resolution, particularly around the methyl appearance energy, and from quantum chemistry computations. However, such tasks appear beyond the scope of this Letter, mainly focused onto the measurement of the total EI crosssection, conventionally carried out with a step of 5 eV, through the correct derivation of the MTBE fragmentsÕ KEDs. The hydrogen ion signals are much weaker than those of heavier ions, involving an even more difficult analysis. The KED of H+ has complex and broad high-energy components superimposed to the thermal one, thus impeding the evaluation of each contribution. We notice that, at high electron energy, the contribution of the fast components in the H+ KED becomes dominant and two rather broad maxima can be distinguished in Fig. 3a at about 4 and 12 eV, respectively; however, at lower electron impact energies (about 35 eV), approaching the H+ appearance energy, the KED does not show well-defined features, which instead could help elucidating the formation mechanism of the fast protons. In our data we find out that, for incident electron energy of 70 eV, the fraction of total ions with kinetic energy above 2 eV is about 10% and increases up to 15% at 150 eV; this finding highlights the importance of careful data correction in the evaluation of KEDs, ion yields and EI ionization cross-sections. In fact, the experimental limitations in the collection of energetic ions may lead to quite different results [12,26]. We have carried out an analysis of our data, not reported here, on ion yield of the fragments, considered without and with the application of the data correction procedure. We find out that the experimental fragmentation mass spectra are quite different from the corrected spectra even at 70 eV electron energy, a typical value in many experi-
Table 1 Values of the thermal energy of the fragment ions, measured at 150 eV electron energy Mass Eth
88 0.039
73 0.15
57 0.081
56 0.12
55 0.057
45 0.141
43 0.45
41 0.201
39 0.45
31 0.201
29 0.165
27 0.045
15 0.45
T.M. Di Palma et al. / Chemical Physics Letters 400 (2004) 191–195
195
Acknowledgement This work was partially supported by Centro Regionale di Competenze ÔAMRAÕ.
References
Fig. 4. Total electron-impact ionization cross-section of MTBE in the electron energy range 20–150 eV.
ments and in some reference databases (e.g. NIST). For example, at 70 eV electron energy, from the experimental mass spectrum, the relative abundance of the CHþ 3 with respect to the t-butyl oxonium ions turns out to be about 10%, but it increases to 40% if the data are corrected for the collection efficiency. These differences become more pronounced at higher electron energy, where a larger amount of non-thermal ions is produced. Finally, we use the absolute ion yield at different electron energies in order to evaluate the ionization crosssection of MTBE. Once the partial pressures of MTBE and Ar are known, the ion yield ratio of a MTBE fragment to that of Ar gives the fragmentÕs (partial) ionization cross-section, relative to that well-known of Ar [27]. The total cross-section is the sum of the partial crosssections of all the fragments and is reported in Fig. 4 over the energy range 20–150 eV, wherein the estimated overall relative errors are within 15%.
5. Conclusions We have performed an experiment of electron impact ionization of the MTBE in the range 20–150 eV. After taking into account the collection efficiency of the spectrometer by means of a Montecarlo simulation of ion trajectories, we have retrieved the kinetic energy distributions of the fragments, among which we have found detectable amounts of fast, non-thermal H+ and CHþ 3. Finally, we have measured the ion yields and the total ionization cross-section of MTBE, previously available only at 70 eV [28]. As of our knowledge, this is the first measurement of the total cross-section of MTBE in the range 20–150 eV.
[1] R.D. Suenram, F.J. Lovas, W. Pereyra, G.T. Fraser, A.R. Hight Walker, J. Mol. Spectrosc. 181 (1997) 67. [2] O.C. Braids, Environ. Forensics 2 (2001) 189. [3] W. Dekant, U. Bernawer, E. Rosner, A. Amberg, Toxicol. Lett. 124 (2001) 37. [4] A. Ciajolo, A. DÕAnna, M. Kurz, Combust. Sci. Technol. 123 (1997) 49. [5] L.N. Gregerson, J.S. Siegel, K.K. Baldridge, J. Phys. Chem. A 104 (2000) 11106. [6] S.D. Chambreau, J. Zhang, J.C. Traeger, T.H. Morton, Int. J. Mass Spectrom. 199 (2000) 17. [7] M. Bu¨chner, H.F. Gru¨tzmacher, Int. J. Mass Spectrom. 199 (2000) 141. [8] M.C. Bisonette, M. George, J.L. Holmes, Int. J. Mass Spectrom. Ion Proc. 101 (1990) 309. [9] J.B. Heywood, Internal Combustion Engine Fundamentals, McGraw-Hill Book Company, New York, 1988. [10] T.D. Ma¨rk, G.H. Dunn (Eds.), Electron Impact Ionization, Springer, Wien, 1985. [11] C. Tian, C.R. Vidal, J. Phys B: At. Mol. Opt. Phys. 31 (1998) 895. [12] C. Tian, C.R. Vidal, J. Chem Phys. 108 (3) (1998) 927. [13] K. Scha¨fer, W.Y. Baek, K. Fo¨ster, D. Gassen, W. Neuwirth, Z. Phys. D 21 (1991) 137. [14] I. Torres, R. Martı´nez, M.N. Sa´nchez Rayo, F. Castan˜o, Chem. Phys. Lett. 328 (2000) 135. [15] I. Torres, R. Martı´nez, M.N. Sa´nchez Rayo, F. Castan˜o, J. Phys. B: At. Mol. Opt. Phys. 33 (2000) 3615. [16] R. Velotta, P. Di Girolamo, V. Berardi, N. Spinelli, M. Armenante, J. Phys. B: At. Mol. Opt. Phys. 27 (1994) 2051. [17] W.C. Wiley, I.H. McLaren, Rev. Sci. Instrum. 26 (1955) 1150. [18] A. Arce, J. Martinez-Ageitos, A. Soto, J. Chem. Eng. Data 41 (1996) 718. [19] SIMION -3D Version 6.0, Idaho National Engineering Laboratory, EG&G Idaho Inc., Idaho Falls, ID 83415, 1995. [20] R.J. Cotter, Time of Flight Mass Spectrometry: Instrumentation and Applications in America Chemical Society, America Chemical Society, Washington DC, 1997. [21] H.U. Poll, V. Grill, S. Matt, N. Abramzon, K. Becker, P. Scheier, T.D. Mark, Int. J. Mass Spectrom. 177 (1998) 143. [22] J. Laskin, C. Lifshitz, J. Mass Spectrom. 36 (2001) 459. [23] W. Beveridge, J.A. Hunter, C.A.F. Jonson, J.E. Parker, Org. Mass Spectrom. 27 (1992) 543. [24] J. Olmsted, III, K. Street Jr., A.S. Newton, J. Chem. Phys. 40 (6) (1964) 2114. [25] K. Gluch, J. Fedor, S. Matt-Leubner, O. Echt, A. Stamatovic, M. Probst, P. Sheier, T.D. Ma¨rk, J. Chem. Phys. 118 (7) (2003) 3090. [26] H. Deutsh, K. Becker, S. Matt, T.D. Mark, Int. J. Mass Spectrom. 197 (2000) 37. [27] R. Rejoub, B.J. Lindsay, R.F. Stebbings, Phys. Rev. A 65 (2002) 42713. [28] A.G. Herrison, E.G. Jones, S.K. Gupta, G.P. Nagy, Can. J. Chem. 44 (1966) 1966.