8692
J. Phys. Chem. 1996, 100, 8692-8696
Appearance Energies of C60 Fragment Ions Revisited S. Matt, D. Muigg, A. Ding,† C. Lifshitz,‡ P. Scheier, and T. D. Ma1 rk* Institut fu¨ r Ionenphysik, Leopold Franzens UniVersita¨ t, Technikerstrasse 25, A-6020 Innsbruck, Austria ReceiVed: January 3, 1996; In Final Form: February 28, 1996X
Following up on a serious discrepancy (factor of 2) in reported appearance energies for fragment ions produced by electron impact ionization of C60, we have measured here the appearance energies as a function of electron current using a crossed beam apparatus dedicated to ionization cross section measurements. Thus, it was possible to conclude that the low-energy peaks in the electron ionization curves of Baba et al. (which have been used to deduce the appearance energies) are very likely caused by consecutive ionization and excitation processes due to the high electron currents employed in these measurements. From this it follows that the true appearance energies are the high appearance energies reported earlier by Ma¨rk and co-workers (and Anderson and co-workers) which were determined at low ionizing electron currents to prevent secondary processes in the ion source. Moreover, we discuss here the crucial role of the kinetic shift in deriving activation energies and other thermochemical properties from measured appearance energies.
1. Introduction The exceptional stability of C60 toward fragmentation and the possibility of energy equipartitioning among electronic and vibrational degrees of freedom after collisional energization was the subject of intensive investigation in the last years.1 Following the various excitation modes (electron impact, singlephoton excitation, heavy particle excitation, and surface collisions), C60 needs to acquire at least an energy of about 45 eV in order to allow ionization and subsequent fragmentation via sequential ejection of C2 units.2 Whereas a large number of ionization studies have been concerned with the determination of appearance and ionization energies for C60 parent ions (with reliable data sets up to charge state 3 from various laboratories3), only a limited number of studies on appearance energies of singly- and multiply-charged C60 (and C70) fragment ions have been reported so far. Besides two electron impact ionization studies from our laboratory (including appearance energies for singly-, doubly-, and triply-charged C60 fragment ions4 and appearance energies for singly-, doubly-, and triply-charged fragment ions for C70 and quadruply-charged fragment ions from C60 and C703) which were in good agreement with results for singly-charged fragment ions reported by Anderson and coworkers5-7 using a charge and energy transfer fragmentation technique, there exists only one other recent study dealing with ionization efficiency curves of singly-, doubly-, and triplycharged C60 fragment ions.8 These recent studies by Baba et al.8 are at variance (by almost a factor of 2!) with the previous studies.3-7 This is even more surprising as the two previous studies (refs 3, 4 and refs 5-7, respectively) are (i) in agreement with each other, (ii) in accordance with theoretical models employing RRKM and finite heat bath (FBH) calculations2,9 and supported by a very recent study of Kolodney et al.10 on the thermal energy dependence of electron impact induced fragmentation of C60. To clarify the origin of this discrepancy, we have studied here ionization efficiency curves under experimental conditions similar to those employed in the study † Guest professor at the Institut fu ¨ r Ionenphysik. Permanent address: Optisches Institut, TU Berlin, Strasse des 17. Juni 135, D-10623 Berlin, Germany. ‡ Permanent address: Dept. of Physical Chemistry and Fritz Haber Research Center For Molecular Dynamics, The Hebrew University of Jerusalem, Jerusalem, 91904 Israel. X Abstract published in AdVance ACS Abstracts, April 15, 1996.
S0022-3654(96)00025-1 CCC: $12.00
of Baba et al. It is possible to demonstrate that the extremely low appearance energies observed by Baba et al. (about a factor of 2 lower than in the other studies) are very likely due to an experimental artifact; that is, using electron currents of 100 µA allows the production of fragment ions via multielectron collision reactions, and therefore the measured appearance energies of Baba et al. are contaminated by these second-order reactions. Moreover, the method used by Baba et al. to deduce the activation energy for the evaporation of C2 from C60+ and other thermochemical considerations are not correct; for example, in the case of polyatomic molecular ions the activation energy cannot be set equal to the simple difference “fragment ion appearance energy minus parent ion ionization energy”. The present paper is structured as follows. After a short description of our experimental setup experimental results concerning electron impact ionization functions for the C60 fragment ions C58+, C56+, and C54+ will be presented for various experimental conditions, including those pertaining to singlecollision conditions (as used in our previous studies3,4) and to “multielectron collision conditions” similar to those used by Baba et al.8 From the dependence of the ionization efficiency curves on the electron current we will be able to show that for the electron currents used by Baba et al. the low-energy portion of the ionization efficiency curve is severely contaminated by multielectron collisions, thereby shifting the appearance energy way below the “true” value. Finally we will discuss additional arguments and evidence for these “true” values and further misconceptions in the paper of Baba et al.8 2. Experimental Section A detailed description of the experimental procedure used has been given previously.2-4,11-13 Briefly, the experimental setup consists of a modified Nier-type electron impact ion source,11 a fullerene molecular beam source,2 and a highresolution double-focusing (reversed geometry) Nier-Johnson sector field mass spectrometer. The performance and operating conditions of this apparatus (in particular those of the Niertype ion source) have been continuously improved over the past 15 years. Today it is possible to measure (absolute) partial ionization cross section functions for atomic and molecular parent ions as well as partial ionization cross section functions for fragment ions formed with excess kinetic energy with high © 1996 American Chemical Society
Appearance Energies of C60 Fragment Ions Revisited accuracy.11-13 Moreover, it has been used to measure appearance and ionization energies reported in refs 3 and 4. The fullerene sample (consisting of C60 powder of 99.99% purity) is heated in a Knudsen-type oven in a temperaturecontrolled mode. Typical oven temperatures under operating conditions range from 500 to 600 °C. The fullerene vapor produced by sublimation effuses through a 0.5 mm diameter orifice under molecular flow conditions. After passing through a skimmer, the fullerene beam enters the ion source via a molecular beam inlet of 3 mm diameter. The fullerene beam (with typical beam densities of 1014 cm-2 s-1 as measured with a piezoelectric deposition gauge) is crossed at right angles by the ionizing electron beam, which is itself collimated by a weak guiding magnetic field of approximately 0.3 T. The beam energy can be varied between close to 0 eV and around 1000 eV. The current-stabilized electron beam (beam currents can be set between 1 µA and 1 mA) has an energy spread of approximately 0.5 eV (fwhm). For calibration and test purposes (e.g. energy scale calibration using Ar as a reference) other gases can be introduced into the ion source either via a capillary leak gas inlet, thereby constituting a static gas target, or as a molecular beam from the opposite side of the fullerene oven via a supersonic nozzle expansion unit. Ions are extracted perpendicular to the electron and fullerene beam from the collision region through a slit in the ion source exit L1 (1.5 mm wide in the y-direction, 8 mm high in the z-direction) by an electric field penetrating into the ion source from an external extraction electrode L2.11 Under typical operating conditions, the ion source exit electrode, the pusher, and the collision chamber are kept at the accelerating voltage (typically +3 kV), whereas L2 is set at about 150-200 V below the accelerating voltage. Further electrodes are used for beam steering and focusing before the ions reach the end of the accelerating region at the grounded slit. After passing the mass spectrometer entrance slit S1 the ions are mass analyzed in a double-focusing mass spectrometer consisting of a 48.5° magnetic sector field (r ) 60 cm), followed by a 90° electric sector field (r ) 18.9 cm). The mass-selected ions are subsequently detected by a channeltron connected to a pulsecounting unit and a laboratory computer. Relative partial ionization cross section functions are obtained by correlating the mass-selected ion signals as a function of electron energy to the number of ions produced in the ion source under constant experimental conditions.11-13
J. Phys. Chem., Vol. 100, No. 21, 1996 8693
Figure 1. Ion current for the production of the fragment ion C56+ produced by electron impact ionization of C60 as a function of electron energy with electron currents of 10 µA (upper part) and 100 µA (lower part). The low-energy curve below 55 eV is also shown enlarged by a factor of 10 (and slightly shifted upward) for better visibility.
3. Experimental Results and Discussion Figure 1 and Figure 2 show as an example ionization cross section curves (in the low-energy region) for the production of the fragment ions C56+ and C54+ by electron impact ionization of C60, measured at two different electron currents. For both fragment ions there is a distinct difference in the apparent shape of the ionization cross section function measured. Similar differences have been observed for other fragment ions such as C58+, C52+, and C50+. Whereas at low electron currents (10 µA) the ionization efficiency curve consists of one peak at around 70 eV, at higher electron currents (100 µA) there exists a second (smaller) peak at approximately 50 eV. It has been shown by many others14,15 that the accurate shape of an ionization curve (including the threshold region and thereby the appearance energy) should be measured (besides taking into account other important ion source conditions, such as proper ion extraction) with electron beam currents in the ion source as low as possible. The general recommendation is to use for well-focused electron beams (guiding magnetic field) electron currents below 10 µA irrespective of whether a stagnant
Figure 2. Ion current for the production of the fragment ion C54+ produced by electron impact ionization of C60 as a function of electron energy with electron currents of 10 µA (upper part) and 100 µA (lower part). The low-energy curve below 55 eV is also shown enlarged by a factor of 10 (and slightly shifted upward) for better visibility.
gas target or a beam target is used. These low electron currents have been proven to be necessary in order to avoid possible influences of electron beam induced space charge effects on the dependence of the measured ion current on the electron energy; that is, at electron currents higher than the usual 10 µA the shape of measured ionization cross section curves is seriously deformed. This effect accounts for the observed change of the main peak (at around 70 eV) in Figure 1 and Figure 2 when going from 10 to 100 µA. Similar changes in the ionization
8694 J. Phys. Chem., Vol. 100, No. 21, 1996
Matt et al.
cross section curves have been observed not only for these fragment ions but also for example for argon ions. In passing we note that therefore our previous appearance energy measurements3,4 and recent absolute ionization cross section measurements16,17 have been carried out with electron currents around 10 µA. Thus, it is no surprise that the present appearance energies deduced from the threshold slope of the main peak (measured at 10 µA) are in good agreement with our previous determinations. As we will prove below, the additional peak at around 50 eV in Figure 1 and Figure 2 for electron currents of 100 µA is caused by an additional artifact. This artifact has not been studied in detail earlier, becausesof the reasons mentioned abovesionization cross section curve measurements were carried out only at electron currents of about or below 10 µA. Because Baba et al.8 measured their ionization cross section curves with electron currents of 100 µA and reported similar additional satellite peaks, from whose onsets they deduced their appearance energies, we felt obliged to clarify the origin of these extra features. For this purpose we have measured and plotted representative measures (i.e. the ion currents at 48 and 72 eV, respectively) of the first peak and the second peak as a function of electron current (Figure 3). As expected the C54+ fragment ion current (representative of the main peak at 72 eV) increases linearly with electron current (see Figure 3b), indicating first-order kinetics in accordance with the simple reaction
C60 + e f C54+ + 2e + 3 C2
(1)
The same behavior can be seen for the parent ion production (see Figure 3a),
C60 + e f C60+ + 2e
(2)
measured at around the main peak position of 48 eV. In contrast, the dependence of the low-energy satellite peak (measured at 48 eV) on electron current is strongly nonlinear (see Figure 3c), indicating higher order kinetics for the production process. Because a plot of the square root of the C54+ ion current (at 48 eV) versus electron current shows a rather linear dependence on electron current, we have to conclude that ions causing this low-energy peak are produced by a sequential stepwise ionization process involving two separate electrons. This conclusion is also supported by the data shown in Figure 4 exhibiting a linear dependence on electron current (in the extended range up to 300 µA) for the ratio between the C58+ ion current measured at the satellite peak position and the main peak position. The occurrence of this second-order reaction sequence clearly violates the requirements necessary for the determination of appearance energies. Thus, it is of no surprise that the apparent appearance energies measured under these conditions are way below the true appearance energies measured under appropriate conditions.3,4 As Baba et al. measured their ionization efficiency curves under experimental conditions similar to the present high-current mode, it is rather likely that also their curves are contaminated by higher order reaction sequences, thereby leading to erroneous appearance energies. It is interesting to point out that their satellite peaks are much more prominent than the present satellite peaks. The reason for this may be different ion extraction conditions in the two experiments (it is well documented that the ion extraction conditions are crucial for the measurements of accurate ionization efficiency curves14,15) and different electron current densities in the interaction region. Using higher electron currents, we
Figure 3. Ion currents versus electron current for C60+ (measured at 48 eV, a), for C54+ (measured at 72 eV, b), and C54+ (measured at 48 eV, c) produced by electron impact ionization of C60. Also shown is the square root of the C54+ ion current measured at 48 eV as a function of electron current (d).
can easily reproduce the relative importance of the satellite to the main peak as reported by Baba et al. After having thus clarified the origin of this low-energy satellite peak (appearing at higher electron currents) as being due to a sequential ionization process, it remains an interesting chore to interpret both the exact mechanism of the reaction sequence and the concomitant appearance energies. For this purpose we compare in Figure 5 for the fragment ion C58+ the threshold region of the low-energy satellite peak (designated by a full line) measured at 100 µA with the threshold region of the main peak (designated by a dotted line), measured at 10 µA, however, normalized and shifted downward to match the low-energy peak measured at 100 µA. From this comparison follows that the low-energy satellite peak appears at approximately half of the energy of that of the
Appearance Energies of C60 Fragment Ions Revisited
J. Phys. Chem., Vol. 100, No. 21, 1996 8695 observed due to the fact that cross sections for these targets are much smaller and therefore reactions with higher order kinetics are not abundant enough to be detected at the electron currents used. Moreover, on the low-energy side of the satellite peak in Figure 5 an additional weak feature can be seen which is interpreted as being due to ionization by three electrons. In accordance with this interpretation the onset of this small feature lies at about one-third of the main peak onset. 4. Conclusions
Figure 4. Ratio of the C58+ ion current measured at 37 eV and the C58+ ion current at 58 eV as a function of electron current.
Figure 5. Low-energy portion of the normalized C58+ ion current versus electron energy measured at 10 µA electron current, designated by the dashed line, and at 100 µA, designated by the solid line. Also shown for comparison is the C58+ ion current measured at 10 µA as a function of electron energy shifted downward to match the onset of the C58+ ion current measured at 100 µA (designated by the dotted line). Furthermore, the threshold region of the C58+ ion current measured at 100 µA is also shown enlarged by a factor of 20, designated also by a solid line.
main peak (a similar relationship has been found by Baba et al. for the appearance energies deduced from the first and second peak). From this and the electron current dependence we may conclude that in a first of two sequential collisions the neutral C60 will be ionized and slightly excited (see reaction 3a), and in a second collision the additional excitation energy (necessary for the production of the fragment ion C58+) is deposited (see reaction 3b). In both steps approximately half of the necessary appearance energy for the production of C58+ is deposited for those C58+ ions produced at the threshold of the reaction sequence 3a and 3b, i.e.,
C60 + e f C60+* + 2e
(3a)
C60+* + e f C58+ + 2e
(3b)
In principle, C58+ could also be produced in a sequential manner different from reaction 3, i.e., where the first electron impact event gives an excited neutral C60*, which then upon the second impact leads to dissociative ionization. In our opinion reaction 3 is the more likely one, because in this case the ion produced in the first step may be trapped in the space charge of the high current electron beam, and thus the probability for a second electron collision is increased. This process has been observed already in the case of the production of highly-charged fullerene ions18 and is, according to these studies, possible because of the very large cross section for the first reaction step (see data given in refs 16, 17) and because of the equally large cross section for the second reaction step.19 It is interesting to note that in both studies, the present and that of Baba et al., for smaller targets such as argon no satellite peaks have been
It was possible in this paper to demonstrate that the lowenergy peaks in the electron ionization efficiency curves of fragments from C60 observed by Baba et al.8 are very likely due to consecutive ionization and excitation processes made possible by the high electron currents employed for determining appearance energies. The “true” values are the “high” appearance energies reported earlier2-4 which were determined at low ionizing electron currents, thereby avoiding secondary processes. Moreover, there are in addition several misconceptions in the paper by Baba et al.8 which lead to wrong conclusions. For small molecules it has been very often assumed that the difference between the appearance energy of a fragment ion and the ionization energy of the molecule is equal to the critical energy of activation, and in the case where there is no reverse activation energy, this difference is equal to the endothermicity of the reaction. In molecules having a large number of degrees of freedom and/or large critical energies of activation these equalities cannot be assumed. Chupka has already shown in 195920 that an ion may have enough energy to dissociate but may not have time enough to do so. The “conventional” kinetic shift is defined as the excess energy required to observe detectable (1%) dissociation within 10 µs, the detection limit and time window being appropriate to conventional mass spectrometer appearance energy measurements.20,21 The “intrinsic” kinetic shift is taken as the energy needed for 10% fragmentation in comparison with radiative relaxation of the excited ion.21 C60 demonstrates large conventional1,2 and intrinsic kinetic shifts. The latter is due to a black body radiative cooling mechanism.22,23 As a result of these kinetic shifts, a simple comparison between experimental and theoretically calculated (by MNDO or MD) appearance energiessas has been done in Table 2 of ref 8sis not possible. Furthermore, appearance energies for C58+ cannot be calculated by adding the ionization energy of C60 to the activation energy for the C2 loss reaction, as has been done in Table 5 of ref 8. The wrong experimental appearance energies and the neglect of kinetic shifts lead to incorrect derivations of enthalpies of formation of neutral C60-2m species (Figure 14 in ref 8) and to the wrong conclusion that the dominant reaction mechanism for C60+ fragmentation is C2m loss rather than m sequential C2 losses. If we employ our own Cn+-C2 binding energies,9 we obtain based on ∆Hf(C60) ) 26 eV8 the following enthalpies of formation for neutral species: ∆Hf(C58) ) 25 eV; ∆Hf(C56) ) 23 eV; and ∆Hf(C54) ) 21 eV. The case for consecutive C2 losses as the dominant C60+* fragmentation mechanism is now well established.24 The conventional kinetic shift for C58+ is ∼34 eV,2,9 while the intrinsic kinetic shift is ∼30 eV.25 Acknowledgment. This work was supported in part by the O ¨ sterreichischer Fonds zur Fo¨rderung der wissenschaftlichen Forschung, by the Bundesministerium fu¨r Wissenschaft, Forschung und Kunst and by the Jubila¨umsfonds, Oesterreichische Nationalbank, Wien, Austria. Furthermore, this research was
8696 J. Phys. Chem., Vol. 100, No. 21, 1996 supported by a grant from the GIF, the German Israeli Foundation for Scientific Research and Development. References and Notes (1) Lifshitz, C. Mass Spectrom. ReV. 1993, 12, 261 and references therein. (2) Foltin, M.; Lezius, M.; Scheier, P.; Ma¨rk, T. D. J. Chem. Phys. 1993, 98, 9624 and references therein. (3) Wo¨rgo¨tter, R.; Du¨nser, B.; Scheier, P.; Ma¨rk, T. D. J. Chem. Phys. 1994, 101, 8674 and references therein. (4) Scheier, P.; Du¨nser, B.; Wo¨rgo¨tter, R.; Lezius, M.; Robl, R.; Ma¨rk, T. D. Int. J. Mass Spectrom. Ion Processes 1994, 138, 77. (5) Christian, J. F.; Wan, Z.; Anderson, S. L. J. Chem. Phys. 1993, 99, 3468. (6) Christian, J. F.; Wan, Z.; Anderson, S. L. J. Chem. Phys. 1992, 96, 10597. (7) Christian, J. F.; Wan, Z.; Anderson, S. L. J. Chem. Phys. 1992, 96, 3574. (8) Baba, M. S.; Narasimhan, T. S. L.; Balasubramanian, R.; Mathews, C. K. J. Phys. Chem. 1995, 99, 3020. (9) Wo¨rgo¨tter, R.; Du¨nser, B.; Scheier, P.; Ma¨rk, T. D.; Foltin, M.; Klots, C. E.; Laskin, J.; Lifshitz, C. J. Chem. Phys. 1996, 104, 1225. (10) Kolodney, E.; Tsipinyuk, B.; Budrevich, A. J. Chem. Phys. 1995, 102, 9263. (11) Stephan, K.; Helm, H.; Ma¨rk, T. D. J. Chem. Phys. 1980, 73, 3763. (12) Poll, H. U.; Winkler, C.; Margreiter, D.; Grill, V.; Ma¨rk, T. D. Int. J. Mass Spectrom. Ion Processes 1992, 112, 1.
Matt et al. (13) Grill, V.; Walder, G.; Margreiter, D.; Rauth, T.; Poll, H. U.; Scheier, P.; Ma¨rk, T. D. Z. Phys. D 1993, 25, 217. (14) Kieffer, L. J.; Dunn, G. H. ReV. Mod. Phys. 1966, 38, 1. (15) Ma¨rk, T. D. In Electron Impact Ionization; Ma¨rk, T. D., Dunn, G. H., Eds.; Springer: Wien, 1995; Chapter 5. Ma¨rk, T. D. In ElectronMolecule Interactions And Their Applications; Christophorou, L. G., Ed.; Academic Press: Orlando, 1984; Vol. 1, Chapter 3, p 251. (16) Du¨nser, B.; Lezius, P.; Scheier, P.; Deutsch, H.; Ma¨rk, T. D. Phys. ReV. Lett. 1995, 74, 3364. (17) Matt, S.; Du¨nser, B.; Lezius, M.; Deutsch, H.; Becker, K.; Stamatovic, A.; Scheier, P.; Ma¨rk, T. D. J. Chem. Phys., in press. (18) Scheier, P.; Ma¨rk, T. D. Phys. ReV. Lett. 1994, 73, 54. Scheier, P.; Du¨nser, B.; Scheier, P. Phys. ReV. Lett. 1995, 74, 3368; J. Phys. Chem. 1995, 99, 15428. (19) Vo¨lpel, R.; Hofmann, G.; Steidl, M.; Stenke, M.; Schlapp, M.; Trassl, R.; Salzborn, E. Phys. ReV. Lett. 1993, 71, 3439. (20) Chupka, W. A. J. Chem. Phys. 1959, 30, 191. (21) Huang, F. S.; Dunbar, R. C. J. Am. Chem. Soc. 1990, 112, 8167. (22) Kolodney, E.; Budrevich, A.; Tsipinyuk, B. Phys. ReV. Lett. 1995, 74, 510. (23) Mitzner, R.; Cambell, E. E. B. J. Chem. Phys. 1995, 103, 2445. (24) Echt, O.; Du¨nser, B.; Muigg, D.; Matt, S.; Scheier, P.; Ma¨rk, T. D. Proceedings International Symposium on Science and Technology of Atomically Engineered Materials; Richmond, 1995. (25) Laskin, J.; Behm, J. M.; Lykke, K. R.; Lifshitz, C. Chem. Phys. Lett., submitted.
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