J. Phys. Chem. 1995,99, 3020-3032
3020
Ionization and Fragmentation of
c60:
An Electron Impact Ionization Study
M. Sai Baba, T. S. Lakshmi Narasimhan, R. Balasubramanian, and C. K. Mathews" Materials Chemistry Division, Chemical Group, Indira Gandhi Centre for Atomic Research, Kalpakkam-603 102, Tamil Nadu, India Received: June 16, 1994; In Final Form: November 28, 1994@
Ionization efficiency curves of both singly and doubly charged fullerene ions (Cm-znrn+,n = 1-5, m = 1 and 2) were determined for the first time, employing electron impact ionization. Appearance energies of all these fragment ions were derived from these data. Relative abundances of the fragment ions were computed from the measured ion intensities and compared with those available in the literature. The results of the present investigation are compared with the data available in the literature on multiply charged and fragment ions. Based on the measured appearance energies of the fragment ions, ionization energies for the formation of doubly charged fragment ions are derived. Enthalpies of formation of Cm-zn (fragment) fullerenes are derived for the first time. The enthalpy data of fullerenes derived in the present work support the loss of C2,, as the mechanism of fragmentation.
Introduction
Ionization of fullerenes (mainly Cm and C70) has been a topic of much study by various workers, ever since these cagelike all-carbon structures were discovered. These molecules are considered as model compounds for the study of ionization as well as photophysical properties of large molecules. Apart from fundamental interest, the study of fragmentation of Cm is important in order to ascertain its survivability in extreme environments. Understanding of the fragmentation mechanism may give an insight into the process that leads to fullerene formation. These molecules upon ionization give rise to multiply charged as well as fragment ions. Fragmentation products are fullerenes whose masses are lower than that of the parent by multiples of C2 units, but whose yields are low. For example, multiply charged ions with a charge as high as +5 have been identified when Cm is ionized,'s2 and fragmentation of Cm yields Cm-2,+, where n = 1,2, ..., n. While the formation of multiply charged ions has been regarded as an indication of aromatic character, higher stability is thought to be the reason for the low yields of fragment ions. Even before the bulk synthesis of Cm, O'Brien et al.3 studied its fragmentation by photoionization and Radi et al.4-6 and Campbell et al.' studied the decomposition of various carbon clusters including c60. After a method for bulk synthesis of c 6 0 was established,8 Lifshitz and co-workers9-' investigated the kinetics of dissociation of Cm. Beck et a1.,12 Bussmann et al.,13 Wysocki et al.,I4 and Whetten et aL15 studied collisioninduced dissociation of c60' on different surfaces. Mowery et al.I6 carried out molecular dynamics simulations on surface collision-induced dissociation of c60. Wurz and Lykkel7-I9 studied various aspects of multiphoton excitation and ionization of C a . Demuro et aL20 have theoretically examined the mechanism of fragmentation. Stanton,21 Wang et a1.,22 and Novoa et al.23 used various theoretical methods to understand the fragmentation behavior of c60. McElvany and CallahanZ4studied the fragmentation process by inducing dissociation through collision with xenon atoms. Anderson and c o - w o r k e r ~ ~employed ~ - ~ ~ a similar method to study the fragmentation of c 6 0 and formation of endohedral complexes but used Ne+,25330 C+,27Li+, Na+, Kf,26s28 and N+ 29 @
Abstract published in Advance ACS Absfracrs, February 1, 1995.
as projectiles. Yo0 et aL31 employed vacuum W photoionization to study the fragmentation of Cm. Foltin et al.32 and Lezius et al.33 have discussed the formation of fragments (Cm-zn+, n = 1, 2, and 3) as a function of electron energy. McElvany et al.,34 in their review of the general aspects of fullerene research, have discussed ionization and fragmentation properties. G r ~ e has n ~ given ~ an overview of the interaction of Cm with surfaces, gas species, and photons. In spite of all these studies, the appearance energy (AE) of the fragments, Cm-zn+, with n > 3, are not yet determined from direct ionization studies. The same is the case with the formation of these fragment ions as a function of energy (either electron or photon). In a recent paper, Volpel et al.36reported the electron impact cross section function for the formation of Cm-znf fragments from Cm+. In our laboratory, we have been studying various aspects of the chemistry of fullerenes over the past 3 years. Employing mass spectrometry, we have determined some of the thermod y n a m i ~ and ~~-~~ properties of fullerenes (mainly Cm and c70). Electron impact ionization cross section of C6040 and C7041 and appearance energies of multiply charged ions of Cm (Cmm+,m = 1, 2, and 3)42and C70 (C70rn+,m = 1, 2, and 3)43 have already been reported. In this paper, we (i) report, for the first time, the appearance energies of singly and doubly charged Cm-zn+ fragments up to n = 5 , based on our experiments on the direct ionization of the equilibrium vapor over pure Cm, (ii) review the available information on both multiply charged and fragment ions in comparison with those determined in the present work, (iii) derive enthalpies of formation of various C60-2n fullerenes from the appearance energy (AE) data, and (iv) discuss the fragmentation mechanism in light of these results. Experimental Section
A VG Micromass MM 30 BK mass spectrometer system was used in the study. The molecular beam effusing out of a Knudsen cell was ionized by electron impact. The ionizing electron beam was set at an emission current (trap source) of 100 pA while the electron energy was kept at a constant value. (The energy could be varied from 0 to 80 eV.) The positive ions thus produced were accelerated to a potential of 3 kV and subsequently mass analyzed by a 90" sector, 30 cm
0022-3654/95/2099-3020$09.00/0 0 1995 American Chemical Society
+
J. Phys. Chem., Vol. 99, No. IO, 1995 3021
Ionization and Fragmentation of Cm
8
0 8 8
8
0
0
8
8 8
0
8 8
.
8 8
I
8
A
0 8
8
e
'
In+
... ..... ..
Ag+
t
*
I
i
Hg+ I
Ar+
b
He+
A
8
**
A
I
A A
8
b
0
8
A 8
--
b
8
8
0
I I
A
8
8
8 8
#
5 10 15
0
A A A A
A
4L A
10 15 20 15 20 25 Electron energy (eV)
25 30 35
Figure 1. Typical ionization efficiency curves of In+, Ag+, Hg+, Ar+, and He+ used for calibrating the electron impact energy scale. The inset shows the calibration curve.
radius, single focusing magnetic analyzer. The effusing molecular beam, the electron beam used for ionization and the extracted ion beam were mutually perpendicular. The focusing conditions were so chosen as to transmit the maximum number of selected ions into the analyzer. The entrance and exit slits of the mass analyzer could be adjusted to choose the resolution of the system. A pneumatically controlled shutter was placed between the effusing molecular beam and the electron impact ion source, thus making it possible to differentiate between the ions produced from the sample and the background gases present in the ion source. Ion currents were measured by a secondary electron multiplier. The multiplier gain could be determined by measuring the ion intensities by using a Faraday cup as well. Data acquisition and processing were carried out by using an IBM-compatible PC. Samples were contained in alumina Knudsen cells (id. 7.5 mm; 0.d. 10.0 mm; height 10.0 mm; orifice diameter 0.51 mm) kept inside a molybdenum chamber which was heated by electron bombardment. The vacuum shroud in which the furnace was located was cooled by chilled water (at a temperature of 280 K) passing through a double annular space built into the shroud. Sample temperatures were measured by a chromel-alumel thermocouple touching the base of the Knudsen cell. This was calibrated against the melting point of silver. The first ionization potentials of In+, Ag+, Hg+, Ar+, and He+ were used to calibrate the electron energy scale. In+,Ag+, and Hg+ ions were produced by the electron impact of the gaseous streams of their atoms effusing out of a Knudsen cell containing the appropriate pure metal. A gas inlet system provided with an inlet probe transducer and two inlet reservoirs of 0.05 and 1 dm3 allowed the admission of permanent gases through a capillary crimp into the ion source assembly. This sample inlet system was used for admitting Ar and He for producing the
ions AI+ and He+. Appearance energies of these reference elements were derived from the respective ionization efficiency (E) curves, and the measured values were plotted against the reported first ionization potentials?6 A least-squares-fitted equation obtained by pooling all these points was used for calibrating the electron impact energy scale. Figure 1 gives the linear portion of the IE curves of these ions. The calibration plot obtained from them is shown as an inset in the same figure. Pure Cm was used in these studies. The samples were prepared by the contact-arc method,8 and fullerene fractions were separated by using column chromatography. The procedure is described in detail e l ~ e w h e r e . ~ ~In. ~a' typical mass spectrometric experiment a sample was taken in a degassed Knudsen cell and heated to a predetermined temperature (generally 800 K) which was kept constant throughout the measurement. Ion intensities were monitored as a function of time to ascertain the constancy of the ion current. After ensuring that the equilibrium was established in the Knudsen cell (as reflected in the constancy of ion intensities), ionization efficiency (IE) curves of various ions were determined. The data were recorded by measuring the ion intensities as a function of electron energy from the threshold to 80 eV while keeping the temperature of the Knudsen cell constant. Appearance energies were derived by linear extrapolation of these IE curves.
Results and Discussion The mass spectrum of the equilibrium vapor consisted of peaks corresponding to Ca-bm+ (n = 0-5, m = 1 and 2), Ca3+, and traces of C ~ O +The . species were identified from their massto-charge ratio and relative abundances of their isotopes. The ratio of Cm+ to that of C ~ Ois+ about 4000. Such a high ratio is an indication of the excellent purity of Cm samples used in
3022 J. Phys. Chem., Vol. 99, No. IO, I995
Sai Baba et al.
J
c&
AEl
1-
, ,
,
w A E2
AE-I
i
1 4
AE1
Electron energy (eV) Figure 2. Ionization efficiency curves of singly charged fragment ions of
(260.
the present study. Among the various ions, the singly and doubly charged Cm ions were the most abundant, and the intensities of C W - ~ and ~ + Cm-2n2+were relatively low. While the fragment ions were relatively less abundant, the summed up intensity of all the C60-2nm+fragments being -30% of Cm+, the multiply charged ions constituted about 50% that of Cm+. After ascertaining the details of the mass spectrum, some runs were carried out at a low resolution to enhance sensitivity. In all cases, the IE curves were similar in shape and the appearance energies derived from them were also similar. In addition, the ratio of I(Cm-2,+) to I(Cm+) at low resolution to that of high resolution was also found to be 1. Studies on Singly Charged Fragment Ions. Shape of the Ionization EfSiciency Curves of &-2n+, n = 1-5. Figure 2 gives a typical set of ionization efficiency (E) curves for Cm-Zn+ ions. Two humps are present in the IE curves, the first one beginning at an energy of AE1 and the second showing up at energy AE2. Absence of such humps in the IE curves of Cm+ as well as those of the ions used for calibrating the electron impact ion source (shown in Figure 3) rules out the possibility of instrumental artifacts being the reason for the observed shape. High purity of the starting material rules out the possibility of higher fullerenes contributing to the intensities of C60-2nf. The
-
appearance of two humps can then be taken as an indication of formation of these ions through two different processes. Wurz and LykkeIg reported that, in multiphoton ionization, a large number of photons are absorbed, and a rapid conversion of electronic excitation to vibrational excitation takes place. Such vibrationally excited molecules can give rise to formation of these fragment ions at higher energies. This may be responsible for the shape of the IE curves and the appearance of the second hump. While the energy AE1 was taken as the threshold required for the formation of the fragments, the second at AE2 was taken as the threshold energy required to access the higher excited states. Appearance Energies of C60-Zn'. Figure 4 gives the linear portion of the IE curves on an expanded scale. The foot of the IE curves was found to be rather flat, with the flatness increasing with increasing n (in C M - ~ ~ +The ) . appearance energies (AEs) were determined by the linear extrapolation of these curves and are listed in Table 1. The estimated error in the measured appearance energy of C60' is f0.5 eV and that of C60-2,' is f 1 . 0 eV. The relatively high magnitude of AEs of C60-2n+ ions as compared to that of Cm+ was taken as an indication of formation of these ions due to fragmentation. As fragmentation involves breakage of bonds, the energy required for their formation should be higher than simple ionization. Comparison with Literature Data. The appearance energies available in the literature are summarized in Table 2 and given in Figure 5. An AE of 22.3 eV for the formation of the C58 fragment was arrived at from the photofragmentation studies of Cm by O'Brien et aL3 Volpel et al.36 determined the ionization cross sections for the formation of various ions from Cm+ due to electron impact by employing electron cyclotron resonance (ECR) coupled with a mass spectrometer. The energy at which the cross section approaches zero was taken as the ionization energy for that process. The ionization energy derived for the formation of c 5 6 + from Cm+ was found to be -15.7. The AE for the formation of c56+ directly from Cm was derived to be 23.3 eV, by adding the ionization energy of Cm+. This is in very good agreement with the AE reported in the present work. Stanton2' calculated, by the MNDO method, enthalpies of various fragmentation reactions of Cm up to the formation of c56. The appearance energies corresponding to the fragmentation reaction via loss of Czn are in reasonable agreement with the appearance energies (AE1) obtained in the present work. Similarly, the appearance energies derived from the theoretically calculated enthalpies of the reaction
by Yi et aL50 and Wang et a1.22are also in very good agreement with the appearance energies obtained in the present work. Anderson and c o - w ~ r k e r s ~studied ~ - ~ ~the collision-induced dissociation of Cm by using various reactants, namely C+, N+, alkali ions (Li+, Na+, K+), and rare gas Ne+. They have listed the appearance energies of fullerene ions in the case of studies with C+, N+, and Ne+. The values are listed in Table 3. In principle, one can obtain the AE for the reaction 2 (R2),
+
+
+
C2, efrom the collision-induced dissociation reaction 3 (R3), c60
= C60-2,
(2)
(3) where n = 1-8, by taking the known first ionization potentials of X from the literature. Accordingly, AE of Cm-znf as per reaction R2 is equal to the sum of AE of reaction R3 and the first ionization potential of X. The AEs calculated by taking the first ionization potential of C+ (11,260 eV), N+ (14.534
J. Phys. Chem., Vol. 99, No. 10, 1995 3023
Ionization and Fragmentation of Cm
0
1 0 2 0 3 0 4 0 5 0 W 7 0 # S O 1
Electron energy (eV) Figure 3. Typical ionization efficiency curves of In+, Ar+, Hg+, Ag+, He+, and Mn+. In+, Ar+, Hg+, Ag+, and He+ were used for calibrating the electron impact energy scale. The ionization efficiency curve of Mn+ was determined during the studies on Mn
+ MnTe system."8
I
Figure 4. Linear portion of the ionization efficiency curves of Cm-2.+ ions on an expanded scale.
eV), and Ne+ (21.560 eV) from ref 46 are also given in Table 3. The AEs derived varied depending upon the nature of the ion used for collision-induced dissociation. Among those, the AEs derived by using N+ are the lowest. The AE for a particular species increased when the collision ion was changed from N+ to C+ to Ne+ (see Table 3). For example, the AE of Csgf derived from their studies was found to be 17.53, 27.3, and 44.6 eV respectively for N+, C+, and Ne+. As the process involved is collision-induced dissociation, the IE values derived
need not be the thermodynamic threshold l9 However. it is difficult to understand the difference in the values obtained by using different ions for collision. The following inferences can be made from these results: (i) the AE of Csg+ can be as low as 17.5 eV, and (ii) probably each of the collisions, depending upon the mass and energetics of the collision ion used, access different channels for the formation of fragment ions. The shapes of IE curves obtained in the present work (see Figure 2) tend to support both these inferences.
3024 J. Phys. Chem., Vol. 99, No. 10, 1995
Sai Baba et al.
TABLE 1: Appearance Energy (AE) of the Cso-h+ Fragments in electronVolts suecies
AEl"
C60+
7.8 20.2 22.5 24.2 26.3 29.7
c58'
c56+ c54+ c52' c50+
TABLE 3: Appearance Energy (AE) of Cso-h+ as per the Reaction Cm X+ = C@-zn+ X, for X = N, C, and Ne, and the Appearance Energy of Fragments Derived from Them by Adding the Ionization Potential of X
+
AE2b
AE1 - AE2
47 54
26
60
36 39 39
AE of Cm-2,,+ as per the reaction CM = c60-2,' C2n
31
66 69
+ x+
species c 5 8
"AE1 is the first threshold energy shown in Figure 2. b A E 2 is threshold for the appearance of the second hump.
c 5 6
c54
c 5 2 c 5 0 c48 c 4 6
TABLE 2: Comparison of Appearance Energies (AE) of Ca-znf Fragments Available in the Literature mode of ionization/ method" PI E1
E1 E1 MNDO LDT MD method CID (N') CID (C+) CID (Ne+)
-
C44
CfT
CSS+ C56+ 40.8 45 50 20.2 22.5 '16 18.2 20.0b 28.3' 21.2 18.2 17.5 26.5 27.3 35.3 44.6 49.6
C54+ 58 24.2
CSZ+ CSO'
26.3
29.7
24.gb 37.0' 29.5 44.3 55.6
37.5 48.3 70.6
44.5 50.3 64.6
reference
Yo0 et al.3' Foltin et al.32 present work Ajie et Stanton2' Yi et al.50 Wang et Christian et al.29 Christian et Christian et
a PI = multiphotonionization, E1 = electron impact, CID = collision -induced dissociation, MNDO = modified neglect of differential overlap method, LDT = local density theory, and MD = molecular dynamics method. Assuming C, loss as per the reaction Cm = C~0-2~' C2,. Assuming nC2 loss as per the reaction c 6 0 = C60-2nf nC2.
+
+
+
X = N' 3 12 15 23 30
+x+
C+
Ne+
16 24 33 37 39 44
23 28 34 49 43 51 55 58
AEa of CW-?~+ as per the Reaction c 6 0 = c60-2,' C2n X = N+ C+ Ne+
+
17.5 26.5 29.5 37.5 44.5
27.3 35.3 44.3 48.3 50.3 55.3
44.6 49.6 55.6 10.6 64.6 72.6 76.6 79.6
a In arriving at these values, AEs of N+ (14.534 eV), C+ (1 1.26 eV), and Ne+ (21.56 eV) were taken from ref 46.
n
.-C Y
3
.-E C Y
.-
Electron energy (eV)
Figure 6. Comparison of the ionization efficiency curve C5g+ obtained in the present work and Foltin et al.32
0 40
42
44
46
48
50
n in
52
54
56
58
60
c&,
Figure 5. Comparison of the appearance energy of C60-2,+ available in the literature: A, Stanton;21B, present work; C, Christian et al.;29 D, Christian et al.;27E, Christian et F, Yo0 et al.;31G , Foltin et a1.32
Yo0 et aL3' reported an appearance energy of 40.8 eV for the formation of (258' based on their photoionization studies. The presence of C58' could be detected by them at 26.95 eV. However, the ratio of c58+to that of c60+was found to increase from 50.003 to 0.07 f 0.04 when the energy was increased from 26.95 to 40.8 eV (which prompted them to suggest the AE to be 40.8 eV). This amounts to an increase of ion intensity by a factor of 24 f 10. For a similar difference in the energy, the increase observed in the present study was by a factor of 8-10. The two are in reasonable agreement, especially since the values quoted by Yo0 et al. are very near the limit of their
detection capability. Such a slow increase of intensity when the energy is increased by more than 13 eV indirectly supports the shape of the IE curves obtained in the present work. The appearance energy derived by Yo0 et aL3' is in reasonable agreement with AE2, the energy at which second hump in the IE curve appeared in the present study (see Figure 2 ) . Foltin et aL3* determined the appearance energies of c58+, C S ~ +and , C54+ as 45, 50, and 58 eV, respectively. The high magnitude of these values prompted the authors to term them as apparent appearance energies. These apparent AEs are in reasonable agreement with the energy AE2 obtained in the present work. Figure 6 gives the comparison of IE curves of C ~ S obtained + in the present study and Foltin et al. The general features from AE2 up to 80 eV are very similar. The same is the case for the ions c56 and c54. The relative intensities of Ca-2n+ to that of Cm+ obtained in both the studies are also in agreement with each other (see Table 4) at higher energies. Vaporization of a fullerite sample (mixture of C a and c70) at 775 K from an oven lined with gold was used as vapor source by Foltin et al., whereas pure C a effusing out of a Knudsen cell at 800 K was used in the present study. We calculated the decrease in the intensity due to the nature of the sample (fullerite) and to the lower temperature (less by 25 deg). The simulated effect of these is shown in Figure 7. These two factors
J. Phys. Chem., Vol. 99,No. 10, 1995 3025
Ionization and Fragmentation of Cm TABLE 4: Comparison of Relative Ion Intensities of Fragments Produced by Direct Electron Impact Ionization of Ch&) in the Present Study and in Foltin et al.j2 E present work Foltin et al.3z (eV) at 800 K at 775 K I(C,+)lZ(C,+) 0.004 I(C5s+)II(Cm+) 50 0.005 I(c56+)/I(c58+) 70 0.8 1.o I(c54+)/I(c58+) 70 0.4 0.5 I(C5s+)Ir(C,+) 70 0.05 U I(c56+)lI(cKl+) 70 0.05 U a I(&+) was not given at 70 eV, and hence the ratio could not be
Cm at 800 K f i
I
calculated. x
TABLE 5: Comparison of Available Information on the Dissociation of CSO+= CSS+f CZand the Appearance Energy of Css+ as per the Reaction Cm = CSS+ CZ eactivation appearance energyb energyd methodmodel" (eV) (eV) ref eren ce photoionization/RRKM 6.0-6.5 13.6-14.1 Yo0 et aL3' 5.6 13.2 Wurz and LykkeI9 multiphoton ionization/RRKM ElmRKM (loose 7.2 14.8 Foltin et al.32 transition state) 14.4 Foltin et al.32 6.8 EIRRKM (tight transition state) EI/EEB 15.0 Foltin et al.32 7.4 -10.0 -17.6 Busman et surface CIDRRKM Radi et a1.6 KERD/statistical 4.6 & 0.5 12.2 phase theory 5.9 f 0.3 13.5 K10ts~~ EEB Lifshitz et aL9 4.6 & 0.5 12.2 KERDEEB 5.23 12.83 Sandler et al." KERDEEB 19.67 to be published53 12.07 estimate 12.4' 20.2 present work AE by E1
+ +
a RRKM = Rice-Ramsperger-Kassel-Marcus model, EEB = evaporate ensemble bath model, KERD = kinetic energy release distribution, AE = appearance energy, and E1 = electron impact. CZ binding energy of C, was taken as the activation energy. Calculated by subtracting the AE of c60+(7.8 eV) from the AE of c58+found as per the reaction Cm c 5 8 + + C2 e-. The appearance energy was calculated by adding the ionization potential of Cm+ (7.6 eV, ref 54).
-
+
can easily result in a decrease in the intensity by a factor of -10. As can be seen from the figure, this decrease in the intensity can easily mask the first hump of the ionization efficiency curves. It is interesting to note that the apparent AEk of C60-2n' reported by Foltin et al.32and Yo0 et al.31are higher than the AE of corresponding C60-2n2+ determined in the present work (see Table 8). Many workers (which include Foltin et al.32and Yo0 et aL3') have reported the activation energies for the formation of C58+ from c60'. Most of these calculations were based on either the Rice-Ramsperger-Kassel-Marcus (RRKM) model for unimolecular decay5I or the evaporation ensemble bath (EEB) model of K l o t ~ .From ~ ~ the binding energy of C2 in Cm, assumed in these calculations, the appearance energy for the dissociation reaction was calculated. The binding energies used by various workers and the AEs derived from them (by adding AE of C60f to the binding energy) are listed in Table 5 along with the details of the methodexperimental technique used. The reported values varied from 4.6 to 10 eV. By using the same model, different binding energies were derived by different workers. For example, application of the EEB model led to activation energies varying from 4.6 to 7.4 eV. Similarly, application of the RRKM model resulted in activation energy varying from 5.6 to 7.2 eV. This perhaps points to the uncertainty of these methods. The minimum energy required for the dissociation of c 6 0 into c58 and C2 as per the reaction 4
z c
.-C -c0
I
0
0
10
20
30 40 50 Electron energy (eV)
60
70
I
Figure 7. Effect of sample being (a) Cm at 800 K and (b) fullerite at 775 K on the ionization efficiency curve. The data given are for the ion C58+. 6 '0
= ' 5 8 + 2'
(4)
cannot be less than the enthalpy of formation of C2. The actual value should be higher by the difference in the enthalpy of formation of C5g and Cm. Most of the binding energies assumed are less than this value. These aspects are discussed e l ~ e w h e r e . ~ ~ Comparison of Relative Abundances. As stated already, a number of researchers have studied the ionization of C a ; the results pertinent to the fragmentation of Cm are given in Table 6. The comparison was restricted only to those studies in which relative intensities are given at least at one energy. The fragmentation yield (relative intensities of the fragment ions with respect to c60+) generally varied from 5% to 10%. Ajie et al.49 and Walter et a1.2 obtained somewhat lower values. It can also be seen that the extent of fragmentation is high when one resorts to collision-induced andor related ionization methods. In such cases the yields varied from 10% to as high as 83%. Takayamam found the fragmentation yield to increase from 5.8% to 83%, when the mode of ionization was changed from electron impact to gas phase fast atom bombardment. It is also interesting to note that Drewello et al. observed fragments (to the extent of 6.8%) while using electron impact i ~ n i z a t i o n , ~ ~ whereas they did not observe any Cm-zn+ fragments while using single photon synchrotron radiati~n.~'Interestingly, C60-2n2+ fragments were detected. In another study, Srivastava et al.61 did not observe any C60-2~+fragments using electron impact ionization up to an energy of 200 eV, though they could detect Ca-2n2+ fragments at that energy. Table 7 gives the comparison of the intensities of the fragments relative to C a f . The order of C M - ~ fragments ~+ according to their abundance of formation was found to be c 6 0 >> C58 L c 5 6 > C54 > C52 1 C50. Studies on Doubly Charged Fragment Ions. Ionization n= EfJiciency Curves and Appearance Energies of C~O-Z,,~', 1-5. Figure 8 gives the linear portion of the IE curves of the doubly charged fragments. The appearance energies derived from them by linear extrapolation are given in Table 8. The estimated error in the measured appearance energy of C6O2+is f0.5 eV, and that of Cm-zn2+ is f l . O eV. Figure 9 gives a comparison of appearance energies of singly and doubly charged fragment ions determined in the present work. The ionization
3026 J, Phys. Chem., Vol. 99, No. IO, 1995
Sai Baba et al.
TABLE 6: Total Abundance of C a - h + Fragments with Respect to Ca+, As Reported by Various Worker@ mass spectrometeP
mode of ionization‘
energy (eV)
vapor sourced
T (K)
relative abundanceg
reference
magnetic sector (DF) magnetic sector hybrid triple stage time-of-flight quadrupole triple quadrupole magnetic sector (DF) magnetic sector (DF) time-of-flight magnetic/quadrupole (DF) magnetic sector (DF) magnetic sector time-of-flight triple sector triple quadrupole magnetic sector
E1 E1 E1 SPI E1 E1 E1 G-FAB E1 E1 E1 PI LI CID CID E1
IO IO
TD TD TD oven TD TD TD TD oven TD oven TD LD oven TD
573 613 648e 903 673 1073 673 673 613 773 775 873
11 1 9
Luffer et al.55 Ajie et al?9 Drewello et aLS6 Drewello et aL5’ Cox et a1.58 Ben-Amotz et al.59 Takayama et aLm Takayama et al.m Srivastava et aL6I Walter et aL2 Foltin et al.32 Yo0 et al.31 Ulmer et a1.62 Christian et aL30 McElvany et aLZ4 present study
70 70 70 70 70 70 100
IO 40.8 70 200 70
KE
600 673 800‘
f
8 10 6 83 d 2 -1 1 2 10 62 13
The sample used by most of the workers is a mixture of Cm and C ~ O* .DF = double focussing. SPI = single photon ionization, G-FAB = gas phase fast atom bombardment, LI = laser ionization (excimer laser 248 or 398 nm), CID = collision-induced dissociation, E1 = electron impact. TD = thermal desorption, LD = laser desorption, KE = Knudsen effusion, and oven = vaporization from an oven. e The sample is pure C60fDid not observe Cm-2,+ fragments. 8 Relative abundance = 100 x W(cm-k+)/l(C~)+);the intensities were either read from the plots of relative intensities or calculated from the measured intensities.
TABLE 7: Comparison of Ion Intensity Ratios of Fragment to that of Ca Obtained by Various Workers mode of energy 100 x I(C,+)/l(Cm+)for n = ionization” (eV) 60 58 56 54 52 50 E1 E1 E1 E1 E1 E1 G-FAB CID sPI PI
70 70 70 50 70 70
100 100 100 100 100 100 100 200 100 70 100 40.8 100
5.0 4.4 0.7 3.8 3.8 4.4 4.4 1.1 0.5 5.0 5.0 2.0 3.3 2.5 26.7 26.7 15.0 30.0 20.0 10.0 3.3 3.7 1.7 7.0 4.0
reference
0.3 0.3 Lufferet Cox et al.58 Ben-Amotz et aLS9 Foltin et al.32 0.8 0.5 present work Takayamam 10.0 5.0 Takayamam 2.2 McElvany et aLZ4 Drewello et al.5’ Yo0 et al.31
a E1 = electron impact, G-FAB = gas phase fast atom bombardment, CID = collision-induced dissociation, SPI = single photon ionization.
energies (as per reaction 5)
can be readily derived by taking the difference in the appearance energies of singly and doubly charged fragment ions. The result is shown in Figure 9 as well as in Table 8. It is observed that the value is more or less constant, except for Cso2+. The only other ionization energy available in the literature, apart from those for the formation of Cm+ and C ~ O +is, that of c 5 6 + determined by McElvany et al.34employing a charge transfer bracketing technique . The agreement between the values derived in the present work and those given by McElvany et al. is very good. The threshold energy for the formation of c56” from Csof due to electron impact derived from the ionization cross section data of Volpel et al.36is 25 eV. This amounts to an AE value of 32.6 eV for the formation of c562+ from Cm. This value is somewhat high. The high value could be due to the degree of curvature near threshold reported by Volpel et a1.36 It is for the first time that the ionization energies for the formation of C60-2n2+,n = 0-5, from the singly charged species are being reported. It was found that the differences between the appearance energies of Cm-zn+ and Cm+ on the one hand and between Cm-zn2+and c602+on the other are approximately equal. This is taken as an indication that the Cln binding energies in both singly and doubly charged CM ions are similar. Relative Abundances Of C60-2,,~+,n = 1-5. Though appearance energies of Ca-zn2+ are being reported for the first time
up to n = 5 in the present work, many reports appeared in which relative intensities of the doubly charged fragment ions are given. Table 9 gives the comparison of relative abundances of these fragments. As is the case with singly charged fragment ions (see Tables 6 and 7), fragmentation yields are very low for electron impact ionization. The total yield is in good agreement among them. Both for multiphoton ionization at higher fluence and for ionization by gas phase fast atom bombardment in the gas phase, the fragmentation yield is very high. A yield as high as 5.56 was obtained by Wurz and LykkeI9 using 212.8 n d a s e r at 193 mJ/cm2. Similarly, the fragmentation yield was found to vary depending upon the ion and its energy, in the gas phase fast atom bombardment studies of Takayama.60.66 These two observations can be taken as confirmation of the involvement of different excited states in the formation of these fragment ions. Available Information on the Formation of Multiply Charged Ions of C ~ OEarlier . we reported42the AEs of Cum+,m = 1, 2, and 3. In all these IE curves were rather flat at the foot, and this led to a considerable error in the AEs derived from them. To overcome this problem, two approaches were adopted. In the first of these, the threshold region of the IE curve was resolved into two straight lines, and the AEs were derived by extrapolation of the lower energy line to the energy axis. A similar method was adopted by Srivastava et aL6’ Alternately, the ion intensities were plotted on a log scale and the electron energies on a linear scale; the appearance energy is taken as that energy at which the ion intensity approaches the background value. This method is similar to the one employed by Foltin et al.32 The AE derived adopting such a method agreed with that derived from the method of linear extrapolation described above. The reevaluated values are listed in the Table 10 along with other available AEs of C m m f ,m = 1, 2, 3, and 4. Appearance Energy of c60’. Rohlfing et ai?’ obtained the AE of Cm+ in 1984 from fluence measurements in their studies on various carbon clusters produced in situ in the mass spectrometer. A number of theoretical methods were employed to obtain the appearance energies. The data varied from 4.5 to 7.6 eV. Zimmerman et al.54 employed the charge transfer bracketing method in combination with the Fourier transform ion cyclotron resonance technique and determined appearance energies for various singly charged carbon clusters C , from n = 50 to 70. With the availability of a method for bulk production of fullerenes,*more data on the ionization properties of these molecules appeared in the literature. There is a fair
J. Phys. Chem., Vol. 99, No. IO, 1995 3027
Ionization and Fragmentation of Cm
1
10 20
30
20
30
30 LO
Electron energy (eV) Figure 8. Linear portion of the ionization efficiency curves of
C60-2:'
TABLE 8: Appearance Energy (AE) of the Cm-h2+ Fragments in electronvolts species AE IE(2)" species AE IE(2)"
32 36
n
3 28-
v
$240
.
C
8C 2
20-
-
16-
0
312: 8-
41
A
-4i3
io
s'2
s'4
e;
sb
do
h
n in C. Figure 9. Appearance energy of singly and doubly charged fragment ions c 6 0 derived from the present work.
agreement among the values determined employing various modes of ionization. A value of 7.6 can be taken as the average value. Figure 10 gives the IE curves of Cm+ obtained by various workers. Though there is a general agreement for AE among various workers, the same is not true for the shape of the ionization efficiency curves. A significant difference between electron impact ionization and photon ionization is the steep
ions on an expanded scale. decrease in the intensity of c60+ in the IE curves above 20 eV in the latter case. Yo0 et a L 3 I in their photoionization experiments observed a steep decrease in the intensity above 25 eV. Hertel et aLg6 also observed a similar drop in the intensity above 20 eV in their studies employing synchrotron radiation (single-photon ionization). The surprising feature, however, is that the ion current drops to almost zero at energies higher than 35 eV. The absence of such a drop in the IE curves obtained by using electron impact is an indication that the cross sections for these two processes are vastly different above 2025 eV. A similar drop was not seen in the IE curve of the doubly charged ion of c 6 0 determined again employing synchrotron single-photon ionization studies by Drewello et alS5' Among the IE curves obtained using electron impact ionization, only Lezius et al.33reported smooth variation of the intensity as a function of energy. Appearance Energy of G o 2 + . Unlike in the case of c60+, there is not much agreement in the AE of Cm2+determined by various workers. Generally, the values can be divided into two groups, one centering around -19 eV (see Table 10) and the other at a lower value of around -16 eV. The data obtained by charge stripping experiments were higher with the exception of that by Mathur et a1.8' They attribute the amount of recoil energy imparted to the target during collisional interaction to be responsible for such high values reported by various workers in charge stripping experiments. This could lead to overestimation by an extent of up to 4 eV in the AE, when the collisional energy used are of the order of 8 kV.g' All the studies in which noticed a flattening at the IE curves were determined3',42,6'.82 the foot of the curve, and perhaps this is characteristic of these ions. Srivastava et al.,61 employing electron impact ionization, observed such a flattening at the foot of the curve and referred to the energy obtained by linear extrapolation of the first part of the IE curve as the appearance energy. Yo0 et aL3' also observed enhanced curvature in their photoionization studies. Steger et aLX2noticed a change of slope in the IE curve 2 eV above the AE in their studies using single-photon ionization (synchrotron radiation). We have also encountered a similar situation.42 We have resolved the initial part of the IE curve
3028 J. Phys. Chem., Vol. 99, No. IO, 1995
Sai Baba et al.
TABLE 9: Relative Abundances of Doubly Charged Ca Fragment Ions relative abundances' of C60-2,~+of n = methodkechnique"
0
1
2
3
4
5
E1 at 70 eV E1 at 70 eV E1 at 70 eV E1 at 70 eV gas phase FABb (i) He, 7 kV (ii) He, 8 kV (iii) Ar 8, kV (iv) Xe, 8 kV MPI 212.8 nm lase$ (i) 29 mJ/cm2 (ii) 69 mJ/cm2 MPI (i)355 nm laser (ii)193 nm laser CID 200 keV with H2
100 100 100 100
5 14 7 15
5 10 1.8
0.7 3.5
0.3 2.7
0.3 3.1
100 100 100 100
59 74 16 15
65 83 17 17
41 69 20 29
44 59 29 20
46 61 39 32
26 1 346 121 113
100 100
65 82
70 114
43 110
35 118
38 132
25 1 556
100
65 21 41
62 29 38
43 18 31
43 21 28
65 37 22
27 8 126 160
total yieldd
reference
11 33 9 15
Luffer and Schrams5 present work Ben-Amotz et al.59 Takayamabo Takayamam Takayamab3 Wurz and Ly!&eI9
100 100
Ding et aLM Hvelplund et al.65
E1 = electron impact, FAB = fast atom bombardment, MPI = multiphoton ionization, and CID = collision-induced dissociation. Fragments ~ O=* 1-5). + up to C32' were detected. 100 x C~-2>+/C60~+. 100 x ~ C ~ - Z , , ~ + / C(n
into two straight lines and taken the lower value obtained by extrapolating the flatter line to the X axis as the AE. Appearance Energy of Cm3+. Very limited data are available for the AE of this ion. Among the available data, the AE recommended in the present work is the lowest. Recent data of Volpel et al.36 are somewhat higher. For the reasons discussed above regarding the AE of Cm2+,we believe the lower AE to be more reliable. Appearance Energy of C604+. For large polycyclic aromatic hydrocarbons a linear relationship between successive ionization energies has been proposed by Smith.88 Walter et aL2 and Javahery et al.85 employed this relation to derive the AE of C604+. We have also employed the same relation to the AEs recommended in the present work for &Om+, m = 1, 2, and 3 , to derive the AE of Cm4+. These are listed in Table 10. We found that the plot of ionization energy versus successive ionization ( m in Cmm+)did not yield a very good linear fit. Javahery et al. raised doubts whether multiply charged C ~ can O be considered large enough to apply Smith's model. Hence, the derived AE should be considered as a rough estimate. The AE derived from the ionization cross section data of Volpel et al.36is the first data based on experimental results. Volpel et al. pointed out that the determination of ionization cross section for the process
c6,3++ e-
= c6:+
+ 2e-
was complicated due to high background rate which resulted in poor signal-to-noise ratio. Thus may lead to considerable error in the AE derived from their data. Probably a more reliable experimental determination is warranted. Figure 11 gives a comparison of the appearance energies of multiply charged ions of c60. For clarity, we have restricted the plot to the AEs obtained by Zimmerman et al.,54 Lifshitz et al.,79 Javahery et al.,85 Walter et a1.,2 Volpel et al.,36 and the present study. Thermochemistry of Fragmentation. Enthalpy of Fonnation of C60-2, Fragments. The observed fragment ions can be formed by two mechanisms: sequential loss of C2 units (as per reaction RM2) or the elimination of a Czn(as per reaction RM1).
From the appearance energies of the fragments derived in the present work, the enthalpy of formation of the fragment
fullerenes can be calculated. The procedure adopted is described below. The enthalpy of any reaction is equal to the difference in the sum of the enthalpies of products and the reactants. If the fragmentation path is as per the reaction RM1, then enthalpy of formation of the fragment C M - is ~ ~given by the relation m(c6i)-2n)
=
AE(RM1) + m(c60)
- M(c2,)
- 1p(c60-2n)
Similarly for reaction RM2, the enthalpy of formation of the fragment C60-2, is given by the relation m(c6(l-2n)
= m(RM2)+ M(c60)- nM(C2) - 1p(c60-2n)
In order to calculate the enthalpy of formation of the fragment ions, the following input data are required: (i) AE for the formation of fragment ions, (ii) enthalpy of formation of CZ,, (iii) ionization potential of C W - ~and ~ , (iv) enthalpy of formation of C a . The appearance energies of the fragments have been determined in the present work. The enthalpies of formation of small carbon clusters available in the literature are given in Figure 12. The data of Martin et al.89 were chosen for the present calculation, as they are in agreement with the experimentally determined enthalpies for C, up to n = 7. The ionization potentials of various Cm-2,+ (n = 0-5) fullerenes were taken as those determined by Zimmerman et al.54 The enthalpy of formation of c6O(g) can be calculated from the enthalpy of formation of Ca(so1id) and the enthalpy of sublimation of Cm. We have earlier determined the enthalpy of sublimation of C60.37 The enthalpy of formation of C a (solid) has been reported by Steele et al.,94 Becchaus et al.,95Kiyobayashi and S a k i ~ a r n a ,and ~ ~ Diogo et al.97 There is good agreement among the values determined by the latter three workers. We have chosen the value recommended by Diogo et al. for the present calculation. The input data selected are shown in Figure 13. The enthalpies of formation for G j 0 - 2 ~ fullerenes calculated for both the reactions (RM1 and RM2) are given in Figure 14. A choice of any other data for AH(C2,) and AH(&) does not alter the variation of the enthalpies as a function of the number of carbon atoms. Vibrational excitation of the fullerene molecules and ions can contribute to the errors in the measured fragmentation energies.
J. Phys. Chem., Vol. 99, No. 10, 1995 3029
Ionization and Fragmentation of Cm
TABLE 10: Comparison of Appearance Energy of Cam+,m = 1,2,3, and 4, Available in the Literature methodkechnique" Cm laser vaporization of graphite/laser ionization PRDDO DV-X method MNDO Hartree-Fock calculation CNDOlS laser vaporization of graphitellaser ionization CNDOlS EM-ZRP charge transfer bracketing experiments using FTICR-MS VUV-photoelectron spectroscopy VUV-PI MS SPI MS EMS EI/MS H2 laser ionization (VUVI-TOF MS EI/MS Cm
AE (eV)
Year
reference
'5.0 4.5 6.38-6.42 7.45-8.41 7.9-8.2 7.35-7.55 6.42-7.87 7.55 7.6 7.61 7.6 7.57 7.58 8.1 7.8 7.8 7.8
1984 1986 1986 1986 1986 1987 1988 1989 1991 1991 1991 1992 1992 1992 1993 1993
Rohlfing et al.67 Marynick et aL6* Hale69 Newton70 Luthi7' Larsson7* Cox et al.73 R~sen~~ Gallup75 Zimmerman et al.54 Lichtenberger et al.76 Yo0 et aL3I deVries et aL7' Sai Baba et al.40 Srivastava et aL61 Zumwalt et aL7* present work
1989
Rosen et al.74
1991 1991 1991 1992 1992 1992 1992 1992 1992 1993 1993 1993 1993
Gallup75 Lifshitz et al.79 Cox et al.58 McElvany et al.34 Caldwell et aLS0 Mathur et a1.*' Steger et a1.82 Yo0 et aL3I Petrie et aLS3 Hrusak et aLa4 Srivastava et aL6' Volpel et Sai Baba et a1."2 present workb
1991 1993 1993 1993
Lifshitz et al.79 Javahery et aLg5 Volpel et a1.36 Sai Baba et al.42 present workb
1992 1993 1993 1993
Walter et aL2 Javahery et aLS5 Volpel et al.36 present work
+ e- = Cm+ + 2e-
+ e- = Cm2++ 3e-
CNDOlS
17.7 18.4 18.4 19.86
3 4 . 0 0 -
t p -
OLifshitz et al.
+
O 30ZI
g c 3 20-
E 5L
+Zimmerman et a!. 0 0
1
2
m in
3
caw
4
5
' 1 ' i ' i ' b n in
Figure 11. Variation of appearance energy of Cmm+as a function of m.
.
lb
'
2
C.
Figure 12. Enthalpies of formation of CZ,, clusters (small) available in the literature.
30). Takayama could detect both even and odd fragments in the low mass range for n 15 with the Czn+l+ being relatively more predominant. Gruen et a1.,lo1using microwave spectroscopy studies on plasmas containing fullerenes, reported the detection of Cz in the fragmentation products. This is the first study in which the presence of C2 alone was detected. Lykke et al.Io2 also reported the detection of C+ in their multiphoton ionization studies. A fragmentation process via sequential loss of C2 necessitates the fragment yield to decrease considerably with increase in the extent of fragmentation. The reported abundances of the fragments are not consistent with this (see Tables 6, 7, and 9). It can be seen from Table 7 that the relative yields are approximately equal for c 5 8 + and C56+ as well as for C52' and CSO+. It is in fact interesting to note that the yield increased with increasing n in some cases for the doubly charged fragment ions (see Table 9).
It is also found that the C2 binding energies ( E ) for various ~ ~Wang et a1.22 fullerenes are not very different. K l o t ~and derived the C2 binding energies f o r various C60-2n fullerenes. These are fitted to an equation of the type E(Cw-2,J = yE(Cw). For n = 0-5, the average value of y obtained from Mots data was 0.92 and that of Wang et a1.,220.8. All these observations point toward the choice of Cz,, loss to be the mode of fragmentation. The enthalpies of formation of C60-2n fullerenes derived in the present study tend to support the CZ,,elimination mechanism. This is clear from the variation of the enthalpy of formation of fullerenes as a function of the number of carbon atoms shown in Figure 14. If sequential loss of C2 is assumed , the enthalpy of formation of these fullerenes should increase linearly with carbon number up to Cs8 and then decrease at Cm. This is at variance with the available information on the stabilities of these molecules. Whereas, if Czn loss is assumed, C ~ has O a lower
Ionization and Fragmentation of Cm
AE
32 2s
of
c+-,
J. Phys. Chem., Vol. 99, No. 10, 1995 3031
40-
fragments
3 24-
.-c
0
g
8 32-
AZO-
5 T -
5 16-
-
E
-
0)
'c
r
t ~
Enthalpy of format n of
h
1 -36-
Enthalpy of formation
-
"\
Present w o k
, C
$28-
128-I
40
z --
\
24
&,,
AE of
I
I
(simple ionisotion) I
I
I
--
1
1 2
n in C.
Figure 13. Enthalpies of formation of C2,, and Cm and appearance energy of Cm-2,+ formed due to simple ionization and fragmentation (input data for calculating the enthalpies of formation of C60-2~ fragments).
-3
32
-
v 20-
Figure 15. Comparison of enthalpies of formation of C@-2,, with those available in the literature.
for m = 1 and 2 and n = 0-5; (ii) the ionization efficiency curves of these ions; (iii) relative intensities of singly and doubly charged fragment ions; (iv) enthalpies of formation of various fullerene molecules C M - ~for ~ n = 0-5; and (v) simple thermodynamic reasoning which enabled us to explain the mechanism of fragmentation which (in our opinion) involves the direct loss of Czn. Acknowledgment. We thank Dr. P. R. Vasudeva Rao and his group for the preparation and the purification of Cm samples. We also thank Dr. R. Viswanathan for useful discussions and assistance in the mass spectrometric measurements. We thank Mr. K. C. Srinivas and his group for the maintenance of the mass spectrometer.
c
.J" 24-
E6 20--
'c
c
16-
-2 -
References and Notes
2 121 5 Y
8-
I
(1) Doyle, R. J.; Ross, M. M. J. Phys. Chem. 1991, 95, 4954. (2) Walter, C. W.; Bae, Y. K.; Lorents, D. C.; Peterson, J. R. Chem. Phys. Lett. 1992, 195, 543. (3) O'Brien, S. C.; Heath, J. R.; Curl, R. F.; Smalley, R. E. J. Chem. Phys. 1988, 88, 220. (4) Radi, P. P.; Bunn, T. L.; Kemper, P. R.; Molchan, M. E.; Bowers, M. T. J. Chem. Phys. 1988, 88, 2809. (5) Radi, P. P.; Hsu, M. T.; Brodbelt-Lustig, J. ; Rincon, M.; Bowers, M. T. J. Chem. Phys. 1990, 92, 4817. ( 6 ) Radi,P. P.; Hsu, M. T.; Rincon, M. E.; Kemper, P. R.; Bowers, M. T. Chem. Phys. Lett. 1990, 174, 223. (7) Campbell, E. E. B.; Ulmer, G.; Busman, H. G. Hertel, I. V. Chem. Phys. Left. 1990, 175, 505. (8) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 354, 347. (9) Lifshitz, C.; Iraqi, M.; Peres, T.; Fischer, J. E. Int. J. Mass Spectrom. Ion Processes 1991. 107, 565. (10) Lifshitz, C.; Gotkis, I.; Sandler, P.; Laskin, J. Chem. Phys. Lett. 1992, 200, 406. (1 1) Sandler, P.; Lifshitz, C.; Klots, C. E. Chem. Phys. Lett 1992, ZOO, 445. (12) Beck, R. D.; St. John, P.; Alvarez, M. M.; Diederich, F.; Whetten, R. L. J. Phys. Chem. 1991, 95, 8402. (13) Busmann, H. G.; Lill, Th.; Reif, B.; Hertel, I. V. Surf: Sci. 1992, 272, 146. (14) Wysocki, V. H.; Ding, J. M.; Jones, J. L.; Callahan, J. H.; King, F. L. J. Am. SOC.Mass Spectrom. 1992, 3, 27. (15) Whetten, R. L.; Yeretzian,C. Int. J. Mod. Phys. E 1992, 6, 3801. (16 ) Mowrey, R. C. ; Brenner, D. W.; Dunlap, D. I.; Mintmire, J. W.; White, C. T.; J. Phys. Chem. 1991, 95, 7138. (17) Lykke, W. R.; Wurz, P. J. Phys. Chem. 1992, 96, 3191. (18) Wurz, P.; Lykke, K. R. J. Phys. Chem. 1991, 95, 7008. (19) Wurz, P.; Lykke, K. R. J. Phys. Chem. 1992, 96, 10129. (20) De Muro, R. L.; Jelski, D. A.; George, T. F. J. Phys. Chem. 1992, 96, 10603.
4 1 0 48
50
52
56
54
n
50
60
~
in C.
Figure 14. Enthalpies of formation of C60-2~calculated assuming nC2 loss and Czn loss in the fragmentation.
enthalpy of formation than other Cm-zn fullerenes. The enthalpies of formation of various fullerenes calculated by Stanton2I and Wang et al.22are consistent with this argument. Therefore, we conclude that Cznelimination is the more probable mechanism for fragmentation. Figure 15 gives the enthalpy of formation of C60-2n data available in the literature. Even though MNDO calculations of StantonZ1 overestimate and Wang et al. calculations** underestimate the enthalpy data, the general variation is in good agreement among all the three. It was found that the difference (m(C60) - m ( C 6 0 - 2 ~ )is, approximately the same. Conclusions
In the present study, we have ionized the equilibrium vapor over c 6 0 solid by electron impact ionization to obtain the following new results: (i) The appearance energies of C60-2nm+
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