J . Phys. Chem. 1990, 94, 4595-4599
4595
0.2% and 1% behave similarly, so such a pathologically large shift is unlikely. Thus, as in the case of the density of the LE phase, our results are more consistent with those of Pallas and Pethica than with Kim and Cannell’s. The mean-field exponents that Kim and Cannel1 measured must now be seriously doubted. Neither our data nor those of Pallas and Pethicals are sufficiently close to the critical point or sufficiently precise to allow distinguishing between mean-field and lsing exponents.
and light-scattering measurements of Winch and Earnshaw.jl
Note Added in ProoJ The first-order nature of the LE-LC transition in PDA has also been demonstrated by the isotherm
(31) Winch, P. J.; Earnshaw, J . C. J . Phys.: Condens. Matter 1989, I , 7 187.
Acknowledgment. This work was supported by National Science Foundation Grants CHE-8902354 and INT-8413698 and by a CNRS-Elf scholarship for S.A. We thank Dr. Keith Stine for helpful discussions and for his assistance with the image analysis of the LE-G transition. We are indebted to Prof. R. S. Williams for the use of the image analysis system.
Carbon Cluster Emission from Polymers under Kiloelectronvolt and Megaelectronvolt Ion Bombardment H. Feld,* R. Zurmiihlen, A. Leute, and A. Benninghoven Physikalisches Institut der Universitat Miinster, Wilhelm-Klemm-Strasse IO, 0 - 4 4 0 0 Miinster, FRG (Received: July 18, 1989; In Final Form: January 11, 1990)
Experiments have been performed to compare secondary ion emission from polymer substrates under kiloelectronvolt ion (secondary ion mass spectrometry, SIMS) and megaelectronvolt ion (plasma desorption mass spectrometry, PDMS) bombardment. The yield of carbon cluster ions has been determined for different polymers. A positive PDMS spectrum of poly(viny1idene fluoride) showing even-numbered carbon clusters to C200is presented. The emission of these carbon cluster ions is explained by the strong degradation of the polymer under megaelectronvolt ion bombardment. This is discussed in comparison with the results in SIMS and laser mass spectrometry.
Introduction During the past years the investigation of carbon clusters has been of increasing interest. Clusters play an important role for transition stage studies from gaseous to solid or liquid state of matter, because they possess properties of an intermediate aggregate state.’ They can be produced by different techniques, e.g., in flames or by SIMS, PDMS, and laser vaporization. There have been interesting developments in recent years in the study of carbon clusters generated by laser vaporization. Up to 1984 only clusters containing 33 carbon atoms had been observed by this technique. Then Kaldor et a1.2 exceeded this limit for the first time by laser vaporization of a graphite rod and subsequent photoionization of the neutral clusters on a time-offlight mass spectrometer. Even-numbered carbon clusters up to C I w +could be detected. The next step was to change the sample and to use polycyclic aromatic hydrocarbons (PAH) or polymers instead of graphite as carbon source. There have been systematic investigations of such samples by laser vaporization and subsequent detection of the produced cluster ions with a LAMMA-1000 TOF-MS3 or a FTMS4 instrument. The highest cluster ions registrated have been about C600+from benzene soot sample^.^ Most spectra taken by laser mass spectrometry show several distributions. Distributions with even- and odd-numbered cluster ions have been observed mainly in the lower mass range whereas in the higher mass range mostly only even-numbered cluster ions are registered. The main difference in these spectra is a strong enhancement of Cm+ for graphite samples compared with PAH or polymer samples. The structure of this superstable Ca+ cluster is assumed to be a polygon with 60 vertices and 32 faces, whereby ~~
( I ) Mfirk, T. D. Int. J. Mass Spectrom. Ion Processes 1987, 79, 1-59. (2) Rohlfing, E. A,; Cox, D. M.; Kaldor, A. J . Cbem. Pbys. 1984, 81, 3322-3330. (3) Lineman, D.; Viswanadham,
S.K.; Sharkey, A . G.; Hercules, D. M. J . Phys. Chem., submitted for publication. (4) So, H. Y.; Wilkins, C. L. J. Pbys. Cbem. 1989, 93, 1184-1187. ( 5 ) Zoeller. J. H. Jr.; Zingaro, R. A,; Macfarlane, R. D. I n i . J. Mass Spectrom. Ion Processes 1987, 77, 21-30. (6) Niehuis, E.; Heller, T.; Feld, H.; Benninghoven. A. J. Vac. Sci. Technol. A 1987, 5, 1243-1 246. 0022-3654/90/2094-4595!$02.50/0
12 faces are pentagonal and 20 hexagonal (truncated icosahedron).’ In the case of atomic particle bombardment of solid substrates the influence of primary ion energy and mass on cluster emission has been investigated. For primary ions of low energy (SIMS) impinging onto a carbon sample a strong emission of negatively charged clusters up to C1f was observed8 The situation is similar for megaelectronvolt particle impact, where positive and negative charged carbonaceous ions occur: (C,H,)- and (C,H,)+ for n < 25 and 0 -< x I 3. The cations could be observed from nearly all PAHs but not from highly aliphatic substrates.s A strong odd-even abundance rule was found in negative spectra of coronene for (C,H)- up to 300 u . ~ Up to now there has been no observation of carbonaceous cations above m = 360 u comparable to those obtained by laser vaporization. We now for the first time report about high mass carbon clusters produced by atomic particle bombardment of a substrate.
Experimental Section All spectra presented have been obtained with a new combination instrument. We developed a time-of-flight instrument that can be operated in the SIMS as well as in the PDMS mode, at the same sample position without breaking the vacuum. The instrument is based upon a design similar to a number of TOF instruments in our laboratory? Both particle sources may irradiate targets from the same side, allowing for the comparison of several substrates. In the SIMS mode the instrument is operated with mass-selected primary ion pulses (pulse width below 1.5 ns, about 5000 ions/pulse, frequency 5 kHz). Primary ion species (Ne, Ar, Xe) (7) Kroto, H. W.; Heath, J . R.; O’Brien, S.C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162-163. (8) Blaise, G. Summer School on Material Characterization Using Ion Beams Aleria, Corsica; Plenum: New York, 1976; p 167. (9) Della-Negra, S . ; Depauw, J.; Joret, H.; LeBeyec, Y. Presented at the Second International Workshop on MeV and keV Ions and Cluster Interactions with Surfaces and Materials, Orsay, France, 1988.
0 1990 American Chemical Society
4596 The Journal of Physical Chemistry, Vol. 94, No. 11, 1990
'
x C3l
"
' ' ' ' ' I "
I " '
Feld et al.
" "
2
.=4
I
4
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t c,t-
12 16
122
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n=12
60
80
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-
160
moss I.[ Figure 1. Negative PDMS spectrum of the lower mass range of polyimide; primary ion (PI) dose density 7.5 X 107/cm2.
and energies (10-30 keV) can easily be changed. In the PDMS mode, fission fragments are delivered from a retractable annular 252Cfsource. Here the primary ions are distributed in mass (80-160 u) and energy (60-1 15 MeV). The source activity is 15 pCi corresponding to a useful primary particle flux of about 300 s-l. Energy focusing is achieved by an one-stage reflector resulting in a routine mass resolution m / A m > 5000. At the moment this resolution cannot be obtained in the low mass range due to the limitation caused by our 100-MHz signal averager. A more detailed description of the instrument will be given in a subsequent report. With this instrument we have carried out comparative investigations with a number of different polymers and determined the yield of fragment and molecular ions. The yield was calculated by dividing the integral value of a secondary ion peak (N(S1)) by the number of primary ions (N(P1)) which were used to take the spectrum: Y = N(SI)/N(PI). The basic interest has focused on the comparison of the desorption modes concerning "fingerprinting", fragmentation, and molecular weight distributions. There have been two ways of sample preparation: ( I ) commercially available polymer foils that were aluminized on one side in order to obtain an equipotential plane and (2) the polymer was dissolved in methanol and then electrosprayed on an aluminum foil. In both cases the sample thickness is sufficient to prevent an influence of metallic substrate/sample interactions on secondary ion emission. Silicon oil contamination due to the production process of the polymer foils was removed by sonication in n-hexane prior to analysis. Two main types of polymers have been used in this investigation: first polyolefins, e.g., poly(tetrafluoroethy1ene) (PTFE), poly(vinylidene chloride) (PVDC), and poly(viny1idene fluoride) (PVDF), and then polymers with an aromatic group, e.g, polyimide, poly(ethy1ene terephthalate) (PET), and polycarbonate (PC). The typical absolute primary ion dose density (PIDD) for a spectrum of these samples was lo7 primary particles/cm2 in the PDMS mode and 1OII primary particles/cm2 in the SIMS mode.
Results The dominant peaks in the mass spectra of the polymers mentioned above are mostly identical in both modes. The yield, however, is a factor of 10-100 higher in the PDMS mode. Details of the comparison will be discussed in a later paper. The subject of this report is carbon cluster emission. In the low mass range ( m < 400 u) we have found carbonaceous ions in the positive and negative spectra of all polymers investigated. The emission of negative ions generally is much stronger in both desorption modes. The clusters are emitted monoatomically or with 1-3 hydrogen atoms. This is demonstrated in the negative PDMS spectrum of polyimide (see Figure 1) where (C,H,)- appear for x I 3. In this mass range all peaks represent carbon cluster ions. The most impressive fact is the odd/even effect of the (C,H)-peaks. In contrast to the other polymers the polyimide spectrum shows an enhancement of the third peak in
\
1Wtl
0
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' \
m',
t
40
\,
PDMS
', /'
3
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SIMS ,
I
5
In
20
cluster number
-
17
I
15
25
n
Figure 2. Yields of the integral values of C; and C,H- peaks in the negative PDMS and SIMS spectra of polycarbonate.
the group of even-numbered clusters: m = ..., 50 u, 74 u, 98 u, 122 u, 146 u, .... The yield of negative cluster ions C; and (C,H)- is shown for a polycarbonate sample (Figure 2) in PDMS and SIMS. The negative PDMS spectra especially show a strong selection rule: The even carbonaceous anions are more abundant than the odd ones. For (C,H)- the yield of odd and even n differs up to 1 order of magnitude. Another point of interest is the change of the selection rule for C,- where from about n = 15 the odd anions start to become more abundant. Generally the cluster ion yield depends on the polymer type: it is higher by about 1 order of magnitude for the polyolefins than for the polymers with an aromatic group. We have not found cluster peaks in the higher mass range for all investigated polymers with the exception of PVDF and PVDC. For these polymers we observed an unusual behavior in the positive PDMS mode; the spectra show bimodal cluster distributions and can be devided into three parts, which is demonstrated for the case of PVDF. 1. In the mass range m < 300 u (fingerprint region) positive PDMS and SIMS spectra show mainly characteristic peaks of the polymer (Figure 3), whereby it can be identified. These peaks on the one hand represent fragment ions with n times the repeat unit, R = (-CH2CF2-), and on the other hand fragment ions of the same structure with several ( k ) H F splittings: ( n / k ) = (R,CF3)+ - k-HF, where n and k are integers. In SIMS these peaks can be found up to n = 4 and k = 2, whereas in PDMS only smaller fragments ( l / l ) , (l/O), and (2/2) are observed. The background peaks in the spectra ( m < 150 u) are due to ions from adsorbed hydrocarbons in SIMS and carbon cluster ions in PDMS. An important difference can be seen in the mass range between 200 and 300 u, where in case of SIMS the ( n / k ) peaks appear and in PDMS a strong carbon cluster emission is observed. By taking the integral values over the C,+ peaks a series of clusters from n = 1 to n = 31 with an odd/even effect is recognized with the odd-numbered clusters being more intense for n I11. From this point on "magic numbers" occur: every fourth peak is enhanced (n = 15, 19, 23, 27) (see Figure 4). 2. In the intermediate mass range 250 u < m < 500 u the first cluster series vanishes below m = 400 u. The carbon cluster intensity is very weak between n = 33 and n = 40. 3. Starting with Ca+ the spectrum contains a second cluster series where the odd cluster numbers are completely missing (see Figure 5). These clusters are accompanied by a series of the form (C,H,)+, x = 1-4. The second series vanishes at about n = 200 but also some higher cluster peaks could be identified (e.g., n = 232). Even by changing energy and mass in the SIMS mode (maximum: m = 129Xeand E = 30 keV) it has not been possible to detect the second cluster series in this mode. Figure 4 shows the integral values of the C,+ peaks for the low and high mass range in positive PDMS spectra of PVDF and PTFE. For PVDF the first distribution varies 1 order of magnitude with a maximum at n = 1 1, whereas the second distribution
The Journal of Physical Chemistry, Vol. 94, No. 11. 1990 4597
Carbon Cluster Emission from Polymers x10
I
l3
u c
hc
0 C
5
5 c ,
2
c
0 3
1
0 50
150
100
-
250
200 mass
/
u
500
1000
1500
2000
2500 mass [u]
3000
Figure 5. Positive PDMS spectrum of the higher mass range of PVDF; PI dose density 1.9 X 109/cm2.
(d) an odd/even effect (selection rule): cluster ions are more intense having odd numbers (positive spectrum) and even numbers (negative spectrum) (e) the selection rule is stronger in the negative spectra, much stronger in PDMS than in SIMS, inverted at about n = 15 only for C;, and stronger for (C,H)- than for C,(f) only in the positive PDMS mode: occurrence of magic numbers, n = 15, 19,23, 27; emission of even-numbered carbon clusters in the high mass range (480-2400 u, Le., n = 40 - n = 200) from PVDF and PVDC. 50
100
-
250
200
150
mass
/
u
Figure 3. Positive S I M S and P D M S fingerprint spectra of PVDF; PI dose density 5.0 X 10I0/cm2 in S I M S and 1.9 X 109/cm2 in PDMS. in-2L1
10 - 5 1 ~
0
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20
30
40
50
60
70
80
90
cluster number n
I
100
-
Figure 4. Yields of the integral values of C , ' peaks in the positive PDMS spectra of PVDF and PTFE.
is very smooth with a maximum about n = 60. We found a similar behavior for PVDC where the ion bombardment leads to HCI splittings. The main difference compared to PVDF is that the splitting of HCI seems to be more difficult so that the cluster emission is less intensive. In the group of the investigated polyolefins only PTFE shows a different emission characteristic. The cluster emission is very small compared to PVDF and PVDC and only the lower cluster distribution is observed. Similarities are the selection rule and the strong emission of Cll+. Summarizing the results we have found that carbon cluster emission from polymer samples under megaelectronvolt/kiloelectronvolt particle bombardment shows the following properties: (a) emission of positive and negative carbon clusters, ranging in size from 1 to a maximum of 33 C atoms (b) cluster ions may be emitted together with hydrogen atoms (CnHx)+/-,x = 0-3 (c) negative cluster ions have a higher yield than positive (in the lower mass region)
Discussion (a) The observed high yields of homonuclear carbon clusters from polymers are astonishing because the carbon content of polymers is only some 10% and the carbon cluster emission is very similar to that of graphite substrates. It is also interesting that the positive PDMS spectra of polymers are nearly identical with laser spectra of graphite in the lower mass range;1° even the selection rule and the magic numbers are nearly the same. The alternation of C; and C,H- is well-known in LDMS spectral1 and is due to the unstable structure of the odd cluster ions. The polyimide and PVDF polymers have been analyzed with megaelectronvolt ions and laser vaporization/desorption. As polyimide is a nitrogen-containing polymer the peaks at m = ..., 50 u, 74 u, 98 u, 122 u, ... in the PDMS spectrum are probably due to the formation of C2w1N-clusters.12 These ions may overlay the usually weaker C2,H2- peaks. The same peaks have been observed with LDMS when looking at N-containing compounds. Because of the limited channel width of the registration system (10 ns) the hydro and nitro carbon peaks cannot be separated in this case. A channel width of 1 ns would be necessary to do that. Laser irradiation of polyimide leads to carbon clusters in the lower mass range as in PDMS, but without any odd/even effect. On the other hand the laser spectrum shows several broad C, distributions between m = 500 u and m = 5000 u.I3 PVDF shows the opposite trend in cluster formation. No carbon cluster ions can be observed by laser vaporization above m = 300 u,14 whereas in PDMS clusters up to m = 2400 u can be detected. In one and the same PDMS spectrum of PVDF the characteristic polymer peaks and both carbon cluster distributions are seen. However, in the laser spectra of polymers the intensities of characteristic peaks as well as the formation of clusters above and below C35depend strongly on laser power densities. Creasy and Brenna measured the higher mass cluster distribution with a power density near the threshold for positive ion formation whereas they needed higher power densities to get the lower mass di~tributi0n.l~We think that there are two sources of carbon (10) Fiirstenau, N.; Hillenkamp, F. fnr. J . Muss Spectrom. fon Processes 1981, 37, 135-151. (1 1) Hercules, D. M. Spectrosc. Lett. 1980, 13, 347-360. (1 2) Hercules, D. M. Private communication. (13) Creasy, W. R.; and Brenna, J. T. Absrr. ASMS 1988, 36, 51-52. (14) Holtkamp, D. Private communication.
4598
The Journal of Physical Chemistry, Vol. 94, No. I I , 1990 w i s e r of ‘epeat u n i t s n 2 J 6 8
,
1
2
-
-,(>TFE)
’‘2
I
l
I
i
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4
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6
7
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18
16
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.,--E
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300
400 530
600
1
1
,
700 800 900 1000 P a s s /u ----c
Figure 6. Yields of the integral values of characteristic peaks in the positive PDMS and S l M S spectra of PVDF and PTFE.
clusters in accordance with Kaldor et al.Is Small clusters are only produced in the case of high energy density and are assumed to be created in the hot inner part of the ion track. The bigger clusters are emitted from the outer regions of this track where the plasma temperature is smaller. So two types of clusters are produced in two spatially separated regions. (b) The emission of high mass carbon clusters from PVDF and PVDC in PDMS can be explained by evaluating the spectra of the polyolefins concerning the emission of characteristic peaks. This is done for PVDF and PTFE in Figure 6. The fragment ion emission from PTFE is similar in both desorption methods. The main fragment ions observed have the structure Rnj2CF+, with the repeat unit R = (-CF,CF,-). The decrease of the fragment ion yield with increasing mass can be described by Y and is parallel for both desorption methods. In contrast, the yields of the corresponding fragments from PVDF, R,CF3+, show an exponential decrease Y e-x that is stronger in PDMS (X(PDMS) = 1.8, X(SIMS) = 1.3). On the other hand the sum of all fragment ions with and without H F splitting (C:=I((R,CF,+ - b H F ) ) has again a decrease similar to PTFE. The reason for this behavior is the degradation and carbonization of PVDF under ion bombardment. The first step of this process is the formation of conjugated polyene
-
-
(-CH2-CF2-),,
--+
(-CH=CF-),
+ oHF
(1)
that is the dehydrohalogenation at one chain side leading to carbon-carbon double bonds. In the second step the polymer loses all side substituents, whereby carbonization takes place: (-CH=CF-), 2n.C + PHF I (2) -+
Whereas in the SlMS spectrum mainly products of the first step can be seen, the PDMS spectrum is dominated by the second step. This is due to the extensive electronic excitation under megaelectronvolt ion bombardment. The produced carbon layer on the PVDF surface under ion bombardment in contrast to PTFE has also been proved with Raman spectroscopy.16 The desorbed H F molecules were observed by residual gas analysis after ion bombardment.” (c) Most theoretical models for fast heavy ion impact on solids18J9agree in that ( 1 ) desorption of large molecules or clusters (15) Cox, D. M.; Reichmann, K . C.; Kaldor, A. J . Chem. Phys. 1988,88, 1588-1597. (16) Tomita, S.; Soeda, F.; Ishitani, A. In Secondary Ion Mass Spectrometry, SIMS VI; Benninghoven, A., Huber,A. M., Werner, H.W., Eds.; Wiley: New York, 1987; p 647. (17) Fina, A.; Le Moel, A.; Duraud, J. P.; Valin, M . T.; Le Gressus, C.; Balanzat, E.: Ramillon, J. M.; Darnez. C. Nucl. Insrrum. Methods 1989, B42, 69-75. (18) Bitenski, 1. S.; Parilis, E. S. Nucl. Insfrum. Methods 1987, B 2 / , 26-36. (19) Sundqvist, B.; Hedin, A.; Salehpour, M.; Save, G.; Johnson, R. E. Nucl. Instrum. Methods 1986, 814, 429-435.
Feld et al. only takes place in the outer region of the ion track and (2) there is a high secondary electron fluence in this region. These lowenergy electrons are responsible for the energy transfer to sample molecules. Therefore, the desorption of H F molecules from PVDF under ion bombardment can be described by the Menzel-Gomer-Redhead mechanism, which is normally used to explain the desorption process in electron stimulated desorption. In the case of PVDF this means that transitions of C-H bonds from bonding to antibonding states are caused by electron impacts. The C-H bond is weakened by the repulsive potential of this excited electronic state; thereby the hydrogen-like bonds between H and F in the polymer backbone could be reinforced, leading to the desorption of HF molecules.20 In summary we assume a degradation of the polymer chain by the high electron density and the subsequent emission of a part of the polymer backbone as a homonuclear carbon cluster. (d) By the same mechanism, the degradation of PVDC can be explained. The observed smaller carbonization of PVDC compared to PVDF is due to the different heats of formation (AHf) for H F and HCI. The reaction enthalpy AHr for the process (-CX?-CHI-)
-+
(-CX=CH-)
+ (H-X)
(3)
whereby X represents either chloride or fluorine atoms, is
AHr = AHf(H-X)
+ AHf(C=C)
- AHf(C-X) -AHf(C-H)
(4) By calculating the enthalpy difference for PVDF and PVDC it is not necessary to estimate the second and fourth term on the right side of the equation, because these values are equal for both. The reaction enthalpy difference is therefore A(AHr) = AH,(PVDC) - AH,(PVDF) = AHf(H-CI) - AHf(C-CI) - (AHf(H-F)
= -92
- AHf(C-F))
- (-398) - (-269 - (-536)) kJ/mol zz 39 kJ/mol
(5)
Thus it takes more energy to carbonize PVDC than PVDF. (e) The differences in carbon cluster emission in PDMS and SlMS are due to the differences in energy transfer from the primary ions to the target atoms. The energy loss of the primary particles in the surface near region (stopping power) mainly determines the features of the desorption process and especially the secondary ion yield. In SIMS the stopping power is given by elastic and inelastic collisions of the primary particles with target atoms (nuclear stopping power (NSP)). In contrast, primary particles with higher energies in the megaelectronvolt range mainly interact with target electrons (electronic stopping power (ESP)). For a dielectric medium like PVDF, the NSP value is more than 1 order of magnitude smaller than the ESP value for the corresponding primary ion energies. Additionally the high cluster yield from a solid substrate under megaelectronvolt particle bombardment is well-known as an effect of the nonlinear part of the sputtering yield.I8 In this case the mean free path for elastic collisions of primary ions with substrate atoms becomes comparable to the mean interatomic distance, so that several elastic collision cascades can overlap. For SIMS the cluster emission can be described by the recombination or the “cluster emission” modeL2’ In the first model clusters consist of atoms which were emitted from neighboring surface lattice sites. In the recombination model the cluster atoms can be emitted independently from lattice sites which are separated by several atomic distances and recombine in the gas phase. Conclusion Due to the similarity of laser desorption mass spectrometry and PDMS concerning carbon cluster emission from polymers, some parts of the desorption processes must have the same nature. This (20) Le Moel, A.; Duraud, J . P.; Balanzat, E. Nucl. Instrum. Methods 1986, B18, 59-63. (21) Benninghoven, A.; Riidenauer; Werner, H.W. Secondary Ion Mass Spectrometry, Wiley: New York, 1987.
J. Phys. Chem. 1990, 94,4599-4610 seems to be valid in time and space; i.e., the energy is deposited in the polymer matrix on the same time and space scale. Especially the small carbon clusters are created in surroundings having the same plasma temperature in both desorption modes. Thus it may be that polymer matrices offer the possibility to get a better understanding of what happens during primary ion impact and secondary ion emission. Additionally degradation effects of polymers could be studied by the comparison of different energy-transfer mechanisms from the primary ions to the target atoms.
4599
Acknowledgment. We thank U. Jurgens, who supported this work by the development and maintenance of the excellent registration computer program. We also thank Prof. D. M. Hercules, Dr. M. P. Chiarelli (University of Pittsburgh) and B. Hagenhoff (University of Munster) for reading the manuscript and for valuable discussions. Registry No. PTFE, 9002-84-0; PVDC, 9002-85- I ; PVDF, 2493779-9; PET, 25038-59-9; C , 7440-44-0.
Infrared Identification of Adsorbed Surface Species on Ni/SiO, and Ni/AI2O3from Ethylene and Acetylene Adsorption Mark P. Lapinskit and John G. Ekerdt* Department of Chemical Engineering, University of Texas at Austin, Austin, Texas 78712 (Received: August 3, 1989; In Final Form: January 3, 1990)
The adsorption of ethylene and acetylene was studied on Ni/Si02 and Ni/A1203catalysts over the temperature range 177-300 K with Fourier-transform infrared spectroscopy (FTIR). The general approach involved adsorbing ethylene or acetylene and monitoring the subsequent surface transformations with FTIR as the catalyst was heated, exposed to hydrogen or deuterium, or subjected to vacuum. For the adsorption of ethylene on Ni/Si02 and Ni/AI2O3at temperatures below 250 K, two types of ?r-bonded species (A and B), a owethylene species (hybridization between sp2 and sp3) and ethylidyne were identified. Over the temperature range 194-240 K, n-bonded species B and the un-ethylene species were observed to decompose while ethylidyne formed. A mechanism is proposed in which adsorbed H inserts into n-bonded species B and the un-ethylene species leading to ethyl and ethylidene intermediates, and finally ethylidyne. The amount of hydrogen adsorbed on the nickel surface and crowding effects are proposed to be responsible for ethylidyne formation on nickel. For temperatures over 250 K, ethylidyne decomposed while several new species were formed. Some ethylidyne remained on the surface at 300 K. Acetylene was very reactive on Ni/Si02 and Ni/A1203 but an acetylene-type species and ethylidyne were identified.
Introduction In heterogeneous catalysis, many studies have been undertaken to identify reaction mechanisms in an effort to understand what factors influence activity and selectivity. The use of vibrational spectroscopies such as infrared (IR), Raman, or electron energy loss (EEL) on transition-metal surfaces greatly simplify mechanistic possibilities by allowing in situ observation of adsorbed species transformations. These methods offer direct monitoring of elementary reactions and reaction intermediates. A wellcharacterized system, which has a limited number of adsorbed species, is required for in situ reaction studies. Adsorption studies of ethylene and acetylene on supported group VI11 metals'-24 and single-crystal surface^^^-^^ have revealed the types of structures which can form during reaction, the sites at which these structures form, and that C2 systems can be good model systems for reaction studies. In this paper we examine the adsorbed species resulting from ethylene and acetylene adsorption on Ni/Si02 and Ni/AI2O3 over the temperature range 177-300 K with FTIR. In a forthcoming paper we will describe the use of IR spectroscopy to follow the rates of surface reactions and determine the kinetics of these reactions. Infrared spectra for C2adsorbed species on Ni/Si02 have been reported mostly at room temperature with good resolution only in the C-H stretching region. The low-temperature studies of ethylene on Ni/Si02 by Morrow and Sheppard'o*" only reported bands in the C-H stretching region. At 195 K, di-u-bonded ethylene (C2H4)was proposed to form upon initial ethylene adsorption. After time at 195 K or at 293 K, di-a-bonded ethylene was thought to form sp3-hybridized acetylene (C2H2)or more likely a C4 species with multiple carbon-metal bonds. The results
* Author to whom correspondence should be addressed. 'Present address: Exxon Research and Development Laboratories, P.O. Box 2226, Baton Rouge, LA 70821. 0022-3654/90/2094-4599$02.50/0
of SomaI6 for C2H4 and C2D4adsorption on Ni/AI2O3 at low temperatures were in doubt because of the possible presence of ~
~
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0 1990 American Chemical Society
