- Email: [email protected]
Comparison of NFIM and FIM of polymer layers with tetracyanoethylene and benzene as image gases
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al:~ie6 surface science
ELSEVIER
Applied Surface Science 87/88 (1995) 146-150
Comparison of NFIM and FIM of polymer layers with tetracyanoethylene and benzene as image gases A. Theiss a, F. Okuyama b, F.W. RSllgen a,, a Institutfftr Physikalische und Theoretische Chemic, UniversitiitBonn, Wegelerstrasse12, D-53115 Bonn, Germany Department of System Engmeermg, Nagoya Institute of Technology, Gokiso-Cho, Showa-ku, Nagoya 466, Japan b
•
•
Received 10 July 1994; acceptedfor publication 27 August 1994
Abstract
Field ion microscopy (FIM) with positive ions and negative ions (NFIM) is compared for polymer layers of two structures grown by field induced anionic polymerization of tetracyanoethylene (TCNE) on a metal tip. The positive and negative ion images of the polymer layer mainly consisting of linear polymer chains are complementary, i.e. positive ion emission is preferentially observed in dark areas of the corresponding negative ion images. For the layer with a erosslinked polymer structure the bright rings of negative ion images are not observed in positive ion images. The dissimilarities between NFIM with TCNE and FIM with benzene and TCNE are attributed to differences in the excess electron and electron hole conductivities of the polymer layers exposed to the image gases.
1. Introduction
The formation of negative ions by field ionization (NFI) without electron emission has been achieved on the surface of polymer layers grown by field induced polymerization of tetracyanoethylene (TCNE) and benzoquinones and has been extensively investigated by negative field ion microscopy (NFIM) [1-6]. The experiments revealed a mechanism of NFI at low field strengths significantly below the onset of electron emission. In this NFI mechanism electrons are transported from the bulk to the surface of the layer by a hopping mechanism between electron acceptor states provided by TCNE molecules absorbed in the layer and adsorbed on the
* Corresponding author. Fax: +49 228 73 25 51.
surface of the layer [5]. A more recent investigation [6] of the mechanism of anionic polymerization of TCNE revealed the initial formation of long linear polymer chains after the onset of polymerization induced by a burst of electron emission from the cathode tip. Thereafter, under conditions of NFI of TCNE, the structure of the layer changes by crosslinking of the polymer chains. While NFIM of the initial stage of the polymer layer only shows large bright and circular dark areas, the final state of the layer is characterized by the appearance of ring structures. In this paper we compare NFIM with FIM for both structures of the TCNE polymer layer, using as image gases TCNE in the negative ion mode and TCNE and benzene in the positive ion mode. Benzene was chosen because of its low ionization energy of 9.24 eV compared to that of TCNE which is 11.77 eV [7]. Since FI and NFI are dependent on different
0169-4332f95f$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00515-X
A. Theiss et aL /Applied Surface Science 8 7 / 8 8 (1995) 146-150
electronic properties of the polymer layer, i.e. NFI on the excess electron conduction and FI on the electron hole conduction in the layer, the question for the differences in the ion images and its relation to the electronic properties of the layer is of main interest.
147
sure was about 2 × 10 - 4 hPa. The gas inlet system and the microscope were heated to about 70°C to achieve a sufficient gas pressure of TCNE. The background pressure was about 10 - 6 hPa. The tip temperature was near 30°C. In the N F I M the ion emission could be distinguished from electron emission by the application of a w e a k magnetic field.
2. Experimental 3. Results The field ion microscope was equipped with a single microchannel plate image intensifier and a video system for recording the ion images on tape. The figures show photographs of ion images taken from a monitor screen. Tungsten tips, prepared by electrochemical etching from 0.127 m m wires, were used. The radii of the metal tips on which the TCNE p o l y m e r layers grow up were between about 100 and 300 nm. The thicknesses of the p o l y m e r layers were between several 100 nm and about 1 /zm. The image gas pres-
Fig. l a shows the negative ion image of a TCNE polymer layer grown on a W tip after a burst o f electron emission and setting the tip potential to zero voltage for about 2 min before raising the negative tip potential to - 3 k V for ion imaging. (The first ions were already observed at - 1 . 2 kV.) For these conditions linear polymers of T C N E are almost exclusively formed by anionic polymerization [6]. The dark areas in the negative ion images of this polymer structure have been attributed to surface areas in
Fig. 1. NFIM and FIM ion images of a TCNE polymer layer in its initial stage: (a) NFIM image of the polymer layer with TCNE at - 3 kV tip voltage after preparation of the layer by anionic polymerization at zero tip voltage, (b) FIM image with benzene at + 11 kV after (a), (c) NFIM image with TCNE at - 3 kV after (b), (d) FIM image with TCNE at + 11 kV after (c). The image gas pressure was 2 X 10 -4 hPa.
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A. Theiss et al. /Applied Surface Science 8 7 / 8 8 (1995) 146-150
which the density of polymer chains is to too low to keep TCNE molecules absorbed in the polymer layer which is a prerequisite of negative ion formation at low field strengths [5,6]. Accordingly, electron emission preferentially starts from dark areas [6]. The formation of dark areas is also promoted by coulomb repulsion between negative space charges of polymer layers with absorbed TCNE molecules. The positive ion image of the polymer layer of Fig. la is shown in Fig. lb with benzene as image gas. In order to avoid major changes of the structure of the layer between the two ion images the tip voltage was set to zero directly after taking the negative ion image, then TCNE was pumped out and benzene introduced before a positive voltage was applied to the tip. In Fig. l b the positive ion emission appears in dark areas o f Fig. la while the bright areas in Fig. la are mostly dark in Fig. lb. The first weak spots due to benzene ion emission could be observed at about + 3 kV. The number of spots strongly increased and changed to nearly homogeneously bright areas with increasing tip potential from about 6 to 11 kV.
The negative ion image obtained after Fig. lb is shown in Fig. lc. The slight change of the ion image compared to Fig. la may be attributed to a small effect of an NFI treatment of the layer [6] rather than to the effect of FI of benzene because similar changes in NFIM were observed after interruptions of negative ion imaging of a polymer layer without intermediate exposure of the layer to FIM conditions. The positive TCNE ion image of Fig. l c is shown in Fig. ld. The same characteristics are observed as with benzene, i.e. positive ion emission preferentially occurs in dark areas of the negative ion image. In contrast to benzene, typically bright flickering spots were observed with TCNE. Single spots were visible between a fraction of a second and several seconds. The spot structure of the TCNE ion images may be related to the high ionization energy of TCNE compared to benzene. A slight disintegration of the polymer layer at + 11 kV was indicated by the increase of the onset potential of positive ion emission subsequently observed. Surprisingly, the onset potentials were about the same for both benzene and TCNE ions.
Fig. 2. NFIM and FIM ion images of a TCNE polymer layer in its final state, i.e. after 6 min NFI treatment of the layer of Fig. ld: (a) NFIM image with TCNE - 3 kV tip potential, (b) FIM image with benzene at + 11 kV after (a), (c) NFIM image with TCNE at - 3 kV after (b), (d) NFIM image after 4 min at - 3 kV after (c), and (e) FIM image with TCNE at + ll kV after (d). Image gas pressure was 2 × 1 0 - 4 hPa.
A. Theiss et aL /Applied Surface Science 8 7 / 8 8 (1995) 146-150
After exposure of the TCNE polymer layer of Fig. ld for about 6 min to NFIM conditions ring structures appeared in the negative ion image as shown in Fig. 2a. The appearance of the ring structures indicates a change of the structure of the layer by crosslinking of the polymer chains which is caused by anionic polymerization under NFI conditions [6]. The positive ion image of this layer with benzene is shown in Fig. 2b. No ring structures are observed in FIM. Benzene ion emission occurs from the whole surface area in which bright ring structures appear in NFIM. The negative TCNE ion image after Fig. 2b is shown in Fig. 2c. The difference between Figs. 2a and 2c indicates a small change of the surface morphology of the layer caused by the application of the high positive potential to the tip. In addition, a loss of brightness and an increase of the negative tip potential for the onset of ion emission was observed between Figs. 2a and 2c. The latter properties of the polymer layer were restored after exposure of the layer to NFI conditions for a few minutes (Fig. 2d), i.e. the image contrast increased and the onset potential decreased again. For FIM with TCNE (Fig. 2e), the same result was obtained as with benzene, i.e. no ring structures were found in the positive ion images. A destruction of the polymer layer was observed at tip potentials > 12 kV. The results of this investigation are supported by similar experiments with different tips.
4. Discussion Both structures of the TCNE polymer layer lead to Significant differences in the image contrast between NFIM and FIM. These differences point to particular electronic properties of the layers. The polymer layer of Fig. 1 is mainly composed of linear polymer chains having the structure of Fig. 3a. A dense layer of this polymer represents an insulator under FIM conditions because the ionization energy of the CN groups is high, as indicated by the high ionization energy of TCNE. Thus, FI via surface charging even with benzene absorbed in the layer is not achieved within the limit of the applied external field. However, in NFIM with TCNE the electron conductivity of this layer is maintained by
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related polymerizationreactions of acrylnitrile [8]).
hopping of excess electrons between low lying electron accepter states of TCNE molecules absorbed in the layer as discussed before [5]. Considering now those surface areas which are nearly free of polymer chains, NFI of TCNE is not possible. However, the formation of positive ions by FI should be possible if the field strength at the metal surface is sufficiently high to ionize polymer chains. The charged polymer chains provide the electron accepter states for FI of molecules arriving from the gas phase. The structure of the TCNE polymer layer in its final state, as indicated by the appearance of the ring structures in the NFIM images (Fig. 2a), is characterized by a crosslinking of the polymer chains and an elastic behaviour under field stress [4-6]. Since the NFI treatment of the TCNE polymer layer, leading to the crosslinked structure of the polymer layer, also causes the formation of a conjugated 7r-electron system along the polymer chains (Fig. 3b) [5] a sufficient electrical conductivity exists for FI of molecules on the surface of the layer. The disappearance of the
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A. Theiss et aL /Applied Surface Science 8 7 / 8 8 (1995) 146-150
ring structures for these conditions (Fig. 2) can be explained by the fact that in F I M the positive charges are shifted to the end of the polymer chains sticking out of the layer while in N F I M the elastic response of the layer is due to a space charge distribution within the bulk of the layer [4,5]. The fact that the measured onset potential was the same for FI of benzene and TCNE indicates the dependence of FI on a critical field required for positive charging of the layer. The origin of this effect is not clear yet. It m a y b e related to the formation of a space charge in the polymer layer at the m e t a l / p o l y m e r layer interface.
5. Conclusions The differences and similarities in image contrast between N F I M and F I M of a polymer layer reflect electronic properties of the layer related to the excess electron conduction and electron hole conduction in the layer. Accordingly the complementary image contrast between N F I M and F I M observed with the T C N E layer composed of linear p o l y m e r chains provides evidence for surface areas with a lack of electron hole conductivity and areas with a lack of conductivity by excess electrons. The similarities between N F I M and F I M of the TCNE polymer layer with a crosslinked structure show that both conductivity mechanisms exist. The disappearance of the
ring structures in F I M reflects a charging of the surface of the layers in contrast to the formation of negative space charges within the bulk of the layers leading to the ring structures in N F I M via an elastic response of the layer.
Acknowledgement The authors are grateful to the Deutsche Forschungsgemeinschaft for financial support of this work.
References [1] R. Schmitz, L. Biitfering and F.W. R611gen,J. Phys. (Paris) 47 (1986) C7-53. [2] R. Schmitz, F. Okuyama and F.W. R611gen,J. Phys. (Paris) 47 (1987) C6-257. [3] R. Schmitz and F.W. R611gen, J. Phys. (Paris) 50 (1989) C8-243. [4] F.W. R611gen, K.H. Ott, S. Raida, R. Schmitz and R. Stoll, Int. J. Mass Spectrom. Ion Processes 100 (1992) 93. [5] F.W. Rfllgen, U. Schmidt, R. Schmitz and A. Theiss, Appl. Surf. Sci. 67 (1993) 128. [6] A. Theiss and F.W. R611gen, Appl. Surf. Sci. 76/77 (1994) 307. [7] R.D. Levine and S.G. Lias, Ionization and Appearance Potential Measurements, NSRD-NSB 71, 1982. [8] E.M. Fettes, Chemical Reactions of Polymers, Vol. 19 (Interscience, New York, 1964).
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.-,..~.:...:::~:!:~
al:~ie6 surface science
ELSEVIER
Applied Surface Science 87/88 (1995) 146-150
Comparison of NFIM and FIM of polymer layers with tetracyanoethylene and benzene as image gases A. Theiss a, F. Okuyama b, F.W. RSllgen a,, a Institutfftr Physikalische und Theoretische Chemic, UniversitiitBonn, Wegelerstrasse12, D-53115 Bonn, Germany Department of System Engmeermg, Nagoya Institute of Technology, Gokiso-Cho, Showa-ku, Nagoya 466, Japan b
•
•
Received 10 July 1994; acceptedfor publication 27 August 1994
Abstract
Field ion microscopy (FIM) with positive ions and negative ions (NFIM) is compared for polymer layers of two structures grown by field induced anionic polymerization of tetracyanoethylene (TCNE) on a metal tip. The positive and negative ion images of the polymer layer mainly consisting of linear polymer chains are complementary, i.e. positive ion emission is preferentially observed in dark areas of the corresponding negative ion images. For the layer with a erosslinked polymer structure the bright rings of negative ion images are not observed in positive ion images. The dissimilarities between NFIM with TCNE and FIM with benzene and TCNE are attributed to differences in the excess electron and electron hole conductivities of the polymer layers exposed to the image gases.
1. Introduction
The formation of negative ions by field ionization (NFI) without electron emission has been achieved on the surface of polymer layers grown by field induced polymerization of tetracyanoethylene (TCNE) and benzoquinones and has been extensively investigated by negative field ion microscopy (NFIM) [1-6]. The experiments revealed a mechanism of NFI at low field strengths significantly below the onset of electron emission. In this NFI mechanism electrons are transported from the bulk to the surface of the layer by a hopping mechanism between electron acceptor states provided by TCNE molecules absorbed in the layer and adsorbed on the
* Corresponding author. Fax: +49 228 73 25 51.
surface of the layer [5]. A more recent investigation [6] of the mechanism of anionic polymerization of TCNE revealed the initial formation of long linear polymer chains after the onset of polymerization induced by a burst of electron emission from the cathode tip. Thereafter, under conditions of NFI of TCNE, the structure of the layer changes by crosslinking of the polymer chains. While NFIM of the initial stage of the polymer layer only shows large bright and circular dark areas, the final state of the layer is characterized by the appearance of ring structures. In this paper we compare NFIM with FIM for both structures of the TCNE polymer layer, using as image gases TCNE in the negative ion mode and TCNE and benzene in the positive ion mode. Benzene was chosen because of its low ionization energy of 9.24 eV compared to that of TCNE which is 11.77 eV [7]. Since FI and NFI are dependent on different
0169-4332f95f$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00515-X
A. Theiss et aL /Applied Surface Science 8 7 / 8 8 (1995) 146-150
electronic properties of the polymer layer, i.e. NFI on the excess electron conduction and FI on the electron hole conduction in the layer, the question for the differences in the ion images and its relation to the electronic properties of the layer is of main interest.
147
sure was about 2 × 10 - 4 hPa. The gas inlet system and the microscope were heated to about 70°C to achieve a sufficient gas pressure of TCNE. The background pressure was about 10 - 6 hPa. The tip temperature was near 30°C. In the N F I M the ion emission could be distinguished from electron emission by the application of a w e a k magnetic field.
2. Experimental 3. Results The field ion microscope was equipped with a single microchannel plate image intensifier and a video system for recording the ion images on tape. The figures show photographs of ion images taken from a monitor screen. Tungsten tips, prepared by electrochemical etching from 0.127 m m wires, were used. The radii of the metal tips on which the TCNE p o l y m e r layers grow up were between about 100 and 300 nm. The thicknesses of the p o l y m e r layers were between several 100 nm and about 1 /zm. The image gas pres-
Fig. l a shows the negative ion image of a TCNE polymer layer grown on a W tip after a burst o f electron emission and setting the tip potential to zero voltage for about 2 min before raising the negative tip potential to - 3 k V for ion imaging. (The first ions were already observed at - 1 . 2 kV.) For these conditions linear polymers of T C N E are almost exclusively formed by anionic polymerization [6]. The dark areas in the negative ion images of this polymer structure have been attributed to surface areas in
Fig. 1. NFIM and FIM ion images of a TCNE polymer layer in its initial stage: (a) NFIM image of the polymer layer with TCNE at - 3 kV tip voltage after preparation of the layer by anionic polymerization at zero tip voltage, (b) FIM image with benzene at + 11 kV after (a), (c) NFIM image with TCNE at - 3 kV after (b), (d) FIM image with TCNE at + 11 kV after (c). The image gas pressure was 2 X 10 -4 hPa.
148
A. Theiss et al. /Applied Surface Science 8 7 / 8 8 (1995) 146-150
which the density of polymer chains is to too low to keep TCNE molecules absorbed in the polymer layer which is a prerequisite of negative ion formation at low field strengths [5,6]. Accordingly, electron emission preferentially starts from dark areas [6]. The formation of dark areas is also promoted by coulomb repulsion between negative space charges of polymer layers with absorbed TCNE molecules. The positive ion image of the polymer layer of Fig. la is shown in Fig. lb with benzene as image gas. In order to avoid major changes of the structure of the layer between the two ion images the tip voltage was set to zero directly after taking the negative ion image, then TCNE was pumped out and benzene introduced before a positive voltage was applied to the tip. In Fig. l b the positive ion emission appears in dark areas o f Fig. la while the bright areas in Fig. la are mostly dark in Fig. lb. The first weak spots due to benzene ion emission could be observed at about + 3 kV. The number of spots strongly increased and changed to nearly homogeneously bright areas with increasing tip potential from about 6 to 11 kV.
The negative ion image obtained after Fig. lb is shown in Fig. lc. The slight change of the ion image compared to Fig. la may be attributed to a small effect of an NFI treatment of the layer [6] rather than to the effect of FI of benzene because similar changes in NFIM were observed after interruptions of negative ion imaging of a polymer layer without intermediate exposure of the layer to FIM conditions. The positive TCNE ion image of Fig. l c is shown in Fig. ld. The same characteristics are observed as with benzene, i.e. positive ion emission preferentially occurs in dark areas of the negative ion image. In contrast to benzene, typically bright flickering spots were observed with TCNE. Single spots were visible between a fraction of a second and several seconds. The spot structure of the TCNE ion images may be related to the high ionization energy of TCNE compared to benzene. A slight disintegration of the polymer layer at + 11 kV was indicated by the increase of the onset potential of positive ion emission subsequently observed. Surprisingly, the onset potentials were about the same for both benzene and TCNE ions.
Fig. 2. NFIM and FIM ion images of a TCNE polymer layer in its final state, i.e. after 6 min NFI treatment of the layer of Fig. ld: (a) NFIM image with TCNE - 3 kV tip potential, (b) FIM image with benzene at + 11 kV after (a), (c) NFIM image with TCNE at - 3 kV after (b), (d) NFIM image after 4 min at - 3 kV after (c), and (e) FIM image with TCNE at + ll kV after (d). Image gas pressure was 2 × 1 0 - 4 hPa.
A. Theiss et aL /Applied Surface Science 8 7 / 8 8 (1995) 146-150
After exposure of the TCNE polymer layer of Fig. ld for about 6 min to NFIM conditions ring structures appeared in the negative ion image as shown in Fig. 2a. The appearance of the ring structures indicates a change of the structure of the layer by crosslinking of the polymer chains which is caused by anionic polymerization under NFI conditions [6]. The positive ion image of this layer with benzene is shown in Fig. 2b. No ring structures are observed in FIM. Benzene ion emission occurs from the whole surface area in which bright ring structures appear in NFIM. The negative TCNE ion image after Fig. 2b is shown in Fig. 2c. The difference between Figs. 2a and 2c indicates a small change of the surface morphology of the layer caused by the application of the high positive potential to the tip. In addition, a loss of brightness and an increase of the negative tip potential for the onset of ion emission was observed between Figs. 2a and 2c. The latter properties of the polymer layer were restored after exposure of the layer to NFI conditions for a few minutes (Fig. 2d), i.e. the image contrast increased and the onset potential decreased again. For FIM with TCNE (Fig. 2e), the same result was obtained as with benzene, i.e. no ring structures were found in the positive ion images. A destruction of the polymer layer was observed at tip potentials > 12 kV. The results of this investigation are supported by similar experiments with different tips.
4. Discussion Both structures of the TCNE polymer layer lead to Significant differences in the image contrast between NFIM and FIM. These differences point to particular electronic properties of the layers. The polymer layer of Fig. 1 is mainly composed of linear polymer chains having the structure of Fig. 3a. A dense layer of this polymer represents an insulator under FIM conditions because the ionization energy of the CN groups is high, as indicated by the high ionization energy of TCNE. Thus, FI via surface charging even with benzene absorbed in the layer is not achieved within the limit of the applied external field. However, in NFIM with TCNE the electron conductivity of this layer is maintained by
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related polymerizationreactions of acrylnitrile [8]).
hopping of excess electrons between low lying electron accepter states of TCNE molecules absorbed in the layer as discussed before [5]. Considering now those surface areas which are nearly free of polymer chains, NFI of TCNE is not possible. However, the formation of positive ions by FI should be possible if the field strength at the metal surface is sufficiently high to ionize polymer chains. The charged polymer chains provide the electron accepter states for FI of molecules arriving from the gas phase. The structure of the TCNE polymer layer in its final state, as indicated by the appearance of the ring structures in the NFIM images (Fig. 2a), is characterized by a crosslinking of the polymer chains and an elastic behaviour under field stress [4-6]. Since the NFI treatment of the TCNE polymer layer, leading to the crosslinked structure of the polymer layer, also causes the formation of a conjugated 7r-electron system along the polymer chains (Fig. 3b) [5] a sufficient electrical conductivity exists for FI of molecules on the surface of the layer. The disappearance of the
150
A. Theiss et aL /Applied Surface Science 8 7 / 8 8 (1995) 146-150
ring structures for these conditions (Fig. 2) can be explained by the fact that in F I M the positive charges are shifted to the end of the polymer chains sticking out of the layer while in N F I M the elastic response of the layer is due to a space charge distribution within the bulk of the layer [4,5]. The fact that the measured onset potential was the same for FI of benzene and TCNE indicates the dependence of FI on a critical field required for positive charging of the layer. The origin of this effect is not clear yet. It m a y b e related to the formation of a space charge in the polymer layer at the m e t a l / p o l y m e r layer interface.
5. Conclusions The differences and similarities in image contrast between N F I M and F I M of a polymer layer reflect electronic properties of the layer related to the excess electron conduction and electron hole conduction in the layer. Accordingly the complementary image contrast between N F I M and F I M observed with the T C N E layer composed of linear p o l y m e r chains provides evidence for surface areas with a lack of electron hole conductivity and areas with a lack of conductivity by excess electrons. The similarities between N F I M and F I M of the TCNE polymer layer with a crosslinked structure show that both conductivity mechanisms exist. The disappearance of the
ring structures in F I M reflects a charging of the surface of the layers in contrast to the formation of negative space charges within the bulk of the layers leading to the ring structures in N F I M via an elastic response of the layer.
Acknowledgement The authors are grateful to the Deutsche Forschungsgemeinschaft for financial support of this work.
References [1] R. Schmitz, L. Biitfering and F.W. R611gen,J. Phys. (Paris) 47 (1986) C7-53. [2] R. Schmitz, F. Okuyama and F.W. R611gen,J. Phys. (Paris) 47 (1987) C6-257. [3] R. Schmitz and F.W. R611gen, J. Phys. (Paris) 50 (1989) C8-243. [4] F.W. R611gen, K.H. Ott, S. Raida, R. Schmitz and R. Stoll, Int. J. Mass Spectrom. Ion Processes 100 (1992) 93. [5] F.W. Rfllgen, U. Schmidt, R. Schmitz and A. Theiss, Appl. Surf. Sci. 67 (1993) 128. [6] A. Theiss and F.W. R611gen, Appl. Surf. Sci. 76/77 (1994) 307. [7] R.D. Levine and S.G. Lias, Ionization and Appearance Potential Measurements, NSRD-NSB 71, 1982. [8] E.M. Fettes, Chemical Reactions of Polymers, Vol. 19 (Interscience, New York, 1964).