Chapter Two

Ionization Energy Measurements

Abstract

The photorefractive effect depends strongly on the magnitude and rate of formation of the space-charge field. The space-charge field is created by the photogeneration, transport and trapping of charges. In polymers these processes are performed by specific molecules which are added to the material. Since these processes involve electron transfer from one molecule to another it is important to have information about the energy levels of the molecular orbitals of origin and destination. A technique is described which allows the determination of the HOMO energy levels of organic molecules. This technique, ultraviolet photoelectron spectroscopy (UPS), can be employed on molecules either in the gas or the solid phase. In this first attempt, the molecules were investigated in the gas phase, since only in this case can their properties be directly compared with each other. By doing so it was possible to obtain values for the HOMO levels of most of the molecules which are used in photorefractive polymers as described in the forthcoming chapters. Some preliminary understanding of the effect that certain molecules have upon the space-charge field formation and the photorefractive behaviour was obtained. 22 Chapter Two

2.1. Introduction

The photorefractive mechanism can roughly be divided into two different regimes, the process of the space-charge field formation and the conversion of this space-charge field into a refractive index grating1. The latter process, although very important, will not be dealt with in this chapter but is thoroughly described in the next chapters. The process of the space-charge field formation is dependent on a number of parameters and can be described by: charge generation, charge transport and charge trapping, as was shown in chapter 1 (figure 1.2). There is one feature that all the processes involved in the space-charge field formation have in common, the transfer of an electron from one molecule to another. This can only be done if the electron overcomes the barrier between the two states2. The barrier that must be overcome depends on geometrical factors, the distance between the two molecules and the positioning of the molecules with respect to one another, but also on the energies of the state of origin and destination3. The total energies of the different states involved in the space-charge field formation are schematically presented in figure 2.1.

Fig. 2.1: Schematic representation of the total energy of the states involved in the creation of a space-charge field. CG* and CG- represent the excited and the negatively charged charge generator molecule, CT*, CT and CT+ represent the excited, the neutral and the positively charged charge transport molecule and Tr+ represents the positively charged trapping site.

It is possible to determine the energies of the different states if information about the ionization energy and electron affinity of the different molecules is available. The main molecular orbitals that are involved in the space-charge field formation are Ionization Energy Measurements 23 the highest occupied molecular orbitals (HOMO’s) of the charge generator, transport and trapping molecule and the lowest unoccupied molecular orbital (LUMO) of the charge generator molecule. One technique that gives information on the energy levels of occupied molecular orbitals is Photoelectron Spectroscopy (PES)4,5,6. In PES a molecule (M) is exited by a monochromatic beam of photons with energy hv, in which process M loses an electron. M + hv ® M+ + e (3.1)

M+ is the resulting ion formed and e is the product photoelectron. In order for this process to occur, the incident photons should have an energy higher than the lowest ionization energy (EI) of the sample. It follows that the energy available after ionization, hv - Ip, must appear as translational energy of the electron. Thus, if mono-chromatic photons are used for ionization and the photon energy is known, a simple determination of the kinetic energy of the photoelectrons provides the ionization energy of the molecules. Depending on the energy of the photons employed, PES is sensitive to different energy ranges of molecular orbitals. For instance when x-ray sources are used to provide the photons, information about the core orbitals of the molecules and atoms under investigation is obtained. In this case the technique is referred to as XPS. When detailed information about the highest occupied molecular orbitals is needed, a source that provides photons with energies comparable to the first ionization energies of the sample is required. Usually a helium gas discharge lamp is used, which provides light in the vacuum-ultraviolet (VUV) region of the electromagnetic spectrum consequently the technique is referred to as Ultraviolet Photoelectron Spectroscopy (UPS). A schematic representation of this process is shown in figure 2.2. Here a molecule with five filled molecular orbitals is depicted, of which only three are accessible by the photons used in this experiment. Electrons can be ejected from these orbitals if photons are absorbed. This results in a molecular ion with three different final states, M+(1), M+(2) and M+(3), and three electrons with different kinetic energies. Usually the features are broadened by various vibrational relaxations. This is schematically depicted as the broadening of the lines in the photoelectron spectrum. 24 Chapter Two

Fig. 2.2: Schematic representation of the processes involved in a UPS experiment. On the left, a molecule with five filled levels, three of which are accessible to the photons. In the middle: the molecular ions M+(1), M+(2) and M+(3) resulting from the ionization of the three highest occupied orbitals. On the right, the corresponding photoelectron spectrum reflecting molecular orbital levels is displayed.

The peaks observed in this spectrum mimic the kinetic energy of the electrons. The energies of the originating molecular orbitals can now be reconstructed by subtracting the kinetic energy of the electrons from the known photon energy. For example, electrons ejected from the highest occupied molecular orbital (which will result in the creation of molecular ion M+(1)) will have the largest kinetic energy and will be observed as the first peak in the photoelectron spectrum. UPS can be performed both on molecules in the solid and in the gas phase6. The main difference between the UPS spectra of molecules measured in the solid and those measured in the gas phase is that in the solid phase interactions between neighbouring molecules play a significant role whereas they do not in the gas phase. These interactions between neighbouring molecules are mainly caused by their polarizabilities. When a molecule is photoionized it can be stabilized by interaction with induced dipoles on surrounding molecules. These interactions decrease the ionization energy and increase the electron affinity resulting in a decrease of the conductivity gap. From the above- described considerations it is not clear in which phase the molecules used in photorefractive polymers should be measured. If the investigated molecules would be used in the pure solid phase, it is obvious that the ionization energy should be measured in the solid phase. However, due to the fact that the molecules will be used in Ionization Energy Measurements 25 combination with an inactive polymer binder and, more importantly, with large concentrations of very polar NLO molecules, the values obtained from the pure solid phase will be inaccurate. This is caused by the difference in polarity of the actual surroundings and that of the investigated molecules, which will shift the energy levels of the investigated molecules, due to a change in dipolar interaction. For this reason the experiments were performed in the gas phase, as the effect of the surroundings is avoided and a more direct comparison of the energy levels can be made. The so obtained energy levels can only be used as an approximation to the actual situation if the highly polar environment has approximately the same effect on all the different molecules investigated. This assumption is a crucial one for the interpretation of the values obtained.

2.2. Experimental section

2.2.1. The ultraviolet photoelectron spectrometer

The essential components of an ultraviolet photoelectron spectrometer are a lamp that produces suitable radiation, an ionization chamber, an electron energy analyser, an electron detector and a recorder5. These components are shown schematically in figure 2.3, and will be briefly discussed.

Fig. 2.3: Essentials of an ultraviolet photoelectron spectrometer. All the electron optics must be contained within a vessel evacuated to 10-6 mbar or less.

The lamp provides the photons which are used to ionize the sample. In order to reach the highest occupied orbitals, which generally have ionization energies between 26 Chapter Two five and fifteen electron volts, photons from a helium discharge lamp are used; more precisely the photons from the He I resonance line at 58.4 nm, equivalent to a photon energy of 21.22 eV. A hemispherical analyser, in combination with a lens system, was used as the electron energy analyser. This system was chosen because it provides a high resolution of the photoelectron spectrum5. Since the molecules are (large) organic molecules and solids at room temperature, some kind of heating device must be constructed to evaporate these molecules. In order to create a sufficiently high density for an acceptable signal-to-noise ratio, large amounts of molecules should be evaporated. Due to the organic nature of these molecules, however, they contaminate the apparatus, which prohibits accurate measurements due to charging effects. Therefore, a source was constructed which provides a collimated beam of gas molecules on which the helium lamp and the analyser are focused. In figure 2.4 a detailed drawing of the employed oven is shown.

Fig. 2.4: A schematic representation of the source used to create a collimated beam of organic gas molecules. 1 is the sample tube, 2 the copper block, 3 the nozzle, 4 the heat shield, 5 the diaphragms, 6 the thermocouple and wire and 7 the heating wires surrounding the oven block. The dashed parts 8 represent the flange to which the source is attached. All components are drawn to scale.

The basic unit of the source is a hollow copper block surrounded by a heating wire; a thermocouple is placed inside the oven to probe the temperature. Directly adjacent to this thermocouple a small glass tube is placed which is sealed on one side and contains the organic sample. On the outlet of the oven towards the ionization chamber a conical nozzle with a 1 mm hole is attached. This nozzle in combination with a diaphragm Ionization Energy Measurements 27 creates a collimated beam of gas molecules. To prevent re-evaporation of sublimed organic material from the diaphragm its temperature is kept as low as possible. This is done by attaching the diaphragm to the main body of the spectrometer using a thick piece of aluminum. Furthermore, to prevent heating of this aluminum block by radiation emitted from the copper oven, a shield is inserted between them. For the same reason a second diaphragm is placed after the first one, which might also be heated by radiation from the oven. The small amount of organic material that now enters the ionization chamber condenses in a very small area on the wall of the chamber. Since the contamination area is so small it can be cleaned easily. The above-described oven construction results in a sufficiently dense molecular beam which provides good signal- to-noise ratios and at the same time minimizes the pollution that occurs during the experiment. One detail concerning the experiment which has not been mentioned yet is the calibration of the spectrum. Even though the energy of the photons is known and the kinetic energy of the emitted electrons is measured, this does not provide an absolute value of the ionization energy of the sample molecules. This is caused by the different workfunctions of the metals used in the analyser, which influences the energy of the photoelectrons. To correct for this effect, a gas of which the ionization energy is accurately known and which is unreactive towards the sample vapour, is introduced into the ionization chamber at the same time. The observed UPS spectra can now be normalized towards the internal standard of the calibration gas. A convenient gas is 2 7 xenon, which has a very distinct P3/2 ionic state at 12.13 eV .

2.2.2. Organic molecules

Apart from the charge generator, transport and trapping molecules, which are directly involved in the space-charge field generation, also some NLO molecules were investigated with UPS. This was done because these molecules are present in very large amounts and might play a role in the trapping of the charges. The synthesis of the investigated molecules is described in the forthcoming chapters and will therefore not be discussed here. Their chemical structure, chemical name and abbreviation are given in figures 2.5, 2.6 and 2.7.

O

O2N NO2 H3C CH3 N N

H3C CH3

NO2

2,4,7-trinitro-9-fluorenone (TNF) N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) 28 Chapter Two

Fig. 2.5 : Chemical structures of the investigated charge generator and charge trapping molecule.

In figure 2.5 a charge generator and a charge trapping molecule are depicted. The main difference between these molecules is the electron density on the ring system. In TNF this electron density is very low because of the three strong electron-withdrawing nitro groups, whereas in TMPD the electron density on the ring system is very large due to the presence of the two strongly electron-donating amine atoms.

N N N N

CH3 CH3 OCH3 OCH3

N,N'-diphenyl-N,N'-bis(4-methylphenyl)-[1,1'-biphenyl]- N,N'-diphenyl-N,N'-bis(4-methoxyphenyl)-[1,1'-biphenyl]- 4,4'-diamine ( p-TPD) 4,4'-diamine (MTPD) CN

CN N N

N

CH3 CH3 N-ethylcarbazole (ECZ) N-(4-(2,2-dicyanoethenyl)phenyl)-N'-phenyl-N,N'-bis(4-methyl phenyl)-[1,1'-biphenyl]-4,4'-diamine (MDCTPD)

N N N

4-(N,N-diethylamino)benzaldehyde diphenyl hydrazone (DEH)

Fig. 2.6 : Chemical structures of the investigated charge transport molecules.

All the charge transport molecules are composed of strong electron donors, nitrogen atoms linked by an aromatic system (figure 2.6). This provides the molecules with a rather large electron density which makes it easier for an electron to escape, and hence facilitate the transport8. Ionization Energy Measurements 29

NO2

N NO2 N

4-(N,N-diethylamino)nitro- 4-(N,N-diethylamino)-b-nitro- benzene (EPNA) styrene (DEANST)

F

NO2 N

FDEAMNST

4-(N,N-diethylamino)-3-fluoro-(Z)-b-methyl- b-nitro-styrene (FDEAMNST)

Fig. 2.7: Chemical structures of the investigated NLO molecules.

The NLO molecules that were investigated are depicted in figure 2.7. NLO molecules are molecules which have a strong electron donor and a strong electron-withdrawing moiety linked by a conjugated bridge9. Viewed with respect to their electron densities they are intermediates between charge generator and charge transport molecules. For such molecules ionization energies are expected which lie between those observed for the charge generator and the charge transport molecules.

2.3. Results and discussion

2.3.1. UPS spectra

In order to understand the influence of the molecules involved in the space- charge field generation on the photorefractive behaviour of a polymer, it is essential to have information about the HOMO levels of these molecules. Investigation with UPS, however, provides information not only about the HOMO but about all orbitals which are accessible to the photons used. This additional information can be used to gain insight in the overall electronic structure of these molecules. Even though this is not discussed in this thesis, the complete photoelectron spectrum as measured is presented. The evaluation of the ionization energy will be described using the UPS spectrum of DEH as an example. In figure 2.8 the photoelectron spectrum of DEH is depicted. The large peak at approximately 12 eV belongs to the xenon gas which was used to calibrate the spectrum, its maximum is set to 12.13 eV. 30 Chapter Two Intensity (arb. units)

6.6 6.4 6.2 6.0 5.8 5.6 Binding energy (eV) Intensity (arb. units)

12 10 8 6 Binding energy (eV)

Fig. 2.8: UPS spectrum of DEH, with xenon as reference gas. The inset shows the magnification of the first peak observed in the spectrum.

The overall resolution in these experiments can be deduced from the width at half height of the reference peak. In all spectra this is approximately 0.15 eV. The first ionization energy is obtained from the onset of the spectrum, as determined through the extrapolation of the initial slope (see inset in figure 2.8), which is found to be approximately 6.1 eV. This results in a first ionization energy of (6.1 + 0.075) = 6.18 eV, after taking the spectral resolution into account. In figure 2.9, 2.10 and 2.11 the UPS spectra are presented for the charge generator and trapping molecule, the charge transport molecules and the electro-optic molecules, respectively. In figure 2.9 it can be clearly seen that TNF has a much higher ionization energy than TMPD. This is most likely caused by the low electron density on the ring system of TNF, whereas TMPD has a large electron density on the ring and consequently a low ionization energy. The UPS spectra of p-TPD, MTPD and MDCTPD are rather similar (figure 2.10). Only the peak positions have slightly shifted. This shift relative to p-TPD is largest for MDCTPD, which is expected because of the strong electron-withdrawing groups attached to one of the two triphenyl moieties. Ionization Energy Measurements 31

Fig. 2.9: UPS spectra of TNF and TMPD.

Fig. 2.10: UPS spectra of the charge transport molecules p-TPD, MTPD, MDCTPD and ECZ. 32 Chapter Two

The difference between the ionization energies of MTPD and p-TPD is rather small, indicating that the presence of the oxygen electron-donors does not have a large effect on the electron density. This is probably due to the presence of the much more electron- donating nature of the amine atoms. One striking thing that can be observed immediately is the large difference in ionization energy between ECZ and the other charge-transporting molecules. From the number of electron-donating groups per aromatic ring, which is similar to that of for instance p-TPD, a comparable electron density and hence ionization energy would be expected. The main difference between ECZ and the TPD-based molecules is the shorter conjugation length in ECZ. Whether this is the reason for the observed difference in ionization energy remains uncertain, as the ionization energy of another very small molecule, TMPD, lies in the range of the values observed for the TPD-based molecules. ECZ is not very often used in photorefractive polymers but it resembles very closely the basic unit of poly(N- vinylcarbazole) (PVK), a very widely used polymer in photorefractive materials. Since it is not possible to measure the gas phase UPS spectrum of PVK, the value obtained for ECZ will be assumed to be approximately equal to that of PVK.

Fig. 2.11: The UPS spectra of the electro-optic molecules EPNA, DEANST and FDEAMNST. Ionization Energy Measurements 33

The ionization energies of the electro-optic molecules are all in the same range (figure 2.11). This is not so surprising as they have the same electron-donating and accepting moieties and only differ with respect to their effective conjugation length. As expected, due to their intermediate electron density caused by the presence of both a strong electron donor and acceptor, their values lie in between those of the charge transport molecules and TNF.

2.3.2. Ionization energies

From the above-described UPS spectra, ionization energies of all the molecules were obtained, using the method described for DEH (table 2.1).

EI (eV) EI (eV) TNF 9.50 ECZ 7.25 TMPD 6.15 DEH 6.20 p-TPD 6.30 EPNA 7.55 MTPD 6.20 DEANST 7.30 MDCTPD 6.50 FDEAMNST 7.20

Table 2.1: Ionization energies of molecules frequently used in photorefractive polymers.

These ionization energies are representative for the HOMO energies of the specific molecules relative to the vacuum level. In order to get an overview of the HOMO energy levels of the different molecules that can be used in a photorefractive material they are depicted in figure 2.12. From the data in this figure several conclusions can be drawn. First of all it can be seen that the energy of the HOMO levels for the charge transport molecules, apart from ECZ, are rather similar. The value for ECZ deviates from the values observed for the other charge transport molecules; the origin for this deviation is not clear, as based on the chemical structure also a rather high electron density would be expected. To a lesser extent the HOMO energy of MDCTPD is also out of range with respect to the other charge transport compounds; this however, is most likely due to the presence of a strong electron-withdrawing group on one of the triphenylamine moieties. This molecule is a combination of a charge transport molecule and an NLO molecule, which is reflected in the HOMO level. The HOMO levels of the NLO molecules are also rather close. This is understandable as the electron-donating and accepting groups that are used are the same for all these molecules. The only variation exists in the way these units are linked, that is, in the extent of conjugation. 34 Chapter Two

Fig. 2.12: Schematic representation of the HOMO energy levels of some molecules frequently used in photorefractive materials, as deduced from gas phase UPS measurements. The value for C60 was obtained from the literature10.

What is obvious, however, is the small difference between the HOMO levels of the NLO molecules and that of ECZ. Therefore, when photorefractive materials consist of ECZ or PVK as the charge transport unit in combination with these NLO molecules, problems may arise in the transport of charges. Due to the small energy difference between the HOMO levels, holes might be temporarily located on NLO molecules, which would decrease the hole mobility. Another observation is the large difference in HOMO energy between C60 and TNF. There is of course a big difference in chemical nature between C60 and TNF: in the latter the presence of three strong electron- withdrawing groups (nitro groups) cause the high ionization energy whereas in the case of C60 there are no electron withdrawing groups and as a consequence the ionization energy is much lower. Another important parameter especially for the charge generator molecules is the electron affinity. This value is necessary in order to determine the actual energy of a the ionized charge transport state (figure 2.1). Using UPS however, the electron affinities can not be determined. Using the values for the HOMO energies of the different molecules together with the value of the LUMO energy for C60, one can now construct a total energy plot of the different species involved in the space-charge field formation (figure 2.1). In figure 2.13 such a total energy plot is depicted for a photorefractive material employing C60 to facilitate the charge generation, ECZ as the charge transport molecule and DEH as the charge trapping molecule. Ionization Energy Measurements 35

Fig. 2.13: A schematic representation of the energies required to obtain a space charge field. The superscripts *, - and + describe the excited, the negatively charged and the positively charged states respectively. - + - + The states C60 /ECZ and C60 /ECZ/ECZ both represent the completely - + separated electron and hole. The C60 /ECZ/ECZ state is depicted to clearify that the energy of the state does not change when the holes moves away from the generation area.

- + The energy required to reach C60 /ECZ is determined by the ionization energy of ECZ minus the electron affinity of C60. The electron affinity of C60 is known from the literature to be 2.65 eV10 . The ionization energy of ECZ was measured to be equal to 7.25 eV; therefore, the energy required to create a hole on a ECZ molecule by the interaction with C60 is 4.60 eV. This is an unrealistic value as it would mean that only with photons of an energy higher than 4.60 eV, which corresponds to a wavelength of 269.5 nm, would a photoexcited electron- hole pair be created. This is in contradiction with what is known from the literature, where usually lasers which emit in the red region of the visible spectrum are used to photogenerate the charges11. In most cases a helium-neon (HeNe) laser which emits at 633 nm is used, the energy of these photons being 1.96 eV. Now we have to bear in mind that the values used to calculate the required energy are obtained from gas phase measurements, whereas, in a working photorefractive polymer the molecules are embedded in a very polar environment which significantly stabilizes the negatively and positively charged molecules. This results in a reduction of the ionization energy and an increase of the electron affinity each by Ep = z e2 a / 2 R4, where a is the polarizability of the surroundings, R is the intermolecular distance and z is the coordination number12. Even though in the case of amorphous photorefractive polymers R and z are unknown it is clear from this equation that the 36 Chapter Two

- polarizability of the medium can have a strong influence on the energy of the state C60 + /ECZ as this is decreased by 2Ep due to the decrease of EI (ECZ) and the increase of the electron affinity (C60). The difference between the ionization energies observed from the gas and the solid phase for triphenylamine, which is a relatively apolar molecule, is already 1.1 eV13, indicating that in the case of very polar molecules this correction can be even higher. Thus the influence of the stabilization can explain the discrepancy between the actual (the working photorefractive polymer) and the calculated energy difference. The migration of the hole away from the location where it was generated results in a positively charged ECZ molecule surrounded only by neutral ECZ molecules. As we consider the first electron-hole pair to be already completely dissociated, the total - + - + energy of the state C60 /ECZ/ECZ is similar to that of C60 /ECZ . From this schematic representation it might seem as if it does not cost any energy to move a hole away from its generation site. This is not the case, as an electron does require additional energy to hop from one molecule to another. This energy however, is much less than what is required to create a free electron and hole, and lies below the thermal activation energy. When a trapping molecule is involved in the electron hopping process the situation differs considerably. Now the final state is different from the initial state because of the difference in ionization energy of the charge transport and the charge trapping molecule. In the case of our example the energy level of the ionized trap molecule is 1.10 eV (the difference between the EI of ECZ and the EI of DEH) lower than that of the ionized charge transport molecule. Only when additional photon energy is available can the hole be released from the trapping site. The energy required to release a hole from a trapping molecule (1.1 eV) can in the case of a photorefractive polymer be provided by photons emitted from a HeNe laser as they have an energy of 1.96 eV. The effect of the polar surroundings can be neglected as this process deals with two ionized species, which will both be stabilized to approximately the same extent by the surrounding molecules.

2.4. Conclusions

Using gas phase UPS, information was obtained about the molecular orbital energy levels of molecules which are frequently employed in photorefractive polymers. In order to do this without immediately contaminating the apparatus, a special source was designed and used to sublime the sample molecules. The photoelectron spectra obtained in this way were used for the determination of the HOMO energy levels. The HOMO energy levels do not vary much from one molecule to another, in the same class, with the exception of ethylcarbazole. ECZ has a remarkably lower HOMO level than its colleague charge transport molecules, so much lower that it is in the same range as the HOMO levels of the NLO molecules. Even though the reason for this difference is not clear it is realized that such a low HOMO level might cause problems in the Ionization Energy Measurements 37 charge transport of holes, when ECZ is used in combination with NLO molecules. The low HOMO level of TNF can be explained on the basis of the relatively low electron density on the ring system. Since the HOMO and LUMO energy levels of C60 are known, an in-depth analysis of the space-charge field formation has been possible. It was shown that the calculated energy necessary to photoexcite an electron and hole is much larger than that observed from experiment. This discrepancy can be accounted for by the absence of dipolar interactions in gas phase experiments, while these interactions are present in experimental circumstances. In spite of the shortcoming of the method used, a reasonable value has been obtained for the energy associated with trapping, i.e. the energy difference between ECZ/ECZ+ and ECZ/DEH+. In this case the dipole stabilization, which is again not taken into account, is assumed to be approximately the same for both states as they are both charged. With this information about the ionization energies of the different molecules, the way to a more fundamental understanding of the influence of these molecules on the photorefractive behaviour has been opened.

2.5. References

[1] W.E. Moerner and S.M. Silence, Chem. Rev. 94, 127 (1994) [2] W.D. Gill in "Photoconductivity and Related Phenomena", J. Mort and D.M. Pai, eds., Elsevier (1976) [3] D.M. Pai and B.E. Springett, Rev. Mod. Phys. 65, 186 (1993) [4] A.D. Bakker and D. Betteridge, “Photoelectron Spectroscopy, Chemical and analytical aspects”, Pergamon Press, Oxford (1972) [5] J.H.D. Eland, “Photoelectron Spectroscopy, an introduction to ultraviolet photoelectron spectroscopy in the gas phase”, Butterworth, London (1974) [6] P.K. Ghosh, “Introduction to Photoelectron Spectroscopy”, Chemical Analysis, vol. 67, Wiley, New York (1983) [7] D.W. Turner, C. Baker, A.D. Baker and C.R. Brundle, “Molecular Photoelectron Spectroscopy”, Wiley, New York, p. 42 (1970) [8] P.M. Borsenberger and D.S. Weiss, eds. "Organic Photoreceptors for Imaging Systems", Optical Engineering vol. 39, Marcel Dekker, Inc. (1993) [9] D.J. Williams, Angew. Chem. Int. Ed. Engl. 23, 690 (1984) [10] L. Wang, J. Conceicao, C. Jin and R.E. Smalley, Chem. Phys. Lett. 182, 5 (1991) [11] Y. Zhang, R. Burzynski, S. Ghosal and M.K. Casstevens, Adv.Mater. 8, 111 (1996) [12] R.W. Lof, M.A. van Veenendaal, B. Koopmans, H.T. Jonkman and G.A. Sawatzky, Phys. Rev. Lett. 68, 3924 (1992). [13] C.B. Duke, J.W.-P. Lin, A. Paton, W.R. Salaneck and K.L. Yip, Chem. Phys. Lett. 61, 402 (1979)