J. Phys. Chem. 1994,98, 10756-10761
10756
Transient Ions and Triplet States in Polymers Containing Phenanthrene Gregory W. Haggquist, Kenji Hisada, Akira Tsuchida, and Masahide Yamamoto* Department of Polymer Chemistry, Faculty of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606, Japan
K. Razi Naqvi* Department of Physics, University of Trondheim, N-7055 Dragvoll, Norway Received: February 18, 1994; In Final Form: July 1, 1994@
A series of phenanthryl and methylmethacrylate copolymers were synthesized and investigated by means of nanosecond laser kinetic spectroscopy. Laser-induced absorption changes (in the 400-500-nm spectral region) were attributed to the triplet state and to radical ions of phenanthrene, whose disappearance could not be explained by conventional homogeneous kinetics. Accordingly, a model was developed in which each transient species (X, say) was divided into two categories: mobile (X,) and napped (Xt). The observations could be interpreted in terms of a mechanism involving, apart from unimolecular decay (if any), two types of encounters: Xm - X m and X, - X,. The bimolecular rate constant for the annihilation process, X, Xm products, was found to increase with rising phenanthrene content. The rate constant for the pseudounimolecular decay of the triplet also increased with loading, an effect attributed to migration of charge and electronic excitation, and quenching of the latter by doublet-state ionic species.
+
I. Introduction Photogenerated excited triplet states and transient ions of many aromatic and heterocyclic compounds have been thoroughly by incorporating the chromophores in diverse media, which range from fluid solvents to rigid polymer matrices. Once produced, the charge (or hole) and electronic excitation can participate in bimolecular interactions brought about by diffusion-mediatedencounters; but even if the rigidity of the medium does not permit appreciable diffusion during the lifetime of the transient species, these photoproducts can nonetheless migrate from one chromophre to another via the electron exchange mechanism and thereby undergo annihilation events leading to the formation of excited-state singlets or other species. There have been extensive studies on the reactions pursued by excited triplet states and photoinduced ions for monomeric chromophores in fluid and frozen solvents. A recent study Haggquist and co-workers3found that both triplets and ions are present after photoexcitation of N-ethylcarbazole in fluid solutions and that the higher the dielectric constant of the solvent the larger the ionsltriplets ratio. They also showed that both triplet-triplet (2'-T) annihilation and ion recombination lead to delayed fluorescence. The kinetic data were analyzed by using a homogeneous model which did not take account of the interactions between triplets and ions. In polymeric systems-where local chromophore concentrations and migration rates are extremely large-the mechanisms for the formation and disappearance of the photoproducts are not necessarily the same as those for the monomeric analogues. One crucial difference, which considerably complicates the analysis, is the heterogeneous nature of the environment surrounding the transients photoproducts. Further, Tsuchida and co-workers4have shown that ions can be trapped within polymer matrices or even by dimeric conformations; similarly, Ito and co-workers5 have shown that excited triplet states can be trapped in copolymer films containing phenanthrene (Phen). @
Abstract published in Advance ACS Abstracts, September 15, 1994.
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The objective of this work is to study, using fluid solutions at ambient temperatures, polymeric system with varying Phen content within copolymers of methyl methacrylate. Nanosecond laser kinetic spectroscopy has been used for monitoring the absorbance of radical ions and triplet states with a view to establishing the connection, if any, between their kinetic paths, and to determining the rates of intra- and interchain annihilation of these species. Since the data are not compatible with conventional homogeneous kinetics, a simple model, incorporating the heterogeneity of the environment, has been developed to rationalize the observations.
11. Experimental Section Purification of Chemicals. Tetrahydrofuran (THF) (Wako Pure Chemical Industry) was distilled from CaH2 just prior to use. 2-Methyltetrahydrofuran (MTHF) (Wako Pure Chemical Industry) was freshly distilled from CaH2 before use. Acetonitrile (Nacalai Tesque) was distilled from CaH2; benzene (Dojindo Laboratories) was dried over CaH2 and then fractionally distilled. N,N-Dimethylformamide (DMF) was distilled under reduced pressure twice before use. Benzophenone (BP) (Nacalai Tesque), trans-stilbene (Wako Pure Chemical Industry), and 1,2-dicyanobenzene (DCNB) were recrystallized three times from ethanol. Synthesis of 9-Vinylphenanthrene (9VPh). A mixture of 40 mL of n-butyl lithium (15 wt % of hexane solution, E. Merck) and 40 mL of anhydrous ether (Dojindo Laboratories) was added dropwise, with vigorous stirring, to a suspension of 16.8 g (4.70 x mol) of triphenylmethylphosphonium bromide (Wako Pure Chemical Industry) in 150 mL of anhydrous ether (Dojindo Laboratories) over a 10-min period. The resultant mixture was stirred for 4 h at room temperature. To this mixture a solution of 9.35 g (4.54 x mol) of phenanthrene-9-carboxaldehyde (Aldrich Chemical Company) in 100 mL of THF was added dropwise. This mixture was then stirred for 3 h at room temperature and refluxed for an additional 5 h. A gentle flow of nitrogen passed through the reaction vessel during the reaction. After the refluxing stage the solvent was distilled off
0022-3654/94/2098-10756$04.50/0 0 1994 American Chemical Society
Polymers Containing Phenanthrene
J. Phys. Chem., Vol. 98, No. 42, 1994 10757
TABLE 1: Monomer Feed, Molecular Weights, Polydispersities, and Composition of Polymers Synthesized
I M
sample 9VPh, % MMA, % Mn, 104 Mw, 104 M w M n Phen, %
1 2 3 4 5
1.4 3.0 9.9 33.7 100.0
98.6 97.0 90.1 66.3
0.0
3.8 2.9 1.4 0.56 0.71
5.5 4.1 2.1 0.80 0.90
1.5 1.4 1.5 1.4 1.3
"
i:\1
1.35 3.09 15.8 47.8 100.0
under reduced pressure. The residue was dissolved in toluene, washed with 25% NaHS03 (aq) three times, and washed with distilled water three times. The organic layer was separated and dried over anhydrous MgS04. The toluene from the organic layer was evaporated until a yellow oil remained, which was purified by passing it through silica gel (Nacalai, 230-400 mesh) with toluene as the eluent, and then recrystallized from ethanol: mp 38-40 "C; yield 1.88 g (20%). Purification of Methyl Methacrylate (MMA). Commercial methyl methacrylate (Wako Pure Chemical Industries) was purified by distillation under reduced pressure before use. Synthesis of the Homopolymer and Copolymers. Weighed quantities of the monomers 9VPh and MMA were dissolved in benzene with a small amount of a,a'-azobisisobutyronitrile. These solutions were sealed off under vacuum after four freezepump-thaw cycles and each sealed ampule was heated to 60 "C. The polymer products were purified by repeated precipitations by dropwise addition of benzene solutions into methanol and dried in vacuo. The characterization of these polymers was determined by 'H N M R , UV-vis absorption, and elemental analysis. The molecular weights and polydispersities, determined by a GPC calibrated with polystyrene standards, are displayed in Table 1. Sample Preparation. The concentration of each sample was adjusted until its absorbance (in a 1-cm quartz cuvette) at the wavelength to be used for excitation was in the range 1 f 0.1. All samples were sealed with a septum and purged with Ar for 20 min before use. Experimental Apparatus. The transient absorption spectra and decay curves were collected by using a cross beam alignment. A Lambda Physik EMG101-MSC excimer laser was used as the excitation source. The excitation wavelength was either 308 nm (XeCl gas), 351 nm (XeF) or 248 nm (KrF). Pulse energies were generally kept at 20 mJlcm2 by using glass or wire attenuation filters. The light from the interrogation source (a Ushio 150 W Xe lamp pulsed by a homemade high voltage unit) was collimated and made to traverse through that portion of the sample which was closest to the face illuminated by the laser; a 2-mm slit, placed on the detection side of the sample, was used to restrict the volume probed by the beam. The monochromator, Ritsu Oyo Kogaku MC-1ON, was placed at a distance of about 0.3 m from the sample in order to reduce the collection of the emission from the sample. A Hamamatsu R1477 photomultiplier tube, with a wiring which enabled it to recover quickly from a dose of intense prompt fluorescence,6 was placed at the exit slit of the monochromator. The data were collected on a Hewlett-Packard 54510A digital oscilloscope (250 M H z ) and transferred to a personal computer. For the 77 K transient absorbance experiments, the samples were immersed into a Pyrex Dewar flask filled with filtered liquid N2. The transient absorption spectra were collected by two methods using the same excitation and interrogation source as in the kinetic measurements. The interrogation beam either was collected by an optical fiber and directed into an Optical Multichannel Analyzer (Unisoku USP-500) or was collected point by point with averaging of 10 decays for every 5 nm from 350 to 600 nm.
300
350
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450
Wavelength /nm
Figure 1. Steady-state fluorescence spectra of samples 1 (solid line), 3 (medium dashed line), and 5 (short dashed line) in benzene solutions excited with 313 nm. 0.20 1
I1
. 000 300
350
400
450
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,.
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Wavelength /nm
Figure 2. Room temperature transient absorbance spectra of samples 1 (solid line), 3 (medium dashed line), and 5 (short dashed line) in DMF solutions 1 ps after a 351-nm excitation pulse (20 mJ/cm2). Collected by using the PMT system.
TABLE 2: Fluorescence Quantum Yields Determined in Benzene sample
df, %
0, ethylphenanthrene 1, 1.35% copolymer 2,3.09% copolymer 3, 15.8% copolymer 4,47.8% copolymer 5, homopolymer
9.7 9.6 8.5 8.8 8.9 6.6
111. Results Fluorescence Spectra. Steady-state fluorescence spectra from benzene solutions of several Phen-containing polymers are shown in Figure 1. No impurity emission was present, and the fluorescence yield decreased with increasing Phen loading within the polymer. Quantum yields, measured against a quinine sulfate standard (excitation at 313 nm), are listed in Table 2. Transient Absorption Spectra. Several authors's7 have reported that the triplet-triplet (T-T) absorption spectrum of Phen has peaks at 485 and 455 nm and a shoulder at 430 nm. The absorbance due to Phen triplets at a wavelength ?, will henceforth be denoted by A d a ) ; the laser-induced absorbance due to some other species U , by Ada). Room temperature transient absorption spectra of several samples in DMF, taken at 1 ps after a 35 1 nm excitation pulse, are shown in Figure 2; one sees that as the Phen content increases, the absorbance decreases and the center of gravity shifts to the blue. Spectra from the low-content samples (1 and 2) resemble the T-T spectrum of monomeric Phen. An unassigned absorbance at 430 nm, A d 4 3 0 ) , is the dominant feature in the homopolymer spectrum and is detectable in all
Haggquist et al.
10758 J. Phys. Chem., Vol. 98, No. 42, 1994 0.30
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1
0.20
d
0.00
'
350
'
'
400
'
'
'
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Wavelength /nm
Wavelength /nm
Figure 3. Room temperature time-resolved transient absorbance spectra of sample 5 in DMF at (a) 100 ns, (b) 250 ns, and (c) 1 ps after a 351-nm excitation pulse (20 mJ/cm2). Collected by using the photodiode array system.
Figure 5. Room temperature transient absorbance spectra of (a) BP (solid line), (b) sample 0 with BP (medium dashed line), and (c) sample 0 (short dashed line) in DMF at 1 ps after a 351-nm excitation pulse (5 mJ/cm2).
7
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0.15
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0.10 ,;
0.05
0.00 300
i 350
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Wavelength /nm
Figure 4. A 77 K transient absorbance spectrum of sample 5 in MTHF at 1 ps after a 351-nm excitation pulse (2 d/cm2). the other samples. Similar results were obtained when benzene was used as the solvent or the oxidation wavelength was changed to 308 nm. Figure 3 gives a series of time-resolved spectra for DMF solution of the homopolymer. At the shortest delay employed, the T-T absorbance of Phen is the most pronounced feature; as the delay increases Ad485) and Ad455) diminish, thereby making Au more distinct; after the disappearance of Ad485) and A7(455), AU(430) also decays. A low-temperature (77 K) transient absorption spectrum for the homopolymer in MTHF was collected (Figure 4) and is assigned to the T-T absorbance of Phen. This spectrum was present at all delay times used in this work, 100 ns to 1 ms. To prevent cracking of the glassy frozen solvent, a 2 mJ/cm2 pulse energy was used and the solute concentration was kept lower than that in the room temperature experiments. On comparing the spectra shown in Figures 2 and 4, one sees that reduction in temperature and a change in the solvent brings about a drastic change in the shape of the spectrum. Sensitization and quenching of Phen and all of the polymers were carried out with BP (5 x M) as the sensitizer and oxygen, trans-stilbene, and DCNB as quenchers. Pulse energies of 5 mJ/cm2 were used for the sensitization experiments and 20 mJ/cm2 for the quenching experiments. Triplet sensitization of Phen with BP shows (Figure 5 ) , at a time long enough to allow for T-T energy transfer, the T-T absorbance of Phen. Since the shape of the transient spectrum produced by direct excitation differs from that of the spectrum obtained by sensitization, we will make the reasonable assumption that the sensitized spectrum arises solely from the Phen triplet and the former contains, by the same token, a contribution from one or more unknown species (denoted collectively by
w.
I
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,.
/
ill
0.00
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Wavelength /nm
Figure 6. Room temperature transient absorbance spectra of (a) transstilbene (solid line, 35 1-nm excitation, 20 mJ/cm2),(b) trans-stilbene (medium dashed line, 248-nm excitation, 100 mJ/cm*),and (c) sample 5 with trans-stilbene (short dashed line, 351-nm excitation, 20 mJ/ cm2)in DMF. The introduction of oxygen quenched the entire absorption signal from the Phen sample, but in the homopolymer Ad430) it was just noticeable and took some 300 ns to come down to the noise level. Quenching of the homopolymer with transstilbene (Figure 6 ) resulted in an absorption band, centered at 500 nm, which could be assigned to the trans-stilbene cation.8 To substantiate this assignment, a transient absorption spectrum of trans-stilbene in acetonitrile (obtained by using a 248 nm, 100 mJlcm2 excitation pulse) was collected. The spectrum recorded under these conditions resembled, apart from a slight (presumably solvatochromic) shift, that obtained through the quenching experiment. It should be pointed out that the oxidative half-wave potential of trans-stilbene (1.51 V vs sce) is lower than that of Phen (1.68 V vs ~ c e ) . Thus ~ it is energetically possible to transfer the hole from Phen to transstilbene. The extinction coefficients for the Phen cation (sample 5) could be obtained by quenching the Phen singlet and triplet with DCNB (see Figure 7). The resulting spectrum revealed bands due to Phen cation and (DCNB)-, the DCNB anion (see discussion below). That the concentrations of the two types of ion are equal is a natural assumption and leads immediately to the extinction coefficient of Phen cation, since the extinction coefficient of (DCNB)- could be determined (1350 M-' cm-' at 380 nm) by performing a similar quenching experiment, using N-ethylcarbazole (Cz) singlet (and triplet) as the electron donor and utilizing the known value10of Cz cation (9600 M-' cm-' at 780 nm). On account of the similarity between the spectrum of Phen cation generated by the electron-transfer process and the spectrum observed at 1 - p delay in the room temperature
Polymers Containing Phenanthrene
0.01
J. Phys. Chem., Vol. 98, No. 42, 1994 10759 ever, no impurities could be detected when the vinyl monomer was examined by means of liquid chromatography, nor did the fluorescence and phosphorescence spectra of the polymer samples reveal any abnormal features. Studies of the homopolymer of phenanthrene15 and related bichromophoric compounds14 have led to the suggestion that the triplet excimer of Phen has an absorption band in the 400500-runrange. The evidence against the presence of triplet excimers in the systems described in this report comes from the quenching experiments. If just the triplet excimer and the monomeric triplet are present in our systems, only triplets of the quencher should be seen when trans-stilbene is used to quench the Phen transients; instead, the cation of trans-stilbene is found, which argues strongly for the presence of the cation of Phen. It is still difficult to completely discount the presence of triplet excimers, but clearly the monomeric triplet of Phen and the cation of Phen are the principal components. In the light of these arguments, Au will be attributed in part to the Phen monocation; the kinetic analysis of the absorbance data will take into account only the triplet and the radical ions of Phen. Ion Analysis. A main observation in this study is that as the content of Phen increases, the absorbance after 1 ys delay goes from mostly T-T absorbance to Ao. A similar change is seen in the time-resolved spectra for the homopolymer, where the familiar phenanthrene peaks at 485 and 455 nm give way to Au within 250 ns. The obvious question is how does the cation form? One can speculate that triplet interactions lead to ionization, a mechanism shown to occur in monomeric Phen samples by Jamagin et al.,16 who measured the current produced from photoexcited Phen and anthracene in THF. This is qute possible, but a substantial rise in the cation concentration was never seen, which might be due to a combination of two conditions, low triplet-triplet ionization yield and a large initial buildup of cations from twophoton ionization andor singlet-singlet ionization before monitoring was possible. In any event, a large concentration of cation is present and more detailed experiments will be required to determine the exact production routes. It is clear that cations are present, but there is no conclusive evidence for the presence of anions or solvated electrons. The detection of Phen anion is rendered difficult by the fact that its absorption spectrum lies in the same region as that of the Phen cation. The presence of the anion, which is taken for granted, will not impair the general form of our kinetic analysis; it will only undermine our estimates of the reported extinction coefficient and concentration of the cation and thereby vitiate the values of the pertinent second-order rate constants. Plots of the reciprocal cation concentration ( l l [ q ) versus time are not linear over the full time range; see Figure 8. Thus, a conventional second-order scheme, -d[q/dt = k[Cj2, is not sufficient to account for ion recombination within this polymeric system, and a more intricate model is clearly needed. It is common in polymer research to classify excitons as mobile and t r ~ p p e d , ' ~ .and ' ~ traps are supposed to be low-energy sites (dimers, etc.). In this paper we will continue to use these terms even though we are working with polymers where the chromophore density per coil covers a wide range, and, in some cases, excitation will be localized simply because like chromophores are few and far between. We postulate now that each polymer coil contains, in general, two types of molecular domains: one where the chromophore density is large (and migration is possible) and another where it is sparse (and migration is highly unlikely). On account of thermal fluctuations, momentary contacts will be established between two
! , 1 1
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Wavelength /m
Figure 7. Room temperature transient absorbance spectrum of sample 5 with DCNB in DMF at 1 ps after a 351-nm excitation pulse (20
d/cm2). homopolymer samples, Ao is tentatively assigned to the Phen cation (see discussion below). Transient Absorbance Kinetics. Absorbance si&als at 485 and 430 nm were measured for all the polymer samples and appeared to implicate two species with overlapping absorption spectra. To apportion the observed absorbance to T and C,the triplet and the cation of Phen, the extinction coefficients, E&) and E&), of T and C had to be determined. The literature" value for ~1(485)is 21 000 M-' cm-'; ~1(430)came out to be 6000 M-' cm-I from the ratio ~1(485)/~1(430), which was taken from the sensitized spectrum of the Phen monomer. The extinction coefficients for the cation, ~ ~ ( 4 8 = 5 )742 M-' cm-' and ~ ~ ( 4 3= 0 )945 M-' cm-', came from experiments in which DCNB was used as the quencher. That the values determined here for the homopolymer are much lower than those reported in literature for the Phen monomer can be attributed to hypochromism, which is a hallmark of polymer systems,'* and to the blurring of the vibrational structure. The concentrations of triplets and cations are determined by inserting the above extinction coefficients into eq 1, with Ai = 485 and 430 nm.
The assumption, implicit in eq 1, that only two species contribute to the measured absorbance is discussed below.
IV. Discussion Assignment of Au. Room temperature transient absorption spectra for the phenanthryl-containingpolymers were composed of excited triplet states of Phen and a second or even third component. Obvious candidates for the species responsible for the additional component(s) are the cation of Phen,I3 the triplet excimer of Phen,14J5 other photoproducts, and an initial impurity. The spectral region in question is 430 f 30 run,where both the triplet excimer and ions of Phen have been stated to absorb. A photoproduct is discounted since ground-state absorbance measurements after prolonged irradiation from the laser did not show any changes in the absorption spectrum. Moreover, the fluorescence spectra measured before and after intensive laser excitation did not indicate any deterioration of the sample. Anthracene, a common impurity in Phen, has sharp T-T absorption bands at 425 and 412 nm.' These bands were absent in the transient spectra, nor was there any trace of anthracene emission in the steady-statefluorescence spectra of the polymers. It is also unlikely that an impurity, if any, came from the solvent since AU was present in two different solvents, DMF and benzene. There remains the possibility that the suspected impurity came from the synthesis of vinylphenanthrene; how-
Haggquist et al.
10760 J. Phys. Chem., Vol. 98, No. 42, 1994
TABLE 3: Rate Constants for the Decay of Mobile and Trapped Cations of Phenanthrene Connected to a Polymer Chain
120000
1 2
3 4 5
30000 0’
0
’
’
5
15 20 Time ips
10
25
I
domains (which may or may not belong to the same category or the same coil), allowing charge and electronic excitation to hop over from one region to another. Since the average number of transient species per coil is considerably smaller than unity, the majority of bimolecular annihilation events would involve partners which were initially formed in different domains. The concentration, can be expressed as = [Xm] [X,],where X = C or T, and the subscripts “m” and “t” designate mobile and trapped species. Assuming the cation and anion concentrations to be equal, the rate equations for [Xm] and [X,] may be written as
[a
+
d[X,]/dt = - a ‘ x ) k ~ ~ [ X , ] 2 p(x)k,$[Xt][Xm] - k\”[X,]
(2)
d[X,]/dt= - a ( x ) k ~ ) [ X t ]-2 p‘x)kg)[Xm][Xt] - k\’)[X,]
(3)
in which k z ) k::), ky’ 0 (since the cation cannot undergo first-order decay), and kjT’ = k\’) ~ Q ~ Q [where Q ] , k‘p’ is the rate constant for true unimolecular deactivation, Q stands for a triplet quencher, and k E , k r ’ , and k z ) are bimolecular rate constants. The terms acX)and Pcx) represent the number of species that disappear for each event; for the triplet a(T) = 2, two mobile triplets are annihilated, and /?(n = 1 because annihilation is between a mobile and a trapped triplet. On the other hand, act) = 1, recombination of a mobile cation and anion, and pee) = 0.5, since the cation has a 50% chance of being mobile. Since the rate constant for Xm-Xm interactions will depend, among other parameters, on the chromophore content of the polymer, will be taken as an adjustable parameter. On the other hand, Xt-Xt encounters will be mediated by material diffusion of polymer chains and the coming into contact of two segments (which may or may not belong the same coil). Unless a special study is undertaken in which the polymer composition is kept unchanged but its concentration is varied systematically, it is difficult to make quantitative statements about the rate constant k::). Pending a detailed study (which is being planned now), we have opted for the simplest choice by taking both rate constants to be independent of the composition and concentration of the polymer; in the absence of any compelling argument in support of our choice, our only defense is the good quality of fits obtained by this simplistic approach. The question of how the data are actually fitted to the rate equations will now be addressed. It should be observed first that, though the initial concentration, [Xjo, can be determined from each decay curve, the quantities [Xmlo and [Xt]ocannot be extracted directly from the data. Let us introduce the ratio $ x ) ( t ) = [X,]/[X] and consider the data pertaining to C. The
+
kg
5.8 6.5
1.6 8.9 25
4.0
24 5 .O 60 82 98
5.0 6.4 11 50
3.6 3.6 3.6 3.6 3.6
list of parameters to be determined by our fitting procedure includes $Q(O), k z ) , and k r ) . One of these, kLc), can be fixed by analyzing the long-time, linear portion of the secondorder plot for the 3% sample, in which the majority of cations are immobile (see below) and [C,]. In order to further reduce the number of adjustable parameters, we will make the simplifying assumption that k z ) = k::)) % since kE’
