J. Phys. Chem. 1995, 99, 1332-1337
1332
XANES Spectroscopic Studies of Evaporated Porphyrin Films: Molecular Orientation and Electronic Structure S. Narioka,* H. Ishii,? Y. Ouchi,? T. Yokoyama? T. Ohta,' and K. Seki*lt Department of Chemistry, Faculty of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan, and Department of Chemistry, Faculty of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan Received: August 30, 1994; In Final Form: October 26, 1994@
Evaporated films of zinc 5,10,15,20-tetraphenylporphyrin(ZnTPP) and 5,10,15,20-tetraphenylporphyrin(H2TPP) on Ag substrates were studied by polarized X-ray absorption near-edge structure (XANES) spectroscopy. With the aid of theoretical simulation by CNDO/S2 calculations with the equivalent core approximation for model compounds, features in the N K-edge XANES spectra were assigned, and the feasibility of probing the N n* partial density of unoccupied states (PDOUS) by the N K-edge XANES was examined. The analysis of polarization dependence of XANES spectra revealed that ZnTPP molecules in the film deposited on Ag substrate at 367 K have a high degree of orientation with the central macrocyclic plane inclined by 28" f 10" to the substrate surface. On the other hand, ZnTPP films evaporated on room-temperature substrates and H2TPP films showed little indication of regular orientation.
I. Introduction Recently, porphyrin thin films have attracted considerable interest for their electric and optical properties, in relation to their possible applications to photovoltaic photocatalysts: memory with photochemical hole burr~ing,~ and so on. In order to attain good electrical and optical characteristics, it is necessary to access and control molecular orientations and arrangements and also to elucidate their electronic structures. The studies of porphyrin films on alkali halides showed that the molecular orientation and arrangement depended on the deposited compound and substrate m o r p h o l ~ g y . ~We . ~ also expect that substrate temperature will affect molecular orientation, as found for metallophthalocyanine evaporated films.8 Polarized X-ray-absorption near-edge structure (XANES) spectroscopy, also called near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, is recognized as a tool for investigating molecular orientation and unoccupied states in organic thin films9 As for porphyrins, N K-edge XANES studies of vanadyl 5,10,15,20-tetraphenylporphyrin(VOTPP) and nickel 2,3,7,8,12,13,17,18-octaethylporphyrin (NiOEP) (Figure le,f were reported,IOin which assignments were carried out by assuming that a metalloporphyrin has two kinds of nitrogens, i.e. a pyridine type (=N-) and a pyrrole type (-N-). However, X-ray photoelectron spectroscopy (XPS) measurements indicate all four nitrogens in a metalloporphyrin molecule are clearly showing that new assignments of their XANES spectra are necessary. On the other hand, the XPS study of free-base porphyrin indicates the existence of these two kinds of nitrogens (=N- and -NH-),11-14 and XANES study of this system should be interesting. From the polarization dependence of reliably assigned XANES spectra, we can also estimate the molecular ~rientation.~ In this work, we applied XANES spectroscopy to evaporated films of two typical porphyrins: 5,10,15,20-~inctetraphenylporphyrin (ZnTPP) (Figure la) and free-base 5,10,15,20-tetraphenylporphyrin (H2TPP) (Figure lb). Our aims in this work are
* To whom correspondence should be addressed. +
@
Nagoya University. The University of Tokyo. Abstract published in Advance ACS Absrracrs, December 15, 1994.
0022-365419512099-1332$09.00/0
0
C2H5
CZH5
(e) (0 Figure 1. Structural chemical formulas of prophyrins: (a) ZnTPP, (b) H*TPP, (c) porphine, (d) Be porphine, (e) VOTPP, and (f) NiOEP.
2-fold. The first is to establish more reliable interpretations of the N K-edge XANES spectra of porphyrins, with the aid of CNDOlS2 MO calculations. We focused our attention on the N K-edge, since it gives information about the electronic structure and orientation of the central macrocyclic ring, which dominates the electric and optical properties. The four phenyl rings are nearly perpendicular to this macrocyclic making C K-edge measurements not useful for orientation studies. We also examined the usefulness of N K-edge XANES spectroscopy for probing the N JC*partial density of unoccupied states (PDOUS) of porphyrins, with the aid of MO calculations.
0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 4, 1995 1333
XANES Spectroscopy of Evaporated Porphyrin Films '
1
ZnTPP
.
1
TABLE 1: Peak Energies and Assignments for the N K-Edge XANES Spectra for ZnTPP and HzTPP
I
D I
feature
ZnTPP excitation energy/eV
assignment
398.2 401.0 403.8 406.4 415.5 -439
' M
n* (eg) n* bu, e,) n* (au) U* U* U*
HzTPP
h
~
H2TPP
feature a b C
d e
f
excitation energy/eV 397.8 400.2 403.0
assignment N without H N with H n* (b3g) n* (blu) n* (b2g)
n*
406.3
U*
-415 -445
U*
U*
For orbital pattems of n* states, see Figure 4.
380
400
420
440
460
Photon energyIeV Figure 2. N K-edge XANES spectra of Z n P P and H2TPP deposited on Ag substrate at room temperature.
Our second aim is the preparation and characterization of highly oriented thin films of porphyrins by vacuum evaporation on a temperature-controlled substrate. We actually succeeded in the case of ZnTPP, and the molecular orientation in the film was determined by the polarization dependence of the XANES spectra. 11. Experimental Section ZnTPP and H2TPP were purchased from Sigma and Tokyo Chemical Industries, Co., Ltd., respectively, and were purified by vacuum sublimation. XANES measurements were performed on the Beamline 11A of the Photon Factory at the National Institute for High Energy Physics (KEK-PF). Synchrotron radiation from the storage ring was monochromatized by a Grasshopper monochromator with a 1200 lines/" grating.18 The total energy resolution was about 0.5 eV. The substrate was vacuum-evaporated Ag film on Cu plate (Ag/ Cu). Porphyrin thin films were prepared by vacuum deposition on these AgKu substrates both at room temperature and at 367 K under vacuum of Pa. The film thickness was 20 nm, as measured by a quartz oscillator. The XANES spectra were measured at the N1, region with the total-electron yield mode using a channeltron. The soft X-ray incidence angle 8 could be varied from normal incidence (8 = 90') to grazing incidence (8 = IS0), without changing the relative arrangement between the sample and the channeltron. Energy calibration was carried out by using the spectra of second standards of hexatriacontane ( ~ - C H ~ ( C H Z ) ~ ~and C Htetracyanoquinodimethane ~) (TCNQ), which were calibrated against the vapor spectra of Ar,19 N2,20 0 2 , 2 1 and SF6.22 111. Results and Discussion A. Peak Assignments. In Figure 2, the N K-edge XANES spectra of unoriented ZnTPP and H2TPP films deposited on Ag/ Cu substrate are shown, and their peak energies are listed in Table 1. Regardless of whether porphyrin has a central metal or not, the general appearance of the spectra is similar; there are three peaks (A, B, and C or a, b, and c) in the lower photon
energy region and three peaks (D, E, and F or d, e, and f) in the higher photon energy region. As described later, the features in these energy regions correspond to excitations to n* and u* orbitals, respectively, as deduced from the polarization dependence of absorption intensity. In the inset of Figure 2, the expanded spectra in the lower energy 1s n* excitation region are shown. We see that the peak positions and relative peak intensities of the two compounds are slightly different between the two compounds. The peak positions of a, b, and c in H2TPP are 0.4-0.8 eV lower than those of A, B, and C in ZnTPP. The peak widths of B and b are broader than those of A and a. The spectral features of ZnTPP are very similar to those reported for VOTPP and NiOEP.l0 From the comparison with the spectra of pyrroles, pyridines, and saturated amines, the authors of ref 10 assigned the peaks corresponding to A and B to the transitions from pyridine type (=N-) and pyrrole type (-N-) nitrogens to a common n* level, respectively. As already mentioned, however, N1, XPS spectra of metalloporphyrins show only a single peak at 391.7 eV," indicating that all four N atoms in a molecule are equivalent. In ref 10, it was also reported that the resonances of free-base octaethylporphyrin (H20EP) are shifted to the lower energy side by -1 eV compared to those of NiOEP, in correspondence with the difference between ZnTPP and H2TPP found in the present study. The authors did not discuss the origin of this shift, seemingly suggesting a common assignment with those for metalloporphyrins. Since two peaks corresponding to -NHand =N- type nitrogen atoms appear at 399.1 and 397.1 eV in the XPS spectrum of free-base HzTPP," we expect a significant difference between the electronic structure of metalloporphyrins and the free-base porphyrin. Clearly, new assignments based on more detailed theoretical analysis are necessary. In order to perform such an analysis, we simulated the n* transitions by the CNDO/S2 XANES spectra for N1, formalism.23 Calculations were carried out for free-base porphine and Be porphine (Figures lc,d) instead of H2TPP and ZnTPP. By replacing the phenyl groups in TPP by four H atoms, the calculation time is much reduced, while the replacement will have little influence on the n electronic structure of the central macrocycle, since phenyl groups are nearly perpendicular to the m a c r ~ c y c l e . ~ ~We - ' ~ also replaced Zn by Be due to the lack of CNDO/S2 parameters for Zn, but this will not be a difficulty for analyzing the difference between free-
-
-
1334 J. Phys. Chem., Vol. 99, No. 4, 1995
-
" I ._
"
5
1
"
Be porphine
'
1
U A
1
Narioka et al. I ~
TABLE 2: Term Values and Assignments for the Calculated Excitations by the CNDO/S2 Method with the Equivalent Core Approximation excitation
Be Porphine term value"/eV
I I1 I11 IV 10
0
6
4
2
7.30
3.95 2.74 1.64
Porphine
0
final state Yf
Term value/eV
Figure 3. Simulated XANES spectra for the transitions from NI, core levels to the n* antibonding orbitals for Be porphine (upper) and porphine (lower). The abscissa is the excitation energy relative to the ionization energy of the NI, level of Be porphine, which is contracted by a factor of 0.95 for a better fit with the observed spectra.
base and metalloporphyrins, since the N K-edge XANES spectra are similar among metalloporphyrins (ZnTPP, VOTPP,'O and NiOEP'O). For the porphine macrocycle, we adapted the molecular geometry based on average bond distances and bond angles from X-ray diffraction of ZnTPP.20 The molecule of Be porphine is assumed to have D4h symmetry, while it is reduced to DZh symmetry for porphine by introducing two H atoms. For simulating the XANES spectra, a restricted Hartree-Fock (RHF) method for doublet states was employed for the NI,excited states. To take into account the relaxation of passive electrons upon core hole creation, the equivalent core approximationZ4was applied to the Nl,-ionized and -excited states; the valence atomic orbital whose N1, core electron is excited was replaced with that of oxygen. In evaluating the energies of higher excited states, a limited configuration interaction (CI) calculation was performed including only single-electron excitation configurations with respect to the doublet ground state. Relative transition probabilities within the one-electron picture were approximately estimated by comparing the population in the components of the 0 z p atomic orbitals among the excited MOs. The N1, ionization energies were also calculated as the difference between the total energies of the ground and ionized states. The simulated spectra were obtained by convoluting 6 functions at each excitation energy with a Gaussian function of 1 eV fwhm, with weights given by the transition probabilities. For free-base porphine, such calculations were carried out for the excitation of two types of N atoms, Le. pyridine type nitrogens (=N-) and pyrrole type nitrogens (-NH-), and these spectra were combined to yield the overall N1, spectrum. We focused on the transitions from N1, levels to TC* orbitals, since the transitions to o* orbitals above the vacuum level may not be adequately treated by the present LCAO type MO calculations. In Figure 3, the simulated N K-edge XANES spectra for transitions from N1, level to z* orbitals in Be porphine and free-base porphine are shown. The abscissa is the excitation energy relative to the calculated ionization energy of the N1, level of Be porphine. For a better fit to the observed spectra, the energy axis is contracted by a factor of 0.95. The three peaks in each simulated spectrum are ascribed to four excitations. The energies and characters of these excitations are listed in Table 2. The simulated spectra correspond well with the observed N1, n* spectra in Figure 2. The second peaks, B and b, in each observed spectrum are broader than the lowest energy peaks, A and a, since the peaks B and b are due to two overlapping transitions. Such a good agreement between the
-
final state Yf
excitation
term value"/eV
N without H
1 2 3 4
7.82 4.24 3.88 2.48
Jc* (b3,)
N with H
~ c (blu) * ~ c *( b g ) ~ c *(blu)
Relative to calculated NI, ionization energy of Be porphine.
observed and simulated spectra supports the reliability of the present calculations. More details of the difference between the spectra of Be porphine and free-base porphine can be analyzed with Figure 4, where energies and orbital pattems of the initial and final states in the ground state are depicted for the eight excitations in Figure 3. The orbital energy differences between the initial and final states are not equal to the excitation energies, but they show parallel trends, as discussed later in more detail. In free-base porphine, the N1, energies for N atoms with and without a hydrogen atom are significantly different, with the energy of the pyridine type nitrogen (=N-) being almost equal to that of four equivalent nitrogens of Be porphine. This corresponds well with the trend in the observed N1, XPS spectra of H2TPP and ZnTPP." The reduction of symmetry from D4h (metalloporphyrin) to D2h (free-base porphyrin) results in a small splitting of the doubly degenerate e2g LUMOs into b3g and b2g orbitals, making the LUMO of Be porphine higher than that of porphine. This is supported by the reported reduction potentials of the macrocycle (- 1.05 V for H2TPP25,26and - 1.35 V for ZnTPP25,27). Thus, excitation I in Be porphine corresponds to excitations 1 and 3 in free-base porphine. Excitation 1 is slightly shifted to lower energy compared to I, mainly due to the difference of LUMO energies. Such a shift corresponds well to the trend in the observed spectra in Figure 2. The excitation 3 appears at significantly higher energy due to the large ionization energy of the pyrrole type (-NH-) nitrogen, forming a part of the second peak in the simulated spectrum (Figure 3). Similarly, excitation I1 in Be porphine corresponds to excitations 2 and 4 in porphine, the latter forming a separated peak in Figure 3. In Be porphine, excitations I11 and IV in Figure 4 are due to transitions to the two orbitals formed by the combination of the 2p, orbital of the macrocyclic ligand and the pz orbital of the central atom. Transitions to the corresponding z* orbitals in metal-free porphyrin are calculated to be negligibly weak. We note that the character of the highest energy peak in the simulated spectrum of free-base porphyrin in Figure 3 does not correspond to the highest energy peak (IV) of Be porphine, but to the second peak (11). Thus, we could properly assign the N1, z* XANES spectra of porphyrins, including the difference between the spectra of metallo porphyrins and free-base porphyrins. The results are much different from simple interpretations based on the apparent similarity between the spectra of these compounds.
-
XANES Spectroscopy of Evaporated Porphyrin Films
J. Phys. Chem., Vol. 99, No. 4, 1995 1335
6% ry.: @&y b2u
Q%3' eg (LUMOs)
,
1
1
-2
-NH-
-4
Be porphine
porphine
Figure 4. Calculated NI, and z* levels of Be porphine (D4h symmetry) and porphine (DZh symmetry) and excitations among them. The orbital energies of the empty levels and the binding energies of NI, core levels (relative to that of ZnTPP) are shown. The energy scale is contracted by a factor of 0.95. The molecular orbital patterns of empty z* states are also shown. Finally, we will examine the feasibility of probing the density of the n* unoccupied states by the measured N K-edge XANES spectra. In XANES spectroscopy, we observe excitations from a core level (or core levels) to various unoccupied states. From the simple nature of the core levels, we can expect that the spectrum will simply reflect the density of unoccupied states (DOUS). In reality, however, the spectrum is modified from DOUS due to the effect of core hole or inner-shell excitonic effects. We should also notice that the XANES spectrum for an element would give information only about the unoccupied states containing contributions from the excited atom. This, in tum, offers a possibility of probing the partial density of states (PDOUS) from the element under measurement. Here we examine the feasibility of probing the N n* PDOUS by N K-edge XANES spectra. In Figure 5,a,b, we show the simulated XANES spectrum and the calculated N ~ d *PDOUS of Be porphine with bars indicating the energies of orbitals with contribution from the 2p, orbitals of N atoms. The simulated PDOUS was obtained by convoluting 6 functions at each orbital energy with a Gaussian function with 1 eV fwhm after contracting the energy scale by a factor of 0.95. The contribution from each orbital is weighted by the sum of squares of the N 2p, atomic orbital coefficients. Note that the four nitrogens are equivalent in such a metalloporphyrin, simplifying the character of initial orbital in the XANES spectrum. We see a correspondence between the simulated XANES spectrum and calculated PDOUS except for the region around 3 eV. The unoccupied states i-iv in (b) correspond to the final states of excitations I-IV in (a), respectively. The weak intensity and energy shifts in XANES spectrum in the region are due to the effect of the core hole. These results indicate that the excitonic effect caused by the interaction between the core hole and the excited electron in Be porphine is not so significant, although not negligible, leading
'
I
'
I
'
I
'
I
'
Be porphine (a) simulated XANES spectrum "
(b) calculated N X* PDOUS
10
a
6
4
2
0
E ne rgyIeV Figure 5. (a) Simulated XANES spectrum of Be porphine. (b) Theoretical partial densities of unoccupied states (PDOUS) of z* orbitals of Be porphine with bars indicating the energies of orbitals with contributions from N 2p, atomic orbitals. The energy scale is contracted by a factor of 0.95. to a reasonable agreement of the XANES spectrum with N n* PDOUS. For porphine, on the other hand, the calculated N JC* PDOUS spectrum does not show correspondence with the simulated XANES spectrum, since the XANES spectrum is the superposition of two sets of PDOUS shifted by the energy separation between the pyridine type and pyrrole type nitrogens. B. Molecular Orientation. In Figure 6, the N K-edge XANES spectra for ZnTPP deposited on AgICu substrate at 367 K are shown as a function of photon incidence angle 9 defined in the inset. All spectra are normalized to the height of the edge-jump. The spectra strongly depend on 8, indicating successful preparation of a highly oriented film. The peaks A, B, and C in the low-energy region are weak at the normal
1336 J. Phys. Chem., Vol. 99, No. 4, 1995 I ZnTPPJAgICu N K-edge
Narioka et al.
Ts=367K
ZnTPP/Ag/Cu
c
h
P=l .o
v) .-c
S I a=28"+1Oo
0 0
I
1
I
1
30
60
90
E-Vector polar angle e/deg. 1
380
,
400
l
420
,
/
440
/
460
Photon energy/eV Figure 6. N K-edge XANES spectra of ZnTPP films deposited on Ag/Cu substrate at 367 K as a function of photon incidence angle 8.
incidence (6' = 90"), while they become strong with decreasing 8. Peaks D and E show the opposite trend of increasing intensity with 8. For a planar n-conjugated system, as in the present case, it is known that Is n* and 1s o* transitions occur for the electric vector of the light E perpendicular and parallel to the molecular plane, respectively .g With these expected polarization dependence and theoretical calculations in section IILA, we can assign peaks A, B, and C in Figure 2 to Nls n* transitions and peaks D and E to NI, u* transitions. The 8 dependence of n* peaks A, B, and C in Figure 6 indicates that the central macrocyclic plane is roughly parallel to the substrate surface. A more quantitative estimation of molecular orientation angle can be performed by fitting the 6' dependence of the peak intensity to the theoretical curves formulated by Stohr and Outka.28 We chose peak A for this analysis, since it corresponds to a single excitation (see section 1II.A) and is well separated in energy from other peaks. When Ip and I, are the resonance intensities for E parallel and perpendicular to the orbital plane of the storage ring, resonance intensity is given by28
-
+
-
-
Figure 7. Observed dependence of the resonance intensity of peak A of ZnTPP in Figure 6 on the photon incidence angle 8. The intensity is normalized at 8 = 15". The real line is the best-fit curve for a = 28" assuming p = 1.0.
28" f IO" to the substrate surface. We give a rather conservative error bar of &lo", due to the uncertainty of background subtraction and limited S/N ratio. This value of a is not much affected by the assumed degree of polarization (a = 26" is obtained for p = 0.8). On the other hand, the 8 dependence of n* peak intensities was small in the XANES spectra for ZnTPP deposited on Ag/ Cu substrate at room temperature. This temperature dependence is probably due to the thermally activated rearrangement of deposit molecules on the substrate surface. We also studied the orientation of H2TPP films deposited on Ag/Cu substrates kept both at room temperature and at 367 K with N K-edge XANES. These films showed only a little 8 dependence of n* peak intensities in both spectra, although it was still sufficient for distinguishingn* and u* excitations. This weak 8 dependence suggests a rather disordered orientation of molecules. Correspondingly, it was reported that the vacuumevaporated H2TPP films on A1 at room temperature are composed of randomly oriented polycrystals.6 It seems that a further search for the correct conditions (substrate material, substrate temperature, film thickness, and so on) is necessary for obtaining well-oriented films of H2TPP.
IV. Summary where p is the degree of polarization of incident light and cl is a constant. When we assume azimuthally random molecular orientation, Ip and I, are claculated, by averaging the Ip and I, over the azimuthal angle, to be 1 zP = c211 + -(3 2
I, = cg sin2 a
cos2 e - 1)(3 cos2 a - I)]
(2a) (2b)
respectively, where a is the angle between the normal of the molecular plane N and the surface normal n (see inset of Figure 7) and c2 and c3 are constants. In Figure 7, the filled circles are observed resonance intensities I(8)/I(8=15") of peak A in Figure 6. The experimental data are best fit by the curve for a = 28" f 10" with p = 1.0, shown by the solid line in Figure 7. This indicates that the central macrocyclic plane is inclined by
Polarized XANES spectroscopic studies were performed for evaporated ZnTPP and H2TPP films. Peaks were assigned by the polarization dependence of the spectra and by simulating the XANES spectra, using CNDO/S2 calculations with the equivalent core approximation for model compounds. From this analysis, different origins were revealed for the n* transitions between H2TPP and ZnTPP, in spite of the apparent similarity of the observed spectra. The correspondence between the simulated XANES spectra and the calculated N x* PDOUS in the model compound of ZnTPP indicates a rather small excitonic effect caused by the interaction between the core hole and the excited electron. Such a correspondence was not found for the model compound of HzTPP due to the overlapping contribution from the two environmentally different nitrogens. Highly oriented ZnTPP films on Ag/Cu substrate were obtained by deposition on a heated substrate at 367 K. From the photonincidence-angledependence of peak intensities for the transition from the NI, core level to the n* LUMO, the central macrocyclic
J. Phys. Chem., Vol. 99, No. 4,1995 1337
XANES Spectroscopy of Evaporated Porphyrin Films plane in this film was deduced to be inclinded by 28" f 10" to the substrate surface. On the other hand, ZnTPP films on the substrate at room temperature and HzTPP films showed little polarization dependence. Acknowledgment. We thank Dr. Y. Takata and Messrs. M. Yoshiki, K. Edamatsu, T. Araki, E. Ito, and K. Oichi for taking part in the XANES measurements and for the help in CNDOI S2 calculations. We are also grateful to Professors K. Yamashita and Y. Harima for helpful discussion. The help of Professor A. Yagishita and Dr. Y. Kitajima of the Photon Factory is gratefully acknowledged. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan (Nos. 05NP0303 and 04404001). This work has been performed under the approval of the Photon Factory Programs Advisory Committee (Proposal Nos. 90-127 and 91-198). References and Notes (1) Kampas, F. J.; Gouterman, M. J . Phys. Chem. 1977, 82, 690. (2) Kampas, F. J.; Yamashita, K.; Fajer, J. Nature 1980, 284, 40. (3) Harima, Y.; Yamashita, K. J. Phys. Chem. 1985, 89, 5325. (4) See for example: (a) Rougee, M.; Ebbesen, T.; Ghettl, F.; Bensasson, R. V. J . Phys. Chem. 1982,86,4404. (b) Sasaki, M.; Mikami, N.; Ikeda, T.; Hachiya, K.; Yasunaga, T. J . Phys. Chem. 1982, 86, 4413. (5) Haarer, D. In Topics in Current Physics; Moemer, W. E., Ed.; Springer-Verlag: Berlin, 1988; Vol. 44, pp 79-125. (6) Yanagi, H.; Takemoto, K.; Hayashi, S.; Ashida, M. J . Cryst. Growth 1990, 99, 1038.
(7) Ashida, M.; Yanagi, H.; Hayashi, S.; Takemoto, K. Acta Crystallogr. B 1991, 47, 87. (8) Uyeda, N.; Ashida, M.; Suito, E. J . Appl. Phys. 1965, 36, 1453. (9) Stohr, I.NEXAFS Spectroscopy; Springer-Verlag: New York, 1992. (10) Mitra-Kirtley, S.; Mullins, 0. C.; van Elp, J.; George, S.;Chen, J. J.; Cramer, S. P. J . Am. Chem. SOC.1993, 115, 252. (11) Zeller, M. V.; Hayes, R. G. J . Am. Chem. Soc. 1973, 95, 3855. (12) Niwa, Y.; Kobayashi, H.; Tsuchiya, T. J . Chem. Phys. 1974, 60, 799. (13) Kanveik, D. H.; Winograd, N. Inorg. Chem. 1976, 15, 2336. (14) Ghosh, A.; Almlof, J.; Gassman, P. G. Chem. Phys. Lett. 1991, 186, 113. (15) Scheidt, W. R.; Kastner, M. E.; Hatano, K. Inorg. Chem. 1978, 17, 706. (16) Fleischer, E. B.; Miller, C. K.; Webb, L. E. J . Am. Chem. Soc. 1964, 86, 2342. (17) Hoard, J. L.; Hamor, M. J.; Hamor, T. A. J . Am. Chem. SOC. 1963, 85, 2334. (18) Brown, F. C.; Bachrach, R. Z . ; Lien, N. Nucl. Instrum. Methods 1978, 152. 13. (19) King, G. C.; Tronc, M.; Read, F. H.; Bradford, R. C. J . Phys. B 1977, IO, 2479. (20) Sodhi, R. N. S.; Brion, C. E. J . Electron Spectrosc. Relat. Phenom. 1984.34, 363. (21) Hitchcock, A. P.; Brion, C. E. J . Electron Spectrosc. Relat. Phenom. 1980, 18, 1. (22) Hitchcock, A. P.; Brion, C. E. Chem. Phys. 1978, 33, 55. (23) Del Bene, J.; Jaffe, H. H. J . Chem. Phys. 1968, 48, 1807, 4050. (24) Davis, D. W.; Shirley, D. A. Chem. Phys. Lett. 1972, 15, 185. (25) Felton, R. H.; Linschitz, H. J . Am. Chem. SOC. 1966, 88, 1113. (26) Peychal-Heiling, G.; Wilson, G. S. Anal. Chem. 1971, 43, 545. (27) Clack, D. W.; Hush, N. S. J . Am. Chem. Soc. 1965, 87, 4238. (28) Stohr, J.; Outka, D. A. Phys. Rev. B 1987, 36, 7891.
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