Theoretical investigation of UV photoelectron spectra of pyridine compound molecules

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopyand Related Phenomena 87 (1997) 141-157

Theoretical investigation of UV photoelectron spectra of pyridine compound molecules Li Yang la'*, Hans Agren a, Vincenzo Carravetta h aInstitute of Physics and Measurement Technology, LinkOping University, $-58183 Linkb'ping, Sweden bIstituto de Chimica Quantistica ed Energetica Molecolare del C.N.R., Via Risorgimento 35, 56100 Pisa, Italy

Received 18 April 1997; accepted 9 June 1997

Abstract A set of pyridine compounds are used to illustrate the possibility of assigning complex UV photoelectron spectra (UPS) by means of independent channel calculations that account for both energies and cross-sections. 2-ethynylpyridine (2EP), 4ethynylpyridine (4EP), 1,4-bis(2-pyridyl)-l,3-butadine (B2PBD), 1,4-bis(4-pyridyl)-l,3-butadine (B4PBD), and their subunits, pyridine and mono-, di- and tri-acetylene, have thus been investigated using the independent channel direct static exchange method. The calculations show that the cross-section of the 4al(n) channel of pyridine at 21.2 eV photon energy is twice those of the lr channels (la2 and 2b2), which supports the reversed order of the 2b2 and 4al(n) Hartree-Fock molecular orbitals (MOs) suggested recently by Moghaddam et al., (Chem. Phys., 207 (1996) 19). Correlations of occupied orbitals between the subunits and their compound molecules, 2EP, 4EP, B2PBD and B4PBD have been predicted by means of population analysis. A simple relation for correcting the calculated binding energies (BEs) for the o MOs has been found for pyridine and for the compound molecules in the region (12 eV < 20 eV) where the pyridine character is dominant. Previous assignments for the ~r bands (BE < 12 eV) have been slightly renewed and assignments for the o bands (BE > 12 eV, up to 20 eV) have been made for the first time. Significant cross-section variations with photoelectron energy are predicted, leading to He I and He II spectra with quite different intensity distributions. © 1997 Elsevier Science B.V.

1. Introduction Although several computational methods have been developed and applied to describe the photoionization processes in small molecules, comparatively little has been accomplished for larger species. UV photoelectron spectroscopy (UPS) is increasingly used for diagnosis of many types of extended species, such as surface adsorbates and polymers [1-5] and it is clear that methods that bridge the gap between

* Corresponding author. i Permanent address: Department of Physics, ZhengzhouInstitute of Surveying and Mapping, 450052 Zhengzhou, People's Republic of China.

theory and experiment in this research field are called for. This goes for experiments using fixed photon frequencies as well as for those with variable photon frequencies generated by synchrotron radiation sources, making recordings of a broad continuous region of photon energies possible. Most analysis in the context of UPS spectra of larger species has been based on level counting using one-particle or an approximate one-particle theory for generating density-of-states plots. Although the one-particle theory as such often can be well motivated for outer valence levels, little is known about the actual dependences of photoelectron cross-sections with respect to level or with respect to photon energy. Especially in the UV range the photon energies may sweep through

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L. Yang et aL/Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

resonances, the shape and position of which might change from channel to channel. Some of the problems of obtaining cross-sections refer to the computation of continuum photoelectron functions in the molecular potential which is both non-local and anisotropic. Using quantum chemistry methods this in turn calls for very large basis sets. The double-basis set technique advocated in the framework of the static-exchange approximation (STEX) in Ref. [6] circumvents much of this problem by computing the potential in a limited basis set while expanding the photoelectron function (and the photoelectron transition matrix) in a very large expanded basis set. The diagonalization of the photoelectron matrix gives eigenvalues and transition moments as input for Stieltjes imaging of the continuous photoabsorption cross-sections. Being an independent particle, independent channel method which is scalable and size-consistent, the STEX/Stieltjes technique is thus suitable to explore UPS spectra of more extended molecules, giving partial channel cross-sections in a well defined way. A recent work on polyenes, up to C 16H 18, indicated that indeed the STEX/Stieltjes approach is pragmatic and gives results that are comparable with available experimental UPS data [7]. In the present work we extend this study to include aromatic systems containing rings, chains and aza-substituents. We explore trends with respect to molecular size, electronic levels and band formation, and investigate the photon energy dependence of the absolute and relative photoionization cross-sections. Except for these particular items we address two general questions: (i) how well do the density-of-state plots represent these spectra; (ii) how can cross-sections help in correlating the level diagrams beween the compound and the constituents (building blocks). In addition, the cross-section calculations provide a means to make new spectral assignments in terms of molecular orbital theory. In Section 2 we first briefly recapitulate some computational facts; we then address each of the molecules in Section 3. Here acetylene and pyridine are considered as subunits for the (2- and 4-) ethynylpyridines and for the bis-pyridyl-butadines. The photoelectron spectra of the chosen molecules are particularly well determined and they represent also models for various polymers and for doping induced effects studied by UPS [8]. In the last section, Section 4, the results are summarized.

2. Computational details The theoretical method applied here is the so-called direct static exchange (STEX) approach, which was originally developed for the core-excitation and ionization processes [6,9] and most recently successfully extended to the UV photoelectron spectroscopy (UPS) calculation of polyenes [7]. In the static-exchange approach a one-hole potential, with appropriate spincoupling of ion electrons and photoelectron, is constructed and diagonalized to yield a primitive photoexcitation spectrum of transition energies and oscillator strengths. The potential can be given by ground state frozen orbitals or by state-specific orbitals which are optimized for each particular hole state. For both computational and formal reasons the former approach is to be preferred in the case of ionization of valence shells. The independent channel approach is assumed in the present work, thus neglecting effects of channel interaction and neglecting photoionization satellites referring to correlation states. In the STEX method the bound orbitals and the potential are constructed in a limited basis set (here DZP basis sets for C and N atoms and DZ for H atoms), while the full spectrum is obtained in a basis set containing many diffuse functions which were well tested in the polyenes studied in Ref. [7]. In order to cover a wider photon energy range, more p and d functions (with exponents 0.1117, 0.0147 and 0.0106) have been added. In addition to the double basis sets technique, the STEX approach is easily interfaced to the Stieltjes imaging technique for the continuous, photoionization, parts of the spectra. The direct SCF program DISCO [10] modified for STEX calculations [6] has been employed throughout.

3. Results and discussion All the pyridine compound molecules (2-ethynylpyridine (2EP), 4-ethynylpyridine (4EP), 1,4-bis(2pyridyl)-l,3-butadine (B2PBD), 1,4-bis(4-pyridyl)1,3-butadine (B4PBD)) can be viewed as built up by pyridine rings and acetylene and/or diacetylene subunits. Thus before the discussion on the pyridine compound molecules, we will discuss the results for pyridine (PY) and for the acetylenes.

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157 hv=21.2 eV . . . .

,

. . . .

i

. . . .

i

. . . .

143

hv=40.8 eV i

. . . .

. . . .

i

. . . .

i

. . . .

,

. . . .

i

. . . .

C2H2

C2H2 1

1

2

3

0

5 l0 15 20 25 30 2 .... , .... , .... , .... , .... C 4 H

2

5

10 . . . .

i

15

. . . .

i

. . . .

04H2

3

1 2

20

25

i

E

. . . .

3

1

2

4 5

0

5

10

15

20

25

30

0

10

5 . . . .

C6H2

4

i

3O

. . . .

. . . .

C8H2

15 i

. . . .

6

20

25

i

i

. . . .

30 . . . .

4

1 2a 1

5

10 15 20 25 Bindingenemy(eV)

30

0

5

10 15 20 25 Bindingenergy(eV)

30

Fig. 1. Computed UPS spectra of acetylene, diacetyleneand triacetylene:(a) He I excitation(left), (b) He II excitation(right).

3.1. A c e t y l e n e s

The calculated UPS spectra of acetylenes are presented in Fig. 1 at both the He I and the He II photon energies. The He I spectrum of acetylene shows three peaks. The lowest one located at 11.27 eV corresponds to the degenerate 7r orbital. The second peak shows up at about 17 eV and the third at 18.7 eV. Both of these are of a symmetry and describe the triple C - C bond and the C - H single bonds. In the He II spectrum a fourth peak, which is mainly built up by the 2s atomic orbitals of carbon atoms, appears at a binding energy (BE) of about 24.3 eV. Passing from the He I photon energy to the He II photon energy, the intensities of the a

orbitals increase while those of the 7r orbitals decrease. Diacetylene shows five peaks in the He I spectrum and seven peaks in the He II spectrum. Comparing with acetylene, diacetylene possesses a different type of b o n d - - t h e single C - C b o n d - - w h i c h corresponds to peak 4 in the spectrum. The intensity of this peak decreases dramatically from He I to He II spectra. For the rest of the ~r and (r channels, a trend similar to acetylene of intensity variation with photon energy is observed, Table 1. Triacetylene fits well into the trend set by acetylene and diacetylene. Owing to a small separation in binding energy between the molecular orbitals corresponding to the C - H bond (with lower BE)

144

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

Table 1 Binding energies and atomic populations of occupied valence orbitals in diacetylene Molecular orbital

lb3g lb2g lb3u lb2u 5ag 4b lu

Atomic population

Binding energy KT

Corr.

Exp. [13]

Cl

Cz

H

10.29 10.29 13.41 13.41 19.16 20.03

--12.70 12.70 17.12 17.79

10.30 10.30 12.71 12.71 17.0 17.5

0.380 0.380 0.632 0.632 0.290 0.062

0.620 0.620 0.368 0.368 0.424 0.620

0.000 0.000 0.000 0.000 0.286 0.320

and the C - C single bond (with higher BE) a strong feature builds up at around 15 eV. The UPS spectrum then shows six and nine structures at the He I and He II photon energies, respectively. The BEs here presented derive from Koopmans theorem, which is a reasonably good approximation for the ionization potential of the outermost valence orbitals, but which can significantly overestimate the BEs of the inner-valence orbitals. As is well known, this exaggeration of the energy region covered by the valence orbital energies can be corrected for by a rescaling procedure [11,12]. In the present case a rescaling factor of 1.3 was adopted. The rescaled BEs match ~quite well the experimental values [13], with an average discrepancy less than 0.2 eV for the three molecules, and bring the calculated spectra in Fig. 1 in excellent agreement with the He I and He II experimental spectra [14,13].

3.2. Pyridine Pyridine (PY) is an important representative of heterocyclic aromatic molecules as it is closely related with benzene. The analysis of its UPS spectrum is suitable for understanding the influence of a heteroatom on the benzene molecular properties. The assignment of the UPS bands, in particular the first three bands, of PY has been debated for a long time on the basis of both experimental and theoretical data [14-16]. The first UPS spectrum of PY was obtained by Kimura et al., and assigned on the basis of SCF calculations [14]. The first three bands in increasing energy order were assigned to the laz(Tr), 2b2(~r) and 1 lal(n) (non-bonding lone pair of N) MOs. An alternative assignment of these orbitals in the order llal(n), laz(Ir) and 2b2(Tr) was proposed in Ref.

[16] (see also references therein). A Green's function calculation with DZP basis set in the 2ph-TDA (twoparticle hole Tamm-Dancoff approximation) assigned the three peaks in a yet different order: la2(Tr), llal(n) and 2b2(Tr) [17,18]. Recently, the issue was reinvestigated both experimentally, by synchrotron photoelectron spectroscopy over a wider photon energy range (50-100 eV), and theoretically, by a more accurate Green's function calculation in the 2ph-TDA extended to the third order and with projection on a larger basis set (TZP) [15], confirming the order la2(Tr), llal(n) and 2b2(Tr). Considering the quality of the calculation and the agreement with the experimental observation that the second peak is more intense than the adjacent features, we adopted this ordering as the base for the interpretation of the spectra of the pyridine compound molecules. The calculated and experimental BEs of the valence orbitals of PY and the corresponding atomic populations have been collected in Table 2. From the table it appears that the valence orbitals are energetically well separated in two groups. Two 7r orbitals (la2(lr), 2b2(Tr)) together with the lone pair tr MOs form the second one. In the first group the Hartree-Fock (HF) BEs agree very well with the experimental data for the two 7r orbitals while the BE of the 1 la l(n) lone pair is about 1.6 eV too low with respect to the experimental value. The set of HF BEs in the second group appears also to be in disagreement with experiment, especially for the inner valence orbitals where relaxation becomes more important. However, the HF values can be easily corrected by rescaling the full group by dividing by a factor of 1.2 and shifting them, as a whole, downwards according to the experimental BE of the first MO of the group. This rescaling and shifting procedure can establish, as shown in Table 2,

145

L Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

Table 2 Binding energies and atomic populations of occupied valence orbitals in pyridine Peak

Molecular orbital

1 2 3 4 5 6 7 8 9 10

la2 1 la l(n) 2b2 7bl lb2 10at 6bl 9al 5bt 8al

Binding energy

Atomic population

KT

Corr.

Exp. [16]

CI

C2

C3

C4

C5

N

9.52 11.36 10.53 14.30 14.86 15.81 16.42 17.92 18.13 19.82

---12.60 13.07 13.86 14.37 15.62 15.80 17.22

9.60 9.75 10.51 12.61 13.1 13.8 14.5 15.6 15.8 17.40 a

0.005 0.013 0.354 0.154 0.102 0.313 0.014 0.034 0.t70 0.176

0.263 0.062 0.142 0.135 0.122 0.064 0.168 0.185 0.151 0.093

0.263 0.062 0.142 0.135 0.122 0.064 0.168 0.185 0.151 0.093

0.233 0.075 0.036 0.099 0.184 0.074 0.164 0.090 0.177 0.143

0.233 0.075 0.036 0.099 0.184 0.074 0.164 0.090 0.177 0.143

0.004 0.616 0.291 0.079 0.287 0.167 0.008 0.024 0.162 0.074

aFrom [15]. a substantial correspondence between corrected BEs a n d e x p e r i m e n t a l v a l u e s a n d it h a s b e e n i n t r o d u c e d in o r d e r to s i m p l i f y the a n a l y s i s o f the s p e c t r a o f pyrid i n e a n d its c o m p o u n d m o l e c u l e s , as will b e s h o w n in the f o l l o w i n g . T h e c a l c u l a t e d H e I a n d H e II p h o t o e l e c t r o n s p e c t r a o f PY, t o g e t h e r w i t h t h e e x p e r i m e n t a l H e I U P S , are c o l l e c t e d in Fig. 2. A s a l r e a d y o b s e r v e d for p o l y e n e s [7], i o n i z a t i o n f r o m a a o r b i t a l h a s a m u c h l a r g e r c r o s s - s e c t i o n t h a n f r o m a 7r orbital. T h i s d i s p a r i t y is s i g n i f i c a n t l y r e d u c e d w h e n the p h o t o n e n e r g y i n c r e a s e s f r o m 21.2 to 40.8 eV. T h e e x p l i c i t e n e r g y

d e p e n d e n c e o f t h e c r o s s - s e c t i o n for the t h r e e c h a n nels, la2(Tr), l l a ] ( n ) a n d lb2(Tr) o v e r a l a r g e e n e r g y r a n g e is s h o w n in Fig. 3, w h e r e t h e v e r t i c a l d o t - d a s h e d lines i n d i c a t e the p o s i t i o n s o f t h e H e I a n d H e II p h o t o n e n e r g i e s . Clearly, all t h r e e c r o s s - s e c t i o n s s h o w a r e s o n a n t b e h a v i o r j u s t a b o v e the i o n i z a t i o n t h r e s h o l d . T h e c r o s s - s e c t i o n s o f t h e t w o ~" c h a n n e l s are q u i t e s i m i l a r in t h e c o n s i d e r e d e n e r g y r a n g e e x c e p t for p h o t o n e n e r g i e s c l o s e to t h e t h r e s h o l d s . T h e r e s o n a n c e in t h e 1 l a l ( n ) c h a n n e l is c o m p a r a t i v e l y m u c h s t r o n g e r a n d g i v e s rise to a m o r e i n t e n s i v e structure in the U P S spectra. A l t h o u g h t h e S C F c a l c u l a t i o n

Table 3 Binding energies and atomic populations of occupied valence orbitals in 4EP Peak

1

10 11

Molecular orbital la2 3b2 15a i(n) 8bl 2b2 7bj lb2 6bl 14al 13al 5bl 12al llaj 4bj 10a~

Binding energy

Atomic population

KT

Corr.

Exp. [16]

C~

C2

C3

C4

C5

N

C6

C7

9.73 9.75 11.60 11.29 12.55 14.72 15.15 16.72 16.84 18.30 18.49 19.56 21.00 23.76 24.23

-----12.82 13.23 14.54 14.63 15.86 16.01 16.90 18.11 20.40 20.80

9.58 9.58 9.99 10.75 11.84 12.8 a 13.3 a 14.3 a 14.7 a 15.8 a

0.005 0.181 0.003 0.010 0.126 0.133 0.153 0.012 0.186 0.025 0.148 0.017 0.116 0.077 0.154

0.262 0.112 0.059 0.016 0.011 0.128 0.134 0.159 0.074 0.205 0.158 0.042 0.069 0.165 0.118

0.262 0.112 0.059 0.016 0.011 0.128 0.134 0.159 0.074 0.205 0.158 0.042 0.69 0.165 0.118

0.234 0.014 0.072 0.003 0.055 0.097 0.153 0.172 0.110 0.069 0.171 0.059 0.119 0.160 0.124

0.234 0.014 0.072 0.003 0.055 0.097 0.153 0.172 0.110 0.069 0.171 0.059 0.119 0.160 0.124

0.004 0.174 0.652 0.000 0.185 0.092 0.216 0.012 0.145 0.040 0.162 0.034 0.015 0.146 0.069

0.000 0.135 0.007 0.453 0.332 0.019 0.045 0.001 0.164 0.021 0.024 0.089 0.111 0.020 0.138

0.000 0.258 0.001 0.494 0.225 0.008 0.013 0.000 0.035 0.011 0.003 0.388 0.174 -0.002 0.096

aEstimated from experimental spectrum in [16].

146

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

UV photoelectron spectra of pyridine a) experimental He I

89 7 12

4.0

~

10

3

i

b) calculated He I

3.0 o 2.0 1.0 0.0

2.0

1.0

0.0

8

|0

12 14 16 Binding energy (eV)

18

20

Fig. 2. Computedand experimentalUPS spectraof pyridine: (a) He I experimentalspectrum,taken from [14], (b) He I calculatedspectrum,(c) He II calculated spectrum. gives a poor BE for the lla](n) lone pair, the calculated intensity of this orbital, as well as those of the other MOs, appears to be in good agrement with the experiment [14]. This agreement and the scaling and shifting procedure previously described for the BEs in the second group, allowed us to rationalize the UPS spectra of pyridine compound molecules. 3.3. 4-ethynylpyridine (4EP)

4-ethynylpyridine (4EP) consists of a pyridine ring and an acetylene unit. The correlation between MOs

of 4EP and its subunits can be readily obtained by considering the symmetry character and the atomic population (see Table 3), and is explicity given in Fig. 4. The MOs of 4EP, just like in pyridine, can be divided in two energetically separated groups. According to the population analysis, the first three MOs are essentially of pyridine origin, although la2 receives some contribution from acetylene through the C - C single bond between the two subunits. Owing to the bonding, the double degeneracy of the 7r MO of acetylene is removed; the in-plane component (Trh) maintains an almost pure acetylene

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

147

Photoionization cross sections of pyridine 30

,

,

,

/ /' / 1

20

--

la2(~ )

.....

2b2(r¢ )

---

11a1(n )

O

"" J

''.

i '\

2 .

10

\'\

'"i., : ",,..

•/

I

5

iI

15

"\

I

"\

I

25 35 Photon energy (eV)

I

45

55

Fig. 3. Energy dependenceof cross-sectionsfor the three lowest MOs of pyridine.

character, while the out-of-plane component (~'v) is mixed with the 7r orbital (2b2) of the pyridine ring and forms the 7r bond of the system. The binding energy of the lone pair, 15al(n), appears to be about 1.7 eV overestimated by the HF calculation compared with experiment. These five MOs form the first group. As seen from the population analysis, all the MOs in the second group, except 12al, have a clear pyridine character. Thus one can expect that the calculated BEs above 14 eV can be well corrected for by the same procedure we adopted for the BEs of pyridine. The corrected BEs for the second group of MOs have been listed in Table 3. As already observed for pyridine, the HF calculation gives reasonably good BEs for the first two 7r MOs (la2 and 3b2) which, being nearly degenerate, build up a single (high but narrow) peak clearly seen at about 9.6 eV in the experimental spectrum, while ionization from 15al(n) results in a strong (lower but much wider) feature centered around 10 eV. This interpretation of the experimental spec-

trum is slightly different from that based on an MNDO calculation in Ref. [16] and which gave a reversed order for the first two 7r MOs (la2 and 3b2) and put the la2 BE at 10 eV. The two well separated features located at 10.75 eV and 11.84 eV in the experimental UPS spectrum (see Fig. 5) have no correspondence in the pyridine spectrum. In addition, the population analysis shows that the two MOs of 4EP with a dominating ethynyl character are well separated from the other MOs. Thus one can unambiguously assign the two peaks to 8bl (~rh) and 2b2, respectively, although the BEs are overestimated (0.4 and 0.7 eV, respectively) by the HF calculation. It should be noted that for an easier comparison with the experimental spectrum in Fig. 5, the experimental BE values have been used in the calculated spectra for energies less than 12 eV (see Fig. 5). The present assignment supports then the previous interpretation [16] of the weak feature located at 11 eV as a vibronic structure of the 10.75 eV band. A tentative assignment of the

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

148

9.0

4 2

4 2

3

N

5 3

5 3

4 2 :~"

w a 2 K

- -

-

-

_ -

-

T¢

x

-

/b b 2 R

11.0

a l or

\

/ /

a"

-~x x xx

,,

n

\ x \\

bl g h a,n

- - - - ~

-

-

-

- -

.....

a'

~h

\~ "~'

- - _

a'n

.

Ku

rt h

a"/¢

xx

13.0 >•o bI o b2

m

"-.

- ~ _

- - - ~ ' - ~

15.0

b~ o' 52R

a'

~r

a"

a I or -

-

-

. . . .

_

a'

or

bl 17.0

a,~ or

\

a I or

-

\ '\

-b,- 2o 2 2 2 2 _ __ a,_- G_

- _ .

b, or

a'

'~

or

, , a 0 t~

a' or

19.0 Fig. 4, Correlation diagram of calculated BEs of 2EP and 4EP with the related compounds pyridine and acetylene. Table 4 Binding energies and atomic populations of occupied valence orbitals in 2EP Peak

1 2 3 4 5 6 7 8 9 10 11 12

Molecular orbital

4a" 22a'(n) 3a" 23a' 2a" 21a' la" 20a' 19a' 18a' 17a' 16a' 15a'

Binding energy

Atomic population

KT

Corr.

Exp. [16]

Ct

C2

C3

C4

C5

N

C6

C7

9.16 11.59 10.50 11.13 11.95 14.86 15.22 16.05 17.07 18.38 18.53 19.44 20.90

-----12.86 13.16 13.85 14.70 15.79 15.92 16.68 17.90

9.26 9.85 10.59 10.80 11.35 12.8 ~ 13.3 a 13.9 a 14.8 a

0.167 0.083 0.003 -0.004 0.062 0.032 0.232 0.044 0.125 0.108 0.105 0.006 0.140

0.180 0.056 0.210 0.017 0.002 0.154 0.124 0.077 0.194 0.145 0.160 0.058 0.061

0.136 0.061 0.100 0.004 0.081 0.083 0.140 0.101 0.203 0.171 0.048 0.063 0.118

0.005 0.010 0.299 0.004 0.080 0.173 0.085 0.309 0.023 0.131 0.058 0.072 0.142

0.229 0.059 0.045 0.000 0.156 0.197 0.087 0.075 0.088 0.191 0.154 0.081 0.021

0.016 0.647 0.301 0.011 0.002 0.073 0.256 0.130 0.018 0.112 0.077 0.034 0.071

0.086 0.013 0.021 0.466 0.343 0.032 0.060 0.013 0.069 0.020 0.090 0.049 0.119

0.182 0.007 0.021 0.497 0.274 0.005 0.016 0.003 0.018 0.008 0.093 0.346 0.146

aEstimated from experimental spectrum in [16].

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

149

U V photoelectron spectra o f 4EP

a) experimental He I

II

1

7 8 3

4

5

v

b) calculated He I

~ 3.0

5

1

,

7

8

12

'~ 2.0

.,~ ¢~ 1.0 0.0 c) calculated He II 2.0

t

1.0

0.0

8

10

12 14 16 Binding energy (eV)

18

20

Fig. 5. Computed and experimental UPS spectra of 4EP: (a) He I experimental spectrum, taken from [16], (b) He I calculated spectrum, (c) He II calculated spectrum. In the calculated spectra, the experimental BEs were used for MOs in the lower group, and the calculated BEs in the second group were rescaled with a factor of 1.2 (relative to the front MO of the group) and shifted downward 1.9 eV as a whole. experimental peaks for BE > 12 eV, which were not assigned previously, is here proposed on the basis of our calculated intensities and corrected BEs. The good agreement o f the so calculated spectra with the experimental He I spectrum [16] over all the energy range (see Fig. 5) supports the present assignment. Comparing with the calculated He II spectrum in Fig. 5, it can be observed that the intensity ratio between the tr and ~r channels is significantly decreased when the photon energy increases from 21.2 eV to 40.8 eV.

3.4. 2-ethynylpyridine (2EP)

2-ethynylpyridine is an isomer of 4EP in which the acetylene unit locates closer to the N atom. The calculated binding energies and the population analysis presented in Table 4 show that, as for pyridine and 4EP, the M O s can be easily divided into two groups. Despite some differences, the sequence of valence shell BEs of 2EP bears a general similarity to that of 4EP. As for pyridine and 4EP, the H F calculation gives good BEs for the outmost valence orbitals, 4a"

150

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

U V photoelectron spectra o f 2EP 3.0 a) experimental He I 2.0

3

5

6

1.0

h 0.0 10

b) calculated He I 6

3.0

9 o

2.0

2

11

"d a~ 1.0 0.0 c) calculated He II

1.0

0.0

10

12 14 16 Binding energy ( e V )

18

20

Fig. 6. Computed and experimental UPS spectra of 2EP: (a) He I experimental spectrum, taken from 116], (b) He I calculated spectrum, (c) He II calculated spectrum. In the calculated spectra, the experimental BEs were used for MOs in the lower group, and the calculated BEs in the second group were rescaled with a factor of 1.2 (relative to the front MO of the group) and shifted downward 2.0 eV as a whole. and 3a", while assuming that as in pyridine and 4EP the BE of the lone pair orbital is overestimated by about 1.7 eV, the 22a'(n) orbital can be considered to correspond to the broad feature centered at 9.85 eV in the experimental spectrum (see Fig. 6). The in-plane Ir component (lrh) of acetylene, 23a', and a 7r orbital originating from the 2b2 M O of pyridine build up a strong feature around 10.6 eV with a shoulder at 10,8 eV in the experimental spectrum. This assignment is slightly different from a previous one, in which the 10.6 eV peak was assigned

as of a2 origin [16]. The 2a" MO, which is dominated by the Try orbital o f the ethynyl group, corresponds to the experimental structure at 11.35 eV. The calculated BEs for the in-plane and out-of-plane 7r orbitals are overestimated by about 0.3 eV and 0.6 eV, respectively. As shown in Table 4, the MOs in the second group bear a general similarity to those of pyridine except for the BE of the first M O which is about 0.5 eV lower than that of pyridine. Indeed, the population analysis shows that most o f the MOs in the second group are of

L. Yang et aL/Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

151

Table 5 Binding energies and atomic populations of occupied valence orbitals in B4PBD Peak

1 2 3

4 5 6 7 8 9 10 11

12 13

Molecular orbital 3b3g lblg lau 15ag(n) 14b lu(n) 8b2g 3b2u 2b3g 8B3u 2b2u 7b2g 7b3u lb3g lb2u 6b2g 6bau 14ag 13blu 13ag 12bl, 5b2g 5b3u 12ag llb~u

Binding energy

Atomic population

KE

Corr.

Exp. [16]

Ci

C2

C3

C4

C5

N

C6

C7

8.92 9.78 9.78 11.80 11.80 10.23 10.68 12.36 13.49 13.93 14.75 14.85 15.13 15.30 16.65 16.65 17.02 17.11 18.34 18.36 18.47 18.49 20.24 20.61

----------12.65 12.68 12.92 13.06 14.18 14.18 14.49 14.54 15.59 15.60 15.70 15.71 17.18 17.48

8.77 9.32 9.32 9.69 9.69 10.0 10.53 11.39 ' 12.13 12.31 12.6 a

0.116 0.004 0.004 0.004 0.002 0.010 0.296 0.182 0.036 0.006 0.128 0.102 0.166 0.172 0.012 0.014 0.168 0.178 0.024 0.012 0.136 0.146 0.106 0.124

0.108 0.264 0.264 0.060 0.060 0.018 0.114 0.020 0.032 0.012 0.124 0.106 0.136 0.118 0.162 0.160 0.078 0.068 0.214 0.226 0.162 0.158 0.068 0.084

0.108 0.264 0.264 0.060 0.060 0.018 0.114 0.020 0.032 0.012 0.124 0.106 0.136 0.118 0.162 0.160 0.078 0.068 0.214 0.226 0.162 0.158 0.068 0.084

0.006 0.232 0.232 0.072 0.074 0.004 0.034 0.066 0.012 0.078 0.104 0.096 0.148 0.110 0.168 0.170 0.128 0.140 0.062 0.058 0.170 0.168 0.134 0.160

0.006 0.232 0.232 0.072 0.074 0.004 0.034 0.066 0.012 0.078 0.104 0.096 0.148 0.110 0.168 0.170 0.128 0.140 0.062 0.058 0.170 0.168 0.134 0.160

0.132 0.004 0.004 0.650 0.650 0.000 0.258 0.238 0.004 0.160 0.096 0.092 0.210 0.146 0.012 0.012 0.148 0.150 0.054 0.060 0.160 0.158 0.032 0.024

0.284 0.000 0.000 0.008 0.004 0.558 0.012 0.288 0.282 0.254 0.020 0.062 0.040 0.120 0.002 0.002 0.128 0.124 0.022 0.014 0.042 0.036 0.160 0.192

0.238 0.000 0.000 0.000 0.002 0.384 0.136 0.118 0.558 0.400 0.004 0.068 0.014 0.108 0.000 0.000 0.034 0.006 0.006 0.008 -0.006 0.006 0.162 0.022

13.0 a 13.9 a 14.4 a

aEstimated from experimental spectrum in [16]. pyridine character. Thus, the procedure adopted to correct the pyridine and 4 E P binding e n e r g y was also e m p l o y e d for 2EP, with a rescaling factor o f 1.3 and an e n e r g y shift o f 2.0 eV. T h e corrected B E s h a v e b e e n listed in T a b l e 4. A l t h o u g h the relative intensity o f the 2 E P s p e c t r u m is s o m e w h a t different f r o m that o f 4EP, the calculated s p e c t r u m at the He I photon e n e r g y seems to reproduce the e x p e r i m e n t a l one quite w e l l up to 16 e V binding energy. The strong broad feature in the 4 E P s p e c t r u m b e t w e e n 14 and 15 eV, w h i c h results f r o m two closely located M O s , 13al and 5bl, corresponds in 2 E P to two well. separated features c e n t e r e d at 13.9 and 14.7 eV, respectively. A n intensive p e a k around 16.0 e V and several w e a k e r structures at higher B E s h a v e been predicted. Unfortunately, no e x p e r i m e n t a l m e a s u r e m e n t is to our k n o w l e d g e available in this e n e r g y range. It is worth noting that at the H e I photon energy, the intensity o f p h o t o i o n i z a t i o n o f a tr partial channel is about t w i c e that o f a 7r channel. T h e two w e a k but

distinct features around 12.3 eV, w h i c h fall into the a part, cannot be related to any isolated p h o t o i o n i z a t i o n channel. 3.5. 1,4-bis(4-pyridyl)-l,3-butadine,

B4PBD

T h e 1,4-bis(4-pyridyl)-l,3-butadine ( B 4 P B D ) can be r e g a r d e d electronically either as a d i m e r o f 4 E P or as a c o m b i n a t i o n o f two pyridine rings and a diacetylene subunit. T h e population analysis g i v e s p r e f e r e n c e to the latter descripton for the outer v a l e n c e M O s (calculated B E < 14 eV, see T a b l e 5) and to the f o r m e r for the inner v a l e n c e M O s . Thus, in the f o l l o w i n g discussion, both points o f v i e w will be adopted according to the c o n s i d e r e d e n e r g y range. The first peak o f the e x p e r i m e n t a l U P S spectrum, b e l o w 9 eV, can be assigned to the 3b3g M O on the basis o f the a g r e m e n t with the calculated BE. C o n sidering that the B E o f the la2 l e v e l in 4 E P was about 0.2 e V overestimated, the c o r r e s p o n d i n g M O s

152

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

UV photoelectron spectra of B4PBD a) experimental He I 4

2

7

5

3

1[

A

3

,°

i

10

8

,

,

i

,

7

b) calculated He I

10

11

.~ 3.0 3

o

o

2.0

/

"-d

,J

1.0

LI

0.0

,

,

c) calculated He II 2.0

1.0

0.0

9

11 13 Binding energy (eV)

15

17

Fig. 7. Computed and experimental UPS spectra of B4PBD: (a) He I experimental spectrum, taken from [16], (b) He I calculated spectrum, (c) He II calculated spectrum. In the calculated spectra, the experimental BEs were used for MOs in the lower group, and the calculated BEs in the second group were rescaled with a factor of 1.2 (relative to the front MO of the group) and shifted downward 2.1 eV as a whole. in B4PBD, l b lg and lau (they are almost degenerate), can be assigned to the second feature (9.32 eV) o f the experimental spectrum. Then the strongest feature centered at about 9.70 eV is readily asigned to the two almost degenerate lone pair MOs, 15ag(n) and 14blu(n), by considering both the intensity and the energy shift of the H F BE already observed for the previous pyridine compound molecules. The MOs 8bEg and 3b2u have, respectively, a diacetylene and a pyridine dominant character. The former corresponds to the in-plane 7r orbital (lbag) of diacetylene, and the

latter to the 2b2 orbital of pyridine. As was shown in the previous discussion, the binding energies of the subunit spectra are well reproduced by H F calculations. It is then reasonable to assign the two MOs to the experimental features located at 10.0 and 10.5 eV, respectively. The 2b3g M O derives from the 2b2 orbital o f 4EP, the BE of which was overestimated by about 0.7 eV. Thus, it is reasonable to assign it to the experimental feature at 11.39 eV. Differently from pyridine and 4EP, it is not easy to distinguish two groups of orbital energies in the set of H F BEs for B4PBD

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

153

U V photoelectron spectra o f B 2 P B D

ik;" b) calculated He I

7

9 10 ll11

"~ 4.0 .,= >.

~ 2.o

0.0 c) calculated He II

2.0

1.0

0.0

7

9

ll 13 Binding energy (eV)

15

17

Fig. 8. Computed and experimental UPS spectra of B2PBD: (a) He I experimental spectrum, taken from [16], (b) He I calculated spectrum, (c) He II calculated spectrum. In the calculated spectra, the experimental BEs were used for MOs in the lower group, and the calculated BEs in the second group were rescaled with a factor of 1.2 (relative to the front MO of the group) and shifted downward 2.2 eV as a whole. because of the two orbitals lying in between. The MOs have a clear diacetylene character and correspond to the experimental features at 12.13 and 12.31 eV, respectively. This interpretation is further supported by the B2PBD case (see Table 6 and Fig. 8), for which the larger calculated energy separation between these two BEs and the following one in energy, that can be considered as the first one of the second group, corresponds, in fact, to a larger experimental gap. The calculated BEs above 14 eV and the atomic populations of the corresponding M O s show a strong

similarity between B4PBD and 4EP, and for this energy range we can regard the former as the double of the latter. W e then applied to this second set of BEs of B4PBD the usual correction procedure with a downward energy shift o f about 2.1 eV and a rescaling by a factor of 1.2. The corrected BEs have been collected in Table 5, and are there compared with data estimated from the experimental spectrum (Fig. 6 of Ref. [16]). As shown in Fig. 7 the correction works well in reproducing the experimental spectrum between 12.6 and 16.0 eV, and four more structures

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

154

Table 6 Binding energies and atomic population of occupied valence orbitals in B2PBD Peak

1 2 3 4 5 6 7 8 9 10 11

12 13

Molecular orbital

4bg 4a. 23ag 22ag(n) 22bu(n) 3bg 3au 2bg 21bu 2au 2lag 20bu lbg lau 20ag 19b. 19ag 18bu 18ag 17b. 17ag 16bu 16ag 15b.

Binding energy

Atomic population

KE

Corr.

Exp. [16]

Ct

C2

C3

C4

C5

N

C6

C7

8.33 9.68 9.83 11.75 11.75 10.50 10.63 11.46 13.18 13.32 14.80 14.82 15.13 15.28 16.03 16.05 16.93 16.98 18.32 18.33 18.54 18.61 20.03 20.45

----------12.60 12.62 12.92 13.04 13.67 13.69 14.42 14.46 15.58 15.58 15.76 15.82 17.00 17.35

8.32 9.07

0.108 0.224 -0.008 0.082 0.076 0.002 0.022 0.122 -0.004 0.004 0.032 0.026 0.236 0.226 0.036 0.030 0.126 0.134 0.124 0.132 0.0086 0.074 0.114 0.136

0.150 0.224 0.018 0.056 0.064 0.252 0.154 0.002 0.012 0.014 0.156 0.182 0.122 0.108 0.074 0.072 0.198 0.200 0.148 0.146 0.193 0.182 0.058 0.078

0.066 0.208 0.006 0.066 0.064 0.158 0.050 0.084 0.004 0.076 0.088 0.082 0.148 0.118 0.116 0.120 0.200 0.208 0.174 0.174 0.034 0.036 0.132 0.160

0.004 0.004 0.004 0.012 0.012 0.244 0.344 0.134 0.002 0.052 0.166 0.176 0.086 0.068 0.302 0.248 0.036 0.030 0.150 0.152 0.054 0.056 0.148 0.176

0.186 0.240 0.000 0.066 0.062 0.014 0.116 0.240 0.000 0.078 0.196 0.192 0.088 0.070 0.074 0.078 0.036 0.070 0.186 0.194 0.216 0.128 0.042 0.024

0.036 0.004 0.012 0.636 0.630 0.282 0.296 0.006 0.018 0.042 0.076 0.074 0.260 0.226 0.136 0.132 0.022 0.024 0.124 0.126 0.080 0.084 0.052 0.072

0.246 0.006 0.584 0.010 0.016 0.034 0.006 0.292 0.322 0.258 0.032 0.028 0.046 0.106 0.010 0.014 0.076 0.056 0.026 0.024 0.070 0.054 0.176 0.196

0.204 0.088 0.380 -0.002 0.002 0.016 0.012 0.122 0.636 0.478 0.006 0.008 0.016 0.076 0.002 0.004 0.056 0.002 -0.002 0.004 0.034 0.010 0.146 0.020

9.60 9.60 10.34 10.7 11.77 12.0 12.6 a 13.0 a 13.8 a 14.6 a

aEstimated from experimental spectrum in [16].

Table 7 Binding energies and atomic populations of occupied a l(n) and rh MOs in pyridine compound molecules Molecular orbital

Binding energy

Atomic population

KT

Exp. [16]

CI

C2

C3

C4

C5

N

C6

C7

11.60 11.29

9.9 10.75

0.003 0.010

0.059 0.016

0.059 0.016

0.072 0.003

0.072 0.003

0.652 0.000

0.007 0.453

0.001 0.494

11.59 11.13

9.85 10.80

0.083 -0.004

0.056 0.017

0.061 0.004

0.010 0.004

0.059 0.000

0.647 0.011

0.013 0.466

0.007 0.497

11.80 11.80 10.23

9.69 9.69 10.0

0.004 0.002 0.010

0.060 0.060 0.018

0.060 0.060 0.018

0.072 0.074 0.004

0.072 0.074 0.004

0.650 0.650 0.000

0.008 0.004 0.058

0.000 0.002 0.384

9.83 11.75 11.75

9.07 9.60 9.60

-0.008 0.082 0.076

0.018 0.056 0.064

0.006 0.066 0.064

0.004 0.012 0.012

0.000 0.066 0.062

0.012 0.636 0.630

0.584 0.010 0.016

0.380 -0.002 0.002

4EP 15a l(n) 8b j

2EP 22a'(n) 23a'

B4BPD 15ag 14b t. 8b2g

B2BPD 23ag 22ag 22bu

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

155

Calculated UPS in DOS approximation (He I)

0 2.0 1.0

J

0.0 •~ 2.0

2EP

1.0

0.0

7

10

13 16 Binding energy (eV)

19

Fig. 9. Calculated UPS spectra with density of state (DOS) approximation. of the experimental spectrum have been tentatively assigned. As for the other molecules, the experimental values of the n-lone pair BE as well as of the low lying MOs (BE < 14 eV), have been used for an easier comparison with the experimental spectrum in Fig. 7. 3.6. 1,4-bis(2-pyridyl)-l,3-butadine,

B2PBD

The photoelectron spectrum of 1,4-bis(2-pyridyl)1,3-butadine (B2PBD) has some similarity with that of B4PBD, although some remarkable differences in the intensity distribution do exist. The first band

extends more into the low BE region, which makes the intensity of the n-band less concentrated. The relative intensity of the n-band decreases with respect to that of B4PBD. On the other hand, a new intensive band between 10 and 11 eV appears. As shown by the calculations (see Table 6), the BE of the 3bg MO, which corresponds to 8b2g in B4PBD, is displaced far from that of the lone pair band but lies close to the BE of another MO, 3au. These two orbitals, together with the 2bg MO, which corresponds to 2b3g in B4PBD, build up the strong feature observed between 10 and 11 eV. The diacetylene dominated

156

L. Yang et aL/Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

MOs, 21bu and 2au, which have BEs close to that of the first peak of the second group in B4PBD, now have lower BEs, as suggested both from the computation and from the experimental spectrum. They form a well separated feature around 11.8 eV. Similar to B4PBD, the calculation predicts an intensive feature around 15.7 eV that seems to be in agreement with the measured UPS spectrum. Unfortunately, the very strong first ionization band of N2 at 15.57 eV, used for calibration, and the cut-off of the experimental spectrum above 16 eV makes the comparison incomplete. Detailed band assignments have been presented in Table 6. From the calculated UPS spectra shown in Fig. 8, a strong feature at about 15.7 eV and a weaker feature at higher energy are predicted. 3. 7. n orbital interaction

It is interesting to consider the interaction between the n orbital of the pyridine group and the in-plane 7rh orbital of the ethynyl group. It was made experimentally observable by Okuba et al. [16] by detecting the slight variation of the energy separation between the n and the 7rh ionization bands passing from 2EP to 4EP where the distance of the two orbitals is larger. The measured splittings were 0.95 and 0.85 eV, for 2EP and 4EP, respectively, indicating a weak n to 7rh orbital interaction [ 16]. The calculated BEs and atomic populations for the n and the ~rh orbitals have been collected in Table 7. The population data indicate for 4EP only a minor mixing between the 7rh orbitals of the ethynyl group and the n orbital. This agrees with the fact that the calculatd BE of the 7rh orbital (11.29 eV) in 4EP is essentially equal to the BE (11.27 eV) of acetylene. For 2EP, however, the mixing is more significant, as is also reflected by the difference of the BEs of the 7rh orbitals in 2EP an 4EP. Our calculated value is -0.15 eV, i.e. of the same order as the experimental value but with opposite sign. A similar observation can be made in the case of B4PBD and B2PBD. The BEs and the atomic populations indicate to interaction in B4PBD, while the n and the 7rh orbitals show some mixing in B2PBD. As for 2EP and 4EP, the BE of the n orbital is rather stable compared with the ~'h orbital. The estimated difference of splitting for B2PBD and B4PBD is - 0.35 eV, again of the same order but opposite in

sign with respect to the experimental observation (0.22 eV). 3.8. Density-of-states spectra

In the density-of-states (DOS) approximation the cross-section of each partial channel is assumed to be the same and energy independent. The UPS spectra are then analyzed simply by counting the number of levels within a certain energy interval. This procedure has been much used for larger systems for which ab initio, let alone cross-section, calculations are not feasible. Fig. 9 presents DOS spectra for the pyridine compounds. We note some similarity with the measured spectra and with computed spectra in the outer valence region where the 7r channel crosssections actually are quite similar. However, the enhanced cross-sections for the n channels are evidently not considered, and the DOS analysis cannot be used for the particular assignment problems posed by the spectral interference of n-levels. Furthermore, the r parts of the spectra are artificially enhanced by the DOS approximation, and the salient differences between He I and He II spectra are evidently not accounted for. The experimental resolution of the a levels is in general poorer and the limits of the DOS approximation are therefore harder to clarify in that part of the spectrum. However, as predicted by the STEX technique, the cross-section variations are larger in this region than in the outer 7r level region.

4. Summary The pyridylacetylenes served in this paper as the first test ground for assignment problems in UPS spectra using the direct STEX technique [6]. It has been demonstrated that the combined use of energies and cross-sections at the independent particle, independent channel, level of approximation can help to assign UV photoelectron spectra of even quite complex systems such as the pyridylacetylenes. An additional tool is the energy dependence of the cross-sections which can be used to assign spectra generated by energy variable photon sources. The pyridylacetylenes studied in the present work show UPS spectra particularly rich in structure which serve as suitable test cases. The calculations show

L. Yang et al./Journal of Electron Spectroscopy and Related Phenomena 87 (1997) 141-157

the dependence of the UPS spectra with respect to molecular structure and size, ionization level and photon energy. Several salient trends can be unraveled, especially for the photon energy- and the molecular size dependences of the cross-sections. As for small molecules and linear polyenes [7] we find that the energy dependence is largest for low, He I, photon energies. A detailed analysis of resolved spectra shows that the independent channel interpretation of the UPS spectra works well both concerning energies (rescaled Koopman energies) and cross-sections (independent channel cross-sections). This seems to hold for the outer valence orbitals (BEs and cross-sections for 7r-, but cross-sections only for n orbitals) and the part of the tr valence that is covered by He I excitation. Towards the inner valence region there are strong hole-mixing and breakdown effects which make state-by-state cross-sections obtained from independent particle or independent channel calculations of less use. For larger species the inner-valence regions of the UPS spectra are often poorly resolved and the independent channel techniques might conceivably be used better for the gross shape of the spectra in this region. Towards the outer valence region, which is still well resolved for larger species like the pyridylacetylenes, the independent or quasi-particle pictures are thus known to hold better. A particular problem posed by the pyridylacetyenes is given by the 7r to n level ordering. The cross-section calculations verify here recent ~" versus n orbital assignment for pyridine based on Green's function computations and make way for assignments of the outer r and n levels as well as for a large energy interval covering o levels of the pyridylacetylenes. Because the al(n) MO (o character) is embedded in the 7r band (lower BEs), the density-of-states approximation (DOS), which seems to work well in reproducing UPS spectra of homogeneous species, like polyenes [7] and diphenylpolyenes [19], is found to break down for the investigated aza substituted compounds. The small interaction between the n orbital at

157

the ortho-position of the pyridine unit and the in-plane ethynyl Irh orbital indicated by experiment [16] is verified here.

References [1] D. Heskett, I. Strathy, E. W. Plummer, R. A. de Paola, Phys. Rev. B 32 (1985) 6222. [2] W. R. Salaneck, in: Skotheim (Ed.), Handbook of Conducting Polymers, Vol. II, Marcel Dekker, New York, 1986. [3] H. Tillborg, A. Nilsson, B. Hero,is, N. M~rtensson, R. E. Palmer, Surf. Sci. 295 (1993) 1. [4] M. Fahlman, M. L6gdlund, S. Stafstr6m, W. R. Salaneck, R. H. Friend, P. L. Bum, A. B. Holmes, K. Kaeriyama, Y. Sonoda, O. Lhost, F. Meyers, J. L. Bredas, Macromolecules 28 (1995) 1959. [5] M. Fahlam, D. Beljonne, M. L6gdlund, R. H. Friend, P. L. Burn, A. B. Holmes, J. L. Bredas, W. R. Salaneck, Chem. Phys. Lett. 214 (1995) 327. [6] H. Agren, V. Carravetta, O. Vahtras, L.G.M. Pettersson, Chem. Phys. Lett. 222 (1994) 75. [7] V. Carravetta, L. Yang, H. ,~gren, Phys. Rev. B 55 (1997) 1004. [8] M. L6gdlund, P. Dannetun, S. Stafstr6m, W. R. Salaneck, M. G. Ramsey, C. W. Spangler, C. Fredricksson, J. L. Bredas, Phys. Rev. Lett. 70 (1993) 970. [9] H..Agren, V. Carravetta, L.G.M. Pettersson, O. Vahtras, Phys. Rev. B 53 (1996) 16074. [10] J. Alml6f, K.Faegri Jr., K. Korsell, J. Comput. Chem. 3 (1982) 385. [11] R. Manne, J. Chem. Phys. 52 (1970) 5733. [12] E. Ortf, J. L. Br6das, J. Chem. Phys. 89 (1988) 1009. [13] G. Bieri, L. ,~,sbrink, J. Electron Spectrosc. Relat. Phenom. 20 (1980) 149. [14] K. Kimura, S. Katsumata, Y. Achiba, T. Yamazaki, S. Iwata (Eds.), Handbook of HeI Photoelectron Spectra of Fundamental Organic Molecules, Halsted Press, New York, 1981, p. 70. [15] M. S. Moghaddam, A. D. O. Bawagan, K. T. Tan, W. yon Niessen, Chem. Phys. 207 (1996) 19. [16] J. Okubo, H. Shinozaki, M. Kubuta, T. Kobayashi, J. Electron Spectrosc. Relat. Phenom. 77 (1996) 265. [17] W. von Niessen, G. H.F. Dierecksen, L. S. Cederbaum, Chem. Phys. 10 (1975) 345. [18] W. von Niessen, W. P. Kraemer, G. H.F. Dierecksen, Chem. Phys. 41 (1979) 113. [19] P. Dannetun, M. L6gdlund, C. W. Spangler, J. L. Bredas, W. R. Salaneck, J. Chem. Phys. 98 (1994) 2853.