He I valence photoelectron spectra of oxomolybdenum(V) complexes containing diolato or alkoxide ligands

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Polyhe&m Vol. 9, No. IS/M, pp. 1965-1973, 1990 Printed in Great Britain

He I VALENCE PHOTOELECTRON SPECTRA OXOMOLYBDENUM(V) COMPLEXES CONTAINING OR ALKOXIDE LIGANDS”

OF DIOLATO

C. S. J. CHANG, A. RAI-CHAUDHURI, D. L. LICHTENRERGER J. H. ENEMARKT

and

Department of Chemistry, University of Arizona, Tucson, AZ 85721, U.S.A. Abstract-The He I valence photoelectron spectra (PES) are reported for several monooxomolybdenum(V) compounds with the general formula of LMoO[O--(CH~),-O] and LMoO(OR)~ (L = hydrotris(3,5dimethyl-1-pyrazolyl)borate ; n = 2-4; R = Me, Et, Tr). The spectra show that the size of the metal-chelate ring in these diolato complexes has a substantial effect on the HOMO ionization. As the chelate ring size in the diolato complexes increases, the HOMO shifts (cu 0.21-0.24 eV) to lower ionization energy. The diolato complexes have their HOMO’s at a higher ionization energy than analogous open chain bis-alkoxide complexes possessing the same number of carbon atoms. The unconstrained bis-alkoxide complexes show much smaller shifts (< 0.11 eV) in the ionization potential associated with the HOMO upon the addition of a CH2 unit to each alkoxide group. These gas-phase results are similar to the results observed by electrochemistry in acetonitrile solution, where both the diolato and bis-alkoxide complexes become easier to oxidize upon increasing the number of CH2 groups, as indicated by their half-wave potentials. The pn orbital ionizations of the oxygen atoms in the alkoxide functions are also shifted (ca 0.12 eV) to lower ionization energy upon adding CH2 residues to the alkoxide chain.

The study of models for the molybdenum cofactor (MO-CO) remains an intriguing topic.‘-* Previous studies of model compounds for MO-COof “oxotype” molybdoenzymes6 have mainly focused on mimicking the enzyme’s catalytic reactionq7** or duplicating parameters of the EPR (electron paramagnetic resonance) spectra, 9 the EXAFS (Extended X-Ray Absorption Fine Structure) spectra 5*‘o and electrochemical behaviour.9d~‘1~‘2To daie, however, little attention has been given to the role of the backbone and side chain of the metalchelate ring and how it can affect the electrochemical and spectroscopic properties of the molybdenum centre. Some systematic investigations of the effect of the backbone of the metalchelate ring on the spectroscopic properties of a metal centre have been reported,‘3 but only a few studies of the spectroscopic properties of monooxo

*Dedicated to Professor Richard F. Fenske for his contributions to theoretical inorganic chemistry and for his inspiration to others. t Author to whom correspondence should be addressed.

metal complexes with chelated ligands have been published. ’ 4 We have recently reported solution electrochemical and spectroscopic measurements of a series of diolato and bis-alkoxide oxomolybdenurn complexes with the general structure shown below in l-6, ’ 5 where compounds 1-3, and 4-6 are diolato and bis-alkoxide complexes, respectively. The reduction potentials of both the MoV’/MoV and the MoV/MoIV pairs and the lowest energy electronic absorption bands (&,a for these complexes are a function of the backbone of the diolato or alkoxide ligands. The shifts of the half-wave potentials of the MoV1/MoV pair for compounds 2 and 3,4 and 5, and 5 and 6 are 0.217,0.030 and 0.019 V, respectively. Clearly, the shifts are significant in the diolato compound series, but not in the bisalkoxide compound series. A similar situation is also observed for the half-wave potentials of the MoV/Mon’ pair, where the shift is 0.168 and 0.218 V between compounds 1 and 2,2 and 3, individually, and the shift for compounds 4 and 5, and 5 and 6 is 0.066 and 0.042 V, respectively. We have

1965

C. S. J. CHANG

1966

(1)

et al.

n =2

(2) n = 3 (3) n = 4 suggested that a steric effect, especially ring strain, could be primarily responsible for this observed pattern of electrochemical behaviour because unconstrained bis-alkoxide complexes show no significant changes in their electrochemical properties. Photoelectron spectroscopy provides primary information about the orbital ionization energies of electrons and about electron distribution in the complexes. In addition, comparison of results obtained from PES of a series of related families of compounds is considered to be highly useful for obtaining secondary information relating to the geometries of compounds. ’6 The relative shifts of the valence ionizations in a series of similar compounds are not only sensitive to charge distributions and relaxation energies, but also to the orbital overlap interactions. The result of the He I valence photoelectron spectroscopy measurements and their significance for these complexes form the basis of this report. EXPERIMENTAL Preparation

Compounds l-7 were prepared by previously reported methods, 3*’ ’ and characterized by elemental analysis, mass spectrometry, cyclic voltammetry, IR, UV-vis spectroscopy and EPR. The neutral ligand, tris(3,5dimethylpyrazolyl)methane [compound (S)] was prepared by a modification of the method of Trofimenko17 and showed a satisfactory C, H, and N analysis. Photoelectron

spectra

All PES were measured with use of a spectrometer which features a 36-cm radius hemispherical analyser (IO-cm gap), and customized sample cells, excitation sources, detection and con-

(5)

R=Et

(6) R =‘h trol electronics, and data collection methods that have been described previously. ‘* The Ar 2P3,2 ionization at 15.759 eV was used as an internal spectral lock for the energy scale. The spectrum of each compound was taken at least twice with no discernable differences between replicate spectra ; no decomposition was observed either before or after the compound sublimed. Data were collected at different sample cell temperatures for different compounds (130°C for 1, 141°C for 2, 158°C for 3, 130°C for 4, 121°C for 5, 120°C for 6, 173°C for 7 and 45°C for 8). The mass spectra recorded at temperatures used in the collection of the spectra indicated that the samples do not decompose. The collected data are represented analytically with the best fit of asymmetric Gaussian peaks by using the program GFIT. I9 The asymmetric Gaussian peaks are defined with the position, the amplitude, the half-width indicated by the high binding energy side of the peak (W,), and the half-width indicated by the low binding energy side of the peak (W,). The confidence limits of the peak positions and deviation for widths are generally +0.02 eV. The confidence limit of the area of a band envelope is about f 5%, with uncertainties introduced in the baseline subtraction and fitting in the tails of the peaks. The parameters of overlapping peaks are not independent and therefore are more uncertain. RESULTS Photoelectron

AND DISCUSSION

spectra

The He I valence PES of the complexes studied in this work are recorded over the range 5.5-15.5 eV, as shown in Fig. 1. The ionization energy region above 11.OeV displays broad and unresolved overlapping ionizations which mainly result from ionization of o-bonding electrons in ligand moieties, C-C, C-H etc., and will not be discussed further.

He I valence photoelectron

heqgy (eV)

Iadzatb 15.0

spectra of oxomolybdenum(V)

7.0

11.0 I

I

1967

complexes

broad band with similar position as observed in compound 8 (band 3 in Fig. 3). The vertical ionization energies of the bands are given in Table 1.

la)

Diolato complexes (b)

(dr (el

Fig. 1. Photoelectron spectra from 5.5 to 15.5 eV of compounds (a) LMoO[O-(CH2),-O]; (b) LMoOW-KH 2)3-01; (4 LMoOP--WH &~I ; (4 LMoO(OMe),; (e) LMoO(OEt),; (f) LMoO(“Pr),; (g) LMoOCl, ; (h) HC{(3,5-Me),Pz} 3.

From Fig. 2 and Table 1, it is very clear that band 1 shifts toward lower ionization energy with the increase in the size of the metal-chelate ring. Band 1 (HOMO) represents the contribution from the unpaired electron in the 4d orbital of the molybdenum atom. This assumption is based on the fact that this ionization is not observed in the PES of dials,” alcohols20*2’ and tris(3,5-dimethylpyrazolyl)methane [compound (8)]. The ranges of I%‘,,and W, for this ionization envelope are 0.620.70 eV and 0.37-0.51 eV, respectively. Such a broad feature reflects the bonding property for this band, and shows that the electron is not absolutely localized in the molybdenum d orbital. Band 2 (SHOMO) in Fig. 2 also has a pronounced asymmetric feature not observed in the spectrum of compound 8. Band 2 is primarily comprised of the 2p, and 2p, orbital ionizations of the terminal 0x0 ligand, which are oriented to interact with the empty 4dXz and 4dyz orbitals of the mol-

. ladzatm 10x)

The close-up PES are. shown in Figs 2-4. For Figs 2 and 4, the region of 5.5-l 1.O eV was analytically represented with six and seven asymmetric Gaussian peaks (labelled l-6 and l-7), respectively, whereas only three peaks (labelled l-3) were used to fit Fig. 3 which is the PES of compound 8. The overall band shapes for the metal and ring n-ionization bands of compounds 14 remain the same throughout the series and are obviously different from those of LMoOCI, [compound (7)], which is the starting material. In Figs 2 and 4, three distinct regions of ionizations can be observed in the close-up spectra of the diolato and bis-alkoxide compounds. The first region comprises the lowest ionization energy band (band l), which is always broad and well isolated from other ionizations, except for that of the ethanediolato compound (1). The second region is a broad, unshifted set of bands (ca 7.75-9.5 eV) with one distinct shoulder on the low ionization energy side (band 2) and one partially resolved shoulder on the high ionization energy side (band 4 in Fig. 2 and bands 4 and 5 in Fig. 4). The last region (band 5 in Fig. 2 and band 6 in Fig. 4) is a

l-3

heqgy 8.0

(eV) 6.0

(bl

Fig. 2. Close-up PES in the range of 5.5-l 1.0 eV of (a) LMoOKWCH,) 2-01; W LMoW-WI 2)~--ol ; (4 LMoO[O--(CH,),4].

1968

C. S. J. CHANG

et al.

Table 1. Ionization data for compounds l-6 and 8” Binding energy (iv)

Relative amplitude

K (eV)

W (eV)

Normalized area -

2 3 4 5

7.09 8.11 8.69 9.36 10.27

1.oo 6.15 12.97 8.32 9.13

0.62 0.37 0.77 1.10 0.72

0.37 0.27 0.61 0.77 0.57

0.06 0.22 1.00 0.87 0.61

2

1 2 3 4 5

6.85 8.06 8.60 9.21 10.16

1.00 5.08 9.09 5.45 6.18

0.70 0.42 0.88 0.88 0.87

0.43 0.27 0.59 0.59 0.71

0.08 0.26 1.oo 0.71 0.73

3

1 2 3 4 5

6.64 8.04 8.59 9.14 10.12

1.00 4.74 8.19 5.28 4.42

0.64 0.50 0.92 1.10 0.82

0.51 0.30 0.53 0.76 0.61

0.10 0.32 1.00 0.83 0.54

4

1 2 3 4 5 6

6.69 8.11 8.72 9.24 9.85 10.18

1.oo 4.92 10.99 6.99 3.62 5.33

0.60 0.49 0.76 0.72 0.48 0.85

0.55 0.23 0.59 0.60 0.43 0.35

0.08 0.24 1.oo 0.62 0.22 0.43

5

1 2 3 4 5 6

6.58 8.09 8.70 9.15 9.73 10.10

1.00 5.91 10.41 6.08 2.44 4.07

0.68 0.73 0.77 0.77 0.66 0.86

0.52 0.25 0.52 0.51 0.37 0.33

0.09 0.43 1.00 0.58 0.19 0.36

6

1 2 3 4 5 6

6.58 8.09 8.65 9.11 9.61 10.06

1.00 6.54 11.22 5.69 3.46 5.72

0.65 0.57 0.96 0.66 0.40 0.77

0.52 0.25 0.50 0.58 0.40 0.40

0.07 0.33 1.oo 0.43 0.17 0.40

8

1 2 3

8.74 9.26 10.24

1.40 1.00 1.17

0.60 0.60 1.37

0.44 0.44 0.61

I .oo 0.71 1.59

Compound

Ionization 1

1

‘For the first ionization, the error bar in the fit analysis is + 0.02 eV for peak position, 0.05-0.2 eV in half-widths, 5-10% in relative amplitudes.

ybdenum atom. This ionization envelope is also observed in compound 7 [Fig. l(g)], with a similar position to that shown in the spectra of compounds l-3 [Fig. I(aHc)]. Since the only oxygen atom involved in compound 7 is the terminal 0x0 ligand, we assign band 2 to ionizations from this oxygen atom. In addition, the lone-pair electrons of chlorine ligands in compound 7 should be well separated from the ionization envelope of lone-pair electrons of the oxygen atom. The Fenske-Hall molecular

orbital calculations on compound 7 showed that the ionization energies of the lone-pair electrons of chlorine differ by at least 0.6 eV from those of the oxygen atom. The experimental result shows that the lone-pair electrons on the chlorine atoms in compound 7 have much higher ionization energy (IP x 11 eV) compared with the lone-pair electrons of the terminal oxygen atom. Similar behaviour is found for many organic compounds2’ containing both a chlorine atom and carbonyl oxygen atom

He I valence photoelectron spectra of oxomolybdenum(V) complexes and which exhibit ionization energies for the lonepair electrons of the oxygen atom that are lower than the lone-pair electrons of the chlorine atom. Band 2 shows almost no shift with change of the number of methylene units in diolato or alkoxide groups (8.048.11 eV and 8.09-8.11 eV for diolato and bis-alkoxide compounds, respectively). The small range of ionization energies is also consistent with the narrow range of molybdenum-oxygen bond lengths in most monooxo complexes.” Thus, the bonding between the molybdenum atom and the terminal 0x0 ligand is quite insensitive to the nature of the ligands which occupy the remaining coordination sites. Comparison of the spectra obtained from lHpyrazoleZ23 1H-3,5-dimethylpyrazole24 and tris(3,5dimethylpyrazolyl)methane (Figs 1(h) and 3) indicate that bands 3 and 4 are probably derived from the pn orbital electron ionizations of the pyrazole rings. The energies of these orbitals will not be significantly affected by a change of the local environment around the metal centre of the complexes. However, comparison of the band widths of the pn ionizations (bands 1 and 2) of compound 8 with bands 3 and 4 of the complexes shows that the band 4 is significantly more broadened than band 3. This result indicates that either the bonding character has increased or that overlapping with other ionizations occurs (as is observed in bands 4 and 5 of compounds &6, vide infru). These pn orbitals do not interact with the d orbitals of molybdenum and show no disturbance of their electronic environment by other ligands around the metal centre, and the first possibility can be ruled out. The broadening of band 4 is assigned to overlapping pn ionizations from oxygen atoms in the diolato ligands that might be buried under band 4. The last band (band 5) for compounds 1-3, is assigned to the lone-pair electrons of nitrogen atoms in the pyrazole rings and has a similar feature to band 3 in compound 8. Ionizations of the oxygen

Ionization

10.0

Fig.

3.

Jheqy

8.0

(eVJ

6.0

Close-up PES in the range of 5.5-l 1.0 eV of compound 8, HC{(3,5-CH,)Pz} 3.

1969

prr electrons might also have some overlap with this band ; however, it is very difficult to confirm this. The ionization energy of this band shifts from 10.27 to 10.12 eV from compound 1 to 3, suggesting that the ionization energy of band 5 is still quite close to that of compound 8, probably also indicating that the interaction with the d orbitals of the metal centre is not as important as that between pn electrons of the diolato oxygen atoms and drc electrons of the molybdenum atom mentioned above. The symmetry of these molecules is no higher than C,, and these compounds contain at least four kinds of chemically different nitrogen atoms. Nevertheless, no splitting of this ionization is observed, in contrast to the case of [Ir@-dimethylpyrazolyl)(CO)2],.24 For the ionization energies within the range of 9-10 eV, compounds l-3 can be represented by one asymmetric Gaussian peak (band 5) whereas compounds &6 are represented by two asymmetric Gaussian peaks (bands 5 and 6). In complexes 4-6, the ionizations of pn electrons of the alkoxide oxygen atoms are observed separately as band 5 which is at lower IP than band 6 which is the nitrogen lone pair ionization. The oxygen pn ionizations are not resolved in complexes l-3 because they are shifted due to the stabilization of the diolato oxygen pn ionizations in these complexes as a result of O(pnk Mo(drr) bonding interactions.

Bis-alkoxide complexes 4-6

The vertical ionization energies of each band for these complexes are listed in Table 1 (spectra in Fig. 4). Bands 2, 3, 4 and 6 for all these compounds maintain similar positions throughout the entire compound series and they are also located in similar positions to those in the diolato complexes. Therefore, they probably arise from those orbitals mentioned in the case of the diolato complexes. The only difference is that the ionizations from px electrons of the orbitals of the oxygen atoms (band 5) of the alkoxide ligands are resolved in these compounds, whereas the ionizations from pn electrons of the diolato oxygen atoms are not. The PES of alkyl alcohol, ROH (R = Me, Et and nPr), have been published.2’ The leading vertical ionization potentials decrease from 10.94 to 10.49 eV from R = Me to R = “Pr due to association with the inductive effect of alkyl groups and reduced oxygen 2p character in the HOMO. The ionization energies of the oxygen pn electrons are initially lowered by forming the alkoxide anion (RO-) from the alcohol molecule due to increase of the electron density of the oxygen atom in the alkoxide ligand.

1970

C. S. J. CHANG bnization

Iheqjy

et al.

(eV)

10.0 I

4

5 (Cl

6 Scheme 1.

Fig. 4. Close-up PES in the range of 5.5-l 1.0 eV of (a) LMoO(OMe),; (b) LMoO(OEt),; (c) LMoO(O”Pr),.

The a-bonding between the oxygen atom in the alkoxide ligand and the molybdenum atom slightly stabilizes this pi ionization, but there is no additional stabilization from n-bonding interactions between the alkoxide oxygen atoms and the molybdenum d electron, as occurs in the didlato complexes (vide supra). In bis-alkoxide complexes, both bands 1 and 5 exhibit shifts to lower ionization energy with the introduction of more methylene units to the linear alkyl groups. For band 1, the maximum shift is only 0.11 eV. The error in the fit analysis is + 0.02 eV for peak position, so the shift in band 1 among the bis-alkoxide compound series is barely significant. Briefly, according to these two facts, in the bisalkoxide complexes, the interaction between pn electrons of the oxygen atoms, in the alkoxide groups and the dn electron of the molybdenum atom is a purely electrostatic repulsion. Consequently, the HOMO is only destabilized by a small degree upon increasing the number of methylene units in the alkyl moieties. This is different from the case in the diolato complex series. Since the HOMO in the diolato complexes is destablized by an overlap interaction betweenpn electrons of the diolato oxygen atoms and the dn electron of the molybdenum

atom, a significant variation of the ionization energy of the HOMO is observed upon changing the number of methylene units in the ring (vide ante). Summary of PES spectra Both the diolato and bis-alkoxide compound series exhibit similar He I PES. Consequently, comparing with the PES of lH-3,5-dimethylpyrazole and tris(3,5-dimethylpyrazolyl)methane, a purely qualitative molecular orbital diagram can be used to assign each band discussed in compounds l-6, as shown in Scheme 1 (for compounds ti) and Scheme 2 (for compounds l-3).

Ionization

4

qp+* Nitrogen

5

[

1

lanepair

Scheme 2.

He I valence photoelectron Correlation

spectra of oxomolybdenum(V)

with electrochemistry

The detailed results of solution electrochemistry have been reported earlier. l5 Within the diolato compound series, the complexes become easier to oxidize with increasing chelate ring size. In the bisalkoxide compound series, complexes become easier to oxidize with extension of alkyl chains in alkoxide ligands. The oxidation potential recorded from solution electrochemistry is the energy difference between the positively charged molecular ion (oxidized species) in its electronic ground state and the neutral molecule and some other energy terms, like solvation energy. The ionization potential for the HOMO is the energy necessary to generate the positive ion in its electronic ground state in the gas phase and can be thought of as a difference of the heat of formation between the molecule and its molecular ion, neglecting vibrational energy. Nevertheless, since we measure a series of similar complexes under similar experimental conditions, value from solution electrochemistry the Ei12 should be directly proportional to the HOMO energy from gas-phase PES, as suggested by Bursten :25 El,2

=

kl(-%OMO)+k2.

The equation for the least-squares line of compounds 2-6 is shown in Fig. 5, where IP(eV) = 0.685(E,,,,V) + 6.25 with a correlation coefficient of 0.919. The half-wave potentials (E,,,) for oxidation obtained from solution phase by electrochemical methods can be thought of as a “solution ionization potential” of the highest occupied orbital.25 Studies on the linear correlation between the HOMO energy and E,,2 for oxidation have been reported

1

I

El/2 CVvsSCE) Fig. 5. Correlation of half-wave oxidation potential (E,,,), in V vs SCE, with lowest energy ionization potential (IP), in eV, for compounds 2-6.

complexes

1971

for many systems.25*26 In the present oxidationreduction reactions, the participating orbital is mainly the Mo(4d,,) orbital, which has the lowest ionization energy. Consequently, the less stable the HOMO of a complex, the easier it is to oxidize, as shown in Fig. 5. The oxidation potentials from solution electrochemistry follow the order 6 < 5 < 4 < 3 < 2 ; however, this trend is altered somewhat inthegasphase,6 N 5 < 3 < 4 < 2 < l.Inthe solution electrochemistry measurement, the redox process is not a Frank-Condon type instantaneous electronic excitation. However, the photoionization of a molecule in the gas phase obeys the FrankCondon principle. Consequently, the difference between these trends from two different phases might reflect a contribution from the solvation energy of various molecular ions. On the basis of arguments mentioned earlier, the ionizations 1, 2, 4 (for compounds l-3), or 5 (for compounds 46) contain some p character from oxygen atoms in the diolato and alkoxide ligands. Therefore, the strength of the interaction between dn electron of the molybdenum atom and p7t electrons of the ligand group orbitals of diolato or alkoxide moieties primarily determines whether or not the HOMO will be destabilized. The larger the interaction between the half-filled dx and filled oxygen px orbitals, the more the HOMO will be destabilized, and the easier it is to oxidize the complex. In the diolato compound series, this interaction becomes significant in a compound with larger ring size because better overlap is possible between 4dJMo) and pn(0) with such a flexible ring conformation. This phenomenon is reflected by increasing the ionization energy difference between band 4 and band 1 from 2.27 to 2.50 eV for compounds 1 to 3. The seven-membered ring diolato compound (3) has the lowest ionization energy for a HOMO in a diolato series, and it also exhibits the lowest half-wave potential and becomes the easiest to oxidize in that series. The unconstrained bis-alkoxide compounds show only minor or no shift of the HOMO to lower ionization energy with longer chain alkyl groups, suggesting that the interaction between 4dJMo) and the alkoxide px(0) orbitals is quite similar in all bis-alkoxide compounds because of similar electronic and steric effects among those alkoxide ligands. However, for the diolato complex series, the steric constraints of the chelate ring will alter the 4dJMo) and&O) interaction for different complexes. CONCLUSION This work has shown that a series of structurally similar diolato and bis-alkoxide complexes exhibit

1972

C. S.

J. CHANG et al.

similar PES. However, there are three differences between these two complex series. (1) The ionization energies of the HOMO of the diolato complexes show significant shifts, but those for the bisalkoxide complexes do not. (2) The ionization from the p7c electrons of the alkoxide oxygen atoms is resolved as band 5 in the bis-alkoxide complexes, but that from analogous diolato oxygen atoms in the diolato complexes is not. (3) The main source of the shifts of the ionization energies of the HOMO of the diolato complexes is the bonding interaction between the pn(0) and &(Mo) orbitals which depends on the steric constraints of the metalchelate ring. Such steric constraints are absent in the bis-alkoxide complexes and the slight destabilization of the HOMO is mainly due to electrostatic repulsion between the prc(0) and &MO) electrons which varies little among the compounds. The electronic effect for diolato and alkoxide ligands having the same number of carbon atoms in their backbones should be similar to one another (as ethanediolato and butanediolato are similar to that of bis-methoxide and bis-ethoxide, respectively), but their steric effects can be considerably different. This work shows that the diolato compound series exhibits a significant shift of the ionization energy of the HOMO, but only a small shift is found among the bis-alkoxide compounds. This result implies that the steric effect resulting from the strain in the diolato chelated ring is primarily responsible for the strength of the interaction between 4d,,(Mo) and 2prc(O). Acknowledgements-Support by the National Institutes of Health (Grant GM37773) to J.H.E. is gratefully acknowledged. D.L.L. acknowledges support by the U.S. Department of Energy (Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, DE-FG0286ERl3501), and the Materials Characterization Program, Department of Chemistry, University of Arizona. We also thank Michael D. Carducci for providing tris(3,5dimethylpyrazolyl)methane, and Roy Hogan for helpful discussions.

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He I valence photoelectron

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21.

22. 23. 24.

25. 26.

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