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Photoelectron studies of trans-polyacetylene using synchrotron radiation
Synthetic Metals, 41--43 (1991) 1365-1368
1.365
PHOTOELECTRON STUDIES OF TRANS-POLYACETYLENE USING SYNCHROTRON RADIATION
J. RASMUSSON, S. STAFSTROM, M. LOGDLUND AND W.R. SALANECK Department of Physics, IFM, University of Link6ping, Link6ping (Sweden) U. KARLSSON MAX-laboratory, University of Lund, Lund (Sweden) D.B. SWANSON AND A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelphia (USA) G.A. ARBUCKLE Department of Chemistry, Rutgers University, Camden, New Jersey 08102 (USA) ABSTRACT We report the results of studies of trans-polyacetylene by photoelectron spectroscopy using synchrotron radiation, in the photon energy in the range from 27 to 125 eV. The relative intensities of the individual peaks in the photoelectron emission spectra are observed to depend significally on the photon energy. These results compare favorably with theoretical calculations of energy-dependent photoionization cross sections.
INTRODUCTION The ultimate goal of this study is to carry out band mapping of the electronic structure of ordered (stretch-aligned films) trans-polyacetylene, or trans-(CH)x, using variable photon energies obtainable from synchrotron radiation (SR). For the present films, the stretching factor was only a factor of 2 to 4, which has proved insufficient, despite an observed optical anisotropy. However, photon-energy-dependent cross section effects must also eventually be taken into account. This portion of the project is reported upon here.
THEORY In order to obtain a theoretical simulation of experimental photoelectron spectra we perform band structure calculations using the Valence Effective Hamiltonian (VEH) method [1,2]. The density of valence states (DOVS) is calculated from the VEH band structure using the method of Delhalle and Delhalle [3]. Since the peaks in the experimental spectra always exhibit finite line widths, it is common to convolute the theoretical I:K)VS curve by a Gaussian functions whose full width of half maximum (FWHM) is adjusted to fit experimental spectra, and has been set here to 1.0 eV. Fun_hermore, as in all Hartree-Fock ab initio results, the spread in energy levels, or the energy band width, is always too wide, and a necessary contraction is performed. In VEH results, it is standard to contract the calculated valence band by a factor of about 1.3 (here 1.28). Solid state 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1366
(polarization energy) effects are approximated by the standard shift of the DOVS towards lower binding energies by 2.0. In comparing photoelectron spectra with calculations of the DOVS, the relative photoionization cross sections of the band states must be included. For the initial and final states in the photoionization process, the VEH wave-functions and plane waves, respectively, are used. The approximation of plane-wave final states ignores the influence of the positively charged molecular ion framework on the escaping photoelectron, and is valid for only sufficiently high electron kinetic energies[4]. This work contains an extension of a method developed by Lohr and Robin [4] and includes stereo regular polymeric systems in the calculation of the cross section. Within the dipole approximation, we derive dOn(k)/d.O oc q5 0~-1 cos2Oqu I< Vn(k)l eiqr>12
(1)
The expression for the differential cross section, don, into the solid angle, d~, depends on the wave vector q of the ejected photoelectron, the angular frequency co of the impinging photons, the angle Oqu between the polarization vector u and the direction q of the ejected photoelectron. The last factor in Eq. 1 contains the overlap between VEH wave functions, Vn(k), describing the initial state in the photoelectron emission process, and plane waves, eiq r, which approximate the state of the ejected electron. EXPERIMENTAL Thin cis-(CH)x films (30 - 50 ~tm thick) with dimensions 10 x 15 mm were produced by the Shirakawa process, and shipped, under vacuum in sealed glass tubes, to the MAX Laboratory for Synchrotron Radiation. The samples were mounted upon sample holders under flowing N2 in a portable glove bag system, and transfered into the Ultra High Vacuum (UHV) spectrometer. The samples were then heated at 160° C for 30 rain. to convert to the trans- form. The heating step did not produce any serious outgasing problems. Measurements were made in UHV with highly polarized SR light at photon energies between 27 and 125 eV. The samples were thin enough that sample charging did not occur. RESULTS AND DISCUSSION The experimental spectra are shown in Fig. 1 (top), and the theoretical results in Fig. 1 (bottom). The agreement between experimental and theoretical spectra is excellent, in terms of the peak positions, heights and widths. The strong peak at the high binding energy side of the experimental spectra taken with 27 eV photons is strongly influenced by the secondary-electron tail (not included in the calculations), which occurs outside of the chosen binding energy region in the remaining experimental spectra. A true photoelectron peak around 20 eV is observed in this and all other spectra. Peak A corresponds to a pure electrons photoemitted from a band derived almost exclusively from C2s atomic states. Peaks B, C, and D all originate from the emission of electrons from o-bands derived from combinations of C(2s) and C(2p) atomic states. Photoelectrons emitted from the only occupied g-band result in the strong shoulder on the low binding energy side of each spectrum in Fig. 1. The width of the occupied part of the g-band is around 5 eV. The total x-band
1367
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Experimental and theoretical photoelectron spectra of trans-(CH)x( hv = 27 - 125 eV ) D C experimental spectra B f ,~.' A
r-
125 eV 90 eV 65 eV 50 eV 40.8 eV
D
.........
B
theoretical spectra
~/
/0-'k
~-----"~f-'~ ~ ~ --_
,'
I
20.0
90 eV ~~.._65eV ~--~ 50 eV
.f I
15.0
27 eV
4°-8ev I
I
10.0 5.0 Binding energy relative Ef
27 eV
Fig.1 Experimental (top) and theoretical (bottom) phott)electron spectra of trans - (CH)x (hv = 27 - 125 eV). On the high energy binding side of the experimental 27 eV spectrum thea'e is a large contribution of secondary electrons as apl~ximated by the handrawn line. In the theoretical 27 eV spectrum, the part of the curve to the left of the asterisk (at highest binding energy) corresponds to photoelectrons with kinetic energy so low (< 12 eV) that the assumption of a plane wave final state is no longer valid.
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width, including the energy gap and the unoccupied part of the x-band, is around 12 eV in the calculation. This result is in close agreement with the value 11 eV obtained from EELS-studies [5]. With increasing photon energy, the intensity of peak D increases relative to the intensities of peaks B and C. The intensity relationship between peaks B and C, however, remains unchanged. Exactly the same behaviour is observed in the theoretical spectra. These chan~es in relative intensities are due to the energy dependence of the cross sections, and reflect the differerlt features of the electronic wavefunctions of the resveetive veaks as derived from Eo. (1). In very well ordered samples, the dispersive nature of the individual bands, especially the isolated C(2s)-derived ~-band at highest binding energy and the C(2p)-derived x-band at lowest binding energy, should be observed. The fact that no dispersion appears in our spectra indicates that the stretching of the present samples (2 - 4x) is insufficient to obtain a high degree of alignment (order) for band dispersion studies. SUMMARY In summary, we present the initial results of a project to employ synchrotron radiation to study the electronic band structure of trans-(CH) x. The present results represent high quality photoelectron spectra of unordered material, recorded for photon energies in the range from 27 eV up to 125 eV. Effects of the photon-energy-dependence of the partial photoionization cross sections are observed. The results of VEH level quantum chemical calculations are in excellent agreement with the photonenergy-dependent results. These spectra will form the basis upon which future spectra, of stretchaligned films of trans-(CH) x, will be analysed. ACKNOWLEDGEMENTS This work is supported by the Swedish Natural Research Council (NFR) and the Swedish National Board for Technical Developement (STU). REFERENCES 1 G. Nicolas and Ph. Durand, $. Chem, Phy~, 70 (1979) 2020. 2 J.M Andre, L.A. Burke, J. Delhalle, G. Nicolas, and Ph. Durand, Int. J. Ouantum Chem. SvmD. 13 (1979) 283. 3 J I)¢lhalle and S. Delhalle, Int.J.Ouantum Chem. 11 (1977) 349. 4 L.L. Lohr & M.B. Robin, L Am. Chem. Soc. 92 (1970) 7241. 5 J. Fink, N. Niicker, B. Scheerer, W. Czerwinski, A. Litzelmann, and A. vom Felde, in H. Kuzmany, M. Mehring, and S. Roth (eds.) Electronic Progerties Qf Con iugated Polymers, Springer Series in Solid State Sciences No. 76, Springer, Berlin, 1987, p. 70.
1.365
PHOTOELECTRON STUDIES OF TRANS-POLYACETYLENE USING SYNCHROTRON RADIATION
J. RASMUSSON, S. STAFSTROM, M. LOGDLUND AND W.R. SALANECK Department of Physics, IFM, University of Link6ping, Link6ping (Sweden) U. KARLSSON MAX-laboratory, University of Lund, Lund (Sweden) D.B. SWANSON AND A.G. MACDIARMID Department of Chemistry, University of Pennsylvania, Philadelphia (USA) G.A. ARBUCKLE Department of Chemistry, Rutgers University, Camden, New Jersey 08102 (USA) ABSTRACT We report the results of studies of trans-polyacetylene by photoelectron spectroscopy using synchrotron radiation, in the photon energy in the range from 27 to 125 eV. The relative intensities of the individual peaks in the photoelectron emission spectra are observed to depend significally on the photon energy. These results compare favorably with theoretical calculations of energy-dependent photoionization cross sections.
INTRODUCTION The ultimate goal of this study is to carry out band mapping of the electronic structure of ordered (stretch-aligned films) trans-polyacetylene, or trans-(CH)x, using variable photon energies obtainable from synchrotron radiation (SR). For the present films, the stretching factor was only a factor of 2 to 4, which has proved insufficient, despite an observed optical anisotropy. However, photon-energy-dependent cross section effects must also eventually be taken into account. This portion of the project is reported upon here.
THEORY In order to obtain a theoretical simulation of experimental photoelectron spectra we perform band structure calculations using the Valence Effective Hamiltonian (VEH) method [1,2]. The density of valence states (DOVS) is calculated from the VEH band structure using the method of Delhalle and Delhalle [3]. Since the peaks in the experimental spectra always exhibit finite line widths, it is common to convolute the theoretical I:K)VS curve by a Gaussian functions whose full width of half maximum (FWHM) is adjusted to fit experimental spectra, and has been set here to 1.0 eV. Fun_hermore, as in all Hartree-Fock ab initio results, the spread in energy levels, or the energy band width, is always too wide, and a necessary contraction is performed. In VEH results, it is standard to contract the calculated valence band by a factor of about 1.3 (here 1.28). Solid state 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1366
(polarization energy) effects are approximated by the standard shift of the DOVS towards lower binding energies by 2.0. In comparing photoelectron spectra with calculations of the DOVS, the relative photoionization cross sections of the band states must be included. For the initial and final states in the photoionization process, the VEH wave-functions and plane waves, respectively, are used. The approximation of plane-wave final states ignores the influence of the positively charged molecular ion framework on the escaping photoelectron, and is valid for only sufficiently high electron kinetic energies[4]. This work contains an extension of a method developed by Lohr and Robin [4] and includes stereo regular polymeric systems in the calculation of the cross section. Within the dipole approximation, we derive dOn(k)/d.O oc q5 0~-1 cos2Oqu I< Vn(k)l eiqr>12
(1)
The expression for the differential cross section, don, into the solid angle, d~, depends on the wave vector q of the ejected photoelectron, the angular frequency co of the impinging photons, the angle Oqu between the polarization vector u and the direction q of the ejected photoelectron. The last factor in Eq. 1 contains the overlap between VEH wave functions, Vn(k), describing the initial state in the photoelectron emission process, and plane waves, eiq r, which approximate the state of the ejected electron. EXPERIMENTAL Thin cis-(CH)x films (30 - 50 ~tm thick) with dimensions 10 x 15 mm were produced by the Shirakawa process, and shipped, under vacuum in sealed glass tubes, to the MAX Laboratory for Synchrotron Radiation. The samples were mounted upon sample holders under flowing N2 in a portable glove bag system, and transfered into the Ultra High Vacuum (UHV) spectrometer. The samples were then heated at 160° C for 30 rain. to convert to the trans- form. The heating step did not produce any serious outgasing problems. Measurements were made in UHV with highly polarized SR light at photon energies between 27 and 125 eV. The samples were thin enough that sample charging did not occur. RESULTS AND DISCUSSION The experimental spectra are shown in Fig. 1 (top), and the theoretical results in Fig. 1 (bottom). The agreement between experimental and theoretical spectra is excellent, in terms of the peak positions, heights and widths. The strong peak at the high binding energy side of the experimental spectra taken with 27 eV photons is strongly influenced by the secondary-electron tail (not included in the calculations), which occurs outside of the chosen binding energy region in the remaining experimental spectra. A true photoelectron peak around 20 eV is observed in this and all other spectra. Peak A corresponds to a pure electrons photoemitted from a band derived almost exclusively from C2s atomic states. Peaks B, C, and D all originate from the emission of electrons from o-bands derived from combinations of C(2s) and C(2p) atomic states. Photoelectrons emitted from the only occupied g-band result in the strong shoulder on the low binding energy side of each spectrum in Fig. 1. The width of the occupied part of the g-band is around 5 eV. The total x-band
1367
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Experimental and theoretical photoelectron spectra of trans-(CH)x( hv = 27 - 125 eV ) D C experimental spectra B f ,~.' A
r-
125 eV 90 eV 65 eV 50 eV 40.8 eV
D
.........
B
theoretical spectra
~/
/0-'k
~-----"~f-'~ ~ ~ --_
,'
I
20.0
90 eV ~~.._65eV ~--~ 50 eV
.f I
15.0
27 eV
4°-8ev I
I
10.0 5.0 Binding energy relative Ef
27 eV
Fig.1 Experimental (top) and theoretical (bottom) phott)electron spectra of trans - (CH)x (hv = 27 - 125 eV). On the high energy binding side of the experimental 27 eV spectrum thea'e is a large contribution of secondary electrons as apl~ximated by the handrawn line. In the theoretical 27 eV spectrum, the part of the curve to the left of the asterisk (at highest binding energy) corresponds to photoelectrons with kinetic energy so low (< 12 eV) that the assumption of a plane wave final state is no longer valid.
1368
width, including the energy gap and the unoccupied part of the x-band, is around 12 eV in the calculation. This result is in close agreement with the value 11 eV obtained from EELS-studies [5]. With increasing photon energy, the intensity of peak D increases relative to the intensities of peaks B and C. The intensity relationship between peaks B and C, however, remains unchanged. Exactly the same behaviour is observed in the theoretical spectra. These chan~es in relative intensities are due to the energy dependence of the cross sections, and reflect the differerlt features of the electronic wavefunctions of the resveetive veaks as derived from Eo. (1). In very well ordered samples, the dispersive nature of the individual bands, especially the isolated C(2s)-derived ~-band at highest binding energy and the C(2p)-derived x-band at lowest binding energy, should be observed. The fact that no dispersion appears in our spectra indicates that the stretching of the present samples (2 - 4x) is insufficient to obtain a high degree of alignment (order) for band dispersion studies. SUMMARY In summary, we present the initial results of a project to employ synchrotron radiation to study the electronic band structure of trans-(CH) x. The present results represent high quality photoelectron spectra of unordered material, recorded for photon energies in the range from 27 eV up to 125 eV. Effects of the photon-energy-dependence of the partial photoionization cross sections are observed. The results of VEH level quantum chemical calculations are in excellent agreement with the photonenergy-dependent results. These spectra will form the basis upon which future spectra, of stretchaligned films of trans-(CH) x, will be analysed. ACKNOWLEDGEMENTS This work is supported by the Swedish Natural Research Council (NFR) and the Swedish National Board for Technical Developement (STU). REFERENCES 1 G. Nicolas and Ph. Durand, $. Chem, Phy~, 70 (1979) 2020. 2 J.M Andre, L.A. Burke, J. Delhalle, G. Nicolas, and Ph. Durand, Int. J. Ouantum Chem. SvmD. 13 (1979) 283. 3 J I)¢lhalle and S. Delhalle, Int.J.Ouantum Chem. 11 (1977) 349. 4 L.L. Lohr & M.B. Robin, L Am. Chem. Soc. 92 (1970) 7241. 5 J. Fink, N. Niicker, B. Scheerer, W. Czerwinski, A. Litzelmann, and A. vom Felde, in H. Kuzmany, M. Mehring, and S. Roth (eds.) Electronic Progerties Qf Con iugated Polymers, Springer Series in Solid State Sciences No. 76, Springer, Berlin, 1987, p. 70.