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Interpretation of photoinduced infrared spectra and doping induced Raman scattering in trans-polyacetylene
Synthetic Metals, 41-43 (1991) 1337-1340
1337
INTERPRETATION OF PHOTOINDUCED INFRARED SPECTRA AND DOPING INDUCED RAMAN SCATTERING IN TRANS-POLYACETYLENE
E.MULAZZI and A.RIPAMONTI Dipartimento di Fisica dell' Universit£, via Celoria 16~ 20133 Milano (Italy) S.LEFRANT Lab.Phys.Cristal., IPCM, Universit~ de Nantes, 44072 Nantes Cedex 03 (France)
ABSTRACT We propose an interpretation of the photoinduced infrared spectra of the cis rich (CH)~ and fully isomerized trans (CH)~ in the framework of the perturbed Green function method in which the lattice dynamics of polyacetylene is considered. The perturbation induced by the photogenerated charge on the lattice is accounted for by a positive parameter A which simulates the changes of the force constants due to the modification of the electron-vibration and electron-electron interactions in the excited states with respect to the ground state. By using the same model and by considering a positive change A of the force constants, the doping induced Raman modes in n doped trans (CH)~ are also well accounted for. INTRODUCTION In recent years, the electronic and vibrational properties of conducting polymers have been studied by using different techniques. In particular the investigation of the photoinduced infrared absorption (PIA) performed on polyaeetylene has provided fundamental information on the photogenerated charges trapped at defect sites, through the photogencrated electronic absorption band peaked at 3500-4200 cm -1. Moreover the photoinduced infrared vibrational bands peaked at 500-600 cm -1, 1282 and 1370 cm -1 are the signature of the perturbation induced by the photogenerated charges on the lattice dynamics of the conjugated segments. More recently, new experimental data obtained from cis rich and trans (CH)= thin films [1] have shown new features in the principal broad band peaked at 500-600 c m - l : the band is structured and presents a well resolved secondary peak at 750 cm -1 in the cis rich samples, while in trans (CH)~ it shows a pronounced shoulder in the same frequency region. The new feature in the cis rich sample is interpreted by the authors 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1338
in [1] to be due to thermal modulation of the absorption of the remnant cis (OH), due to the (C-H) out-of-plane deformation. Very recently, PIA spectra have been investigated with particular care on oriented (CH)z samples with different cis content [2], with a pump laser polarized perpendicular and the probe i.r. beam polarized parallel to the chain axis. In that work the changes of intensities of the vibrational and electronic bands and their ratios as function of the cis content and the pump laser frequency together with an important observation concerning the structure at -.~ 750 cm -1, are presented. It is derived from the experimental data that this feature recorded in the PIA spectra of samples with different cis content, cannot be due to a thermal modulation of the absorption of cis (CH)®, because of polarized and thermal shift arguments. Nevertheless no clear interpretation is proposed to explain the origin of this feature as in the cis rich sample as well in the isomerized trans (CH),. In this paper we report the theoretical interpretation of the vibration feature concerning the principal band of the PIA spectra (at 500-600 cm -1 ) of the (CH), samples with different cis content and of the fully trans (CH), samples. Our calculations are in the framework of the model developed previously [3] in order to interpret the photoinduced infrared bands in trans polyacetylene. In this model the photoinduced infrared new bands are calculated by considering the lattice dynamics of trans polyacetylene perturbed by the photogenerated charges trapped on conjugated segments of different lengths. By using the same model also the two bands at ,., 1260 and ,v 1590 cm -1 in the Raman spectra of K doped trans (CH)= for a dopant concentration y>_16 % are interpreted. Following [4], we show that they are due to the doping induced vibrational modes of the polymeric chain perturbed by the dopant. RESULTS In order to explain the shape of the broad principal band in the PIA spectrum and the frequencies observable in the Raman scattering of K-doped polyacetylene we follow the model developped in [3] and [4]. In the framework of that model which uses the perturbed Green's function formalism, the perturbation induced by the trapped charges on the lattice dynamics of the conjugated segments is taken into account through A. This represents the change of the force constant with respect to the unperturbed lattice of the polymeric chain. The new vibrational modes induced by the perturbation are calculated by considering that the trapped charges determine a positive change of the force constants of the lattice dynamics of the conjugated segments because of the change of electron-electron and electron-vibration interactions. By following the perturbed Green's function method and the approximation explained in [3] and [4], the density of the perturbed vibrational states p(oJ~) is written in terms of A, ~°(w 2) and p°(0fl) which are the real and imaginary parts
1339
of the unperturbed Green's function, respectively, as given in [3]. Then by following [3] and [4] it is possible to find that the new vibrational modes which are photogenerated or doping induced are determined by the zeros of 1/A=~3°(~02). The evaluation of the shape of the broad band in the PIA spectra in the cis rich and in trans (CH)= is performed by considering the diagram of t3°(0J2) as function of w as given in Fig.1 in [3] for trans (CH)=, which is derived for long segment lattice dynamics and analogous diagrams for short and finite segments. The contributions to the broad band are coming from various frequencies of the perturbed long and short segment lattice dynamics, by considering different values of A (starting from A = 2.10Scm-2), the intensities of the related i.r. transitions evaluated through the dipole moments for the different conjugated segments and the derivative of t3°(~02) as given in [3]. These different contributions are then weighted by the bimodal distribution of the conjugated segments which has been introduced to interpret and reproduce the resonant Raman and the optical absorption spectra of trans (CH)~ [5]. All the details will be given in a forthcoming publication. In this way, by considering a bimodal distribution with N1=50,c'1 =10,N2=10,#2=5,G=0.3 (see [5] for the meaning of the parameters) we are able to reproduce the band shape peaked at 550-600 cm -1 with a sideband peaked at 750 cm -1, as found in [1] and [2] for cis rich. While, by considering Nl=50,~rl=10, N2=14,#2=5,G=0.5 the band shape given in Fig. 1 is evaluated for the isomerized trans (CH)~. Then in the proposed model, the structures found at ~ 750 cm -1 are due to the perturbed short segment frequencies. The bigger the relative weight of the short segments and shorter the segments entering the distribution, the higher is the intensity of the side-
500.
dot).
7O0.
800.
900.
1000.
~{cm-1)
Fig. 1 Calculated shape of the main photoinduced band of trans (CH), in the frequency region 400-1000 cm -1. The parameters are given in the text.
1340 (CHK
0.15 ) x
1600--
1267
7 IY\
/
\ I
1000
1200
1400 Av (cm "1)
1600 ID
Fig. 2 Raman spectra of K doped (CH)=,y=16~,T=20°C;a)AL =457.9nm;b)AL=676.4nm
band whose shape would present a better resolution from the m ~ n band. For a very high quality samples (G = 0.7) the shoulder at ~740 cm -1 would be less evident than in Fig 1. By using the same model and following symmetry arguments given in [4], the Raman active mode frequencies induced by K dopant are found at win ~1260 cm -1 and w2a ~1550-1600 cm -1 by considering a perturbation A--2- 4.6.10 e cm -~ (see [4]). These values are in good agreement with those shown in Fig.2 where the Raman spectra of the K doped polyacetylene are given. It is worth while noting that the values of wla are not dependent on A and on A, while
W2R
is depending on both [4]. The pre-resonance condition of the incident laser
frequency with those of the doping induced electronic transitions relative to different length doped segments is responsible of the change of W2a with A, and consequently of its change with A [4]. REFERENCES [1] H.E.Schaffer, R.H.Friend and A.J.Heeger Phys. Rev. B ~6 (1987) 7537 [2] K.Pihler and G.Leising ~
(1990) 533
[3] G.P.Brivio and E.Mula~zi Solid State ~omm. 60 (1986) 203 [4] E.Mulazzi et al. Synth.Met. 24(1988)365 ; E.Mulaszi et al. Svnth.Met. 28(1989)D323 [5] G.P.Brivio and E.Mula~zi Phys. Rev. B ~0 (1984) 676
1337
INTERPRETATION OF PHOTOINDUCED INFRARED SPECTRA AND DOPING INDUCED RAMAN SCATTERING IN TRANS-POLYACETYLENE
E.MULAZZI and A.RIPAMONTI Dipartimento di Fisica dell' Universit£, via Celoria 16~ 20133 Milano (Italy) S.LEFRANT Lab.Phys.Cristal., IPCM, Universit~ de Nantes, 44072 Nantes Cedex 03 (France)
ABSTRACT We propose an interpretation of the photoinduced infrared spectra of the cis rich (CH)~ and fully isomerized trans (CH)~ in the framework of the perturbed Green function method in which the lattice dynamics of polyacetylene is considered. The perturbation induced by the photogenerated charge on the lattice is accounted for by a positive parameter A which simulates the changes of the force constants due to the modification of the electron-vibration and electron-electron interactions in the excited states with respect to the ground state. By using the same model and by considering a positive change A of the force constants, the doping induced Raman modes in n doped trans (CH)~ are also well accounted for. INTRODUCTION In recent years, the electronic and vibrational properties of conducting polymers have been studied by using different techniques. In particular the investigation of the photoinduced infrared absorption (PIA) performed on polyaeetylene has provided fundamental information on the photogenerated charges trapped at defect sites, through the photogencrated electronic absorption band peaked at 3500-4200 cm -1. Moreover the photoinduced infrared vibrational bands peaked at 500-600 cm -1, 1282 and 1370 cm -1 are the signature of the perturbation induced by the photogenerated charges on the lattice dynamics of the conjugated segments. More recently, new experimental data obtained from cis rich and trans (CH)= thin films [1] have shown new features in the principal broad band peaked at 500-600 c m - l : the band is structured and presents a well resolved secondary peak at 750 cm -1 in the cis rich samples, while in trans (CH)~ it shows a pronounced shoulder in the same frequency region. The new feature in the cis rich sample is interpreted by the authors 0379-6779/91/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1338
in [1] to be due to thermal modulation of the absorption of the remnant cis (OH), due to the (C-H) out-of-plane deformation. Very recently, PIA spectra have been investigated with particular care on oriented (CH)z samples with different cis content [2], with a pump laser polarized perpendicular and the probe i.r. beam polarized parallel to the chain axis. In that work the changes of intensities of the vibrational and electronic bands and their ratios as function of the cis content and the pump laser frequency together with an important observation concerning the structure at -.~ 750 cm -1, are presented. It is derived from the experimental data that this feature recorded in the PIA spectra of samples with different cis content, cannot be due to a thermal modulation of the absorption of cis (CH)®, because of polarized and thermal shift arguments. Nevertheless no clear interpretation is proposed to explain the origin of this feature as in the cis rich sample as well in the isomerized trans (CH),. In this paper we report the theoretical interpretation of the vibration feature concerning the principal band of the PIA spectra (at 500-600 cm -1 ) of the (CH), samples with different cis content and of the fully trans (CH), samples. Our calculations are in the framework of the model developed previously [3] in order to interpret the photoinduced infrared bands in trans polyacetylene. In this model the photoinduced infrared new bands are calculated by considering the lattice dynamics of trans polyacetylene perturbed by the photogenerated charges trapped on conjugated segments of different lengths. By using the same model also the two bands at ,., 1260 and ,v 1590 cm -1 in the Raman spectra of K doped trans (CH)= for a dopant concentration y>_16 % are interpreted. Following [4], we show that they are due to the doping induced vibrational modes of the polymeric chain perturbed by the dopant. RESULTS In order to explain the shape of the broad principal band in the PIA spectrum and the frequencies observable in the Raman scattering of K-doped polyacetylene we follow the model developped in [3] and [4]. In the framework of that model which uses the perturbed Green's function formalism, the perturbation induced by the trapped charges on the lattice dynamics of the conjugated segments is taken into account through A. This represents the change of the force constant with respect to the unperturbed lattice of the polymeric chain. The new vibrational modes induced by the perturbation are calculated by considering that the trapped charges determine a positive change of the force constants of the lattice dynamics of the conjugated segments because of the change of electron-electron and electron-vibration interactions. By following the perturbed Green's function method and the approximation explained in [3] and [4], the density of the perturbed vibrational states p(oJ~) is written in terms of A, ~°(w 2) and p°(0fl) which are the real and imaginary parts
1339
of the unperturbed Green's function, respectively, as given in [3]. Then by following [3] and [4] it is possible to find that the new vibrational modes which are photogenerated or doping induced are determined by the zeros of 1/A=~3°(~02). The evaluation of the shape of the broad band in the PIA spectra in the cis rich and in trans (CH)= is performed by considering the diagram of t3°(0J2) as function of w as given in Fig.1 in [3] for trans (CH)=, which is derived for long segment lattice dynamics and analogous diagrams for short and finite segments. The contributions to the broad band are coming from various frequencies of the perturbed long and short segment lattice dynamics, by considering different values of A (starting from A = 2.10Scm-2), the intensities of the related i.r. transitions evaluated through the dipole moments for the different conjugated segments and the derivative of t3°(~02) as given in [3]. These different contributions are then weighted by the bimodal distribution of the conjugated segments which has been introduced to interpret and reproduce the resonant Raman and the optical absorption spectra of trans (CH)~ [5]. All the details will be given in a forthcoming publication. In this way, by considering a bimodal distribution with N1=50,c'1 =10,N2=10,#2=5,G=0.3 (see [5] for the meaning of the parameters) we are able to reproduce the band shape peaked at 550-600 cm -1 with a sideband peaked at 750 cm -1, as found in [1] and [2] for cis rich. While, by considering Nl=50,~rl=10, N2=14,#2=5,G=0.5 the band shape given in Fig. 1 is evaluated for the isomerized trans (CH)~. Then in the proposed model, the structures found at ~ 750 cm -1 are due to the perturbed short segment frequencies. The bigger the relative weight of the short segments and shorter the segments entering the distribution, the higher is the intensity of the side-
500.
dot).
7O0.
800.
900.
1000.
~{cm-1)
Fig. 1 Calculated shape of the main photoinduced band of trans (CH), in the frequency region 400-1000 cm -1. The parameters are given in the text.
1340 (CHK
0.15 ) x
1600--
1267
7 IY\
/
\ I
1000
1200
1400 Av (cm "1)
1600 ID
Fig. 2 Raman spectra of K doped (CH)=,y=16~,T=20°C;a)AL =457.9nm;b)AL=676.4nm
band whose shape would present a better resolution from the m ~ n band. For a very high quality samples (G = 0.7) the shoulder at ~740 cm -1 would be less evident than in Fig 1. By using the same model and following symmetry arguments given in [4], the Raman active mode frequencies induced by K dopant are found at win ~1260 cm -1 and w2a ~1550-1600 cm -1 by considering a perturbation A--2- 4.6.10 e cm -~ (see [4]). These values are in good agreement with those shown in Fig.2 where the Raman spectra of the K doped polyacetylene are given. It is worth while noting that the values of wla are not dependent on A and on A, while
W2R
is depending on both [4]. The pre-resonance condition of the incident laser
frequency with those of the doping induced electronic transitions relative to different length doped segments is responsible of the change of W2a with A, and consequently of its change with A [4]. REFERENCES [1] H.E.Schaffer, R.H.Friend and A.J.Heeger Phys. Rev. B ~6 (1987) 7537 [2] K.Pihler and G.Leising ~
(1990) 533
[3] G.P.Brivio and E.Mula~zi Solid State ~omm. 60 (1986) 203 [4] E.Mulazzi et al. Synth.Met. 24(1988)365 ; E.Mulaszi et al. Svnth.Met. 28(1989)D323 [5] G.P.Brivio and E.Mula~zi Phys. Rev. B ~0 (1984) 676