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Perturbation of the nonlinear optical response of a conjugated polymer by an adsorbate-induced electronic state
Volume 177, number 3
CHEMICAL PHYSICS LETTERS
22 February 1991
Perturbation of the nonlinear optical response of a conjugated polymer by an adsorbate-induced electronic state G.J. Blanchard and J.P. Heritage Bell Communications Research Inc.. 331 Newman Springs Road, Red Bank, NJO7701, USA
Received 24 September 1990;in final form I9 November 1990
The coupling between ground state vibrations and excited electronic states in the polydiacetylene PTS has been investigated using stimulated inverse Raman scattering. Both Raman resonances and optical Stark effects have been measured and the interference between them has been used to determine several exciton-phonon coupling constants. An unexpected narrow (7.5 meV half-width) electronic feature at s 2.23 eV in PTS is observed. An even parity exciton in this spectral region has been postulated before by others. Our work demonstrates that this new electronic feature is resident on the pnlymer backbone, the same as the odd-parity exciton. This new feature must also possess some odd-parity character. The cause of this odd-parity contribution is likely a complex between molecular oxygen and the diacetylenic backbone.
1. Introduction Solid-state systems possessing large third-order optical nonlinearities have been examined extensively because they are potentially useful for optical signal processing applications [ 1,21. Two classes of materials have received the most attention: conjugated polymers and GaAs/AlGaAs multiple quantum well structures (MQWS). While the understanding of the nonlinear optical response of III-V systems is, in general, more complete than it is for conjugated polymers, these latter materiais are more amenable to chemical modification and thus continue to attract substantial research effort. We will focus here on one type of conjugated polymer,.a crystalline polydiacetylene. Polydiacetylenes possess large nonresonant xC3) values and have useful physical properties (solubility, crystallinity) which are determined by the identity of the side-group functionalities. For PTS, a polydiacetylene with p-toluene sulfonate side groups and a highly crystalline structure, Ix”’ 1has been measured in the range 5 X lo-” esu 220 meV below the exciton resonance to 9 x 1Oe9 esu 8 1 meV below resonance [ 3 1. In addition to potential applications in optical signal processing, these large nonlinearities can be used to reveal certain fundamental spectroscopic properties of these mate-
rials. We report here on a series of two-color experiments designed to probe Irn{X(”(o,,; co,,to_,, copr)} and thereby understand more completely the coupling between the electronic (including vibronic) and vibrational states of PTS. Both the off-resonance and on-resonance nonlinear optical properties of PTS have been studied and modeled very well over several relatively narrow spectral ranges [ 4-7 1. The on-resonance response is described well by the phase-space filing formalism [4] and the off-resonance response, in the regions studied, is modeled well using Schmitt-Rink’s theory of phonon-mediated optical nonlinearities in a coupled three-level system [ 81. In PTS, the off-resonance nonlinearities are dominated by strong coupling between the exciton and ground state phonons. This coupling is probed directly and sensitively by resonance Raman scattering [9 ]_ We have used stimulated inverse Raman scattering, a technique where both w, and ws are provided as coherent fields, wpumpc w,,,,, and IDump > I,,,. Inverse Raman spectroscopy is similar in concept to stimulated Raman loss spectroscopy and is more convenient to use than spontaneous resonance Raman measurements for optically dense materials. We use different exciton sidebands in the ‘B, exciton manifold as Raman intermediate states and measure the displace-
0009-2614/91/$ 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
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ment between the ground-state and excitonic potential wells by comparing the sizes of the nonlinear response for a particular phonon and several different intermediate states. We noted previously [ 6,7] three principal energy-dependent contributions to our spectra: ( 1) inverse Raman scattering, dominant when the probe is off-resonance; (2) the phononmediated optical Stark effect, dominant when the probe is on-resonance, and (3) the excitonic or twolevel optical Stark effect, small in all cases because of the large detuning of the pump from the exciton resonance. We achieve excellent agreement between experiment and theory [6,7] except when the probe is tuned near 2.2 eV. The agreement can be recovered only when we add a spectral feature at a2.23 eV which is k: l/4 the width of the ‘B, exciton. This feature does not correspond to any sideband of the ‘B, exciton and is narrower than the lB,, exciton; we believe it to be what has been previously termed the ‘$ exciton [ 11. Batchelder and co-workers have reported a spectral feature at 2.25 eV in PTS which arises from a charge-transfer complex between molecular oxygen and the polydiacetylene backbone [ 10,111 but its symmetry assignment remains unclear [lo]. Our observation of this feature demonstrates that it must possess at least some odd-parity character. In addition, we observe strong coupling of ground-state phonons to this line, indicating that this electronic feature, and, hence, the adsorbed molecular oxygen, is resident on the polymer backbone instead of the side groups.
2. Experimental We have discussed our experimental configuration in detail before [ 121. A mode-locked Ar+ laser (Spectra Physics model 171-09) pumps synchronously two dye lasers (Coherent model 701-3). The pump dye laser is operated in the range 1.476- 1.687 eV using LDS-821, LDS-751 or LDS-722 dye (Exciton). The probe dye laser is operated in the range 1.968-2.260 eV using rhodamine 610, rhodamine 6G or disodium fluorescein ( Exciton) . For all measurements the pump peak intensity is 10’W/cm’ and the probe peak intensity is lo5 W/cmZ, the same as in previous stimulated inverse Raman experiments re288
22Febrwy
1991
ported elsewhere [ 6,7]. All inverse Raman signals reported here follow the instrumental cross-correlation function (10 ps fwhm), and all measurements were made in air. Thin samples of poly- [ 2,4-hexadiyn-1,6diol-bis(ptoluene sulfonate ) 1, PTS, were polymerized thermally between quartz plates [ 131. We estimate the thickness of the crystals we used in this series of experiments to be 700 A [ 71. While there is thickness variation between individual crystals, apparent both optically and via electron microscopy, we have approximated a single sample thickness for use in our calculations. This has been demonstratedas a valid approximation before [ 6,7 1.
3. Results and discussion We have used stimulated inverse Raman scattering to measure the two-color nonlinear optical response in PTS, a system possessing strong excitonphonon coupling. The two-color experiments are designed to detect interference between the inverse Raman effect and the phonon-mediated optical Stark effect [6,7]. Measurement of the degree and character of this interference allows determination of exciton-phonon coupling constants. The phonon-mediated optical Stark effect dominates the nonlinear response of PTS when w,,~~ is near an electronic resonance and the pump-probe detuning corresponds to a vibrational resonance. The signature of the phonon-mediated optical Stark effect is a transmissive response due to splitting of the exciton line when upvmp is in resonance with the excitonc phonon transition. In spectral regions where the phonon-mediated optical Stark effect does not contribute significantly, the inverse Raman vibrational resonances appear as absorptive features. Interference between the inverse Raman response and the phonon-mediated optical Stark effect in spectral regions where both effects contribute is determined by the coupling between the various states (transition cross sections). We model this interference phenomenon using Schmitt-Rink’s theory of phonon-mediated optical nonlinearities [8]; the excitonc phonon transition cross sections are considered adjustable parameters to be optimized. The theory of phonon-mediated optical nonlin-
Volume 177, number 3
22 February 1991
CHEMICAL PHYSICS LETTERS
earities describes the optical response of a coupled three-level system. We have shown this theory to be in excellent agreement with experimental stimulated inverse Raman data for PTS [ 6,7 1. Inclusion of more than one phonon or exciton in the calculation is accomplished by simple linear superposition. In earlier experiments [ 71 we observed that coupling between ground-state combination mode phonons and the excited-state C=C stretching mode is stronger than coupling to the v=O exciton. This is expected from overlap integral calculations; comparison of the transition cross sections for these two families of transitions offers a means of determining the displacement between the ground state and excitonic potential wells. We have observed this enhancement in ImCyt3’) experimentally for a series of combination modes involving the (ground-state) C=C stretch [ 71 and report here the same effect in figs. la and lb for the 20, and w1+3+4 phonons. (This phonon numbering scheme follows Batchelder and Bloor’s original designations [ 9 1.) The v= 0 exciton is used as the Raman intermediate state in fig. 1a and the excited-state C=C stretch is the intermediate state in fig. lb. The best-fit cross sections for these and several other transitions are presented in table 1. In general, the ground-state vibrations are coupled more strongly to the excited-state C=C stretch than to the excited-state C=C stretch. Perhaps surprisingly, this is true for the ground-state C=C stretching mode combinations and overtones as well (table 1). From the cross sections which we report here we can determine the potential well displacements. Using the 204 mode as an example, we obtain the potential well displacement [ 7,9] DxO.0022 nm for the C-C excited-state well and 0~~0.0036 nm for the C=C excited-state well. In order to model these data accurately, more than one excitonic resonance needs to be considered. The exciton manifold can be modeled as the sum of twelve exciton sidebands, each weighted appropriately according to its Franck-Condon factor [ 91. We obtain good agreement with the experimental spectrum near the v= 0 exciton at x 2 eV and for the lower-energy vibrational sidebands. At higher energy, however, there appears to be a “tail” or background in the experimental data not accounted for by this calculation [5,9]. Thus we expect best agreement between ex-
a
57 2-
-2 -
3w2
up= 1.476eV
-3 1.90
“.
“.
1.95
.
“.
2.00
‘.
2.05
.’
2.10
Yfxob@ lev)
25
o - 1.642eV
m-15
I0 = 10 t G5
2.20 Opmbe
2.25
2.30
(*'
Fig. 1. (a) Stimulated inverse Raman spectnrmof PTS for w,= 1.476eV. (b) Stimulated inverse Raman spectrum for o,= 1.642eV.
periment and theory in the region of the v=O exciton, and that the quality of the agreement will decrease as higher exciton sidebands are used as intermediate states. We observe this trend in our fits of the model to the experimental data. The calculated inverse Raman spectra for probe energies near 2.2 eV are dominated by three eleo tronic intermediate states. Two of these resonances correspond to vibrational sidebands of the ‘B, exciton; the excited-state C=C stretch at 2.1601 eV and the excited state C=C stretch at 2.237 eV. Both are modeled as a Lorentzian line with 2y,= 60 meV, the same linewidth as the u=O exciton. The third resonance in this region is much narrower (2y= 15 meV) than the ‘B, exciton sidebands and is centered at 2.2275 + 0.0010 eV. We find that the inclusion of this feature is necessary in order to achieve agreement with our data. We present fits of the theory to the 289
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CHEMICAL PHYSICS LETTERS
Table 1 Selected transition cross sections for overtone and combination modes in PTS. The mode numbering scheme is consistent with ref. [ 91. The cross sections are best fit values obtained from the model of phonon-mediated optical nonlinearities. The uncertainty in these determinations is estimated to be k 10%.See text for a discussion. - indicates that the cross section for this transition was not determined Mode w
Energy (eV)
00~10
W)
~Oov-ll (ev)
bnev
exciton
C=C str. exciton
C=C str. exciton
1+1 2t2 2+2+2
0.5173 0.3680 0.5523
0.09 0.12 0.16
0.75 0.55 1.40
0.10 0.55 0.70
1+3+4 3t3t2
0.5259 0.4825
0.09
0.55 0.90
0.05 0.10
data both with and without this latter feature included. Fig. 2a shows the agreement without this sharp feature and fig. 2b shows the agreement with the feature included. There are two important dif-
a
'I.10
2.15
2.20
2.25
2.50
25 20
b
0 I
2w3+w2j
-5. "P"*687'v. -10 2.10 2.13
I
; ?+3+4 . 2.20
T
,
.
2.25
, 2.30
uprobe (ev)new feature
Fig. 2. (a) Fit of theory to data without inclusion of a state at 2.2275 eV. (b) Fit of theory to data including the state at 2.2275 eV. See text for a discussion.
290
(W
new state
tl=o
1.25 _ 0.75
ferences between the two calculated spectra: ( 1) the 20, and the ol +3+4lines are calculated to be transmissive in fig. 2a but are observed experimentally to be absorptive: (2) the feature observed experimentally near 2.23 eV is not reproduced in calculations shown in fig. 2a. Adding the narrow feature at 2.2275 eV not only restores the calculated 20, and w~+~+~ lines to an absorptive shape but also generates a feature near 2.23 eV, corresponding to the excitonic (two-level) optical Stark effect on this line [ 141. We note that even with the inclusion of this line we do not obtain exact agreement with the data. We believe that this is because of our approximation of this feature as a single Lorentzian line and also that we do not account for the band tail beneath the ‘B, exciton resonance (vide ante). We now turn our attention to the physical grounds for including this narrow line in our calculation. Electroabsorption experiments have identified an excitonic feature in PTS at 2.39 eV, coincident with the onset of the conduction band. Bloor et al. [ 1 ] have noted that this 2.39 eV feature is a vibrational sideband of a ‘A, exciton whose origin is near 2.2 eV. If it is this excitonic feature which we observe experimentally then it must possess some odd-parity character. The state located at 2.39 eV, and consequently the one near 2.2 eV, has not been identified positively as a ‘$ state [ lo] and additionally there are ‘B. exciton sidebands near 2.2 eV with which the narrow feature can mix. Thus it is not clear whether this state, located at 2.2275 eV, is of predominantly odd parity or even parity. This new electronic feature is, however, substantially narrower than the ‘B, exciton and is best tit with a Franck-Condon inten-
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CHEMICAL PHYSICS LETTERS
sity factor which is within 10% of the C=C stretching exciton-sideband Franck-Condon factor [ 9 I. Despite the prominence of this feature in the inverse Raman spectra we fail to observe it with linear absorption measurements, consistent with its mixed parity character and spectral congestion around 2.2 eV. If this is a ‘Apstate, then it is possible that we are accessing different (B,) vibrations than are allowed using the ‘B, exciton as the intermediate state. This possibility is suggested by the data for w, = 1.687 eV, where there is a strong absorptive resonance at 2.20 eV (see fig. 2). This resonance occurs at Aw=O.5130 eV, whereas 20,=0.5173 eV and 0,+~+~=0.5259 eV. We observe experimentally, however, that the 2+, and w1+3+4modes are coupled to this new feature. The couplings between these ground-state phonons and the new state are strong and of the same order as the couplings to the v= 1 ‘B, exciton reported earlier (see table 1) [ 6,7]. The observation that some of the vibrations are coupled to both the new electronic state and the ‘B, exciton is compelling evidence for at least some odd-parity character in the new state. It also demonstrates that both the ‘B, exciton and this new feature occupy the same physical region of the polymer backbone. Batchelder and collaborators [ 10,11,15 ] have used surface enhanced as well as spontaneous resonance Raman spectroscopy to study the effect of adsorbed molecular oxygen on the spectroscopy of PTS. There is a maximum in the excitation profile of the molecular oxygen vibration at 2.39 eV [ 151 coincident with the onset of the conduction band in PTS, and another maximum near 2.25 eV [ 111. The maxima in this excitation profile are thought to arise from an adsorbate-induced symmetry perturbation of an even-parity exciton and a charge-transfer complex between the molecular oxygen and the PTS backbone is postulated [ 11,151. The formation of such a complex has been shown to give rise to small blueshifts in the C=C and C=C stretching frequencies. The size and direction of these frequency shifts depend on the particular backbone vibration involved and its relationship to the adsorbed oxygen. We observe spectral shifts in our data also (see fig. 2 ) . The shifts Seen in the data reported here are either to the red or blue, depending on the specific vibrational resonance. In addition, these same data contain some
22 February 199I
information as to the penetration depth of the oxygen into the sample. The spectra which we report here are transmission measurements. The sample thickness of 700 k, suggeststhat surface effects would not contribute substantially to the experimental signal [ 6 1. Thus our observation of a prominent feature at x 2.23 eV indicates that molecular oxygen has somehow penetrated the sample. This finding is in agreement with recent resonance Raman measurements of PTS, although it is not clear whether the oxygen penetration is actually into the lattice or only into “micro-cracks” in the individual crystallites [ 111.
4. Conclusions We have studied the stimulated inverse Raman spectroscopy of the polydiacetylene PTS in a spectral region where the ‘B, exciton manifold fails to describe quantitatively the electronic details of this material. We detect a spectral feature at ~2.23 eV which is significantly sharper than the ‘B, exciton sidebands. This electronic feature is located at an energy where a ‘A8exciton has been detected before [ I 1. If this is the feature we observe, then its assignment as a ‘$ state must be called into question: We would not observe it if it were of purely even parity. At present our experiments are not capable of determining the origin of the odd-parity contribution to this new state, but a symmetry perturbation of the ‘$ state due to a charge-transfer complex between adsorbed molecular oxygen and the polydiacetylene backbone is the likely explanation. Our work reveals previously unknown details regarding the nature and localization of this molecular complex.
Acknowledgement We would like to thank Professor D.N. Batchelder for communicating his work to us prior to its publication, Dr. S. Etemad for several insightful and stimulating discussions and Dr. G.L. Baker for providing the samples. 291
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References [ 1] D. Bloor, D,J, Ando, P.A. Norman, J.S. Obhi, P.V. Kolinsky and B. Mow&an. Physics Scripta T 19 (1987) 226, and references therein. [2] S. Schmitt-Rink, D.S. Chemla and D.A.B. Miller, Advan Phys. 38 ( 1989) 89, and references therein. [3] G,M. Carter, M.K.Thakur, Y.J. Chen and J.V. Hryniewicz, Appl. Phys. Letters47 (1985) 457. [4] B.I. Greene, J. Orenstein, R.R. Millard and L.R. Williams, Phys. Rev. Letters 58 (1987) 2750. [ 5lB.I. Greene, J.F. Mueller, J. Orenstein, D.H. Rapkine, S. Schmitt-Rink and M. Thakur, Phys. Rev. Letters 61 (1988) 325. [6] G.J.Blanchard, J.P. Heritage,A.C. vonLelnnen,M.K Kelly, G.L. Baker and S. Etemad, Phys. Rev. Letters 63 (1989) 887.
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[7] G.J.Blanchardand J.P. Heritage, J. Chem. Phys. 93 (1990) 4377. [ 81B.I. Greene, J. Orenstein, S. Schmitt-Rink and M.K. Thakur, in: Proceedings of the NATO Workshop on Optical Switching in Low-Dimensional Systems, Marbella, Spain, 1988,cds. H. Haug and L. Banyal,NATO Advanced Studies Institutes, Ser. B, Vol. 194 (Plenum Press, NewYork,1989). [ 91 D.N. Batchelder and D. Bloor, J. Phys. C 15 (1982) 3005. [lo] N.J. Poole and D.N. Batchelder, Mol. Cryst. Liquid Cryst. 105 (1984) 55. [ 111B.J.E. Smith and D.N. Batchelder, Polymer, in preparation. 1121G.J. Blanchard, J. Chem. Phys. 87 (1987) 6802. [ 131M.K. Thakur and SE. Meyler, Macromolecules 18 (1985) 2341. [ 14] A. von Lehmen, D.S. Chemla, J.E. Zucker and JP. Heritage, Opt, Letters 11 ( 1986) 609. [ 151D.N. Bat&elder, N.J. Poole and D. Bloor, C&an. Phys. Letters 81 (1981) 560.
CHEMICAL PHYSICS LETTERS
22 February 1991
Perturbation of the nonlinear optical response of a conjugated polymer by an adsorbate-induced electronic state G.J. Blanchard and J.P. Heritage Bell Communications Research Inc.. 331 Newman Springs Road, Red Bank, NJO7701, USA
Received 24 September 1990;in final form I9 November 1990
The coupling between ground state vibrations and excited electronic states in the polydiacetylene PTS has been investigated using stimulated inverse Raman scattering. Both Raman resonances and optical Stark effects have been measured and the interference between them has been used to determine several exciton-phonon coupling constants. An unexpected narrow (7.5 meV half-width) electronic feature at s 2.23 eV in PTS is observed. An even parity exciton in this spectral region has been postulated before by others. Our work demonstrates that this new electronic feature is resident on the pnlymer backbone, the same as the odd-parity exciton. This new feature must also possess some odd-parity character. The cause of this odd-parity contribution is likely a complex between molecular oxygen and the diacetylenic backbone.
1. Introduction Solid-state systems possessing large third-order optical nonlinearities have been examined extensively because they are potentially useful for optical signal processing applications [ 1,21. Two classes of materials have received the most attention: conjugated polymers and GaAs/AlGaAs multiple quantum well structures (MQWS). While the understanding of the nonlinear optical response of III-V systems is, in general, more complete than it is for conjugated polymers, these latter materiais are more amenable to chemical modification and thus continue to attract substantial research effort. We will focus here on one type of conjugated polymer,.a crystalline polydiacetylene. Polydiacetylenes possess large nonresonant xC3) values and have useful physical properties (solubility, crystallinity) which are determined by the identity of the side-group functionalities. For PTS, a polydiacetylene with p-toluene sulfonate side groups and a highly crystalline structure, Ix”’ 1has been measured in the range 5 X lo-” esu 220 meV below the exciton resonance to 9 x 1Oe9 esu 8 1 meV below resonance [ 3 1. In addition to potential applications in optical signal processing, these large nonlinearities can be used to reveal certain fundamental spectroscopic properties of these mate-
rials. We report here on a series of two-color experiments designed to probe Irn{X(”(o,,; co,,to_,, copr)} and thereby understand more completely the coupling between the electronic (including vibronic) and vibrational states of PTS. Both the off-resonance and on-resonance nonlinear optical properties of PTS have been studied and modeled very well over several relatively narrow spectral ranges [ 4-7 1. The on-resonance response is described well by the phase-space filing formalism [4] and the off-resonance response, in the regions studied, is modeled well using Schmitt-Rink’s theory of phonon-mediated optical nonlinearities in a coupled three-level system [ 81. In PTS, the off-resonance nonlinearities are dominated by strong coupling between the exciton and ground state phonons. This coupling is probed directly and sensitively by resonance Raman scattering [9 ]_ We have used stimulated inverse Raman scattering, a technique where both w, and ws are provided as coherent fields, wpumpc w,,,,, and IDump > I,,,. Inverse Raman spectroscopy is similar in concept to stimulated Raman loss spectroscopy and is more convenient to use than spontaneous resonance Raman measurements for optically dense materials. We use different exciton sidebands in the ‘B, exciton manifold as Raman intermediate states and measure the displace-
0009-2614/91/$ 03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)
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ment between the ground-state and excitonic potential wells by comparing the sizes of the nonlinear response for a particular phonon and several different intermediate states. We noted previously [ 6,7] three principal energy-dependent contributions to our spectra: ( 1) inverse Raman scattering, dominant when the probe is off-resonance; (2) the phononmediated optical Stark effect, dominant when the probe is on-resonance, and (3) the excitonic or twolevel optical Stark effect, small in all cases because of the large detuning of the pump from the exciton resonance. We achieve excellent agreement between experiment and theory [6,7] except when the probe is tuned near 2.2 eV. The agreement can be recovered only when we add a spectral feature at a2.23 eV which is k: l/4 the width of the ‘B, exciton. This feature does not correspond to any sideband of the ‘B, exciton and is narrower than the lB,, exciton; we believe it to be what has been previously termed the ‘$ exciton [ 11. Batchelder and co-workers have reported a spectral feature at 2.25 eV in PTS which arises from a charge-transfer complex between molecular oxygen and the polydiacetylene backbone [ 10,111 but its symmetry assignment remains unclear [lo]. Our observation of this feature demonstrates that it must possess at least some odd-parity character. In addition, we observe strong coupling of ground-state phonons to this line, indicating that this electronic feature, and, hence, the adsorbed molecular oxygen, is resident on the polymer backbone instead of the side groups.
2. Experimental We have discussed our experimental configuration in detail before [ 121. A mode-locked Ar+ laser (Spectra Physics model 171-09) pumps synchronously two dye lasers (Coherent model 701-3). The pump dye laser is operated in the range 1.476- 1.687 eV using LDS-821, LDS-751 or LDS-722 dye (Exciton). The probe dye laser is operated in the range 1.968-2.260 eV using rhodamine 610, rhodamine 6G or disodium fluorescein ( Exciton) . For all measurements the pump peak intensity is 10’W/cm’ and the probe peak intensity is lo5 W/cmZ, the same as in previous stimulated inverse Raman experiments re288
22Febrwy
1991
ported elsewhere [ 6,7]. All inverse Raman signals reported here follow the instrumental cross-correlation function (10 ps fwhm), and all measurements were made in air. Thin samples of poly- [ 2,4-hexadiyn-1,6diol-bis(ptoluene sulfonate ) 1, PTS, were polymerized thermally between quartz plates [ 131. We estimate the thickness of the crystals we used in this series of experiments to be 700 A [ 71. While there is thickness variation between individual crystals, apparent both optically and via electron microscopy, we have approximated a single sample thickness for use in our calculations. This has been demonstratedas a valid approximation before [ 6,7 1.
3. Results and discussion We have used stimulated inverse Raman scattering to measure the two-color nonlinear optical response in PTS, a system possessing strong excitonphonon coupling. The two-color experiments are designed to detect interference between the inverse Raman effect and the phonon-mediated optical Stark effect [6,7]. Measurement of the degree and character of this interference allows determination of exciton-phonon coupling constants. The phonon-mediated optical Stark effect dominates the nonlinear response of PTS when w,,~~ is near an electronic resonance and the pump-probe detuning corresponds to a vibrational resonance. The signature of the phonon-mediated optical Stark effect is a transmissive response due to splitting of the exciton line when upvmp is in resonance with the excitonc phonon transition. In spectral regions where the phonon-mediated optical Stark effect does not contribute significantly, the inverse Raman vibrational resonances appear as absorptive features. Interference between the inverse Raman response and the phonon-mediated optical Stark effect in spectral regions where both effects contribute is determined by the coupling between the various states (transition cross sections). We model this interference phenomenon using Schmitt-Rink’s theory of phonon-mediated optical nonlinearities [8]; the excitonc phonon transition cross sections are considered adjustable parameters to be optimized. The theory of phonon-mediated optical nonlin-
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CHEMICAL PHYSICS LETTERS
earities describes the optical response of a coupled three-level system. We have shown this theory to be in excellent agreement with experimental stimulated inverse Raman data for PTS [ 6,7 1. Inclusion of more than one phonon or exciton in the calculation is accomplished by simple linear superposition. In earlier experiments [ 71 we observed that coupling between ground-state combination mode phonons and the excited-state C=C stretching mode is stronger than coupling to the v=O exciton. This is expected from overlap integral calculations; comparison of the transition cross sections for these two families of transitions offers a means of determining the displacement between the ground state and excitonic potential wells. We have observed this enhancement in ImCyt3’) experimentally for a series of combination modes involving the (ground-state) C=C stretch [ 71 and report here the same effect in figs. la and lb for the 20, and w1+3+4 phonons. (This phonon numbering scheme follows Batchelder and Bloor’s original designations [ 9 1.) The v= 0 exciton is used as the Raman intermediate state in fig. 1a and the excited-state C=C stretch is the intermediate state in fig. lb. The best-fit cross sections for these and several other transitions are presented in table 1. In general, the ground-state vibrations are coupled more strongly to the excited-state C=C stretch than to the excited-state C=C stretch. Perhaps surprisingly, this is true for the ground-state C=C stretching mode combinations and overtones as well (table 1). From the cross sections which we report here we can determine the potential well displacements. Using the 204 mode as an example, we obtain the potential well displacement [ 7,9] DxO.0022 nm for the C-C excited-state well and 0~~0.0036 nm for the C=C excited-state well. In order to model these data accurately, more than one excitonic resonance needs to be considered. The exciton manifold can be modeled as the sum of twelve exciton sidebands, each weighted appropriately according to its Franck-Condon factor [ 91. We obtain good agreement with the experimental spectrum near the v= 0 exciton at x 2 eV and for the lower-energy vibrational sidebands. At higher energy, however, there appears to be a “tail” or background in the experimental data not accounted for by this calculation [5,9]. Thus we expect best agreement between ex-
a
57 2-
-2 -
3w2
up= 1.476eV
-3 1.90
“.
“.
1.95
.
“.
2.00
‘.
2.05
.’
2.10
Yfxob@ lev)
25
o - 1.642eV
m-15
I0 = 10 t G5
2.20 Opmbe
2.25
2.30
(*'
Fig. 1. (a) Stimulated inverse Raman spectnrmof PTS for w,= 1.476eV. (b) Stimulated inverse Raman spectrum for o,= 1.642eV.
periment and theory in the region of the v=O exciton, and that the quality of the agreement will decrease as higher exciton sidebands are used as intermediate states. We observe this trend in our fits of the model to the experimental data. The calculated inverse Raman spectra for probe energies near 2.2 eV are dominated by three eleo tronic intermediate states. Two of these resonances correspond to vibrational sidebands of the ‘B, exciton; the excited-state C=C stretch at 2.1601 eV and the excited state C=C stretch at 2.237 eV. Both are modeled as a Lorentzian line with 2y,= 60 meV, the same linewidth as the u=O exciton. The third resonance in this region is much narrower (2y= 15 meV) than the ‘B, exciton sidebands and is centered at 2.2275 + 0.0010 eV. We find that the inclusion of this feature is necessary in order to achieve agreement with our data. We present fits of the theory to the 289
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Table 1 Selected transition cross sections for overtone and combination modes in PTS. The mode numbering scheme is consistent with ref. [ 91. The cross sections are best fit values obtained from the model of phonon-mediated optical nonlinearities. The uncertainty in these determinations is estimated to be k 10%.See text for a discussion. - indicates that the cross section for this transition was not determined Mode w
Energy (eV)
00~10
W)
~Oov-ll (ev)
bnev
exciton
C=C str. exciton
C=C str. exciton
1+1 2t2 2+2+2
0.5173 0.3680 0.5523
0.09 0.12 0.16
0.75 0.55 1.40
0.10 0.55 0.70
1+3+4 3t3t2
0.5259 0.4825
0.09
0.55 0.90
0.05 0.10
data both with and without this latter feature included. Fig. 2a shows the agreement without this sharp feature and fig. 2b shows the agreement with the feature included. There are two important dif-
a
'I.10
2.15
2.20
2.25
2.50
25 20
b
0 I
2w3+w2j
-5. "P"*687'v. -10 2.10 2.13
I
; ?+3+4 . 2.20
T
,
.
2.25
, 2.30
uprobe (ev)new feature
Fig. 2. (a) Fit of theory to data without inclusion of a state at 2.2275 eV. (b) Fit of theory to data including the state at 2.2275 eV. See text for a discussion.
290
(W
new state
tl=o
1.25 _ 0.75
ferences between the two calculated spectra: ( 1) the 20, and the ol +3+4lines are calculated to be transmissive in fig. 2a but are observed experimentally to be absorptive: (2) the feature observed experimentally near 2.23 eV is not reproduced in calculations shown in fig. 2a. Adding the narrow feature at 2.2275 eV not only restores the calculated 20, and w~+~+~ lines to an absorptive shape but also generates a feature near 2.23 eV, corresponding to the excitonic (two-level) optical Stark effect on this line [ 141. We note that even with the inclusion of this line we do not obtain exact agreement with the data. We believe that this is because of our approximation of this feature as a single Lorentzian line and also that we do not account for the band tail beneath the ‘B, exciton resonance (vide ante). We now turn our attention to the physical grounds for including this narrow line in our calculation. Electroabsorption experiments have identified an excitonic feature in PTS at 2.39 eV, coincident with the onset of the conduction band. Bloor et al. [ 1 ] have noted that this 2.39 eV feature is a vibrational sideband of a ‘A, exciton whose origin is near 2.2 eV. If it is this excitonic feature which we observe experimentally then it must possess some odd-parity character. The state located at 2.39 eV, and consequently the one near 2.2 eV, has not been identified positively as a ‘$ state [ lo] and additionally there are ‘B. exciton sidebands near 2.2 eV with which the narrow feature can mix. Thus it is not clear whether this state, located at 2.2275 eV, is of predominantly odd parity or even parity. This new electronic feature is, however, substantially narrower than the ‘B, exciton and is best tit with a Franck-Condon inten-
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sity factor which is within 10% of the C=C stretching exciton-sideband Franck-Condon factor [ 9 I. Despite the prominence of this feature in the inverse Raman spectra we fail to observe it with linear absorption measurements, consistent with its mixed parity character and spectral congestion around 2.2 eV. If this is a ‘Apstate, then it is possible that we are accessing different (B,) vibrations than are allowed using the ‘B, exciton as the intermediate state. This possibility is suggested by the data for w, = 1.687 eV, where there is a strong absorptive resonance at 2.20 eV (see fig. 2). This resonance occurs at Aw=O.5130 eV, whereas 20,=0.5173 eV and 0,+~+~=0.5259 eV. We observe experimentally, however, that the 2+, and w1+3+4modes are coupled to this new feature. The couplings between these ground-state phonons and the new state are strong and of the same order as the couplings to the v= 1 ‘B, exciton reported earlier (see table 1) [ 6,7]. The observation that some of the vibrations are coupled to both the new electronic state and the ‘B, exciton is compelling evidence for at least some odd-parity character in the new state. It also demonstrates that both the ‘B, exciton and this new feature occupy the same physical region of the polymer backbone. Batchelder and collaborators [ 10,11,15 ] have used surface enhanced as well as spontaneous resonance Raman spectroscopy to study the effect of adsorbed molecular oxygen on the spectroscopy of PTS. There is a maximum in the excitation profile of the molecular oxygen vibration at 2.39 eV [ 151 coincident with the onset of the conduction band in PTS, and another maximum near 2.25 eV [ 111. The maxima in this excitation profile are thought to arise from an adsorbate-induced symmetry perturbation of an even-parity exciton and a charge-transfer complex between the molecular oxygen and the PTS backbone is postulated [ 11,151. The formation of such a complex has been shown to give rise to small blueshifts in the C=C and C=C stretching frequencies. The size and direction of these frequency shifts depend on the particular backbone vibration involved and its relationship to the adsorbed oxygen. We observe spectral shifts in our data also (see fig. 2 ) . The shifts Seen in the data reported here are either to the red or blue, depending on the specific vibrational resonance. In addition, these same data contain some
22 February 199I
information as to the penetration depth of the oxygen into the sample. The spectra which we report here are transmission measurements. The sample thickness of 700 k, suggeststhat surface effects would not contribute substantially to the experimental signal [ 6 1. Thus our observation of a prominent feature at x 2.23 eV indicates that molecular oxygen has somehow penetrated the sample. This finding is in agreement with recent resonance Raman measurements of PTS, although it is not clear whether the oxygen penetration is actually into the lattice or only into “micro-cracks” in the individual crystallites [ 111.
4. Conclusions We have studied the stimulated inverse Raman spectroscopy of the polydiacetylene PTS in a spectral region where the ‘B, exciton manifold fails to describe quantitatively the electronic details of this material. We detect a spectral feature at ~2.23 eV which is significantly sharper than the ‘B, exciton sidebands. This electronic feature is located at an energy where a ‘A8exciton has been detected before [ I 1. If this is the feature we observe, then its assignment as a ‘$ state must be called into question: We would not observe it if it were of purely even parity. At present our experiments are not capable of determining the origin of the odd-parity contribution to this new state, but a symmetry perturbation of the ‘$ state due to a charge-transfer complex between adsorbed molecular oxygen and the polydiacetylene backbone is the likely explanation. Our work reveals previously unknown details regarding the nature and localization of this molecular complex.
Acknowledgement We would like to thank Professor D.N. Batchelder for communicating his work to us prior to its publication, Dr. S. Etemad for several insightful and stimulating discussions and Dr. G.L. Baker for providing the samples. 291
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References [ 1] D. Bloor, D,J, Ando, P.A. Norman, J.S. Obhi, P.V. Kolinsky and B. Mow&an. Physics Scripta T 19 (1987) 226, and references therein. [2] S. Schmitt-Rink, D.S. Chemla and D.A.B. Miller, Advan Phys. 38 ( 1989) 89, and references therein. [3] G,M. Carter, M.K.Thakur, Y.J. Chen and J.V. Hryniewicz, Appl. Phys. Letters47 (1985) 457. [4] B.I. Greene, J. Orenstein, R.R. Millard and L.R. Williams, Phys. Rev. Letters 58 (1987) 2750. [ 5lB.I. Greene, J.F. Mueller, J. Orenstein, D.H. Rapkine, S. Schmitt-Rink and M. Thakur, Phys. Rev. Letters 61 (1988) 325. [6] G.J.Blanchard, J.P. Heritage,A.C. vonLelnnen,M.K Kelly, G.L. Baker and S. Etemad, Phys. Rev. Letters 63 (1989) 887.
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[7] G.J.Blanchardand J.P. Heritage, J. Chem. Phys. 93 (1990) 4377. [ 81B.I. Greene, J. Orenstein, S. Schmitt-Rink and M.K. Thakur, in: Proceedings of the NATO Workshop on Optical Switching in Low-Dimensional Systems, Marbella, Spain, 1988,cds. H. Haug and L. Banyal,NATO Advanced Studies Institutes, Ser. B, Vol. 194 (Plenum Press, NewYork,1989). [ 91 D.N. Batchelder and D. Bloor, J. Phys. C 15 (1982) 3005. [lo] N.J. Poole and D.N. Batchelder, Mol. Cryst. Liquid Cryst. 105 (1984) 55. [ 111B.J.E. Smith and D.N. Batchelder, Polymer, in preparation. 1121G.J. Blanchard, J. Chem. Phys. 87 (1987) 6802. [ 131M.K. Thakur and SE. Meyler, Macromolecules 18 (1985) 2341. [ 14] A. von Lehmen, D.S. Chemla, J.E. Zucker and JP. Heritage, Opt, Letters 11 ( 1986) 609. [ 151D.N. Bat&elder, N.J. Poole and D. Bloor, C&an. Phys. Letters 81 (1981) 560.