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Femtosecond impulsive vibrational spectroscopy in conjugated polymers☆
MOLSTR 11234
Journal of Molecular Structure 521 (2000) 261–270 www.elsevier.nl/locate/molstruc
Femtosecond impulsive vibrational spectroscopy in conjugated polymers 夽 G. Cerullo a,b, G. Lanzani c, L. Pallaro b, S. De Silvestri a,b,* a
INFM, Dipartimento di Fisica del Politecnico, Piazza L. da Vinci 32, 20133 Milan, Italy b Centro di Elettronica Quantistica e Strumentazione Elettronica-C.N.R., Milan, Italy c INFM-Istituto di Matematica e Fisica, Universita’ di Sassari, Sassari, Italy Received 5 April 1999; accepted 26 July 1999
Abstract We apply the impulsive coherent vibrational spectroscopy technique to the study of the vibrational structure of conjugated polymers and oligomers. Molecular vibrations are directly observed in the time domain using tunable 10 fs light pulses from a recently developed optical parametric amplifier. We present results obtained with the prototypical systems sexithiophene, polidiacetylene and poly( p-phenylenevinylene), observing modes with frequencies up to 2100 cm ⫺1 (16 fs period). We directly measure vibrational dephasing times and gain hints on the potential energy surface anharmonicity. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Conjugated polymers; Vibronic structure; Femtosecond spectroscopy
1. Introduction Resonant Raman scattering (RRS) is a technique widely employed to study the optical transition and the electronic properties of p-conjugated polymers. This technique is used to investigate fundamental issues, such as electron–phonon coupling, disorder and nonlinear optics. The former plays a major role in the photophysics of semiconducting polymers, due to the large nuclear re-adjustment of the carbon backbone upon photoexcitation. p-electrons are strongly coupled to only a few normal modes of the carbon 夽
In honour of Professor Giuseppe Zerbi on the occasion of his 65th birthday. * Corresponding author. INFM, Dipartimento di Fisica del Politecnico, Piazza L. da Vinci 32, 20133 Milan, Italy. Tel.: ⫹39-0223996151; fax: ⫹39-02-23996126. E-mail address: [email protected] (S. De Silvestri).
chain, which have the right symmetry to induce dimerization. The coupling of p electrons to these modes drives the dimerization that contributes to open a gap in the electronic density of states. The gap is thus modulated by total-symmetric vibrations of the dimerization amplitude. The phenomenon can be described by the effective conjugation coordinate (ECC) model [1,2], which identifies the coupled modes as those showing dispersion with the effective conjugation force constant. RRS strongly enhances total symmetric modes, and, in addition, those modes that contain a sizable contribution from the ECC coordinate. Consequently, the theory predicts that RRS spectra contain a few strongly enhanced lines, which can be used as the probe of the electronic structure. The same modes are in principle present in the vibronic structure of the optical transition too [3]. The experimental observation of the latter, related to the excited state, is however extremely difficult, and
0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00441-X
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Fig. 1. Layout of the experimental setup. LBO, lithium triborate crystal; BBO, b-barium borate crystal; DF, dichroic filter; S, sapphire plate; A, attenuator; IF, interference filter; PD, photodiode.
in most cases impossible, due to the large inhomogeneous broadening caused by chain length distribution. Even in systems with low intrachain disorder and controlled chain length, such as the oligomeric model-compounds, the direct observation of ECC modes in the excited state is very complicated and ambiguous, because of the rich vibronic structure caused by vibronic coupling among the lowest excited states [4]. In this report we discuss the application to conjugated polymers of a rather new spectroscopic technique, namely the Impulsive Coherent Vibrational Spectroscopy (ICVS) [5]. ICVS can be still considered a Raman event, but with several peculiarities: (i) the process is now studied in the time domain; (ii) excitation occurs in a time range shorter than the characteristic time for nuclear motion, in other words this means that the excitation is broad-band, adding in phase several levels of the coupled modes; (iii) the macroscopic observable is the collective coherent vibration of an ensemble of excited molecules (or chains). ICVS has some distinct advantages with respect to
the more conventional spectroscopic techniques, such as resonance Raman and high-resolution absorption and photoluminescence. In the latter, experiments must be performed on highly ordered samples at very low temperatures and their assignment is complicated by the Franck–Condon (FC) activity of overtones, the combination of pure modes, as well as the Herzberg–Teller vibronic coupling between different electronic states. In contrast ICVS can be applied to disordered materials at room temperature, it avoids the problem of overlap of the low frequency modes with the pump line and selectively detects the frequencies of pure total-symmetric modes coupled to the electronic transition. In addition, ICVS provides important information on the vibrational dynamics. It allows measuring the frequencies and damping times of the coupled vibrational modes both in the ground and in the excited state, to discriminate between the contributions of the two states [6] and to measure anharmonicity of the vibrational motion directly in the time domain. By analyzing the probe wavelength dependence of the amplitude and phase of the coupled modes, it provides insight on the structure of the
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Fig. 2. Typical amplified pulse spectra obtained at different wavelengths.
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excited state potential energy surfaces (PES) and, with the support of adequate quantum mechanical modeling, it allows reconstructing its shape [7,8]. In order to perform ICVS, ultrashort pulses, with duration of the order of 10 fs and center frequency in resonance with the optical transition of the sample, are needed. Such extremely short pulses have been obtained in the past using amplified dye lasers [9,10], but the considerable complication and difficulty of operation of these systems limited their extensive use in ultrafast spectroscopy, particularly for organic materials with absorption in the blue-green spectral range [11]. Recently, we developed a new source of ultrashort pulses, the Noncollinear Optical Parametric Amplifier (NOPA) that allows us to easily generate 10 fs pulses tunable in the visible region with an high degree of stability and reproducibility [12,13]. Using this source, we were able to perform ICVS experiments on a number of conjugated systems with both fundamental and applicative interest.
Fig. 3. Autocorrelation traces of the pulses corresponding to the spectra reported in Fig. 2 (dots) together with their best fits assuming a sech 2 pulse shape (solid lines) and the FWHM pulsewidths.
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Fig. 4. Schematic of the two mechanisms involved in the impulsive excitation of ground and excited state oscillations. S0 and S1 are the ground and excited state potential energy surfaces, and the wavepackets are nuclear probability distributions.
In the following paragraphs we will describe in some detail the experimental set-up which allows us to perform ICVS in conjugated systems. Then we will discuss in general the mechanism underlying ICVS and lastly we will present several cases, showing experimental results obtained by our group in prototype conjugated systems.
2. Experimental The experiments were performed using a novel NOPA source, the experimental layout of which is shown in Fig. 1. We start with a conventional amplified Ti:sapphire laser, generating 150 fs pulses at 780 nm at 1 kHz repetition rate. A fraction of the light is frequency doubled in a lithium triborate (LBO) crystal, yielding pulses at 390 nm that are used to pump the NOPA; another small fraction of the light is focused into a thin sapphire plate, to generate a single-filament white light continuum, used to seed the NOPA. Both the pump and the seed beams are focused into a b-barium borate (BBO) crystal, where the parametric amplification takes place. Using a noncollinear geometry with a suitable pump-seed angle ( ⬇ 3.7⬚), the phase-matching angle becomes essentially independent of wavelength over most of the visible region, so that very broad gain bandwidths can be obtained. The amplified pulses
have bandwidths broader than 50 THz and center wavelengths tunable from 490 to 700 nm. Tuning is achieved by varying the pump-seed pulses delay: a typical sequence of amplified pulse spectra is shown in Fig. 2. The pulses have an energy of 1–2 mJ, which is more than enough for spectroscopic applications, and display good amplitude stability, with peak-topeak fluctuations of less than 7%. The amplified pulses are then sent to a pulse compressor, which provides the negative dispersion necessary to cancel the positive frequency chirp introduced by the continuum generation and the amplification processes. The simplest compressor consists of a pair of Brewster-cut fused silica prisms; alternatively, chirped dielectric mirrors, reflecting different frequency components of the incident radiation at different positions within the multilayer structure, can be used [14]. After compression, pulse durations of 10–12 fs can be obtained across the whole tuning range, as shown in Fig. 3. The compressed pulses are sent to a standard setup for pump–probe experiments, also shown in Fig. 1. The delay of the probe beam is controlled by a stepper motor with a time resolution of 0.6 fs; the probe beam is selected with an iris and its differential transmission is measured with a photodiode followed by a lock-in amplifier. Measurements at selected wavelengths are made by passing the probe beam through a 10 nm bandwidth interference filter after it traverses the
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sample. In all the measurements, we checked whether the differential transmission signal was linear with respect to the pulse energy. Since extremely short pulses can be temporally broadened even by small amounts of glass, we used the same setup for performing pump–probe and pulse duration measurements.
3. Impulsive coherent vibrational spectroscopy ICVS can be understood using either a time-domain or an eigenstate picture. In the time domain description, the molecule is excited essentially without nuclear motion, due to the short pump pulse-width. Therefore, it is left in a nonequilibrium position in the excited state and starts to oscillate: we experimentally detect the modulations of the absorption spectrum induced by these oscillations. In the eigenstate description, the short pulse excites in phase many different eigenstates, separated by the oscillator frequency, which form oscillating wavepackets [15,16]. In Fig. 4 it is shown that in our experiments we can detect wavepackets both on the ground and the excited state PES. Pump–probe spectroscopy can be understood as a three field interaction with the sample [17]; two field interactions are with the pump, creating a population which is then interrogated by the probe field. For short pump pulses, excited state oscillations are observed when the two fields in the pump pulse excite a population wavepacket on the excited state PES, which then subsequently oscillates back and forth leaving and returning to the FC region. Ground state oscillations are generated when the first field induces a polarization wavepacket on the excited state PES, which is then allowed to propagate for some time so that the second field brings the wavepacket back down to the ground state, displaced from the hole left behind. In order to significantly excite ground state oscillations, the time between the first and the second interaction must be long enough to allow the polarization wavepacket to propagate on the excited state PES so as to be displaced from the hole in the ground state. The relative contributions of excited and ground state to the oscillatory pattern are strongly dependent on the ratio between pulse length and oscillation period. For very short pump pulses, the pump–probe signal is dominated by excited state
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dynamics, because the polarization wavepacket created by the first field does not move significantly from the FC region upon arrival of the second field. As the pump pulse length increases, the polarization wavepacket has time to propagate further out of the FC region within the pump pulse, thus increasing the amplitude of ground state oscillations. This analysis can be made more quantitative with the help of a numerical solution of the Schro¨dinger equation for two displaced harmonic oscillators [16]. For 10 fs pulses, simulations show that for a two-level system the ground state contribution is negligible for oscillation periods longer than ⬇ 60 fs.
4. Results and discussion In this section, we show results obtained by applying ICVS to three different conjugated systems, namely sexithiophene (T6), polydiacetylene (PDA) and poly( p-phenylenevinylene) (PPV). Each of them is paradigmatic of a class of compounds aimed to peculiar applications. Thiophene oligomers, and T6 in particular, are model compounds for the understanding of the electronic structure and the photophysics of conjugated polymers [18]. In addition, T6 is one of the best candidates for molecular electronic devices, having good transport properties and very high field effect mobility [19]. This is due to the supramolecular organization shown in the solid phase, caused by efficient p–p stacking. PDA can be used to produce very high quality optical waveguides and it is the most promising organic for third order nonlinear optical applications [20]. Its conjugated backbone has alternance of single, double and triple bonds, showing large delocalization and small p–p ⴱ gap. It is unique also from a basic physical point of view, because it is the only polymer semiconductor for which the one-dimensional Wannier exciton picture is commonly accepted. PPV is the most extensively used material in electroluminescent devices, and it is the first for which LEDs were demonstrated [21]. It shows strong emission with high quantum yield even in the solid phase, and under high excitation density a clear phenomena of line narrowing assigned to ASE along the film plane [22]. Blended with high electron affinity
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Fig. 5. Differential transmission of a T6 film at 510 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
molecules, such as fullerene, it shows fast and efficient photoinduced charge transfer, which promise applications in photovoltaic conversion devices [23]. 4.1. Sexithiophene The experiments were performed on polycrystalline T6 films with approximate thickness of 100 nm,
vacuum deposited at standard conditions. The 10 fs pump pulses were centered at 510 nm, close to the first peak of the absorption spectrum of the T6 film. Fig. 5 shows the differential transmission (DT ) of the T6 film vs. pump–probe delay at the probe wavelength of 510 nm. The features at negative and near-zero delays at this and other wavelengths are due to pump-perturbed free induction decay and coherent coupling and thus do not carry any information on the excited state dynamics [24]. For positive delays we observe a negative DT signal, growing in approximately 200 fs, corresponding to photoinduced absorption (PA). The primary photoexcitations in T6 films are tightly bound Frenkel excitons [25]; therefore we attribute the signal to competition of ground-state bleaching and absorption originating from the exciton level. The initial competition between bleaching and PA cannot be due to coherent artifacts because it takes place at time delays exceeding the pulse duration. We interpret this phenomenon as due to exciton intraband relaxation that populates the lower lying states, from where absorption has stronger oscillator strength. The PA signal is modulated by a complex oscillatory pattern, which is caused by the motion of the wavepacket launched by the pump pulse on the multidimensional excited state PES. The inset of Fig. 5 shows the Fourier transform of the oscillatory component of the signal, after a slowly varying background has been subtracted. We clearly identify five modes, at
Fig. 6. Differential transmission of a T6 film at 540 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
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Fig. 7. Differential transmission of a PDA film at 600 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
frequencies v1 112 cm⫺1 ; v2 299 cm⫺1 ; v3 702 cm⫺1 ; v4 1040 cm⫺1 ; and v5 1454 cm⫺1 ; respectively. Measurements at other wavelengths show either bleaching or PA; in all cases, however, the signal is modulated by an oscillatory pattern due to the same
Fig. 8. Differential transmission of a PDA film at 610 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
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five characteristic frequencies. Fig. 6 shows for instance DT at 540 nm, close to the absorption edge in the T6 film; again we observe a PA signal, modulated by an oscillatory pattern, the Fourier transform of which is shown in the inset. The relative intensities of the same five characteristic frequencies depend on the probing wavelength. The reason for this difference is that, by changing the probe wavelength, we probe different regions of the multidimensional excited state potential energy surface [7,8]. Because of the low ground-state absorption cross-section at 540 nm, we assign the rapidly decaying positive signal observed at early times to stimulated emission from the excited state, which is then masked by the growth of PA. In both Figs. 5 and 6 it is shown that the oscillations persist almost undamped for delays longer than 1 ps. The characteristic coherence time decay of the lowest frequency mode v 1, was determined to be 3 ps, corresponding to a linewidth of 3 cm ⫺1. Note that the measured timescale for dephasing of v 1 is considerably longer than that generally assumed for vibrational energy redistribution, which is of the same order of magnitude of the vibrational period (see previous discussion on exciton relaxation). This may indicate that relaxation takes place without destruction of vibrational coherence. Let us now discuss the assignment of the observed vibrational modes. The lowest energy modes at v1 112 cm⫺1 and v2 299 cm⫺1 correspond to the 126 and 297 cm ⫺1 ones proposed in Ref. [4] to interpret the absorption of the single crystal. We therefore assign v 1 and v 2 to coherent motion in the excited PES of vibrational wavepackets built on the corresponding FC manifold. ICVS thus gives complementary information about the role of such modes in the vibronic dynamics and consistently supports their assignment to total symmetric (ag) intramolecular modes. The modes at v3 702 cm⫺1 ; v4 1040 cm⫺1 ; and v5 1454 cm⫺1 correspond to the modes measured by fluorescence and Raman spectroscopy in a T6 single crystal [26] at 699, 1056, and 1460 cm ⫺1, respectively. In agreement with our previous estimate, they can be assigned to ground state modes. We find that higher frequency modes have faster decay, as it is observed for v 5 with t5 0:4 ps: We speculate that modes with larger dispersion are more sensitive to diagonal disorder and thus undergo faster
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Fig. 9. Fourier transforms of the oscillatory component of the signal in Fig. 8 for the time windows: (a) 0–200 fs; and (b) 200–400 fs.
dephasing. Note that we are measuring the dephasing time of the collective vibrational coherence, and not the dephasing time of the polarization of the vibronic transition, which is expected to be much shorter. 4.2. Polydiacetylene The experiments were performed on blends of poly[1,6-bis(3,6-didhexadecyl-N-carbazolyl)-2,4-
Fig. 10. Differential transmission of a PPV film at 580 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
hexadiyne] (polyDCDH-HS) and polyethylene. The excitation pulse was centered at 540 nm, close to the peak of ground state absorption. Differential transmission signals vs. pump–probe delay are shown in Figs. 7 and 8 for the probing wavelengths of 600 and 610 nm, respectively. At both wavelengths we see a positive DT, which we assign to bleaching of the optical transition. A formation time is clearly seen in the temporal traces. We assign it to population build-up at the bottom of the excited state manifold due to thermalization, determining for the first time in a direct way that the time scale for this process is 100 fs. We observe strong coherent excitation in the CyC v1 ⬇ 1500 cm⫺1 and CxC v2 ⬇ 2100 cm⫺1 stretching vibrations. The CxC strectching mode, which has a period of approximately 16 fs, is to our knowledge the highest molecular oscillation frequency ever observed in the time domain. These are the most strongly coupled amongst the eight resonantly enhanced phonon lines [27] in the RRS spectrum of PDA. Interestingly these two modes are the only ones showing phonon-dispersion behavior, i.e. strong shifting of the Raman frequencies upon changing the excitation photon energy in resonance. The latter effect is attributed to disorder in the polymer sample, which generates a distribution of Raman frequencies associated with different electronic energy gaps. The peculiar role of the double and triple stretching modes indicates their higher sensitivity to the electronic wavefunction delocalization. Because of the very short period of these modes, almost comparable to the pulse duration, we can assign them to ground state oscillations. Vibrational dephasing is extremely rapid for the CxC mode, as indicated by the large line-width of the Fourier spectrum. The frequency of this mode increases in time, as shown in Fig. 9, in which the Fourier transforms of different time windows of the oscillatory signal are reported. The coherent excitation process, which leads to ground state wavepacket oscillation, as described in Section 3, is expected to act through a displacive mechanism. This means that initially the wavepacket is placed out of equilibrium at high level in the ground state PES. Frequency change may indicate that the wavepacket is sliding down an anharmonic potential well, with higher frequencies at the bottom. It
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is this anharmonicity, which can be responsible for the quick dephasing of the mode. 4.3. Polyparaphenylene The experiments were performed on spin-coated DMO-PPV films with an optical density of about 0.5 at the first absorption peak. In addition, in this case the pump pulse was centered at 540 nm, close to the absorption peak. The differential transmission signal at the probe wavelength of 580 nm is shown in Fig. 10: we assign it to a superposition of ground state bleaching and stimulated emission. In addition, in this case the signal is modulated by a complex oscillatory pattern, in which the beating of different frequencies is apparent. The Fourier transform of the oscillatory component indicates two dominant modes, with frequencies of 1300 and 1570 cm ⫺1. Measurements performed at different probe wavelengths always show the same two frequencies, although with different intensities and relative weights. Comparison with previous RRS studies of the vibrational structure of PPVs [28] allows us to assign the two frequencies to the trans-vinylene C– C stretching mode (1300 cm ⫺1) and to the ring C–C quadrant stretching mode (1570 cm ⫺1). It is interesting to note that the 1300 cm ⫺1 band shows a double-peaked structure: this can be attributed to differences in the ground and excited state frequencies relative to this vibration, the excited state being, as expected, at a lower frequency compared to the ground state. 5. Conclusions In this work we have presented the first results of the application of ICVS to the study of the vibrational structure of conjugated polymers. This technique allows a direct time-domain observation of molecular vibrations and has several advantages over conventional spectroscopic techniques; however, it requires the availability of extremely short light pulses (⬇10 fs duration) with frequency tunable in the visible spectral range. The recent development of a stable and reliable source of such pulses has now paved the way for a systematic time-domain study of the vibrational dynamics of organic materials. Time-domain techniques provide a wealth of new information on
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the vibronic dynamics, such as direct measurement of vibrational damping and mode anharmonicity. A full exploitation of their potential, however, will require additional work, both theoretical and experimental. In particular, a quantum-mechanical modeling of the pump–probe measurements at different wavelengths will allow us to obtain important information on the shapes and relative positions of the potential energy surfaces in configuration space.
Acknowledgements We are indebted to Dr M. Muccini and Prof. C. Taliani for preparing the T6 sample, to Dr D. Comoretto and Prof. G. Dellepiane for preparing the PDA sample, and to Dr C. Brabec for preparing the PPV sample.
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[16] W.T. Pollard, H.L. Fragnito, J.-Y. Bigot, C.V. Shank, R.A. Mathies, Chem. Phys. Lett. 168 (1990) 239. [17] S. Mukamel, Principles of Nonlinear Optical Spectroscopy, Oxford University Press, New York, 1995. [18] C. Taliani, L.M. Blinov, Adv. Mater. 8 (1998) 353. [19] F. Garnier, G. Horowitz, X. Peng, D. Fichou, Adv. Mater. 2 (1990) 592. [20] T. Kobayashi (Ed.), Relaxation in Polymers, World Scientific, Singapore, 1993. [21] J.H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burn, A. Kraft, A.B. Holmes, Nature 347 (1990) 539. [22] G.J. Denton, N. Tessler, M.A. Stevens, R. Friend, Adv. Mater. 9 (1997) 547.
[23] N.S. Sariciftci, L. Smilowitz, A.J. Heeger, F. Wudl, Science 258 (1992) 1474. [24] C.H. Brito Cruz, J.P. Gordon, P.C. Becker, R.L. Fork, C.V. Shank, IEEE J. Quantum Electron. QE-23 (1997) 4222. [25] G. Lanzani, S. Frolov, P.A. Lane, Z.V. Vardeny, M. Nisoli, S. De Silvestri, Phys. Rev. Lett. 79 (1997) 3066. [26] M. Muccini, E. Lunedei, A. Bree, G. Horowitz, F. Garnier, C. Taliani, J. Chem. Phys. 108 (1998) 7327. [27] L.X. Zheng, R.E. Benner, Z.V. Vardeny, G.L. Baker, Phys. Rev. B 42 (1990) 3235. [28] K.F. Voss, C.M. Foster, L. Smilowitz, D. Mihailovic, S. Askari, G. Srdanov, Z. Ni, S. Shi, A.J. Heeger, F. Wudl, Phys. Rev. B 43 (1991) 5109.
Journal of Molecular Structure 521 (2000) 261–270 www.elsevier.nl/locate/molstruc
Femtosecond impulsive vibrational spectroscopy in conjugated polymers 夽 G. Cerullo a,b, G. Lanzani c, L. Pallaro b, S. De Silvestri a,b,* a
INFM, Dipartimento di Fisica del Politecnico, Piazza L. da Vinci 32, 20133 Milan, Italy b Centro di Elettronica Quantistica e Strumentazione Elettronica-C.N.R., Milan, Italy c INFM-Istituto di Matematica e Fisica, Universita’ di Sassari, Sassari, Italy Received 5 April 1999; accepted 26 July 1999
Abstract We apply the impulsive coherent vibrational spectroscopy technique to the study of the vibrational structure of conjugated polymers and oligomers. Molecular vibrations are directly observed in the time domain using tunable 10 fs light pulses from a recently developed optical parametric amplifier. We present results obtained with the prototypical systems sexithiophene, polidiacetylene and poly( p-phenylenevinylene), observing modes with frequencies up to 2100 cm ⫺1 (16 fs period). We directly measure vibrational dephasing times and gain hints on the potential energy surface anharmonicity. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: Conjugated polymers; Vibronic structure; Femtosecond spectroscopy
1. Introduction Resonant Raman scattering (RRS) is a technique widely employed to study the optical transition and the electronic properties of p-conjugated polymers. This technique is used to investigate fundamental issues, such as electron–phonon coupling, disorder and nonlinear optics. The former plays a major role in the photophysics of semiconducting polymers, due to the large nuclear re-adjustment of the carbon backbone upon photoexcitation. p-electrons are strongly coupled to only a few normal modes of the carbon 夽
In honour of Professor Giuseppe Zerbi on the occasion of his 65th birthday. * Corresponding author. INFM, Dipartimento di Fisica del Politecnico, Piazza L. da Vinci 32, 20133 Milan, Italy. Tel.: ⫹39-0223996151; fax: ⫹39-02-23996126. E-mail address: [email protected] (S. De Silvestri).
chain, which have the right symmetry to induce dimerization. The coupling of p electrons to these modes drives the dimerization that contributes to open a gap in the electronic density of states. The gap is thus modulated by total-symmetric vibrations of the dimerization amplitude. The phenomenon can be described by the effective conjugation coordinate (ECC) model [1,2], which identifies the coupled modes as those showing dispersion with the effective conjugation force constant. RRS strongly enhances total symmetric modes, and, in addition, those modes that contain a sizable contribution from the ECC coordinate. Consequently, the theory predicts that RRS spectra contain a few strongly enhanced lines, which can be used as the probe of the electronic structure. The same modes are in principle present in the vibronic structure of the optical transition too [3]. The experimental observation of the latter, related to the excited state, is however extremely difficult, and
0022-2860/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00441-X
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Fig. 1. Layout of the experimental setup. LBO, lithium triborate crystal; BBO, b-barium borate crystal; DF, dichroic filter; S, sapphire plate; A, attenuator; IF, interference filter; PD, photodiode.
in most cases impossible, due to the large inhomogeneous broadening caused by chain length distribution. Even in systems with low intrachain disorder and controlled chain length, such as the oligomeric model-compounds, the direct observation of ECC modes in the excited state is very complicated and ambiguous, because of the rich vibronic structure caused by vibronic coupling among the lowest excited states [4]. In this report we discuss the application to conjugated polymers of a rather new spectroscopic technique, namely the Impulsive Coherent Vibrational Spectroscopy (ICVS) [5]. ICVS can be still considered a Raman event, but with several peculiarities: (i) the process is now studied in the time domain; (ii) excitation occurs in a time range shorter than the characteristic time for nuclear motion, in other words this means that the excitation is broad-band, adding in phase several levels of the coupled modes; (iii) the macroscopic observable is the collective coherent vibration of an ensemble of excited molecules (or chains). ICVS has some distinct advantages with respect to
the more conventional spectroscopic techniques, such as resonance Raman and high-resolution absorption and photoluminescence. In the latter, experiments must be performed on highly ordered samples at very low temperatures and their assignment is complicated by the Franck–Condon (FC) activity of overtones, the combination of pure modes, as well as the Herzberg–Teller vibronic coupling between different electronic states. In contrast ICVS can be applied to disordered materials at room temperature, it avoids the problem of overlap of the low frequency modes with the pump line and selectively detects the frequencies of pure total-symmetric modes coupled to the electronic transition. In addition, ICVS provides important information on the vibrational dynamics. It allows measuring the frequencies and damping times of the coupled vibrational modes both in the ground and in the excited state, to discriminate between the contributions of the two states [6] and to measure anharmonicity of the vibrational motion directly in the time domain. By analyzing the probe wavelength dependence of the amplitude and phase of the coupled modes, it provides insight on the structure of the
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Fig. 2. Typical amplified pulse spectra obtained at different wavelengths.
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excited state potential energy surfaces (PES) and, with the support of adequate quantum mechanical modeling, it allows reconstructing its shape [7,8]. In order to perform ICVS, ultrashort pulses, with duration of the order of 10 fs and center frequency in resonance with the optical transition of the sample, are needed. Such extremely short pulses have been obtained in the past using amplified dye lasers [9,10], but the considerable complication and difficulty of operation of these systems limited their extensive use in ultrafast spectroscopy, particularly for organic materials with absorption in the blue-green spectral range [11]. Recently, we developed a new source of ultrashort pulses, the Noncollinear Optical Parametric Amplifier (NOPA) that allows us to easily generate 10 fs pulses tunable in the visible region with an high degree of stability and reproducibility [12,13]. Using this source, we were able to perform ICVS experiments on a number of conjugated systems with both fundamental and applicative interest.
Fig. 3. Autocorrelation traces of the pulses corresponding to the spectra reported in Fig. 2 (dots) together with their best fits assuming a sech 2 pulse shape (solid lines) and the FWHM pulsewidths.
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Fig. 4. Schematic of the two mechanisms involved in the impulsive excitation of ground and excited state oscillations. S0 and S1 are the ground and excited state potential energy surfaces, and the wavepackets are nuclear probability distributions.
In the following paragraphs we will describe in some detail the experimental set-up which allows us to perform ICVS in conjugated systems. Then we will discuss in general the mechanism underlying ICVS and lastly we will present several cases, showing experimental results obtained by our group in prototype conjugated systems.
2. Experimental The experiments were performed using a novel NOPA source, the experimental layout of which is shown in Fig. 1. We start with a conventional amplified Ti:sapphire laser, generating 150 fs pulses at 780 nm at 1 kHz repetition rate. A fraction of the light is frequency doubled in a lithium triborate (LBO) crystal, yielding pulses at 390 nm that are used to pump the NOPA; another small fraction of the light is focused into a thin sapphire plate, to generate a single-filament white light continuum, used to seed the NOPA. Both the pump and the seed beams are focused into a b-barium borate (BBO) crystal, where the parametric amplification takes place. Using a noncollinear geometry with a suitable pump-seed angle ( ⬇ 3.7⬚), the phase-matching angle becomes essentially independent of wavelength over most of the visible region, so that very broad gain bandwidths can be obtained. The amplified pulses
have bandwidths broader than 50 THz and center wavelengths tunable from 490 to 700 nm. Tuning is achieved by varying the pump-seed pulses delay: a typical sequence of amplified pulse spectra is shown in Fig. 2. The pulses have an energy of 1–2 mJ, which is more than enough for spectroscopic applications, and display good amplitude stability, with peak-topeak fluctuations of less than 7%. The amplified pulses are then sent to a pulse compressor, which provides the negative dispersion necessary to cancel the positive frequency chirp introduced by the continuum generation and the amplification processes. The simplest compressor consists of a pair of Brewster-cut fused silica prisms; alternatively, chirped dielectric mirrors, reflecting different frequency components of the incident radiation at different positions within the multilayer structure, can be used [14]. After compression, pulse durations of 10–12 fs can be obtained across the whole tuning range, as shown in Fig. 3. The compressed pulses are sent to a standard setup for pump–probe experiments, also shown in Fig. 1. The delay of the probe beam is controlled by a stepper motor with a time resolution of 0.6 fs; the probe beam is selected with an iris and its differential transmission is measured with a photodiode followed by a lock-in amplifier. Measurements at selected wavelengths are made by passing the probe beam through a 10 nm bandwidth interference filter after it traverses the
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sample. In all the measurements, we checked whether the differential transmission signal was linear with respect to the pulse energy. Since extremely short pulses can be temporally broadened even by small amounts of glass, we used the same setup for performing pump–probe and pulse duration measurements.
3. Impulsive coherent vibrational spectroscopy ICVS can be understood using either a time-domain or an eigenstate picture. In the time domain description, the molecule is excited essentially without nuclear motion, due to the short pump pulse-width. Therefore, it is left in a nonequilibrium position in the excited state and starts to oscillate: we experimentally detect the modulations of the absorption spectrum induced by these oscillations. In the eigenstate description, the short pulse excites in phase many different eigenstates, separated by the oscillator frequency, which form oscillating wavepackets [15,16]. In Fig. 4 it is shown that in our experiments we can detect wavepackets both on the ground and the excited state PES. Pump–probe spectroscopy can be understood as a three field interaction with the sample [17]; two field interactions are with the pump, creating a population which is then interrogated by the probe field. For short pump pulses, excited state oscillations are observed when the two fields in the pump pulse excite a population wavepacket on the excited state PES, which then subsequently oscillates back and forth leaving and returning to the FC region. Ground state oscillations are generated when the first field induces a polarization wavepacket on the excited state PES, which is then allowed to propagate for some time so that the second field brings the wavepacket back down to the ground state, displaced from the hole left behind. In order to significantly excite ground state oscillations, the time between the first and the second interaction must be long enough to allow the polarization wavepacket to propagate on the excited state PES so as to be displaced from the hole in the ground state. The relative contributions of excited and ground state to the oscillatory pattern are strongly dependent on the ratio between pulse length and oscillation period. For very short pump pulses, the pump–probe signal is dominated by excited state
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dynamics, because the polarization wavepacket created by the first field does not move significantly from the FC region upon arrival of the second field. As the pump pulse length increases, the polarization wavepacket has time to propagate further out of the FC region within the pump pulse, thus increasing the amplitude of ground state oscillations. This analysis can be made more quantitative with the help of a numerical solution of the Schro¨dinger equation for two displaced harmonic oscillators [16]. For 10 fs pulses, simulations show that for a two-level system the ground state contribution is negligible for oscillation periods longer than ⬇ 60 fs.
4. Results and discussion In this section, we show results obtained by applying ICVS to three different conjugated systems, namely sexithiophene (T6), polydiacetylene (PDA) and poly( p-phenylenevinylene) (PPV). Each of them is paradigmatic of a class of compounds aimed to peculiar applications. Thiophene oligomers, and T6 in particular, are model compounds for the understanding of the electronic structure and the photophysics of conjugated polymers [18]. In addition, T6 is one of the best candidates for molecular electronic devices, having good transport properties and very high field effect mobility [19]. This is due to the supramolecular organization shown in the solid phase, caused by efficient p–p stacking. PDA can be used to produce very high quality optical waveguides and it is the most promising organic for third order nonlinear optical applications [20]. Its conjugated backbone has alternance of single, double and triple bonds, showing large delocalization and small p–p ⴱ gap. It is unique also from a basic physical point of view, because it is the only polymer semiconductor for which the one-dimensional Wannier exciton picture is commonly accepted. PPV is the most extensively used material in electroluminescent devices, and it is the first for which LEDs were demonstrated [21]. It shows strong emission with high quantum yield even in the solid phase, and under high excitation density a clear phenomena of line narrowing assigned to ASE along the film plane [22]. Blended with high electron affinity
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Fig. 5. Differential transmission of a T6 film at 510 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
molecules, such as fullerene, it shows fast and efficient photoinduced charge transfer, which promise applications in photovoltaic conversion devices [23]. 4.1. Sexithiophene The experiments were performed on polycrystalline T6 films with approximate thickness of 100 nm,
vacuum deposited at standard conditions. The 10 fs pump pulses were centered at 510 nm, close to the first peak of the absorption spectrum of the T6 film. Fig. 5 shows the differential transmission (DT ) of the T6 film vs. pump–probe delay at the probe wavelength of 510 nm. The features at negative and near-zero delays at this and other wavelengths are due to pump-perturbed free induction decay and coherent coupling and thus do not carry any information on the excited state dynamics [24]. For positive delays we observe a negative DT signal, growing in approximately 200 fs, corresponding to photoinduced absorption (PA). The primary photoexcitations in T6 films are tightly bound Frenkel excitons [25]; therefore we attribute the signal to competition of ground-state bleaching and absorption originating from the exciton level. The initial competition between bleaching and PA cannot be due to coherent artifacts because it takes place at time delays exceeding the pulse duration. We interpret this phenomenon as due to exciton intraband relaxation that populates the lower lying states, from where absorption has stronger oscillator strength. The PA signal is modulated by a complex oscillatory pattern, which is caused by the motion of the wavepacket launched by the pump pulse on the multidimensional excited state PES. The inset of Fig. 5 shows the Fourier transform of the oscillatory component of the signal, after a slowly varying background has been subtracted. We clearly identify five modes, at
Fig. 6. Differential transmission of a T6 film at 540 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
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Fig. 7. Differential transmission of a PDA film at 600 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
frequencies v1 112 cm⫺1 ; v2 299 cm⫺1 ; v3 702 cm⫺1 ; v4 1040 cm⫺1 ; and v5 1454 cm⫺1 ; respectively. Measurements at other wavelengths show either bleaching or PA; in all cases, however, the signal is modulated by an oscillatory pattern due to the same
Fig. 8. Differential transmission of a PDA film at 610 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
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five characteristic frequencies. Fig. 6 shows for instance DT at 540 nm, close to the absorption edge in the T6 film; again we observe a PA signal, modulated by an oscillatory pattern, the Fourier transform of which is shown in the inset. The relative intensities of the same five characteristic frequencies depend on the probing wavelength. The reason for this difference is that, by changing the probe wavelength, we probe different regions of the multidimensional excited state potential energy surface [7,8]. Because of the low ground-state absorption cross-section at 540 nm, we assign the rapidly decaying positive signal observed at early times to stimulated emission from the excited state, which is then masked by the growth of PA. In both Figs. 5 and 6 it is shown that the oscillations persist almost undamped for delays longer than 1 ps. The characteristic coherence time decay of the lowest frequency mode v 1, was determined to be 3 ps, corresponding to a linewidth of 3 cm ⫺1. Note that the measured timescale for dephasing of v 1 is considerably longer than that generally assumed for vibrational energy redistribution, which is of the same order of magnitude of the vibrational period (see previous discussion on exciton relaxation). This may indicate that relaxation takes place without destruction of vibrational coherence. Let us now discuss the assignment of the observed vibrational modes. The lowest energy modes at v1 112 cm⫺1 and v2 299 cm⫺1 correspond to the 126 and 297 cm ⫺1 ones proposed in Ref. [4] to interpret the absorption of the single crystal. We therefore assign v 1 and v 2 to coherent motion in the excited PES of vibrational wavepackets built on the corresponding FC manifold. ICVS thus gives complementary information about the role of such modes in the vibronic dynamics and consistently supports their assignment to total symmetric (ag) intramolecular modes. The modes at v3 702 cm⫺1 ; v4 1040 cm⫺1 ; and v5 1454 cm⫺1 correspond to the modes measured by fluorescence and Raman spectroscopy in a T6 single crystal [26] at 699, 1056, and 1460 cm ⫺1, respectively. In agreement with our previous estimate, they can be assigned to ground state modes. We find that higher frequency modes have faster decay, as it is observed for v 5 with t5 0:4 ps: We speculate that modes with larger dispersion are more sensitive to diagonal disorder and thus undergo faster
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Fig. 9. Fourier transforms of the oscillatory component of the signal in Fig. 8 for the time windows: (a) 0–200 fs; and (b) 200–400 fs.
dephasing. Note that we are measuring the dephasing time of the collective vibrational coherence, and not the dephasing time of the polarization of the vibronic transition, which is expected to be much shorter. 4.2. Polydiacetylene The experiments were performed on blends of poly[1,6-bis(3,6-didhexadecyl-N-carbazolyl)-2,4-
Fig. 10. Differential transmission of a PPV film at 580 nm probe wavelength vs. pump–probe delay. Inset: Fourier transform of the oscillatory component.
hexadiyne] (polyDCDH-HS) and polyethylene. The excitation pulse was centered at 540 nm, close to the peak of ground state absorption. Differential transmission signals vs. pump–probe delay are shown in Figs. 7 and 8 for the probing wavelengths of 600 and 610 nm, respectively. At both wavelengths we see a positive DT, which we assign to bleaching of the optical transition. A formation time is clearly seen in the temporal traces. We assign it to population build-up at the bottom of the excited state manifold due to thermalization, determining for the first time in a direct way that the time scale for this process is 100 fs. We observe strong coherent excitation in the CyC v1 ⬇ 1500 cm⫺1 and CxC v2 ⬇ 2100 cm⫺1 stretching vibrations. The CxC strectching mode, which has a period of approximately 16 fs, is to our knowledge the highest molecular oscillation frequency ever observed in the time domain. These are the most strongly coupled amongst the eight resonantly enhanced phonon lines [27] in the RRS spectrum of PDA. Interestingly these two modes are the only ones showing phonon-dispersion behavior, i.e. strong shifting of the Raman frequencies upon changing the excitation photon energy in resonance. The latter effect is attributed to disorder in the polymer sample, which generates a distribution of Raman frequencies associated with different electronic energy gaps. The peculiar role of the double and triple stretching modes indicates their higher sensitivity to the electronic wavefunction delocalization. Because of the very short period of these modes, almost comparable to the pulse duration, we can assign them to ground state oscillations. Vibrational dephasing is extremely rapid for the CxC mode, as indicated by the large line-width of the Fourier spectrum. The frequency of this mode increases in time, as shown in Fig. 9, in which the Fourier transforms of different time windows of the oscillatory signal are reported. The coherent excitation process, which leads to ground state wavepacket oscillation, as described in Section 3, is expected to act through a displacive mechanism. This means that initially the wavepacket is placed out of equilibrium at high level in the ground state PES. Frequency change may indicate that the wavepacket is sliding down an anharmonic potential well, with higher frequencies at the bottom. It
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is this anharmonicity, which can be responsible for the quick dephasing of the mode. 4.3. Polyparaphenylene The experiments were performed on spin-coated DMO-PPV films with an optical density of about 0.5 at the first absorption peak. In addition, in this case the pump pulse was centered at 540 nm, close to the absorption peak. The differential transmission signal at the probe wavelength of 580 nm is shown in Fig. 10: we assign it to a superposition of ground state bleaching and stimulated emission. In addition, in this case the signal is modulated by a complex oscillatory pattern, in which the beating of different frequencies is apparent. The Fourier transform of the oscillatory component indicates two dominant modes, with frequencies of 1300 and 1570 cm ⫺1. Measurements performed at different probe wavelengths always show the same two frequencies, although with different intensities and relative weights. Comparison with previous RRS studies of the vibrational structure of PPVs [28] allows us to assign the two frequencies to the trans-vinylene C– C stretching mode (1300 cm ⫺1) and to the ring C–C quadrant stretching mode (1570 cm ⫺1). It is interesting to note that the 1300 cm ⫺1 band shows a double-peaked structure: this can be attributed to differences in the ground and excited state frequencies relative to this vibration, the excited state being, as expected, at a lower frequency compared to the ground state. 5. Conclusions In this work we have presented the first results of the application of ICVS to the study of the vibrational structure of conjugated polymers. This technique allows a direct time-domain observation of molecular vibrations and has several advantages over conventional spectroscopic techniques; however, it requires the availability of extremely short light pulses (⬇10 fs duration) with frequency tunable in the visible spectral range. The recent development of a stable and reliable source of such pulses has now paved the way for a systematic time-domain study of the vibrational dynamics of organic materials. Time-domain techniques provide a wealth of new information on
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the vibronic dynamics, such as direct measurement of vibrational damping and mode anharmonicity. A full exploitation of their potential, however, will require additional work, both theoretical and experimental. In particular, a quantum-mechanical modeling of the pump–probe measurements at different wavelengths will allow us to obtain important information on the shapes and relative positions of the potential energy surfaces in configuration space.
Acknowledgements We are indebted to Dr M. Muccini and Prof. C. Taliani for preparing the T6 sample, to Dr D. Comoretto and Prof. G. Dellepiane for preparing the PDA sample, and to Dr C. Brabec for preparing the PPV sample.
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