- Email: [email protected]
Probing phase relaxation by measuring fluorescence interference: polarization beating and electron–phonon coupling in conjugated polymers
ARTICLE IN PRESS
Journal of Luminescence 108 (2004) 205–209
Probing phase relaxation by measuring fluorescence interference: polarization beating and electron–phonon coupling in conjugated polymers$ . b, L. Kunaa, F. Milotaa,*, J. Sperlinga,1, A. Tortschanoffa, V. Szocs H.F. Kauffmanna b
a Institute of Physical Chemistry, University Vienna, Wahringerstrasse 42, A-1090 Vienna, Austria . Institute of Chemistry, Comenius University, Mlynska Dolina CH 2, SK-84215 Bratislava, Slovakia
Abstract We report on the measurement of coherent optical-excitation transients in the low-energy states of the strongly disordered multi-chromophore poly(para-phenylene-vinylene) at low phonon temperatures. Interferometric femtosecond (fs) excitation/probing measurements, employing freely propagating 70 fs pulses have been used to generate spatially overlapping electronic and nuclear wavepackets. High-resolution detection in combination with the unique spectral features of the sample was used to project-out two vibronic fluorescence transitions with discrete arrival states a and b: By monitoring the correlations of the a- and b-photons on the detector, the coherent superposition can be probed as a typical polarization beating. The recorded fluorescence interferograms show strong damping on a timescale of 200 fs: The relaxation of the coherent signal is indicative of, mostly, homogeneous site-dephasing and hence, gives rise to the observation of the structural relaxation of the initial Franck–Condon excitation, i.e. the formation of the dressed state. r 2004 Elsevier B.V. All rights reserved. PACS: 78.47.+p; 42.50.Md; 42.25.Kb Keywords: Conjugated polymers; Time-resolved fluorescence spectroscopy
1. Introduction The unique semiconducting properties of pconjugated organic molecules have made polymers $ This work was done within the ADLIS special research program (F.M. and A.T.) and in the framework of the Austrian Science Foundation project P14884-CHE (L.K. and V.S.). *Corresponding author. Tel.: +43-1-4277-52445; fax: +431-4277-9524. E-mail address: [email protected] (F. Milota). 1 A Doctoral Scholarship Program of the Austrian Academy of Science (DOC) is gratefully acknowledged.
like poly(para-phenylene-vinylene) (PPV) powerful vehicles in recent opto-electronic applications [1,2]. Although these technological progresses rest upon incoherent excitations/relaxations, there is great technical and academic interest in the investigation of the ultrafast photophysical mechanisms that create coherent pathways and quantum-stochastic [3] relaxation channels in these disordered many-body systems. A widely accepted model [4] to describe the ground-state of PPV assumes the all-trans conjugated chain configuration to be disrupted by
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.044
ARTICLE IN PRESS 206
F. Milota et al. / Journal of Luminescence 108 (2004) 205–209
statistical breaks [5], which results in an ensemble of segmental sites with a fluctuation of selfenergies and a dispersion of inter-site, electronic coupling strengths. The excited state of the polymer splits into a broad density-of-states (DOS) of S1 -levels, NðeÞ: Due to ultrafast structural relaxation of the Franck–Condon (FC) excitations [6,7] and large segmental site-disorder, the electronic/nuclear decoherence in PPV is supposed to proceed on a sub-200 fs scale. In this work, we use femtosecond interferometry (fs-IF) in combination with energy-specific excitation and fluorescence detection tuning to probe electronic/nuclear coherence in the low-vibrational S1 ’S0 intra-DOS regime of PPV. The observation of this ultrafast coherence relies on the correlation of two wavepackets (WPs) that are prepared by double-pulse excitation and probed via second-order fluorescence interference [8,9] using tuned excitation/probing pulses.
Ti:S-Amplifier / OPA PC S Mono
tp
L1
L2
PM Amp Ct
Comp
Delay
Fig. 1. Schematics of the fs-fluorescence Michelson-type interferometer based upon a Ti : S-200 kHz Amplifier/OPA system with 70 fs freely propagating excitation/probing pulses of equal intensity and collinear arrangement; S—sample, L1 and L2—lenses, Mono—monochromator. Data processing via single channel single photon counting: Amp—amplifier, PM— photomultiplier, Ct—counter, PC—Peltier Cooler, Comp— personal Computer for data acquisition and control of experiment (tp ; monochromator, etc.); see Ref. [9] for further details.
Fluorescence (arb.units)
All experiments were performed at 1:4 K with 70 fs excitation pulses from an amplifier/OPAsystem (Coherent Inc.), with the center wavelength of the pulses tuneable over the whole range of the low-energy edge of the absorption spectrum. A PPV film on a sapphire substrate was prepared by the precursor method [10]. A scheme of the setup is depicted in Fig. 1, which basically consists of a light source, a Michelson-type interferometer, and the detection unit. A recapitulation of PPV’s spectral fluorescence properties is required, before the fluorescence interferograms (Fig. 3) are described. Fig. 2 shows the low-temperature time-integrated fluorescence spectrum of PPV (right-hand ordinate) recorded with a CCD-spectrometer (S2000; Ocean Optics) and 70 fs pulsed excitations at lL ¼ 520 nm: The spectrum features the typical vibronic progression from a phenyl-stretching mode with a 0:18 eV spacing [11]. In the bottom-state excitation regime, the shapes of the bands are no longer the results of energy funnelling [12], but distinctly depend on the energy of excitation [11]. For excitation into the red absorption edge the bands shift parallel with
Absorption (arb.units)
2. Experimental coherences 1
1
500
525
550
575
600
625
650
675
λ [nm]
Fig. 2. Absorption edge of the long-wavelength S1 ’S0 transition (left-hand ordinate) and time-integrated fluorescence spectrum (right-hand ordinate) of PPV at 1:4 K; pulsed excitation (70 fs width) at center wavelength lL ¼ 520 nm: Bimodal S1dr ðn ¼ 0Þ-S0 ðn ¼ 1Þ fluorescence transition with vibronic arrival-states 1a ; 1b centered at 572 and 564 nm; respectively, with splitting energy Oa;b C400 cm1 :
the exciting laser-pulse carrier wavelength lL and their widths directly reflect the target-area of S1 populations launched by the broadband excitation pulse. Recall that the fluorescence is subject to a weak Stokes-shift ðdeC0:02 eVÞ [4] which indicates that the fluorescent state in PPV-optical dynamics is dressed ðdrÞ by some vibrational/torsional
ARTICLE IN PRESS F. Milota et al. / Journal of Luminescence 108 (2004) 205–209
(a)
I det (arb.units)
states—S1dr ðn ¼ 0Þ—and hence to be contrasted from the vertical, coherent FC-excitation, S1 : The key-issue of spectroscopic data which will have central impact on the interferometric measurements is the discrete, bimodal structure of the S1dr ðn ¼ 0Þ-S0 ðn ¼ 1Þ fluorescence transition (in the following simply referred to as 0-1). Two transitions with different fluorescence photons a; b are considered in the literature to activate strong Raman modes, the latter being assigned to C–H bending modes [13,14]. The energy difference between the two vibronic levels is about Oa;b ¼ 400 cm1 and we label them as n ¼ 1a and 1b in the forthcoming description. In the interferometric experiments, the correlated fluorescence I det ðtp Þ was detected by using a narrow-band monochromator and a photomultiplier tube. Wavelength-selective detection (fluorescence detection wavelength lD ) is essential for the observation of coherences and we note that the width of the detection window in the experiments must be smaller than the width of the homogeneous lines to extract the coherent transient! This criterion can be easily managed, as, due to the ultrafast structural relaxation process (homogeneous site dephasing), the spectral widths of the segmental bottom-site-Lorentzians are extremely broad. The resolution of the monochromator was set to 2 nm which is a reasonable compromise to achieve sufficient fluorescence intensity and, at the same time, quite satisfactory spectral resolution. Fig. 3 shows two experimental interferometric scans for pulse excitation at lL ¼ 541 nm at different detection energies tuned across the bimodal 0-1 fluorescence transition comprising a; b photons and discrete arrival-states 1a and 1b : In the interferograms (IF) the convolution-type fluorescence interference intensity I det is plotted as a function of the pulse-to-pulse delay tp : No special effort was undertaken to stabilize the interferometer, so the IFs have been not always perfectly symmetric for positive and negative pulse-to-pulse delays and the carrier frequency(ies) cannot be obtained accurately by direct Fourier inversion. However, this technical complication is not of concern here. Rather, the relevant information is the significant oscillatory structure of the relaxation profiles, which constitute high-fre-
207
(b)
(c)
0
50
100
150
200
250
300
tp [fs]
Fig. 3. One-sided interferometric scans for excitation at centerwavelength lL ¼ 541 nm and two detection wavelengths tuned across the bimodal 0-1 fluorescence transition: (a) ldet ¼ 570 nm; (b) ldet ¼ 580 nm; and (c) is the pulse-to-pulse autocorrelation.
quency carrier waves and low-frequency modulation, in terms of beating oscillation. Further, a pronounced dependence on the fluorescence detection wavelength has been observed (Fig. 3). The fluorescence IFs go beyond the temporal length of the pulse-to-pulse autocorrelation function and thus the IFs show, quite naturally, a substantially longer-lived relaxational behavior. Nevertheless, the entirety of beats in Fig. 3 dephases in the limit of a critically damped wave, where damping-time and beating oscillation period have the same order of magnitude. Therefore, for the majority of scans, only half-cycles of the beating oscillation can be observed. However, for detection at ldet ¼ 580 nm; a full period of a single oscillation has been observed in these patterns. Quite obviously, the sites with different self-energies in the target area of the bottom-states are subject to different rates of homogeneous dephasing which seems to be affected by the conjugation (coherence) length of the segments [15]. In particular, for the asymptotic part of single-cycle oscillation interferograms, the relaxation is mainly controlled by homogeneous dephasing. This allows a crude estimate of the electronic coherence time and thus a preliminary access to the time-scale of the structural relaxation process (homogeneous dephasing) to be of the order of T20 C130 fs; in accordance with interferometric measurements on resonant Rayleigh scattering [16].
ARTICLE IN PRESS 208
F. Milota et al. / Journal of Luminescence 108 (2004) 205–209
Lower-energy excitations tuned, for example, at lexc ¼ 543 nm show similar behavior (not shown here). In general, for excitations well below the migrational threshold [15,17], a shift in excitation wavelength goes hand-in-hand with a corresponding shift, both in the single-pulse fluorescence spectrum and in the fluorescence interferograms. The internal structure of the oscillations has turned-out to be very similar for different excitation energies. They show nearly the same oscillatory features at shifted detection energies, the shift being equal to the excitation shift. This phenomenological dependence indicates that similar processes are involved and this confirms that the patterns are a result of the relative detuning of the detection from the excitation energy.
3. Summary Novel electronic-excitation oscillations have been measured for the low-energy wing-regime in the DOS of the strongly disordered, polymeric multi-chromophore poly(p-phenylenevinylene), PPV, at low phonon temperatures. Phase-sensitive fluorescence interferometry with freely propagating pairs of tunable 70 fs excitation pulses has been employed where (a) double-pulse excitation generates two spatially overlapping wavepackets and (b) the interference term of sequential fluorescence directly reflects coherence loss, i.e. follows the relaxing electronic dipole oscillation in the optical free-induction decays of the siteLorentzians. To overcome destructive, inhomogeneous site-dephasing caused by the initial pulse excitation of intra-DOS target sites, two discrete, vibronic 0-1 S1 -S0 fluorescence transitions with (Raman-active) arrival-states 1a ; 1b ; characteristic of the low-temperature behavior and resolvable by narrow-band detection, have been used as particular target states. The latter have enabled sub-sets of electronic wavepackets corresponding to energetic sub-spaces of the excited site-ensembles to be projected out from the inhomogeneous pulse-excitation spectrum. Inphase superposition of pairs of coherent siteLorentzians has been recorded as typical site-tosite polarization beatings from superimposed
optical free-induction decays by monitoring the interference of (quasi-)iso-energetic fluorescence photons on the detector. The beating oscillations observable by this measuring concept are, in general, free from inhomogeneous site dephasing, but, clearly, convolved by the (Gaussian type) pulse-to-pulse auto-correlation. The oscillatory relaxations of the fluorescence interferograms show critical damping behavior, with typically half-to-single cycle waves. The period of the beating seems to be slightly affected by the spectral position of the excitation and detection window, just as the damping varies with the excitation and fluorescence energy. Furthermore, the beating wave vanishes (i) for excitations into the ultimate, low-energy regime of the bottom-states and (ii) for excitations into the transport states above the localization threshold. The strong damping reflects, predominantly, homogeneous dephasing under very narrow-band detection conditions and hence reveals, for the first time, the doorway for looking into C100 fs structural relaxation of the initial Franck–Condon state, i.e. the formation of the dressed excitation (torsional–vibrational clothes) as a special form of molecular electronphonon coupling.
References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. dos Santos, . J.L. Bredas, M.Loglund, W.R. Salaneck, Nature 397 (1999) 121. [2] R.H. Friend, J.H. Burroughes, T. Shimoda, Phys. World 6 (1999) 35. [3] S. Nakajima, Prog. Theor. Phys. 20 (1959) 948. [4] U. Rauscher, H. B.assler, D.D.C. Bradley, M. Hennecke, Phys. Rev. B 42 (1990) 9830. [5] G. Rossi, R.R. Chance, R. Silbey, J. Chem. Phys. 90 (1989) 7594. [6] R. Kersting, U. Lemmer, R.F. Mahrt, K. Leo, H. Kurz, H. . B.assler, E.O. Gobel, Phys. Rev. Lett. 70 (1993) 3820. [7] M. Yan, L.J. Rothberg, E.W. Kwock, T.M. Miller, Phys. Rev. Lett. 75 (1995) 1992. [8] N.F. Scherer, R.J. Carlson, A. Matro, M. Du, A.J. Ruggiero, V. Romero-Rochin, J.A. Cina, G.R. Fleming, S.A. Rice, J. Chem. Phys. 95 (1991) 1487. [9] A. Tortschanoff, K. Brunner, C. Warmuth, H.F. Kauffmann, J. Chem. Phys. 110 (1999) 4493.
ARTICLE IN PRESS F. Milota et al. / Journal of Luminescence 108 (2004) 205–209 . [10] G. Kopping-Grem, G. Leising, M. Schimetta, A. Huber, Synth. Met. 76 (1996) 53. [11] S. Heun, R.F. Mahrt, A. Greiner, U. Lemmer, H. B.assler, D.A. Halliday, D.D.C. Bradley, P.L. Burn, A.B. Holmes, J. Phys.: Condens. Matter 5 (1993) 247. [12] R. Kersting, B. Mollay, M. Rusch, J. Wenisch, G. Leising, H. Kauffmann, J. Chem. Phys. 106 (1997) 2850. [13] D. Rakovic, R. Kostic, L.A. Gribov, I.E. Davidova, Phys. Rev. B 41 (1990) 10744.
209
[14] L. Orion, J.P. Buisson, S. Lefrant, Phys. Rev. B 57 (1998) 7050. [15] S.P. Kennedy, N. Garro, R.T. Phillips, Phys. Rev. B 64 (2001) 115206. [16] S.P. Kennedy, N. Garro, R.T. Phillips, Phys. Rev. Lett. 86 (2001) 4148. [17] J. Sperling, F. Milota, A. Tortschanoff, C. Warmuth, B. Mollay, H. B.assler, H.F. Kauffmann, J. Chem. Phys. 117 (2002) 10877.
Journal of Luminescence 108 (2004) 205–209
Probing phase relaxation by measuring fluorescence interference: polarization beating and electron–phonon coupling in conjugated polymers$ . b, L. Kunaa, F. Milotaa,*, J. Sperlinga,1, A. Tortschanoffa, V. Szocs H.F. Kauffmanna b
a Institute of Physical Chemistry, University Vienna, Wahringerstrasse 42, A-1090 Vienna, Austria . Institute of Chemistry, Comenius University, Mlynska Dolina CH 2, SK-84215 Bratislava, Slovakia
Abstract We report on the measurement of coherent optical-excitation transients in the low-energy states of the strongly disordered multi-chromophore poly(para-phenylene-vinylene) at low phonon temperatures. Interferometric femtosecond (fs) excitation/probing measurements, employing freely propagating 70 fs pulses have been used to generate spatially overlapping electronic and nuclear wavepackets. High-resolution detection in combination with the unique spectral features of the sample was used to project-out two vibronic fluorescence transitions with discrete arrival states a and b: By monitoring the correlations of the a- and b-photons on the detector, the coherent superposition can be probed as a typical polarization beating. The recorded fluorescence interferograms show strong damping on a timescale of 200 fs: The relaxation of the coherent signal is indicative of, mostly, homogeneous site-dephasing and hence, gives rise to the observation of the structural relaxation of the initial Franck–Condon excitation, i.e. the formation of the dressed state. r 2004 Elsevier B.V. All rights reserved. PACS: 78.47.+p; 42.50.Md; 42.25.Kb Keywords: Conjugated polymers; Time-resolved fluorescence spectroscopy
1. Introduction The unique semiconducting properties of pconjugated organic molecules have made polymers $ This work was done within the ADLIS special research program (F.M. and A.T.) and in the framework of the Austrian Science Foundation project P14884-CHE (L.K. and V.S.). *Corresponding author. Tel.: +43-1-4277-52445; fax: +431-4277-9524. E-mail address: [email protected] (F. Milota). 1 A Doctoral Scholarship Program of the Austrian Academy of Science (DOC) is gratefully acknowledged.
like poly(para-phenylene-vinylene) (PPV) powerful vehicles in recent opto-electronic applications [1,2]. Although these technological progresses rest upon incoherent excitations/relaxations, there is great technical and academic interest in the investigation of the ultrafast photophysical mechanisms that create coherent pathways and quantum-stochastic [3] relaxation channels in these disordered many-body systems. A widely accepted model [4] to describe the ground-state of PPV assumes the all-trans conjugated chain configuration to be disrupted by
0022-2313/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2004.01.044
ARTICLE IN PRESS 206
F. Milota et al. / Journal of Luminescence 108 (2004) 205–209
statistical breaks [5], which results in an ensemble of segmental sites with a fluctuation of selfenergies and a dispersion of inter-site, electronic coupling strengths. The excited state of the polymer splits into a broad density-of-states (DOS) of S1 -levels, NðeÞ: Due to ultrafast structural relaxation of the Franck–Condon (FC) excitations [6,7] and large segmental site-disorder, the electronic/nuclear decoherence in PPV is supposed to proceed on a sub-200 fs scale. In this work, we use femtosecond interferometry (fs-IF) in combination with energy-specific excitation and fluorescence detection tuning to probe electronic/nuclear coherence in the low-vibrational S1 ’S0 intra-DOS regime of PPV. The observation of this ultrafast coherence relies on the correlation of two wavepackets (WPs) that are prepared by double-pulse excitation and probed via second-order fluorescence interference [8,9] using tuned excitation/probing pulses.
Ti:S-Amplifier / OPA PC S Mono
tp
L1
L2
PM Amp Ct
Comp
Delay
Fig. 1. Schematics of the fs-fluorescence Michelson-type interferometer based upon a Ti : S-200 kHz Amplifier/OPA system with 70 fs freely propagating excitation/probing pulses of equal intensity and collinear arrangement; S—sample, L1 and L2—lenses, Mono—monochromator. Data processing via single channel single photon counting: Amp—amplifier, PM— photomultiplier, Ct—counter, PC—Peltier Cooler, Comp— personal Computer for data acquisition and control of experiment (tp ; monochromator, etc.); see Ref. [9] for further details.
Fluorescence (arb.units)
All experiments were performed at 1:4 K with 70 fs excitation pulses from an amplifier/OPAsystem (Coherent Inc.), with the center wavelength of the pulses tuneable over the whole range of the low-energy edge of the absorption spectrum. A PPV film on a sapphire substrate was prepared by the precursor method [10]. A scheme of the setup is depicted in Fig. 1, which basically consists of a light source, a Michelson-type interferometer, and the detection unit. A recapitulation of PPV’s spectral fluorescence properties is required, before the fluorescence interferograms (Fig. 3) are described. Fig. 2 shows the low-temperature time-integrated fluorescence spectrum of PPV (right-hand ordinate) recorded with a CCD-spectrometer (S2000; Ocean Optics) and 70 fs pulsed excitations at lL ¼ 520 nm: The spectrum features the typical vibronic progression from a phenyl-stretching mode with a 0:18 eV spacing [11]. In the bottom-state excitation regime, the shapes of the bands are no longer the results of energy funnelling [12], but distinctly depend on the energy of excitation [11]. For excitation into the red absorption edge the bands shift parallel with
Absorption (arb.units)
2. Experimental coherences 1
1
500
525
550
575
600
625
650
675
λ [nm]
Fig. 2. Absorption edge of the long-wavelength S1 ’S0 transition (left-hand ordinate) and time-integrated fluorescence spectrum (right-hand ordinate) of PPV at 1:4 K; pulsed excitation (70 fs width) at center wavelength lL ¼ 520 nm: Bimodal S1dr ðn ¼ 0Þ-S0 ðn ¼ 1Þ fluorescence transition with vibronic arrival-states 1a ; 1b centered at 572 and 564 nm; respectively, with splitting energy Oa;b C400 cm1 :
the exciting laser-pulse carrier wavelength lL and their widths directly reflect the target-area of S1 populations launched by the broadband excitation pulse. Recall that the fluorescence is subject to a weak Stokes-shift ðdeC0:02 eVÞ [4] which indicates that the fluorescent state in PPV-optical dynamics is dressed ðdrÞ by some vibrational/torsional
ARTICLE IN PRESS F. Milota et al. / Journal of Luminescence 108 (2004) 205–209
(a)
I det (arb.units)
states—S1dr ðn ¼ 0Þ—and hence to be contrasted from the vertical, coherent FC-excitation, S1 : The key-issue of spectroscopic data which will have central impact on the interferometric measurements is the discrete, bimodal structure of the S1dr ðn ¼ 0Þ-S0 ðn ¼ 1Þ fluorescence transition (in the following simply referred to as 0-1). Two transitions with different fluorescence photons a; b are considered in the literature to activate strong Raman modes, the latter being assigned to C–H bending modes [13,14]. The energy difference between the two vibronic levels is about Oa;b ¼ 400 cm1 and we label them as n ¼ 1a and 1b in the forthcoming description. In the interferometric experiments, the correlated fluorescence I det ðtp Þ was detected by using a narrow-band monochromator and a photomultiplier tube. Wavelength-selective detection (fluorescence detection wavelength lD ) is essential for the observation of coherences and we note that the width of the detection window in the experiments must be smaller than the width of the homogeneous lines to extract the coherent transient! This criterion can be easily managed, as, due to the ultrafast structural relaxation process (homogeneous site dephasing), the spectral widths of the segmental bottom-site-Lorentzians are extremely broad. The resolution of the monochromator was set to 2 nm which is a reasonable compromise to achieve sufficient fluorescence intensity and, at the same time, quite satisfactory spectral resolution. Fig. 3 shows two experimental interferometric scans for pulse excitation at lL ¼ 541 nm at different detection energies tuned across the bimodal 0-1 fluorescence transition comprising a; b photons and discrete arrival-states 1a and 1b : In the interferograms (IF) the convolution-type fluorescence interference intensity I det is plotted as a function of the pulse-to-pulse delay tp : No special effort was undertaken to stabilize the interferometer, so the IFs have been not always perfectly symmetric for positive and negative pulse-to-pulse delays and the carrier frequency(ies) cannot be obtained accurately by direct Fourier inversion. However, this technical complication is not of concern here. Rather, the relevant information is the significant oscillatory structure of the relaxation profiles, which constitute high-fre-
207
(b)
(c)
0
50
100
150
200
250
300
tp [fs]
Fig. 3. One-sided interferometric scans for excitation at centerwavelength lL ¼ 541 nm and two detection wavelengths tuned across the bimodal 0-1 fluorescence transition: (a) ldet ¼ 570 nm; (b) ldet ¼ 580 nm; and (c) is the pulse-to-pulse autocorrelation.
quency carrier waves and low-frequency modulation, in terms of beating oscillation. Further, a pronounced dependence on the fluorescence detection wavelength has been observed (Fig. 3). The fluorescence IFs go beyond the temporal length of the pulse-to-pulse autocorrelation function and thus the IFs show, quite naturally, a substantially longer-lived relaxational behavior. Nevertheless, the entirety of beats in Fig. 3 dephases in the limit of a critically damped wave, where damping-time and beating oscillation period have the same order of magnitude. Therefore, for the majority of scans, only half-cycles of the beating oscillation can be observed. However, for detection at ldet ¼ 580 nm; a full period of a single oscillation has been observed in these patterns. Quite obviously, the sites with different self-energies in the target area of the bottom-states are subject to different rates of homogeneous dephasing which seems to be affected by the conjugation (coherence) length of the segments [15]. In particular, for the asymptotic part of single-cycle oscillation interferograms, the relaxation is mainly controlled by homogeneous dephasing. This allows a crude estimate of the electronic coherence time and thus a preliminary access to the time-scale of the structural relaxation process (homogeneous dephasing) to be of the order of T20 C130 fs; in accordance with interferometric measurements on resonant Rayleigh scattering [16].
ARTICLE IN PRESS 208
F. Milota et al. / Journal of Luminescence 108 (2004) 205–209
Lower-energy excitations tuned, for example, at lexc ¼ 543 nm show similar behavior (not shown here). In general, for excitations well below the migrational threshold [15,17], a shift in excitation wavelength goes hand-in-hand with a corresponding shift, both in the single-pulse fluorescence spectrum and in the fluorescence interferograms. The internal structure of the oscillations has turned-out to be very similar for different excitation energies. They show nearly the same oscillatory features at shifted detection energies, the shift being equal to the excitation shift. This phenomenological dependence indicates that similar processes are involved and this confirms that the patterns are a result of the relative detuning of the detection from the excitation energy.
3. Summary Novel electronic-excitation oscillations have been measured for the low-energy wing-regime in the DOS of the strongly disordered, polymeric multi-chromophore poly(p-phenylenevinylene), PPV, at low phonon temperatures. Phase-sensitive fluorescence interferometry with freely propagating pairs of tunable 70 fs excitation pulses has been employed where (a) double-pulse excitation generates two spatially overlapping wavepackets and (b) the interference term of sequential fluorescence directly reflects coherence loss, i.e. follows the relaxing electronic dipole oscillation in the optical free-induction decays of the siteLorentzians. To overcome destructive, inhomogeneous site-dephasing caused by the initial pulse excitation of intra-DOS target sites, two discrete, vibronic 0-1 S1 -S0 fluorescence transitions with (Raman-active) arrival-states 1a ; 1b ; characteristic of the low-temperature behavior and resolvable by narrow-band detection, have been used as particular target states. The latter have enabled sub-sets of electronic wavepackets corresponding to energetic sub-spaces of the excited site-ensembles to be projected out from the inhomogeneous pulse-excitation spectrum. Inphase superposition of pairs of coherent siteLorentzians has been recorded as typical site-tosite polarization beatings from superimposed
optical free-induction decays by monitoring the interference of (quasi-)iso-energetic fluorescence photons on the detector. The beating oscillations observable by this measuring concept are, in general, free from inhomogeneous site dephasing, but, clearly, convolved by the (Gaussian type) pulse-to-pulse auto-correlation. The oscillatory relaxations of the fluorescence interferograms show critical damping behavior, with typically half-to-single cycle waves. The period of the beating seems to be slightly affected by the spectral position of the excitation and detection window, just as the damping varies with the excitation and fluorescence energy. Furthermore, the beating wave vanishes (i) for excitations into the ultimate, low-energy regime of the bottom-states and (ii) for excitations into the transport states above the localization threshold. The strong damping reflects, predominantly, homogeneous dephasing under very narrow-band detection conditions and hence reveals, for the first time, the doorway for looking into C100 fs structural relaxation of the initial Franck–Condon state, i.e. the formation of the dressed excitation (torsional–vibrational clothes) as a special form of molecular electronphonon coupling.
References [1] R.H. Friend, R.W. Gymer, A.B. Holmes, J.H. Burroughes, R.N. Marks, C. Taliani, D.D.C. Bradley, D.A. dos Santos, . J.L. Bredas, M.Loglund, W.R. Salaneck, Nature 397 (1999) 121. [2] R.H. Friend, J.H. Burroughes, T. Shimoda, Phys. World 6 (1999) 35. [3] S. Nakajima, Prog. Theor. Phys. 20 (1959) 948. [4] U. Rauscher, H. B.assler, D.D.C. Bradley, M. Hennecke, Phys. Rev. B 42 (1990) 9830. [5] G. Rossi, R.R. Chance, R. Silbey, J. Chem. Phys. 90 (1989) 7594. [6] R. Kersting, U. Lemmer, R.F. Mahrt, K. Leo, H. Kurz, H. . B.assler, E.O. Gobel, Phys. Rev. Lett. 70 (1993) 3820. [7] M. Yan, L.J. Rothberg, E.W. Kwock, T.M. Miller, Phys. Rev. Lett. 75 (1995) 1992. [8] N.F. Scherer, R.J. Carlson, A. Matro, M. Du, A.J. Ruggiero, V. Romero-Rochin, J.A. Cina, G.R. Fleming, S.A. Rice, J. Chem. Phys. 95 (1991) 1487. [9] A. Tortschanoff, K. Brunner, C. Warmuth, H.F. Kauffmann, J. Chem. Phys. 110 (1999) 4493.
ARTICLE IN PRESS F. Milota et al. / Journal of Luminescence 108 (2004) 205–209 . [10] G. Kopping-Grem, G. Leising, M. Schimetta, A. Huber, Synth. Met. 76 (1996) 53. [11] S. Heun, R.F. Mahrt, A. Greiner, U. Lemmer, H. B.assler, D.A. Halliday, D.D.C. Bradley, P.L. Burn, A.B. Holmes, J. Phys.: Condens. Matter 5 (1993) 247. [12] R. Kersting, B. Mollay, M. Rusch, J. Wenisch, G. Leising, H. Kauffmann, J. Chem. Phys. 106 (1997) 2850. [13] D. Rakovic, R. Kostic, L.A. Gribov, I.E. Davidova, Phys. Rev. B 41 (1990) 10744.
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