Stark Spectroscopy of Excited-State Transitions in a Conjugated Polymer C. Gadermaier, 1,2, * F. Grasse, 3 S. Perissinotto, 1 M. Graf, 3 F. Galbrecht, 4 U. Scherf, 4 E. J. W. List, 3 and G. Lanzani 1 1 National Laboratory of Ultrafast Science, Dipartimento di Fisica, Politecnico di Milano, P.zza L. da Vinci 32, 20133 Milano, Italy 2 Department of Complex Matter, Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia 3 Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State Physics, Graz University of Technology, A-8010 Graz, and Institute of Nanostructured Materials and Photonics, JOANNEUM RESEARCH A-8160 Weiz, Austria 4 Department of Chemistry and Institute of Polymer Chemistry, Wuppertal University, Gauss-Str. 20, 42097 Wuppertal, Germany (Received 15 February 2007; published 4 February 2008) Stark spectroscopy, which is well established for probing transitions between the ground and excited states of many material classes, is extended to transitions between transient excited states. To this end, it is combined with femtosecond pump-probe spectroscopy on a conjugated polymer with appropriately introduced traps which harvest excitation energy and build up a sufficient excited state population. The results indicate a significant difference in the effective dipole moments between two short lived excited states. DOI: 10.1103/PhysRevLett.100.057401 PACS numbers: 78.66.Qn, 42.65.Re, 78.20.e, 78.47.p Stark (electroabsorption, EA) spectroscopy has eluci- dated the electronic structure of many materials classes [1]. It reveals the changes of polarizability p and/or dipole moment m f that accompany electronic transitions. Polarizability is a measure of electron delocalization, while a dipole moment indicates charge transfer. Yet, except in simple gas molecules, EA studies have been limited to transitions from the ground to an excited state. Given the importance of excited states in optoelectronics and non- linear optics, we extend EA to transitions between transient excited states. This requires a high density of excited states and generation and probing of these states with fs time resolution. To reach high densities of an excited state with a distinct spectral feature, we chose polyfluorene (PF), one of the best studied conjugated polymers [2,3], in which 5% of the fluorene units are replaced by fluorenone (FLO) [4 – 8]. FLO does not destroy the conjugation along the PF back- bone and barely affects the primary excited singlet state S 1 , but adds an excited state F 1 of lower energy [9]. Following photoexcitation in the FLO-free segments, energy is fun- neled to FLO-containing sites [10], which builds up sub- stantial F 1 populations. We probe their EA via pump-probe spectroscopy with a modulated electric field. Besides the well-known field-induced S 1 dissociation [11–13], we find a clear EA signature of F 1 , which is discussed in terms of an effective dipole moment. In isotropic media, the change in the absorption coeffi- cient  due to electric field F is   1 2 pF 2 @ @E  1 6 m f F 2 @ 2  @E 2  c   0 (1) where E is energy and the last two terms describe transfer of oscillator strength from allowed (c, c< 0) to forbid- den transitions ( 0 ). In fs pump-probe spectroscopy, a pump pulse excites the sample and the relative change T=T in optical trans- mission is measured with a probe pulse at a defined delay t. T=T is proportional to the change N i in the popula- tion of states i, their effective absorption or emission cross sections  i , and the sample thickness d [14]: T T E; t X i  i EN i td: (2) Field-induced changes in T=T are  2 T T  X i  i EN i td  X i  i EN i td  X i  i E 2 N i td: (3) The first term describes changes in the cross sections due to Stark effect. The second term regards field-induced changes in the populations. The square differential ac- counts for the two perturbing factors: the pump beam and the electric field. The chemical structure of the 95%/5% fluorene/ fluorenone random copolymer (k-PF), is shown in Fig. 1. k-PF films of 100 nm thickness are sandwiched between Al and indium-tin oxide coated with poly(3,4- ethylenedioxythiophene) poly(styrenesulfonate). The fs pump-probe setup is described in [14]. An electric field modulated between 0 and 1:5  10 6 V cm 1 is applied in reverse bias to avoid charge injection. The T=T spectrum of k-PF is shown in Fig. 1 for different pump-probe delays. The positive signal above 2.4 eV is a combination of photobleaching (PB) due to depletion of S 0 and stimulated emission (SE) from S 1 [15,16]. The negative part (photoinduced absorption PA) changes with time, which indicates the contribution of at least two electronic states. The simplest model that cor- rectly describes this behavior assumes three relaxation paths for S 1 : directly to S 0 , bimolecular recombination of two S 1 , and migration to F 1 : PRL 100, 057401 (2008) PHYSICAL REVIEW LETTERS week ending 8 FEBRUARY 2008 0031-9007= 08=100(5)=057401(4) 057401-1 © 2008 The American Physical Society