Multi-Dimensional Optical Spectroscopy

This project presents new strategies and efforts towards disentangling excitonic coherences and intersite coupling in molecular many-bodies, with special emphasis to be placed on the electronic coherence problem in conjugated polymeric/oligomeric site-arrays. Modern, third-order optical photon-echo (PE) spectroscopies - made possible by recent advances in femtosecond pulse technology and phase stabilization - in tandem with new interferometric measuring techniques for weak, diffracted - electric fields will be proposed which provide new windows and snap- shots into the nature of intermolecular couplings between molecular segments in main chain polymers. A new generation of transient photon-echoes generated by a sequence of three sub-20 fs pulses (3-pulse stimulated photon echo, 3-PPE) and time-gated by a fourth heterodyning pulse carries, intrinsically, the specific signatures of site-to- site couplings, delocalization/exciton localization, and electronic interactions to vibrations and phonons. The exact time-order of the echo-experiment with regard to the incoming sequence of the three pulses implies a rigorous correlation of multiple dipole-excitations and thus the expansion of the polarization on higher-dimensionalities via a systematic variation of the two, independent pulse-to-pulse delays T12 and T23, respectively. It is conjectured, therefore, that 3-D representations and, in particular, 2D contour lines of the frequency-dispersed echo-field infrequency-time space or, alternatively, in 2D frequency-frequency plots from double Fourier transform echo- fields have the potential to reveal the typical cross-correlations between molecular interactions and thus may capable to visualize electronic site-to-site coupling along the antidiagonal directions characterized by the off- diagonal elements in the frequency (energy) matrix. The big challenge in this proposal is that novel Fourier-Transform photon echoes of complexity are capable to probe the multi-dimensional p(3) response on very short temporal and spatial scales of the disordered and, partly, coupled segmental ensemble in a conjugated polymer. The main objective is to demonstrate excitation delocalization brought about by the dispersion of segmental miniexcitons and to measure the time dependence of the delocalized exciton on its way to localization, due to conformational site disorder and electron-vibrational coupling. Such measurements will give rise to a crude estimate of (some average of) exciton coherence size in conjugated systems and thus rationalize the microscopic, chemical model of distributed segmental sites and spatially restricted, segmental excitons along the main chain of conjugated polymers. These experimental results provide essential input for theoretical models that may describe exciton dynamics in conjugated polymers, more rigorously. Detailed simulations of the nD-echoexperiments and a more quantitative interpretation will be worked out in a joint-program with Shaul Mukamel`s group in Rochester, USA. The numerical computational background of this type of spectroscopy is still in its infancy, in particular, for the complex polymer morphology, so considerable joint experimental and theoretical effort will be required in order to develop the polymer-intrinsic features of exciton motion into a routine structural and dynamical tool . Preparatory work on the development of inversion algorithms has to be done that could directly yield the inter-chromophoric couplings from the signals. Furthermore, the writing of efficient sampling and simulation algorithms for multiple- time correlation functions of the third-order, optical response functions, directly proportional to the echo-signal, is a necessary goal within the 3-years frame-work of this project.

Relaxation of the electronic phase coupled to nuclear dynamics is one of the great unknowns in the mystery of physical and chemical pathways in the early femtosecond-excitation of complex molecular systems. In project P14884 n-D Optical Spectroscopy - Electronic Coupling in Polymers (2001 - 2004), new concepts and experiments were developed and technical strategies devised to measure molecular coherence and, from coherence loss (decoherence), to tackle electronic inter-site coupling and electron-phonon coupling in molecular many bodies, with special emphasis to be placed on the electronic coherence problem in conjugated polymeric/oligomeric site arrays. Aside from seminal, phase-sensitive wavepacket interference studies and preliminary insights into the details of structural relaxation in the regime of homogeneous site dephasing (< 120 fs) (cf. Sec.2), systematic experimental work has been concerned with the up & downs in the experimental realization of homodyned two dimensional (2-D) 3-pulse photon-echo techniques, a time-integrated technique with inter-pulse delays T12 , T23 that probes the square of the third -order polarization signal pulse | P2 (T 12 , T23 )|2 as a function of two-delay time channels on a time scale of tens of hundred fs. Starting practically from the scratch, three years ago, a great deal of expertise has been, meanwhile, acquired that has enabled realistic access to the true difficulties and the most relevant problems dealing with the particulars & details in the installation of true 2-D, time-resolved (TR) photon- echo 2-D techniques.(see, follow-up program P18233). Novel, because tunable 3-Pulse stimulated PE peak-shift techniques have been realized on the basis of both the Coherent 200 kHZ/OPA-sub-70 fs (Vienna) and the 1 kHz Clark /NOPA-sub 20 fs system (Munich, cooperation with E. Riedle) and applied to probe excitonic interaction and dephasing in low-molecular weight dye-type oligomers and conjugated polymers, in liquid solution (cf. Sec.2). Finally, in the last stage of the program, the Coherent RegA kHZ system could be successfully equipped by a specially adapted 200 KHz non-collinear optical parametric amplifier, NOPA. In this quite novel combination, the width of the high-repetitive nJ-pulses, meanwhile, gives rise to deliver pulses below 20 fs ! Experimental progress has paved the way into a new perception of the situation with regard to forthcoming 2-D molecular excitonic research and development and ensures the experimental feasibility of these strategies in a difficult, but exciting field. Despite the fact, that the experimental level of (phase-sensitive) 2-D optical photon- echo spectra could not be reached in this program, as an ultimate objective, the diversity of achievements made in project P14884 will have strong impact on molecular nano-scale physics and thus has substantial, long-term significance. Also, from a fundamental point-of-view, basic science measures sub-100fs site-to-site coupling and electron-phonon scatter in disordered materials, applying state-of-the art coherence techniques is of great relevance, being highly exciting and novel for molecular excitonics.