A. Tortschanoff

Studying vibrational wavepacket dynamics by measuring fluorescence interference fluctuations

The principle of coherence observation by interference noise [COIN, Kinrot et al., Phys. Rev. Lett. 75, 3822 (1995)] has been applied as a new approach to measuring wavepacket motion. In the COIN experiment pairs of phase-randomized femtosecond pulses with relative delay time τ prepare interference fluctuations in the excited state population, so the correlated noise of fluorescence intensity—the variance varF(τ)—directly mimics the dynamics of the propagating wavepacket. The scheme is demonstrated by measuring the vibrational coherence of wavepacket motion in the B-state of gaseous iodine. The COIN interferograms obtained recover propagation, recurrences and spreading as the typical signature of wavepackets. The COIN measurements were performed with precisely tuned excitation pulses which cover the bound part of the B-state surface up to the dissociative limit. In combination with preliminary numerical calculations, comparison has been made with results from previous phase-locked wavepacket interferometr...

Molecular quantum dynamics in a thermal system: Fractional wave packet revivals probed by random-phase fluorescence interferometry

The method of coherence observation by interference noise (COIN) [Kinrot et al., Phys. Rev. Lett. 75, 3822 (1995)] has been shown to be a useful tool for measurements of wave packet motion at the quantum-classical border. We present the first systematic interferometric study of fractional vibrational revivals in the B state of thermal iodine (I2) vapor. Experimental COIN interferograms ranging from 200 fs to 40 ps are presented for various excitation wavelengths. The complex temporal structure of the observed fluorescence includes rapid initial damping in the short-time regime and the appearance of quarter- and half-revivals on the quantum-mechanical long-time scale. These features arise from a delicate balance between rotational and vibrational molecular coherences. The clear observation of the wave packets on the long time scale is possible due to the long-time stability of the COIN interferometer. Lowest-order perturbative solutions nicely recover the experimental results, and closed-form analytical ex...

Femtosecond excitation tuning and site energy memory of population transfer in poly(p-phenylenevinylene): Gated luminescence experiments and simulation

We present a comprehensive experimental and computational study on fs-relaxational dynamics of optical excitations in the conjugated polymer poly(p-phenylenevinylene) (PPV) under selective excitation tuning conditions into the long-wavelength, low-vibrational S1ν=0-density-of-states (DOS). The dependence of single-wavelength luminescence kinetics and time-windowed spectral transients on distinct, initial excitation boundaries at 1.4 K and at room temperature was measured applying the luminescence up-conversion technique. The typical energy-dispersive intra-DOS energy transfer was simulated by a combination of static Monte Carlo method with a dynamical algorithm for solving the energy-space transport Master-Equation in population-space. For various, selective excitations that give rise to specific S1-population distributions in distinct spatial and energetic subspaces inside the DOS, simulations confirm the experimental results and show that the subsequent, energy-dissipative, multilevel relaxation is hier...

Correlation of femtosecond wave packets and fluorescence interference in a conjugated polymer: Towards the measurement of site homogeneous dephasing

Probing electronic femtosecond (fs) coherence among segmental sites that are congested by static and dynamic site disorder and subject to structural relaxation is a big, experimental challenge in the study of photophysics of poly(p-phenylenevinylene). In this work, fs-wave-packet fluorescence interferometry experiments are presented that measure macroscopic coherent kernels and their phase-relaxation in the low-temperature, bottom-state regime of the density-of-states below the migrational threshold energy where downhill site-to-site transfer is marginal. By using freely propagating and tunable 70 fs excitation/probing pulses and employing narrow-band spectral filtering of wave packets, fluorescence interferograms with strongly damped beatings can be observed. The coherences formally follow the in-phase superpositions of two site-optical free-induction-decays and originate from distinct pairs of coherent doorway-states, different in energy and space, each of them being targeted, by two discrete quantum-arrival-states 1(alpha) and 1(beta), via independent, isoenergetic 0-->1 fluorescence transitions. The coherent transients are explained as site-to-site polarization beatings, caused by the interference of two fluorescence correlation signals. The numerical analysis of the damping regime, based upon second-order perturbational solutions, reveals the lower limit value of homogeneous dephasing in the range from T(2) approximately 100 fs to T(2) approximately 200 fs depending on the site-excitation energy of the bottom-states. The experiments enable to look into the formation of the relaxed state as a special molecular process of electron-phonon coupling and hence open-up a quite new perspective in the puzzle of multichromophore optical dynamics and structural relaxation in conjugated polymers.

Excitonic fs-luminescence rise in poly(phenylenevinylene), PPV

Abstract In the so-called molecular approach a realistic PPV chain is considered to be a collection of segmental subunits separated by a distribution of breaks, which arises by statistical disorder and predicts (i) a variation of conjugation lengths and (ii) a density of states (DOS) of localized S 1 -levels. In this picture, the asymptotic optical dynamics is triggered by energy relaxation toward the low-energy tail states of the DOS and gives rise (i) to transient red shifts and (ii) to fs-luminescence patterns dependent on the spectral position of the luminescence window. In this work the delicate fs-interplay of both evolving and decaying luminescence photons caused by interference, due to inhomogeneous broadening (IHB), and the vibrational spacings in the Franck-Condon (FC) progression, has been studied. High-precision fs-luminescence rise -profiles have been probed at e = 2.25 and 2.10 eV , respectively, that provide, for the first time, direct experimental evidence for the sequential dynamics of molecular hopping excitations in the segmental environment of PPV.

Coherence from fluorescence correlations: Oscillatory femtosecond fluorescence in pentacene/p-terphenyl

Spontaneous fluorescence has been used to measure the coherent femtosecond response of the organic, binary crystal pentacene/p-terphenyl. By using two-pulse excitation with phase-randomized pulses in an interferometric setup and analyzing the variance of the fluctuating intensity of correlated fluorescence photons, femtosecond beatings have been observed. The pattern of these terahertz oscillations is strongly dependent on the detuning frequency range of the exciting pulses, but is rather invariant with regard to the spectral position of the fluorescence probe window. In the interferometric regime of freely propagating pulses novel, ultrafast fluorescence carrier-wave oscillations superimposed to the beat structure have been obtained. The oscillatory signals evolve from a coherent superposition of optical free induction decays, caused by the different electronic transition energies of the pentacene absorber sites O1, O2, O3, and O4, respectively, are monitored as intrinsic, heterodyne beats by the fluores...

Ultrafast optical dynamics of spiro-compounds

We report on time-resolved measurements of spiro-type molecules in the ps- and fs-timescale. Due to the orthogonality of the transition dipoles of two identical moieties connected through a spiro-atom, one expects these compounds to show two perpendicular, degenerate states that may enter into a complex free evolution upon fs excitation. On ps scales the bichromophoric units behave as isolated absorbers, thus their fluorescence stems from localized site populations. Level relaxation gives rise to slightly non-exponential relaxation that can be quantified in terms of underlying life-time distributions by employing numerical Laplace inversion. Fs fluorescence upconversion experiments have been employed to elucidate, for the first time, nuclear and electronic dynamics subsequent to broad-band fs-preparation.

Electronic coupling and coherences in disordered polymers: Femtosecond 2D-photon echo correlation spectroscopy, signatures of an excitonic two-segmental site system: A theoretical study

The two-dimensional (2D) temporal shape of the (degenerate) three-pulse photon-echo (3-PPE) polarization P(3)(t′,τ12,τ23=0) has been theoretically analyzed for a weakly interacting two-site system (TSS) in the presence of site inhomogeneous broadening. The TSS has been modeled in terms of two identical, energy-degenerate, excitonically coupled pairs of electronic states in the site-representation (i.e., two S0 and S1 states forming a 4-level system). The 2D time-domain signal S(t′,τ12,τ23=0) has been derived in a closed-form solution which allowed to formulate the analytical Fourier transform analog under sequential δ-pulse excitation. The resulting 2D PE signal in frequency–frequency space S(ωt′,ωτ12) is formed in the region of the resonant, optical carrier frequency by, generally, four peaks, arranged at the edges of a square with distances determined by the excitonic coupling. The two diagonal peaks are of one-exciton origin, while the two off-diagonal (cross) peaks involve two-exciton effects, too. Th...

Fluorescence interferometry with random phased pulses: Diagonal site disorder and vibrational effects in molecular solids—A theoretical treatment

The technique of COIN (coherence observation by interference noise) relies on the measurement of correlated fluorescence fluctuations subsequent to two-pulse excitation with randomized relative phase [Kinrot et al., Phys. Rev. Lett. 75, 3822 (1995)]. In this paper, a comprehensive theoretical study on the use of random-phase fluorescence interferometry in the measurement of solid-state, molecular coherence is given. Optical response functions in terms of populations expanded to second order in a perturbational approach are presented for a guest/host system that are based on a generalized Bloch–Liouville formalism including inhomogeneous site broadening and harmonic vibrational modes. The theoretical treatment is intended to explain the coherence and coherence loss mechanisms measured, very recently, in the pentacene/p-terphenyl mixed crystal at low phonon temperatures. The expressions derived within the limits of the usual approximations are quite general and valid for both δ- and finite-width pulses. The...

Coherence from Femtosecond Fluorescence Interference Noise-Terahertz Electronic Beatings in Pentacene

By measuring fluctuations of fluorescence correlations in Pentacene/p-Terphenyl novel fs-beats are observed. They arise from polarisation interferences due to descrete diagonal disorder between spatially different Pentacene sites.