Two-dimensional Photon Echo Spectra Research Articles

Efficient tool to calculate two-dimensional optical spectra for photoactive molecular complexes.

We combine the coherent modified Redfield theory (CMRT) with the equation of motion-phase matching approach (PMA) to calculate two-dimensional photon-echo spectra for photoactive molecular complexes with an intermediate strength of the coupling to their environment. Both techniques are highly efficient, yet they involve approximations at different levels. By explicitly comparing with the numerically exact quasiadiabatic path integral approach, we show for the Fenna-Matthews-Olson complex that the CMRT describes the decay rates in the population dynamics well, but final stationary populations and the oscillation frequencies differ slightly. In addition, we use the combined CMRT+PMA to calculate two-dimensional photon-echo spectra for a simple dimer model. We find excellent agreement with the exact path integral calculations at short waiting times where the dynamics is still coherent. For long waiting times, differences occur due to different final stationary states, specifically for strong system-bath coupling. For weak to intermediate system-bath couplings, which is most important for natural photosynthetic complexes, the combined CMRT+PMA gives reasonable results with acceptable computational efforts.

Nonlinear spectroscopy of photon-dressed Dirac electrons in a quantum dot

We study the localization of dressed Dirac electrons in a cylindrical quantum dot (QD) formed on monolayer and bilayer graphene by spatially different potential profiles. Short-lived excitonic states which are too broad to be resolved in linear spectroscopy are revealed by cross-peaks in the photon-echo nonlinear technique. Signatures of the dynamic gap in the two-dimensional photon-echo spectra are discussed.

Unraveling the Correlated Dynamics of the Double Hydrogen Bonds of Nucleic Acid Base Pairs in Solution

Quantum mechanics/molecular mechanics hybrid simulations (CPMD/GROMOS) of nonlinear infrared spectra are performed for a modified adenine-uracil base pair in CDCl(3) solution. Employing a mapping between hydrogen bond distances and fundamental as well as overtone transition frequencies on the basis of on-the-fly snapshot potential energy curves, energy gap correlation functions are established. These correlation functions are utilized to determine pump-probe and two-dimensional photon echo spectra. Analysis of the latter yields off-diagonal peaks signifying correlated fluctuations of the N-H···N and N-H···O hydrogen bonds.

Optical coherence and theoretical study of the excitation dynamics of a highly symmetric cyclophane-linked oligophenylenevinylene dimer

Optoelectronic properties of a polyphenylenevinylene-based oligomer and its paracylophane-linked dimer are studied using a variety of experimental and theoretical techniques. Despite the symmetrical structure and redshifted absorption of the dimer versus the monomer, an exciton picture is not the most appropriate. Electronic structure calculations establish changes in charge density upon optical excitation and show localized excitations that cannot be accounted for by a simple Frenkel exciton model. Visible frequency pump-probe anisotropy measurements suggest that the dimer should be considered as a three-level system with a fast, approximately 130 fs, internal conversion from the higher to lower energy excited electronic state. Signatures of nuclear relaxation processes are compared for electric field-resolved transient grating and two-dimensional photon echo spectra. These measurements reveal that nuclear relaxation occurs on similar time scales for the monomer and dimer. The connection between the spectral phase of four-wave mixing signals and the time dependent width of a nuclear wave packet is discussed. Semiempirical electronic structure and metropolis Monte Carlo calculations show that the dominant line broadening mechanisms for the monomer and dimer are associated with inter-ring torsional coordinates. Together, the theoretical calculations and electric field-resolved four-wave mixing experiments suggest that while the structure of dimer is more rigid than that of monomer, the difference in their rigidities is not sufficient to slow down excited state relaxation of dimer with respect to the monomer.

Propagation effects on the peak profile in two-dimensional optical photon echo spectroscopy

The effect of pulse propagation on the lineshape in a two-dimensional photon echo spectrum is investigated using the coupled Maxwell Bloch equations. Reshaping of excitation and signal pulses due to propagation results in broadening of peak profiles. The amount of broadening is not symmetric along the two dimensions and depends on pulse intensity and relaxation mechanism. It is found that relaxation mechanisms that give the same lineshape at low optical density (OD) can give different lineshapes at high OD. Even at relatively low OD the reshaping can be quite severe, and might result in misinterpretation of the obtained spectrum.

Effect of pulse propagation on the two-dimensional photon echo spectrum of multilevel systems

The effect of pulse propagation on the two-dimensional photon echo (2DPE) spectrum of multilevel systems is investigated using a perturbative method. At high optical densities (OD) peak profiles are broadened asymmetrically, in most cases more strongly along the ω2 direction than along the ω1 direction. The amount of broadening is determined both by the OD and by the dynamics of the system. In addition, especially if the different transitions in the system are of unequal strength, the relative intensity of the peaks changes with OD. But even if the transition strengths are the same, the behavior of the cross peaks is different from the diagonal peaks. Since peak shape and relative intensity are important parameters in the interpretation of 2DPE spectra, such OD effects should be taken into account.