Vibrational coherence transfer characterized with Fourier-transform 2D IR spectroscopy
Two-dimensional infrared (2D IR) spectroscopy of the symmetric and asymmetric C[Triple Bond]O stretching vibrations of Rh(CO)(2)acac in hexane has been used to investigate vibrational coherence transfer, dephasing, and population relaxation in a multilevel vibrational system. The transfer of coherence between close-lying vibrational frequencies results in extra relaxation-induced peaks in the 2D IR spectrum, whose amplitude depends on the coherence transfer rate. Coherence transfer arises from the mutual interaction of the bright CO stretches with dark states, which in this case reflects the mutual d-pi(*) back bonding of the Rh center to both the terminal carbonyls and the acetylacenonate ligand. For 2D IR relaxation experiments with variable waiting times, coherent dynamics lead to the modulation of peak amplitudes, while incoherent population relaxation and exchange results in the growth of the relaxation-induced peaks. We have modeled the data by propagating the density matrix with the Redfield equation, incorporating all vibrational relaxation processes during all three experimental time periods and including excitation reorientation effects arising from relaxation. Coherence and population transfer time scales from the symmetric to the asymmetric stretch were found to be 350 fs and 3 ps, respectively. We also discuss a diagrammatic approach to incorporating all vibrational relaxation processes into the nonlinear response function, and show how coherence transfer influences the analysis of structural variables from 2D IR spectroscopy.
[Show abstract] [Hide abstract] ABSTRACT: We utilize two-color two-dimensional infrared spectroscopy to measure the intermolecular coupling between azide ions and their surrounding water molecules in order to gain information about the nature of hydrogen bonding of water to ions. Our findings indicate that the main spectral contribution to the intermolecular cross-peak comes from population transfer between the asymmetric stretch vibration of azide and the OD-stretch vibration of D(2)O. The azide-bound D(2)O bleach/stimulated emission signal, which is spectrally much narrower than its linear absorption spectrum, shows that the experiment is selective to solvation shell water molecules for population times up to ~500 fs. The waters around the ion are present in an electrostatically better defined environment. Afterwards, ~1 ps, the sample thermalizes and selectivity is lost. On the other hand, the excited state absorption signal of the azide-bound D(2)O is much broader. The asymmetry in spectral width between bleach/stimulated emission versus excited absorption has been observed in very much the same way for isotope-diluted ice Ih, where it has been attributed to the anharmonicity of the OD potential.0Comments 10Citations
- "The intensity of that band rises very quickly and is essentially fully established already after 250 fs (i.e., the earliest population time for which artifacts from the pulse overlap are sufficiently small). At a first sight, this result is very counter-intuitive since typical population transfer times between vibrations are many picoseconds up to hundreds of picoseconds [13,353637. However, the time it takes to establish a quasi-equilibrium of population and depopulation of the D 2 O excited state is dictated by the fastest rate in the level scheme ofFig. "
[Show abstract] [Hide abstract] ABSTRACT: The description of excited state dynamics in energy transfer systems constitutes a theoretical and experimental challenge in modern chemical physics. A spectroscopic protocol that systematically characterizes both coherent and dissipative processes of the probed chromophores is desired. Here, we show that a set of two-color photon-echo experiments performs quantum state tomography (QST) of the one-exciton manifold of a dimer by reconstructing its density matrix in real time. This possibility in turn allows for a complete description of excited state dynamics via quantum process tomography (QPT). Simulations of a noisy QPT experiment for an inhomogeneously broadened ensemble of model excitonic dimers show that the protocol distills rich information about dissipative excitonic dynamics, which appears nontrivially hidden in the signal monitored in single realizations of four-wave mixing experiments.0Comments 29Citations
- "The reduced density matrix of the system is ρðTÞ ¼ Tr B ρ total ðTÞ, where the trace is over the degrees of freedom of the bath. If the initial state is a product, ρ total ð0Þ ¼ ρð0Þ ⊗ ρ B ð0Þ (always with the same initial bath state ρ B ð0Þ), then the evolution of the system may be expressed as a linear transformation (13): ρ ab ðTÞ ¼ ∑ cd χ abcd ðTÞρ cd ð0Þ:  The central object of this article is the process matrix χðTÞ, which is independent of the initial state ρð0Þ. As opposed to master equations that are written in differential form, Eq. 1 can be regarded as an integrated equation of motion for every T. It holds both for Markovian and non-Markovian dynamics of the bath, and it always leads to positive density matrices. "
[Show abstract] [Hide abstract] ABSTRACT: Is it possible to infer the time evolving quantum state of a multichromophoric system from a sequence of two-dimensional electronic spectra (2D-ES) as a function of waiting time? Here we provide a positive answer for a tractable model system: a coupled dimer. After exhaustively enumerating the Liouville pathways associated to each peak in the 2D-ES, we argue that by judiciously combining the information from a series of experiments varying the polarization and frequency components of the pulses, detailed information at the amplitude level about the input and output quantum states at the waiting time can be obtained. This possibility yields a quantum process tomography (QPT) of the single-exciton manifold, which completely characterizes the open quantum system dynamics through the reconstruction of the process matrix. This is the first of a series of two articles. In this manuscript, we specialize our results to the case of a homodimer, where we prove that signals stemming from coherence to population transfer and viceversa vanish upon isotropic averaging, and therefore, only a partial QPT is possible in this case. However, this fact simplifies the spectra, and it follows that only two polarization controlled experiments (and no pulse-shaping requirements) suffice to yield the elements of the process matrix which survive under isotropic averaging. The angle between the two site transition dipole moments is self-consistently obtained from the 2D-ES. Model calculations are presented, as well as an error analysis in terms of the angle between the dipoles and peak overlap. In the second article accompanying this study, we numerically exemplify the theory for heterodimers and carry out a detailed error analysis for such case. This investigation provides an important benchmark for more complex proposals of quantum process tomography (QPT) via multidimensional spectroscopic experiments.0Comments 0Citations
- "Alternatively, by going back to the time-domain picture provided by the authors in their previous work , and applying novel concepts of QPT for initially correlated states949596, a coarse grained and consistent tomographic protocol could be designed to address this problem. Finally, it might be worth considering additional nonlinear optical spectroscopic techniques such as considering the analysis of both rephasing and non-rephasing signals [59, 97], transient grating , pump probe , or phase cycling of multipulse induced fluorescence  to investigate if they provide additional information for a more robust QPT. Albeit this article not being exhaustive, we hope to have convinced the reader that the QPT approach follows the spirit of MDOS in a very natural way. "