A coherent nonlinear optical signal induced by electron correlations

The correlated behavior of electrons determines the structure and optical properties of molecules, semiconductors, and other systems. Valuable information on these correlations is provided by measuring the response to femtosecond laser pulses, which probe the very short time period during which the excited particles remain correlated. The interpretation of four-wave-mixing techniques, commonly used to study the energy levels and dynamics of many-electron systems, is complicated by many competing effects and overlapping resonances. Here we propose a coherent optical technique, specifically designed to provide a background-free probe for electronic correlations in many-electron systems. The proposed signal pulse is generated only when the electrons are correlated, which gives rise to an extraordinary sensitivity. The peak pattern in two-dimensional plots, obtained by displaying the signal versus two frequencies conjugated to two pulse delays, provides a direct visualization and specific signatures of the many-electron wave functions.     

Two-dimensional infrared surface spectroscopy for CO on Cu(100): detection of intermolecular coupling of adsorbates

Surface-specific infrared signals obtained by subjecting the system to two infrared laser pulses are calculated for an admixture of CO and isotopic CO on Cu(100) by using molecular dynamics simulation based on a stability matrix formalism. The two-dimensional profiles of the signals in the frequency domain show both diagonal and cross peaks. The former peaks mainly arise from the overtones of the CO and isotopic CO, while the latter represent the couplings between those. As temperature is increased, the phases of cross peaks in a second-order infrared response function change significantly, while those of diagonal peaks are unchanged. The authors show that the phase shifts are originated from the potential anharmonicities due to the electronic interaction between adsorbates. Using a model with two dipole moments, they find that the frustrated rotational mode activated with temperature has effects on the anharmonicity. These results indicate that two-dimensional infrared surface spectroscopy reveals the anharmonic couplings between adsorbates and surface atoms or between adsorbates which cannot be observed in first-order spectroscopy.     

First-principles simulation of amide and aromatic side chain ultraviolet spectroscopy of a cyclic dipeptide

By combining time-dependent density functional theory (TDDFT) and molecular dynamics (MD) simulations, we calculate the ultraviolet absorption and circular dichroism (CD) of a cyclic dipeptide, cyclo(L-Pro-D-Tyr), in the 185-300 nm region. The absorption is dominated by the phenol chromophore of tyrosine. The CD spectrum shows both phenol and amide units transitions. A crude coherent two-dimensional ultraviolet spectrum (2DUV) calculated by neglecting the two-excitation states shows a cross-peak between two transitions of the phenol in the tyrosine side chain. Additional cross-peaks between the side chain and the backbone are observed when using a chirality-induced pulse polarization configuration.     

Novel coherent two-dimensional optical spectroscopy probes of chirality exchange and fluctuations in molecules

We demonstrate how stochastic transitions between molecular configurations with opposite senses of chirality may be probed by 2D optical signals with specific pulse polarization configurations. The third-order optical response of molecular dimers (such as biphenyls) with dynamical axial chirality is calculated to order of k(2) in the wavevector of light. Spectroscopic signatures of equilibrium chirality fluctuations are predicted for three dynamical models (Ornstein-Uhlenbeck, two-state jump, and diffusion in double well) of the dihedral angle that controls the chirality.     

Sequential decay involving multiple continua

We have utilized the Green's function method to derive an explicit solution for the problem of sequential decay involving multiple continua with constant coupling between adjacent continua. This model system is applicable for theoretical studies of dynamics of photodissociation, predissociation and electronic quenching of polyatomic molecules.     

A Gallavotti-Cohen-Evans-Morriss Like Symmetry for a Class of Markov Jump Processes

We investigate a new symmetry of the large deviation function of certain time-integrated currents in non-equilibrium systems. The symmetry is similar to the well-known Gallavotti-Cohen-Evans-Morriss-symmetry for the entropy production, but it concerns a different functional of the stochastic trajectory. The symmetry can be found in a restricted class of Markov jump processes, where the network of microscopic transitions has a particular structure and the transition rates satisfy certain constraints. We provide three physical examples, where time-integrated observables display such a symmetry. Moreover, we argue that the origin of the symmetry can be traced back to time-reversal if stochastic trajectories are grouped appropriately.     

Phase diagram and density large deviations of a nonconserving ABC model

The effect of particle-nonconserving processes on the steady state of driven diffusive systems is studied within the context of a generalized ABC model. It is shown that in the limit of slow nonconserving processes, the large deviation function of the overall particle density can be computed by making use of the steady-state density profile of the conserving model. In this limit one can define a chemical potential and identify first order transitions via Maxwell's construction, similarly to what is done in equilibrium systems. This method may be applied to other driven models subjected to slow nonconserving dynamics.     

Two-dimensional vibrational lineshapes of amide III, II, I and A bands in a helical peptide

An effective exciton Hamiltonian for all amide bands is used to calculate the absorption and photon echo spectra of a 17 residue helical peptide (YKKKH17). The cross peak bandshapes are sensitive to the inter-band couplings. Fluctuations of the local amide frequencies of the all amide fundamental and their overtone and combination states are calculated using the multipole electric field induced by environment employing the electrostatic DFT map of N-methyl acetamide. Couplings between neighboring peptide units are obtained using the anharmonic vibrational Hamiltonian of glycine dipeptide (GLDP) at the BPW91/6-31G(d,p) level. Electrostatic couplings between non-neighboring units are calculated by a fourth rank transition multipole coupling (TMC) expansion including 1/R³ (dipole–dipole), 1/R⁴ (quadrupole–dipole), and 1/R⁵ (quadrupole–quadrupole and octapole–dipole) interactions.