Spectral diffusion at the water/lipid interface revealed by two-dimensional fourth-order optical spectroscopy: a classical simulation study

Using a classical simulation protocol for nonlinear optical signals, we predict the two-dimensional (2D) spectra of water near a monolayer of [1,2-dimytristoyl-sn-glycero-3-phosphatidylcholine] (DMPC) generated by three IR probe pulses followed by one visible probe pulse. Sum-frequency-generation 1D spectra show two peaks of the OH stretch representing two environments: near-bulk water nonadjacent to DMPC and top-layer water adjacent to DMPC. These peaks create a 2D pattern in the fourth-order signal. The asymmetric cross-peak pattern with respect to the diagonal line is a signature of coherence transfer from the higher- to the lower-frequency modes. The nodal lines in the imaginary part of the 2D spectrum show that the near-bulk water has fast spectral diffusion resembling that of bulk water despite the orientation by the strong electrostatic field of DMPC. The top-layer water has slower spectral diffusion.     

Simulation and visualization of attosecond stimulated x-ray Raman spectroscopy signals in trans-N-methylacetamide at the nitrogen and oxygen K-edges

The stimulated Raman component of the pump-probe spectrum of trans-N-methylacetamide obtained in response to two soft x-ray pulses is calculated by treating the core excitations at the Hartree-Fock static-exchange level. The signal reveals the dynamics of valence-electron wave packets prepared and detected in the vicinity of a selected atom (either nitrogen or oxygen). The evolving electronic charge density as well as electronic coherence of the doorway and the window created by the two pulses are visualized using a time-dependent basis set of natural orbitals, which reveals that the wave packets consist of several entangled valence particle-hole pairs.     

Coherent control protocol for separating energy-transfer pathways in photosynthetic complexes by chiral multidimensional signals

Adaptive optimizations performed using a genetic algorithm are employed to construct optimal laser pulse configurations that separate spectroscopic features associated with the two main energy-transfer pathways in the third-order nonlinear optical response simulated for the Fenna-Matthews-Olson (FMO) photosynthetic complex from the green sulfur bacterium Chlorobium tepidum. Superpositions of chirality-induced tensor components in both collinear and noncollinear pulse configurations are analyzed. The optimal signals obtained by manipulating the ratios of various 2D spectral peaks reveal detailed information about the excitation dynamics.     

Two-dimensional near-ultraviolet spectroscopy of aromatic residues in amyloid fibrils: a first principles study

We report a first principles study of two dimensional electronic spectroscopy of aromatic side chain transitions in the 32-residue β-amyloid (Aβ(9-40)) fibrils in the near ultraviolet (250-300 nm). An efficient exciton Hamiltonian with electrostatic fluctuations (EHEF) algorithm is used to compute the electronic excitations in the presence of environmental fluctuations. The through-space inter- and intra-molecular interactions are calculated with high level quantum mechanics (QM) approaches, and interfaced with molecular mechanics (MM) simulations. Distinct two dimensional near ultraviolet (2DNUV) spectroscopic signatures are identified for different aromatic transitions, and the couplings between them. 2DNUV signals associated with the transition couplings are shown to be very sensitive to the change of residue-residue interactions induced by residue mutations. Our simulations suggest that 2DNUV spectra could provide a useful local probe for the structure and kinetics of fibrils.     

Q-switching an all-fiber laser using acousto-optic null coupler

A new method for Q-switching an all-fiber laser is presented. It is based on induced acoustic long period grating operating on a null coupler, which acts as acoustically controlled tunable output coupler. Q-switching is achieved by switching on and off the acoustic wave in a burst mode, thereby generating laser pulses that are ~400 times shorter than the acoustically controlled coupler’s rise time. Output pulse energy of 22 μJ and temporal width of ~100 ns were measured at a wavelength of 1.54 μm.     

The Formation of Pd Nanocrystals from Pd2(dba)3 Microcrystals

As‐grown platelets formed from tris‐(dibenzylideneacetone) dipalladium(0) [Pd₂(dba)₃] precursor in the presence of Pd17 RNA are investigated before and after thermal annealing. Results show that as‐grown platelets are disordered crystals of Pd₂(dba)₃ containing 1?2 nm Pd clusters and platelets grown in the absence of RNA are morphologically and structurally similar to those formed with RNA. The initially formed crystals are so sensitive to environmental variables that the degree of crystallinity can not be determined accurately by electron diffraction. X‐ray crystallography on as‐grown platelets gives a crystal structure consistent with Pd₂(dba)₃, but reveals a composition of ≈Pd_1.07(dba)₃, indicating one Pd atom in Pd₂(dba)₃ is lost from the structure. Both electron beam and thermally induced decomposition of as‐grown Pd₂(dba)₃ platelets having a hexagonal habit on the micrometer scale produces elemental Pd platelets having a hexagonal habit on the nanometer scale. These hexagonal platelets are composed of a partially sparse form of Pd₂(dba)₃ that is initially crystalline but rapidly degrades due to the loss of Pd atoms from organic ligand cages. Once released, Pd atoms aggregate to form Pd clusters, which grow and transform into well‐formed Pd nanocrystals under electron‐beam irradiation or through thermal annealing.     

Stochastic Liouville equation simulation of multidimensional vibrational line shapes of trialanine

The line shapes detected in coherent femtosecond vibrational spectroscopies contain direct signatures of peptide conformational fluctuations through their effect on vibrational frequencies and intermode couplings. These effects are simulated in trialanine using a Green's function solution of a stochastic Liouville equation constructed for four collective bath coordinates (two Ramachandran angles affecting the mode couplings and two diagonal energies). We find that fluctuations of the Ramachandran angles which hardly affect the linear absorption can be effectively probed by two-dimensional spectra. The signal generated at k(1)+k(2)-k(3) is particularly sensitive to such fluctuations.     

Ultrasonic transcutaneous energy transfer for powering implanted devices

This paper investigates ultrasonic transcutaneous energy transfer (UTET) as a method for energizing implanted devices at power level up to a few 100 mW. We propose a continuous wave 673 kHz single frequency operation to power devices implanted up to 40 mm deep subcutaneously. The proposed UTET demonstrated an overall peak power transfer efficiency of 27% at 70 mW output power (rectified DC power at the load).The transducers consisted of PZT plane discs of 15 mm diameter and 1.3 mm thick acoustic matching layer made of graphite. The power rectifier on the implant side attained 88.5% power transfer efficiency.The proposed approach is analyzed in detail, with design considerations provided to address issues such as recommended operating frequency range, acoustic link matching, receiver’s rectifying electronics, and tissue bio-safety concerns. Global optimization and design considerations for maximum power transfer are presented and verified by means of finite element simulations and experimental results.     

Rigid backbone polymers. X. Transitions in bulk polyisocyanates

Data obtained by hot-stage cross-polarized light microscopy and thermogravimetric analysis were used to plot a transition map for the poly(n-alkylisocyanate) homologous family. Most members had a low-temperature transition (or relaxation). The middle members of the family, with side-chain length of 4 ≤ n < 13 carbon atoms, exhibit a mesomorphic range of temperatures. Members with sidechains of n ≥ 13 in length, go directly from the low-temperature transition to the melting point without passage through a liquid crystalline state. A transition map for two homopolymers and a series of their copolymers, poly[x mol% n-butyl + (100-x)mol% p-anisole-3-propyl]isocyanate, was also prepared. The range of mesomorphicity as a function of composition and temperature is clearly indicated.