Review of the Theoretical Description of Time‐Resolved Angle‐Resolved Photoemission Spectroscopy in Electron‐Phonon Mediated Superconductors

We review recent work on the theory for pump/probe photoemission spectroscopy of electron‐phonon mediated superconductors in both the normal and the superconducting states. We describe the formal developments that allow one to solve the Migdal‐Eliashberg theory in nonequilibrium for an ultrashort laser pumping field, and explore the solutions which illustrate the relaxation as energy is transferred from electrons to phonons. We focus on exact results emanating from sum rules and approximate numerical results which describe rules of thumb for relaxation processes. In addition, in the superconducting state, we describe how Anderson‐Higgs oscillations can be excited due to the nonlinear coupling with the electric field and describe mechanisms where pumping the system enhances superconductivity.     

Coherent and turbulent processes in the bistable regime around a tandem of cylinders including reattached flow dynamics by means of high-speed PIV

The turbulent flow around two cylinders in tandem at the sub-critical Reynolds number range of order of 10⁵ and pitch to diameter ratio of 3.7 is investigated by using time-resolved Particle Image Velocimetry (TRPIV) of 1 kHz and 8 kHz. The bi-stable flow regimes including a flow pattern I with a strong vortex shedding past the upstream and the downstream cylinder, as well as a flow pattern II corresponding to a weak alternating vortex shedding with reattachment past the upstream cylinder are investigated. The structure of this “reattachment regime” has been analyzed in association with the vortex dynamics past the downstream cylinder, by means of POD and phase-average decomposition. These elements allowed interconnection among all the measured PIV planes and hence analysis of the reattachment structure and the flow dynamics past both cylinders. The results highlight fundamental differences of the flow structure and dynamics around each cylinder and provide the ‘gap’ flow nature between the cylinders. Thanks to a high-speed camera of 8 kHz, the shear-layer vortices tracking has been possible downstream of the separation point and the quantification of their shedding frequency at the present high Reynolds number range has been achieved. This issue is important regarding fluid instabilities involved in the fluid–structure interaction of cylinder arrays in nuclear reactor systems, as well as acoustic noise generated from the tandem cylinders of a landing gear in aeronautics.     

Combination of LFP‐TRIR spectroscopy and DFT computation as a tool to determine the intermediate during the photooxidation of triarylphosphine

Laser flash photolysis‐time‐resolved infrared spectroscopy (LFP‐TRIR) was performed on an acetonitrile or dichloromethane solution of triarylphosphines, Ar₃P, in air. A transient spectrum consisting of several absorption bands appeared in the region of 1050–1300 cm^?1 on the TRIR on a microsecond timescale, which disappeared on a millisecond timescale. To identify the observed transient intermediate, the IR spectra of possible intermediates of the photoreaction were simulated by theoretical calculations based on density functional theory (DFT). The IR spectrum simulated for the phosphine peroxidic radical cation, Ar₃P⁺OO^?, well predicted the observed IR spectrum, showing that Ar₃P⁺OO^? is formed as a transient intermediate upon the LFP of Ar₃P in air. Copyright © 2014 John Wiley & Sons, Ltd.     

Analyzing ultrafast multiplex coherent anti‐Stokes Raman spectra with picosecond probing

This article reports an efficient method to simulate time and frequency resolved coherent anti‐Stokes Raman scattering spectra measured with picosecond pump and probe fields and ultrashort Stokes pulses. A systematic comparison of measured and simulated time and frequency dependent data is presented for carbon tetrachloride, chloroform, cyclohexane, octane, and poly(methyl methacrylate). While the first compound exhibits no Raman active modes in the considered spectral region of the CH‐stretch vibrations, the other ones show Raman spectra of increasing complexity. Vibrational frequencies and homogeneous dephasing rates are extracted by fitting explicit analytical formulas to the recorded data. Interference between nonresonant and resonant contributions to the nonlinear polarization is taken fully into account. The ability to measure the influence of inhomogeneous broadening is discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

基于单模光纤的交叉相位调制型频率分辨光学开关超短脉冲测量

利用单模光纤中的光弹效应和交叉相位调制(XPM)效应,提出了一种频率分辨光学开关法测量超短脉冲的新方案.在本方案中,单模光纤的前一部分产生可变延迟,后一部分作为非线性介质产生非线性效应.该方案只需一根单模光纤,无须复杂的光路校准,结构简单,损耗低;光纤中的XPM效应易发生,无须相位匹配.对提出的方案进行了数值模拟,采用基于矩阵的主元素广义投影算法,恢复出待测脉冲的幅度和相位信息,并研究了光纤长度和待测超短脉冲的脉冲宽度对测量结果的影响.结果表明:测量准确度随着光纤长度的增加而提高,选取长度为2 km的光纤,就可以实现对超短脉冲的准确测量;本文方案适用于脉冲宽度不小于80 fs的超短光脉冲的测量.     

New experimental measurement of natSe(n, γ) cross section between 1 eV to 1 keV at the CSNS Back-n facility

The ⁷⁴Se is one of 35 p-nuclei, and ⁸²Se is a r-process only nucleus, and their (n, γ) cross sections are vital input parameters for nuclear astrophysics reaction network calculations. The neutron capture cross section in the resonance range of isotopes and even natural selenium samples has not been measured. Prompt γ-rays originating from neutron-induced capture events were detected by four C₆D₆ liquid scintillator detectors at the Back-n facility of China Spallation Neutron Source (CSNS). The pulse height weighting technique (PHWT) was used to analyze the data in the 1 eV to 100 keV region. The deduced neutron capture cross section was compared with ENDF/B-VIII.0, JEFF-3.2, and JENDL-4.0, and some differences were found. Resonance parameters were extracted by the R-matrix code SAMMY in the 1 eV-1 keV region. All the cross sections of ^natSe and resonance parameters are given in the datasets. The datasets are openly available at http://www.doi.org/10.11922/sciencedb.j00113.00019.     

Mechanism of UV-induced formation of Dewar lesions in DNA

The importance of a backbone: The mechanism of formation of Dewar lesions has been investigated by using femtosecond IR spectroscopy and ab?initio calculations of the exited state. The 4π?electrocyclization is rather slow, occurs with an unusual high quantum yield, and--surprisingly--is controlled by the phosphate backbone.     

Two‐Photon Absorption and Time‐Resolved Stimulated Emission Depletion Spectroscopy of a New Fluorenyl Derivative

The synthesis, comprehensive linear photophysical characterization, two‐photon absorption (2PA), steady‐state and time‐resolved stimulated emission depletion properties of a new fluorene derivative, (E)‐1‐(2‐(di‐p‐tolylamino)‐9,9‐diethyl‐9H‐fluoren‐7‐yl)‐3‐(thiophen‐2‐yl)prop‐2‐en‐1‐one ( 1 ), are reported. The primary linear spectral properties, including excitation anisotropy, fluorescence lifetimes, and photostability, were investigated in a number of aprotic solvents at room temperature. The degenerate 2PA spectra of 1 were obtained with open‐aperture Z‐scan and two‐photon induced fluorescence methods, using a 1 kHz femtosecond laser system, and maximum 2PA cross‐sections of ～400–600 GM were obtained. The nature of the electronic absorption processes in 1 was investigated by DFT‐based quantum chemical methods implemented in the Gaussian 09 program. The one‐ and two‐photon stimulated emission spectra of 1 were measured over a broad spectral range using a femtosecond pump–probe‐based fluorescence quenching technique, while a new methodology for time‐resolved fluorescence emission spectroscopy is proposed. An effective application of 1 in fluorescence bioimaging was demonstrated by means of one‐ and two‐photon fluorescence microscopy images of HCT 116 cells containing dye encapsulated micelles.     

Hepatic lipid composition analysis using 3.0-T MR spectroscopy in a steatotic rat model

Purpose

To investigate the feasibility of in vivo assessment of hepatic lipid composition using 3.0-T proton magnetic resonance spectroscopy (¹H-MRS) in a steatotic rat model and compare it to histopathological and biochemical assessment.

Materials and Methods

Hepatic steatosis was induced by feeding rats with a methionine/choline-deficient (MCD) diet for 1, 2, 3, 5 or 7 weeks (n=5 per group). At the end of the diet period, ¹H-MRS of the liver was performed, and rats were sacrificed for histopathological and biochemical assessment of the liver. Spectra were acquired in a single voxel (1.2 cc) using a point-resolved spectroscopic sequence with TE/TR=35/2000 ms and 64 signal acquisitions. From the MR spectra, peak area ratios were calculated to estimate hepatic lipid composition.

Results

During MCD diet periods, hepatic steatosis significantly increased on histopathology (P<.001). The ¹H-MRS measurements of total hepatic fat content [1.3/(1.3+4.65) ppm] correlated strongly with histological macrovesicular hepatic steatosis (r=0.93, P<.001) and with the biochemical total hepatic fatty acids (r=0.94, P<.001). Total unsaturated fatty acids [TUFA, 5.4/(1.3+4.65) ppm] estimated with ¹H-MRS strongly correlated with the biochemical unsaturated fatty acids (r=0.90, P<.001). Polyunsaturated fatty acids [PUFA, 2.8/(1.3+4.65) ppm] estimated with ¹H-MRS strongly correlated with biochemical PUFA (r=0.91, P<.001). The proportion of total unsaturated fatty acids relative to the amount of total fatty acids (rTUFA, 5.4/1.3ppm) measured with ¹H-MRS strongly correlated with the biochemical amount of unsaturated relative to total hepatic fatty acids (r=0.81, P<.001). The proportion of PUFA relative to the amount of total fatty acids (rPUFA, 2.8/1.3 ppm) measured with ¹H-MRS correlated with the biochemical amount of PUFA relative to total fatty acids (r=0.59, P=0.005,) and with the biochemical amount of omega-6 PUFA relative to total fatty acids (r=0.73, P<.001).PUFA at ¹H-MRS correlated with the histopathologically assessed degree of lobular inflammation in the liver (r=0.57, P=.001).

Conclusion

3.0T ¹H-MRS is able to measure poly- and unsaturated hepatic fatty acids and this strongly correlates with biochemical assessment. This study provides evidence that 3.0-T ¹H-MRS is a noninvasive technique to assess hepatic lipid composition.