Brownian dynamics in Fourier space: membrane simulations over long length and time scales

A simulation algorithm for elastic membrane sheets is presented. Overdamped stochastic dynamics including hydrodynamic coupling to surrounding solvent and arbitrary external forces are generated by employing Fourier modes of the sheet as the primary dynamic variables. Simulations over the micron length scale and second time scale are easily achieved. The dynamics of a lipid bilayer attached to an underlying network of cytoskeletal filaments is used to estimate the diffusion constant of membrane-bound proteins on the surface of the red blood cell.     

傅里叶变换实时红外光谱复原的FPGA实现

为实时获取战场激光、大气污染物气体、毒气等待测物光谱信息的光谱复原,设计了基于现场可编程门阵列(FPGA) 的实时光谱采集分析系统。该系统选用迈克尔逊干涉具和碲镉汞探测器获取待测光谱信息,将采集到的数据传入FPGA。利用硬件描述语言VerilogHDL在Xilinx FPGA芯片上依据傅里叶变换(FFT)实现干涉条纹到光谱数据的实时处理。实验结果表明,FPGA实际计算1 024点基2-FFT频谱分布信息与Matlab理论计算结果相同,可满足实时光谱探测的要求。     

Relaxation dynamics of Mn2+ intraion excitation in Cd0.5Mn0.5Te: Dependence on the optical pumping level

A study is reported of the Mn²⁺ intracenter 3d luminescence in a dilute Cd_0.5Mn_0.5Te magnetic semiconductor at pulsed excitations of up to 3.5 MW/cm². At high excitation levels and at a temperature of 77 K, the kinetics varies strongly over the emission band profile. The luminescence decay curve can be resolved into a fast and a delayed component, which correspond to the excitation of extended and localized states in the manganese ion system. The fast relaxation of the extended states is largely determined by the up-conversion. As the temperature is lowered, the contribution of the fast component at the center of the emission band and in its low-energy wing decreases because of the weakening role of the extended states lying above the mobility edge.     

High resolution Fourier transform spectrum of HDO in the 7500-8200 cm region: revisited

The HDO absorption FT spectrum is recorded and analyzed in the 7500-8200 cm⁻¹ spectral region. The high accuracy ab initio calculation of Schwenke and Partridge was successfully applied for spectrum assignment that resulted in derivation of 508 precise rovibrational energy levels for the (3 0 0), (0 3 1), (1 1 1), (0 6 0), (2 2 0), and (0 0 2) states, with 295 of them being reported for the first time. In particular, eight new energy levels, including the band center at 7914.3170 cm⁻¹, were derived for the highly excited bending (0 6 0) state from transitions borrowing their intensities through local high-order resonance coupling with the (3 0 0) and (0 3 1) states.     

Analysis of the Fourier transform spectrum of 13C18O2 in the 4.3- and 16-μm regions

Fourier transform spectra have been obtained for ¹³C¹⁸O₂ in the regions of ν₂ (16 μm) and ν₃ (4.3 μm) at a resolution of 0.04 cm^?1. From a least-squares fit of the P- and R-branch lines for the transitions, values have been calculated for the rotational constants B, the centrifugal distortion constants D, and the band origins. Based on the derived constants, the calculated wavenumbers for the P and R lines of both the 01¹0-00⁰0 and 00⁰1-00⁰0 transitions agree with the observed positions within ±0.003 cm^?1. We have also observed the difference bands ν₁-ν₂ and the hot bands (ν₃ + ν₂) ? ν₂. From an analysis of these transitions we have determined values for the l-doubling constants for ν₂ and the location of the inactive fundamental ν₁.     

Complex local dynamics in DNA on the picosecond and nanosecond time scales

Time-resolved Stokes shift measurements of the local structural relaxation of three DNA oligonucleotides are presented. Logarithmic relaxation is seen for over three decades in time (40 ps-40 ns), indicating a complex relaxation among a large number of conformational substates. The observed relaxation is the same in all the sequences. Sequence dependence of the localized dynamics of DNA does not appear within this time range. We infer that 30%-50% of the relaxation is faster than 40 ps, has a nonlogarithmic decay and has a sequence dependent amplitude.     

Effect of the nonlinearity of CCD in Fourier transform profilometry on spectrum overlapping and measurement accuracy

In Fourier transform profilometry (FTP), we must restrain spectrum overlapping caused by the nonlinearity of charge coupled device (CCD) and increase the measurement accuracy of object shape. Firstly, the causes of producing higher-order spectrum components and inducing spectrum overlapping are analysed theoretically, and simple physical explanation and analytical deduction are given. Secondly, aiming to suppress spectrum overlapping and improve measurement accuracy, the influence of spatial carrier frequency of projection grating on them is analysed. A method of increasing the spatial carrier frequency of projection grating to restrain or reduce the spectrum overlapping significantly is proposed. We then analyze the mechanism of how the spectrum overlapping is reduced. Finally, the simulation results and experimental measurements verify the correction of the theory and method proposed.     

激光单稳频指标的激光计量红外光谱仪精度分析

在傅里叶变换红外光谱仪中,采用激光计量光程差可提高光谱的测量精度。激光波长作为光程差测量的基准单位,其线宽和稳频特性直接决定了光程差的误差,进而影响光谱的测量精度。根据光谱仪的干涉原理,通过傅里叶逆变换和误差分析方法,建立了光谱测量相对误差与激光单稳频指标之间的理论模型,并对该模型进行了仿真计算。结果证明,该模型可作为设计激光计量光程差光谱仪系统的理论依据。     

Effect of the nonlinearity of the CCD in Fourier transform profilometry on spectrum overlapping and measurement accuracy

In Fourier transform profilometry (FTP), we must restrain spectrum overlapping caused by the nonlinearity of the charge coupled device (CCD) and increase the measurement accuracy of the object shape. Firstly, the causes of producing higher-order spectrum components and inducing spectrum overlapping are analysed theoretically, and a simple physical explanation and analytical deduction are given. Secondly, aiming to suppress spectrum overlapping and improve measurement accuracy, the influence of spatial carrier frequency of projection grating on them is analysed. A method of increasing the spatial carrier frequency of projection grating to restrain or reduce the spectrum overlapping significantly is proposed. We then analyze the mechanism of how the spectrum overlapping is reduced. Finally, the simulation results and experimental measurements verify the correction of the proposed theory and method.     

High-resolution Fourier transform spectrum of H2S in the region of 8500-8900 cm

High-resolution Fourier transform infrared spectrum of H₂S was recorded and analyzed in the region of the polyad. More than 450 transitions were assigned to the 3ν₁ + ν₂ and 2ν₁ + ν₂ + ν₃ bands with the maximum values of quantum numbers J and K_a equal to 14, 7, and 14, 9 for these two bands, respectively. The theoretical analysis was fulfilled with the Hamiltonian which takes into account strong resonance interactions among the studied vibrational states (3 1 0), (2 1 1), and also “dark” states (0 3 2) and (2 3 0). The rms deviation is 0.0019 cm⁻¹. The intensity borrowing effect in the doublets in the P-branch transitions of the 3ν₁ + ν₂ and 2ν₁ + ν₂ + ν₃ bands is observed and discussed.     

Phase-field-crystal models for condensed matter dynamics on atomic length and diffusive time scales: an overview

Here, we review the basic concepts and applications of the phase-field-crystal (PFC) method, which is one of the latest simulation methodologies in materials science for problems, where atomic- and microscales are tightly coupled. The PFC method operates on atomic length and diffusive time scales, and thus constitutes a computationally efficient alternative to molecular simulation methods. Its intense development in materials science started fairly recently following the work by Elder et al. [Phys. Rev. Lett. 88 (2002), p. 245701]. Since these initial studies, dynamical density functional theory and thermodynamic concepts have been linked to the PFC approach to serve as further theoretical fundamentals for the latter. In this review, we summarize these methodological development steps as well as the most important applications of the PFC method with a special focus on the interaction of development steps taken in hard and soft matter physics, respectively. Doing so, we hope to present today's state of the art in PFC modelling as well as the potential, which might still arise from this method in physics and materials science in the nearby future.     

Analysis of the microwave rotation spectrum of silyl cyanide,SiH3CN,in its ground and vibrationally excited states

The rotational spectrum of silyl cyanide, SiH₃CN, has been studied by microwave spectroscopy. A detailed analysis of the transitions in the excited states of the ν₃, ν₄, ν₇, and ν₈ modes has been carried out. In particular the v₈ = 1, 2, 3, and 4 vibrational satellites have been subjected to a complete vibration-rotation analysis. Ground state transitions of ²⁸SiH₃¹²C¹⁴N, ²⁹SiH₃¹²C¹⁴N, ³⁰SiH₃¹²C¹⁴N, ²⁸SiH₃¹³C¹⁴N, and ²⁸SiH₃¹²C¹⁵N have all been studied in natural abundance. The overall study has yielded a large amount of detailed vibration-rotation data. The effects of quadrupole interactions have been taken into account. To confirm the analysis, computer simulated spectra have been compared with those observed.     

Femtosecond to nanosecond relaxation time scales in electronically excited tetrakis(dimethylamino)ethylene: identification of the intermediates

In the TDMAE molecule (title molecule), the time evolution has been analyzed from the very initial excitation step down to a fluorescent state, over widely different time scales. Pump probe measurements have been performed at 3 different excitation wavelengths 400, 266 and 200 nm. The decay has been followed over the femtosecond and subnanosecond ranges with this method and the decay of the final charge transfer state has been detected by its fluorescence emission. This allows an overview of the complete decay mechanism. The initial relaxation pathway is interpreted in a similar way to ethylenic molecules, where the initial wavepacket is quickly trapped in a doubly excited state Z with charge transfer character. Then the Z state decays slowly (10-100 picoseconds) into the final state. In difference to monoalkenes the final stage of this evolution is a charge transfer state. The decay of transient Z state to the charge transfer state is a further assessment of the partial ionic character of the Z state. This type of molecule with low ionization potential can be viewed as a demonstrative example of the interrelation between the charge induced forces and the deformations in excited state reaction dynamics. Received 17 January 2001 and Received in final form 23 February 2001     

Study of femtosecond dynamics in vibrationally excited free bis(trifluoromethyl)ketene and metal carbonyl molecules induced by CO bond resonant excitation

The dynamics of relaxation processes in free bis(trifluoromethyl)ketene (CF₃)₂CCO and Fe(CO)₅ and Cr(CO)₆ metal carbonyl molecules after multiphonon excitation of the C=C=O and C=O vibrations by femtosecond laser infrared radiation was studied. The temporal and spectral dependences of the relaxation of the excited vibrational states were measured. Kinetics with a characteristic decay time of about 5 ps was obtained for (CF₃)₂CCO molecules. Its behavior is interpreted as a manifestation of the intramolecular relaxation of the excited vibration states of the resonance mode. Kinetic curves with characteristic times of about 250–500 fs were observed for Fe(CO)₅ and Cr(CO)₆. The behavior of these curves depends on the mutual orientation of the polarizations of the pump and probe pulses.     

Fourier transform spectroscopy of the low-lying excited electronic states of FeO: A new perturbing Σ state and improved ground state constants

A small rotational perturbation has been found in the ⁵Δ_i ground state of FeO. This occurs in the v = 2 level of the $\text{Ω}\text{= 2}$ substate, and only one Λ-boudling component of one rotational level is affected. The perturbing state, which lies about 2100 cm^?1 above the lowest spin-orbit level of the molecule, is orbitally nondegenerate, and is likely to belong to the ⁷Σ⁺ state arising from the configuration (4sσ)¹(3dδ)²(3dπ)²(3dσ)¹. The new state, a⁷Σ⁺, is possibly responsible for anomalies in the intensity pattern of the FeO^? photodetachment spectrum of Engelking and Lineberger [J. Chem. Phys., 66, 5054–5058 (1977)]. Improved vibrational and rotational constants are presented for the ground state, combining new Fourier transform measurements of the Λ-doubling in X⁵Δ₁ with the recent microwave data of Endo et al. [Astrophys. J., 278, L131–132 (1984)].     

Studies of the mid- and far-infrared laser magnetic resonance spectra of the CD radical: information on vibrationally excited levels

Two separate studies of the CD radical in vibrationally excited levels of its X²Π ground state have been made by the technique of laser magnetic resonance. The first of these studies was in the far-infrared; rotational transitions of CD in the v=1 and 2 levels have been detected. The second study was carried out in the mid-infrared using a carbon monoxide laser magnetic resonance spectrometer. In these experiments, transitions in the (1,0), (2,1), and (3,2) bands have been detected. All the available data on CD in its X²Π state have been used to determine an improved set of molecular parameters for the CD radical. In addition to the above data sets, previous far-infrared laser magnetic resonance on the CD radical in the v=0 level and FTIR observations of the (1,0) and (2,1) bands have been included. The principal molecular parameters determined are: , , , , where the figures given in parentheses are one standard deviation from the least squares fit. A small but significant dependence of the orbital contribution to the magnetic dipole moment on the vibrational quantum number is detected. This may reflect the mixing between the X²Π and a⁴Σ^- states of CD.     

准直系统热光学效应对静态傅里叶变换红外光谱仪光谱复原的影响研究

在以多级微反射镜为核心器件的静态傅里叶变换红外光谱仪中, 由于准直系统距离红外光源较近, 光源的热辐射会导致其局部温度升高, 从而引起材料折射率发生改变, 使得由准直系统出射的光束存在一定的发散角, 进而影响光谱仪系统复原光谱所能达到的分辨率水平. 本文研究了光谱仪系统正常工作状态下准直系统各区域的温度分布情况, 由此计算出了相应的离焦量. 通过计算准直光束发散角在光程差采样区域内的分布, 分析了由此引入的附加光程差对光谱复原的影响. 通过计算光谱结构误差随准直系统温度的变化, 得到了准直系统温度控制的合理范围. 最后, 对基于SiC光源的光谱仪进行了实验, 结果显示制冷光源复原光谱的光谱结构误差与非制冷光源的光谱结构误差相比有明显改善. 因此, 降低光源温度对减小准直系统热光学效应的影响是非常有效的. 本文的研究结果将为解决同类问题提供参考.     

Modelling transient heat conduction in solids at multiple length and time scales: A coupled non-equilibrium molecular dynamics/continuum approach

A method for controlling the thermal boundary conditions of non-equilibrium molecular dynamics simulations is presented. The method is simple to implement into a conventional molecular dynamics code and independent of the atomistic model employed. It works by regulating the temperature in a thermostatted boundary region by feedback control to achieve the desired temperature at the edge of an inner region where the true atomistic dynamics are retained. This is necessary to avoid intrinsic boundary effects in non-equilibrium molecular dynamics simulations. Three thermostats are investigated: the global deterministic Nosé–Hoover thermostat and two local stochastic thermostats, Langevin and stadium damping. The latter thermostat is introduced to avoid the adverse reflection of phonons that occurs at an abrupt interface. The method is then extended to allow atomistic/continuum models to be thermally coupled concurrently for the analysis of large steady state and transient heat conduction problems. The effectiveness of the algorithm is demonstrated for the example of heat flow down a three-dimensional atomistic rod of uniform cross-section subjected to a variety of boundary conditions.