Trends in the electronic and vibrational contributions to the dipole moment,polarizabilities, and first and second hyperpolarizabilities of the hydrides of Li,Na and K

The dipole moments μ, polarizabilities α, and first and second hyperpolarizabilities, β and γ of LiH, NaH and KH (MeH) have been computed using Hartree-Fock, MP2 and CCSD(T) theories. The static electronic and vibrational contributions to these properties are presented. The vibrational properties have been analysed into contributions due to zero-point vibrational averaging and pure vibrational terms. An alternative dissection of the vibrational properties into nuclear relaxation and curvature terms has also been considered. KH has been selected as a model system to study how the number of electrons, which are correlated (2, 10 and 20) affect both electronic and vibrational properties. The 10 electron approximation gives results that are practically the same as those computed by taking into account all 20 electrons of KH. The double-harmonic approximation has been shown to give satisfactory results for the pure vibrational contributions to the polarizability and the first hyperpolarizability, while this approximation is useful for demonstrating, qualitatively, the significance of the pure vibrational contributions to the second hyperpolarizability. In many cases the vibrational contributions are rather small percentages of the corresponding electronic contribution. However, several exceptions to the above observation have been noted. In all the cases considered the vibrational properties should be computed if reasonably accurate property values are sought. Electron correlation is important for both electronic and vibrational contributions to the electrical properties of the hydrides considered. The results are in satisfactory agreement with most of the best theoretical and experimental data concerning bond lengths, vibrational frequencies and electrical properties.     

Effects of vibrational averaging on coupled cluster calculations of spin–spin coupling constants for hydrocarbons

We present vibrationally corrected nuclear spin–spin coupling constants for four hydrocarbons with different types of carbon–carbon bonds calculated with coupled cluster (CC) theory. First, we perform a systematic basis set investigation on acetylene for all of the four contributions (Fermi-contact, spin-dipole, para- and diamagnetic spin–orbit) to the spin–spin coupling constants and subsequently choose basis sets of sufficient flexibility to describe converged electronic properties. Then, in order to describe the effects of vibrational motion for the studied molecules we perform a Taylor expansion in the normal coordinates up to second order – a method that is well known for both its quality and efficiency – and rigorously estimate the resulting contribution for all types of spin–spin coupling constants. Combined, this allows us to obtain highly accurate benchmark estimates of the spin–spin coupling constants for acetylene, ethylene, ethane, and cyclopropane. This work provides one of the first systematic benchmarks of zero-point vibrational contributions to spin–spin coupling constants in poly-atomic molecules using the reliable CC theory and it is thus an important reference for further research within in-silico spin–spin coupling constant determination. We note that earlier computational estimates of zero-point vibrational effects agree well with those presented here (for acetylene, ethylene, and cyclopropane) while vibrational corrections for ethane are reported for the first time.     

Coherent dynamics of the localized vibrational modes of hydrogen in CaF2

We report the observation of giant quantum coherence effects in the localized modes of ionized hydrogen in synthetic fluorite. Infrared free induction decay experiments on the substitutional H- center show dramatic modulations at negative delay times due to interference between multiple vibrational levels. Spectrally resolving the degenerate four wave mixing signal allows unambiguous assignments of the participating vibrational states. The dependence of the signal intensity upon the delay path between the exciting free electron laser pulses can be accounted for in terms of the resonant third order polarization with a common dephasing time for the excited states.     

First-principles calculation of nuclear resonance vibrational spectra

We present a ‘first-principles’ methodology for the calculation of the parameters that are required for the simulation of nuclear resonance vibrational spectra (NRVS) of molecular systems. Formulae are given for the intensities of vibrational transitions corresponding to the so-called single- and double-phonon contributions to the NRVS signal. The method is also valid for those vibrations that are not in the high-frequency/low-temperature limit. We have rigorously treated the issue of orientational averaging of the Lamb–Mössbauer factor and the effect of the neglect of its anisotropy on the calculated NRVS pattern. Normal mode composition factors are determined in a compact form as appropriate components of an orthogonal matrix that diagonalizes the Hessian matrix. The method is illustrated by simulating the NRVS spectra and the partial vibrational density of states of [FeO(H₂O)5]²⁺ on the basis of vibrational frequencies and normal mode composition factors obtained from density functional theory (DFT) calculations.     

Spot-size reduction in terahertz apertureless near-field imaging

We show measurements and calculations of the terahertz (THz) near field of a metal tip with a specially formed, semicircular apex that allows us to identify the separate contributions of the tip apex and shaft to the measured signal. We find that when the tip-crystal distance is not modulated the measured near-field signal is overwhelmed by contributions from the tip shaft, resulting in a relatively large THz spot size. When the tip-crystal distance is modulated, with subsequent lock-in detection at the modulation frequency, only the near-field distribution of the semicircular apex is observed, resulting in a much smaller THz spot size and thus improved spatial resolution.     

Vibrational corrections to electric properties of weakly bound systems

The pure electronic and vibrational contributions to electric dipole moments, dipole polarizabilities, and first hyperpolarizabilities have been evaluated for the HF and H₂O dimers. The zero-point vibrational average corrections to dipole moments and dipole polarizabilities turn out to be relatively small. However, the corresponding contributions to the first hyperpolarizability are found to be of the same magnitude as the pure electronic values. The so-called pure vibrational corrections to the dipole polarizability and first hyperpolarizability of hydrogen bonded dimers are exceptionally large and indicate that the perturbation theory method used for their evaluation fails to account properly for the high mechanical and electric anharmonicities present in these systems. The analysis of different harmonic and anharmonic contributions to the pure vibrational correction to the first hyperpolarizability shows explicitly the importance of the low frequency intermolecular modes.     

Diagrammatic representation of the third-order polarization in transient CARS

Summary A diagrammatic perturbative approach is adopted to derive the expression for the third-order polarization,P ⁽³⁾ which originates the anti-Stokes emission in a time-resolved CARS experiment. The various contributions toP ⁽³⁾ are calculated assuming that laser fields are off-resonance with respect to any electronic level of material system. The resonant part of the polarization consists of two terms, in which the roles of the pump and probe pulses at frequency ω₁ are exchanged. The global expression allows the direct calculation of the signal time profile in a transient CARS experiment once a model is assumed for the laser pulse shape.     

The spectral structure of the low level excitation in the process of the collisionless dissociation of polyatomic molecules

The resonant characteristics of the energy absorbed by SF₆ gas which is deeply cooled and exposed to IR laser irradiation of moderate intensity have been investigated experimentally. It was found that the gas cooling changes the dependence of the absorbed energy on the laser intensity and allows to reveal the structure of the spectrum. The experimental results are explained by a dominating influence of the two-photon absorption on the excitation of low vibrational levels in the laser fields of moderate intensity.     

Novel non-linear optical techniques for diagnostics: laser-induced gratings and two-colour four-wave mixing

Electrostriction, population transfer caused by absorption of laser energy and subsequent thermalization are the most important mechanisms responsible for the formation of laser-induced gratings. Applying time resolved light diffraction from electrostrictive gratings allows for the measurements of thermodynamic properties like temperature, gas composition, and flow velocity. Thermal gratings can be employed as a very sensitive spectroscopic tool.The Two-Colour Four-Wave Mixing (TCFWM) signal formed by a non-resonant pump laser and a resonant probe laser in an acetylene/air diffusion flame comprises of contributions due to soot and C₂, the latter is generated by vaporising the soot. Profile measurements across the flame have shown that the signal composition changes within the flame.Intermediate level labelling by Two-Colour Resonant Four-Wave Mixing Spectroscopy (TC-RFWM) is applied to reduce the spectral complexity of molecular spectra. In addition, we observe extra resonances forbidden by a conventional three-level scheme in OH- and NH-containing flames. A detailed investigation shows that the newly observed TC-RFWM resonances are induced by collisional energy transfer within the population, alignment and orientation gratings formed in both electronic states that are coupled to the laser fields.     

运用球高斯分布极化势研究低能电子与H2 分子碰撞的振动激发动量迁移散射截面

用最近改进的球高斯分布(distributed spherical Gaussian,DSG)极化势模型,在振动密耦合框架下和基于量子力学从头计算的静电势、交换势的基础上,研究了低能电子与H₂分子碰撞振动激发的动量迁移散射截面(momentum transfer cross section,MTCS).通过包含18个振动波函数、5个分波和16个分子对称性,得到了0<E≤10 eV时收敛性很好的ν=0→ν'=0,1,2,3等几个振动跃迁通道和总的MTCS,并与其他实验和理论得到的值进行了比较和分析. 关键词： 2分子碰撞')" href="#">H₂分子碰撞 动量迁移散射截面 振动激发 球高斯分布极化势     

Controlling vibrational wave packet motion with intense modulated laser fields

Intense, nonresonant laser fields produce Stark shifts that strongly modify the potential energy surfaces of a molecule. A vibrational wave packet can be guided by this Stark shift if the laser field is appropriately modulated during the wave packet motion. We modulated a 70 fs laser pulse with a period on the time scale of the vibrational motion (approximately 10 fs) by mixing the signal and idler of an optical parametric amplifier. We used ionization of H2 or D2 to launch a vibrational wave packet on the ground state of H2(+) or D2(+). If the laser intensity was high as the wave packet reached its outer turning point, the Stark shift allowed the molecule to dissociate through bond softening. On the other hand, if the field was small at this critical time, little dissociation was measured. By changing the modulation period, we achieved control of the dissociation yield with a contrast of 90%.     

Impact of intermolecular vibrational coupling effects on the sum-frequency generation spectra of the water/air interface

ABSTRACT

We have examined the impact of intermolecular vibrational coupling effects of the O-H stretch modes, as obtained by the surface-specific velocity-velocity correlation function approach, on the simulated sum-frequency generation spectra of the water/air interface. Our study shows that the inclusion of intermolecular coupling effects within the first three water layers, i.e. from the water/air interface up to a distance of 6?Å towards the bulk, is essential to reproduce the experimental SFG spectra. In particular, we find that these intermolecular vibrational contributions to the SFG spectra of the water/air interface are dominated by the coupling between the SFG active interfacial and SFG inactive bulk water molecules. Moreover, we find that most of the intermolecular vibrational contributions to the spectra originate from the coupling between double-donor water molecules only, whereas the remaining contributions originate mainly from the coupling between single-donor and double-donor water molecules.     

High-sensitivity vibrational imaging with frequency modulation coherent anti-Stokes Raman scattering (FM CARS) microscopy

We demonstrate a new approach to coherent anti-Stokes Raman scattering (CARS) microscopy that significantly increases the detection sensitivity. CARS signals are generated by collinearly overlapped, tightly focused, and raster scanned pump and Stokes laser beams, whose difference frequency is rapidly modulated. The resulting amplitude modulation of the CARS signal is detected through a lock-in amplifier. This scheme efficiently suppresses the nonresonant background and allows for the detection of far fewer vibrational oscillators than possible through existing CARS microscopy methods.     

Nuclear magnetic resonance in ferromagnetic terbium

The temperature dependence of the magnetic dipole and electric quadrupole hyperfine fields has been measured by NMR in ferromagnetic terbium metal. The results are in excellent agreement with a general theoretical relation connecting moments of the magnetization. This relation allows us to separate ionic and crystalline contributions to the nuclear electric field gradient.     

Specific heat of small vanadium particles in the normal and superconducting state

The specific heat C of ultrafine vanadium particles of various diameters (2.9–13 nm) has been measured in the temperature interval 1.5–12 K and in magnetic fields up to 3.5 T. Both the vibrational and electronic contributions to C in the normal state are strongly enhanced as compared to the bulk behavior. For not too small particles (> ～10nm), the vibrational specific heat can be interpreted in terms of the discrete phonon spectrum of free elastically vibrating small spheres while, at low temperatures, the vibrational specific heat of the smallest particles is predominantly due to Einstein modes which are attributed to low-frequency vibrations of weakly bound surface atoms. Level quantization does not appear to play a detectable role in the electronic specific heat of the normal state. Rather, the observed enhancement must be attributed to an increased electronic density of states at (100) surfaces of bcc metals or to electronic states of substoichiometric V-oxides. The transition range to superconductivity progressively broadens with decreasing particle size due to fluctuations. In this temperature range, the electronic specific heat behaves in qualitative agreement with theoretical predictions.     

Multiple inelastic electron scattering processes in thin molecular films: a simple model

Electron energy loss spectra of multilayer molecular films contain bands resulting from inelastic collisions at two or more sites within the film. The contributions of twofold loss processes are analysed for selected vibrational spectra of benzene and SF₆ using a simple model that was suggested previously and is extended here. It is shown that this analysis can be used not only to detect dipole scattering contributions to particular vibrational excitations. Combined with an estimate of the twofold loss spectrum based on the pattern of the fundamental bands the model also enables the identification of overtone and combination bands which is important for the detection of resonant processes.     

二维格子神经元网络的振动共振和非线性振动共振

本文采用数值模拟的方法, 在通过电突触耦合或化学突触耦合的二维格子神经元网络中, 研究了FitzHugh-Nagumo神经元受到双频信号输入时神经元网络对低频信号的响应特性. 结果表明:当固定受到双频输入信号的神经元在体系中所占的比例且FitzHugh-Nagumo神经元参数处于可激发区域时双频信号中的高频部分可诱导出动作电位产生, 而且随着高频输入信号强度的增加, 神经元网络对低频输入信号响应先增大后减小, 出现了极大值, 即发生了振动共振现象. 另外本文还研究了神经元网络对低频输入信号的二次谐波的响应, 同样发现了非线性振动共振现象, 并且体系对低频信号的响应随着其频率ω 的增加也产生共振现象, 即发生了双共振现象. 上述共振现象在以电突触耦合的二维格子神经元网络中和以化学突触耦合的二维格子神经元网络中都可以观察到. 当固定双频输入信号中高频输入信号强度时, 随着受到双频输入信号的神经元在体系中所占比例的变化, 电突触耦合的二维格子神经元网络对低频输入信号的响应与化学突触耦合的二维格子神经元网络对低频输入信号的响应相比有很大的不同.     

Effects of a modulation of the pump-polarization in a degenerate pump/probe experiment

The effects of the pump-polarization in a degenerate pump/probe experiment are analyzed. It is shown that modulating the pump polarization from linear to circular induces a modulation in the probe absorption change that allows to separate various contributions that are mixed in the signal. An experimental demonstration is carried out in a laser dye (Coumarin 500). Received: 21 July 1998 / Revised: 28 October 1998 / Accepted: 20 November 1998     

The combined use of SCF-MO calculations and experimental data in the evaluation of quadratic force fields: MOCIC potential functions for fluoroform,methyl acetylene,and methyl cyanide

A semiempirical method combining SCF-MO calculations and limited vibrational data has been employed to evaluate the completely general quadratic potential fields of fluoroform, methyl acetylene, and acetonitrile. MOCIC (molecular orbital constraint using interaction coordinates) potential fields are presented for gas phase molecules of intermediate size. Here general harmonic force fields or excellent approximations utilizing extensive experimental data are available as standards. A statistical evaluation of the interaction potentials shows that there is some improvement in going from MNDO or ab initio SCF-MO force fields to the MOCIC functions which reliably reproduce the off-diagonal vibrational potential constants in most instances. The MOCIC primary compliants are excellent approximations of their vibrational counterparts, as expected. Comparison of the calculated isotopic frequencies, Coriolis coupling constants, and centrifugal distortion constants for the SCF-MO, MOCIC, and vibrational spectroscopic potential fields with the corresponding experimental values also shows MOCICs reliability for molecules with many interaction potentials. There is substantial improvement in the calculated isotopic frequency shifts and centrifugal distortion constants in going from SCF-MO to MOCIC functions.