Cooling and alignment of ethylene molecules in supersonic seeded expansions: diagnostic and application to gas phase and surface scattering experiments

Rotational cooling and collisional alignment of ethylene molecules is induced by seeding effects in supersonic expansions with lighter gas carriers such as He and Ne. The dependencies of the degree of alignment on the rotational state, on the final speed of the molecules and on the diffusion angular cone have been characterized by coupling two different experimental methodologies. An application to surface scattering is then demonstrated by measuring stereo-dynamical effect in the adsorption on metallic surfaces.     

Interaction potentials and rotational scattering for H2 and MuH with He

Elastic and inelastic differential cross-sections for the rotational scattering of p-H₂ and MuH by He at a single collision energy are calculated with potential surfaces obtained by varying the parameters in a model potential. The importance of the attractive and repulsive terms in the potential, and of the available kinetic energy in the different channels is demonstrated, and it is shown that the whole potential surface determines the scattering cross-sections. Inelastic and elastic scattering are compared, and the differences between the rotational scattering of homonuclear and heteronuclear homopolar molecules are ascribed to the molecular constants rather than to the numbers of open channels.     

Highest rotational and vibrational excitation of swift diatomic molecules scattered by solid surface

It is shown that at nondissociative scattering of swift molecules by solid surface and inverse population of the highest rotational and some vibrational states unattainable at thermal excitation could be achieved. For single crystal surface a specific polarization and orientational effects in rotational and vibrational excitation of the scattered molecules could be observed.     

Laser spectroscopy of desorbing molecules

In this article the application of tunable dye lasers to desorption phenomena is illuminated. These lasers provide radiation continuously tunable from 105 nm in the vacuum ultraviolet to about 10 m in the mid-IR. By employing either laser induced fluorescence (LIF) or resonance enhanced multiphoton ionization (REMPI) spectroscopy almost all diatomic and many polyatomic molecules can be probed with the sensitivity required to detect desorbing molecules under UHV conditions. The spectral resolution of the lasers is sufficiently high that rotational state selectivity is achieved. Recent developments permit in addition the velocity distributions of molecules to be determined with internal quantum state resolution. Therefore very detailed information about the molecular dynamics has been obtained. In most experiments so far reactive recombinations off surfaces have been investigated. In this paper special emphasis will be given to the recombination of hydrogen on copper and palladium surfaces. For these systems very detailed data about the internal state populations at various surface temperatures have been obtained. The rotational cooling previously observed in molecular beam scattering has also been established for desorption. Strong vibrational excitation has been observed, which in the case of desorption from copper may be associated with the recombination dynamics, whereas for desorption of D₂ from Pd(100) a molecular precursor state might be responsible. By measuring the velocity distribution in each quantum state, the complete energetics of the desorbing molecules has been determined. Some first experiments on laser induced desorption with state selective detection of the desorbing molecules will also be discussed. Finally, making use of the polarization analysis of the signal, alignment effects in the desorption can be observed, permitting observation of molecular dynamics with a magnifying glass.Heisenberg fellow of the Deutsche Forschungsgemeinschaft     

Differential cross sections from quantum calculations on coupled Ab initio potential energy surfaces and scattering experiments for Cl(2P)+H2 reactions

To assess the relative reactivity of the spin-orbit excited state of atomic Cl with molecular hydrogen, we have measured differential cross sections using an atomic Cl beam with a known concentration of the ground and excited spin-orbit states. These are compared with the first determination of the cross sections from quantum mechanical scattering calculations on a set of coupled ab initio potential energy surfaces. The comparison suggests that these surfaces may underestimate the degree of rotational excitation of the HCl products and that the excited spin-orbit state plays a minor role in the reaction.     

Derivation of a temperature-dependent accommodation coefficient for use in modeling laser-induced incandescence of soot

This paper presents a derivation of an expression to estimate the accommodation coefficient for gas collisions with a graphite surface, which is meant for use in models of laser-induced incandescence (LII) of soot. Energy transfer between gas molecules and solid surfaces has been studied extensively, and a considerable amount is known about the physical mechanisms important in thermal accommodation. Values of accommodation coefficients currently used in LII models are temperature independent and are based on a small subset of information available in the literature. The expression derived in this study is based on published data from state-to-state gas-surface scattering experiments. The present study compiles data on the temperature dependence of translational, rotational, and vibrational energy transfer for diatomic molecules (predominantly NO) colliding with graphite surfaces. The data were used to infer partial accommodation coefficients for translational, rotational, and vibrational degrees of freedom, which were consolidated to derive an overall accommodation coefficient that accounts for accommodation of all degrees of freedom of the scattered gas distributions. This accommodation coefficient can be used to calculate conductive cooling rates following laser heating of soot particles.     

Fingerprints for spin-selection rules in the interaction dynamics of O2 at Al(111)

We perform mixed quantum-classical molecular dynamics simulations based on first-principles potential-energy surfaces to demonstrate that the scattering of a beam of singlet O2 molecules at Al(111) will enable an unambiguous assessment of the role of spin-selection rules for the adsorption dynamics. At thermal energies we predict a sticking probability that is substantially less than unity, with the repelled molecules exhibiting characteristic kinetic, vibrational and rotational signatures arising from the nonadiabatic spin transition.     

Thermal Hall-Senftleben effect

A new effect is predicted, namely, the occurrence of a heat flow perpendicular to both the temperature gradient and the magnetic field in solid dielectrics with unfrozen rotational degrees of freedom of molecules. The method of moments is developed with allowance made for the processes of phonon scattering by molecules with a change in their rotational state.     

Rotational state distribution of NO molecules scattered from surfaces

Quantum mechanical calculations of final rotational state distributions are carried out for a simple model of the scattering of NO from surfaces. The dependence of the results on parameters of the potential such as sign and size of the anisotropy and asymmetry, the well depth etc. is discussed. Certain difficulties in explaining experimental results for NO scattered from Ag (111) are pointed out. The dependence of the results on an initial rotational temperature is discussed also. Its main effect — which is well known from the literature — is to average out the quantum oscillations of the cross sections around the rainbow structures. The oscillations may become visible at rotational temperatures slightly lower than used up to now.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday     

在银、金和铂基底上银纳米粒子诱导的单层有机分子的表面增强拉曼散射(英文)

Noble metallic nanostructures exhibit a phenomenon known as surface-enhanced Raman scattering （SERS） in which the scattering cross sections are dramatically enhanced for molecules adsorbed thereon. Thanks to the enormously large enhancement factor on the order of 106～1015, one can readily acquire thevibrational spectra from adsorbates on roughened surfaces of Ag, Au, and Cu. However, SERS has not developed to be as powerful a surface technique as many people had hoped initially because of two specific obstacles.     

Quantum theory of energy exchange in direct encounters of polyatomic molecules with nonrigid surfaces

Recently developed effective mass theory of vibrational, rotational and translational energy exchange in molecular collisions is extended naturally to polyatomic molecules colliding with nonrigid surfaces. Considering the processes where the short-range forces are of prevailing importance, we transform the Hamiltonian into the form which enables us to eliminate the coordinates which are cyclic in the limit of zero range forces and can thus be expected to be approximately ignorable when forces are of short, but finite, range. Instead of three rotational and three translational coordinates we are thus left with one only relevant generalized coordinate. The mass corresponding to this coordinate can easily be calculated; it depends on the mass and the moments of inertia of the molecule, and on the segment of the potential surface on which the system evolves. The motion of the only relevant generalized coordinate is then quantized. The inelastic scattering problem is treated within first-order in the distorted wave expansion. In the cases where this may be expected to be inadequate, more advanced approximation schemes developed in scattering theory could be applied as well. The excitations of bulk phonons and also of vibrational modes localized on the surface are included in the theory. As an application, energy transfer in collisions of CO₂ molecules with Pt and Ag surfaces is studied (since this is the experimentally most studied case involving molecules of more than two atoms), and the results are compared with the experiments and with the recent semiclassical stochastic trajectory calculations in which the transfer of vibrational energy to rotations is neglected and the solid's vibrations are treated by a quasiclassical approximation. Excellent agreement was found between the present theory and the trajectory calculations mentioned for all three gas temperatures considered (290, 580 and 1160 K) in the case of small change in vibrational energy and high surface temperature, i.e., under the conditions where the agreement between the two methods could possibly be expected because of the approximations involved.     

气体-表面相互作用的分子动力学模拟研究

采用分子动力学模拟方法研究了气体分子Ar在光滑和粗糙Pt表面上的散射规律.提出了一种速度抽样方法,计算了不同温度条件下气体分子对光滑和粗糙表面的切向动量适应系数和吸附概率.结果显示：光滑表面条件下,气体分子的切向动量系数和吸附概率都随着温度的升高而降低;粗糙度对气体分子切向动量与表面的适应具有极大的促进作用,当粗糙度足够大时,切向动量适应系数的大小趋近于1.0,对温度的敏感性也逐渐降低.采用粒子束方法对气体分子在光滑和粗糙表面上的散射规律进行了定量分析.总结了散射过程中气体分子的典型轨迹和动量变化规律,将气体分子在光滑表面的散射分为两种类型：单次碰撞后散射和多次碰撞后散射.单次碰撞后散射的气体分子平均切向动量有所减小,而经过多次碰撞后散射的气体分子则倾向于保持原有的平均切向动量.对于粗糙表面,粗糙度的存在使气体分子与表面间的动量和能量适应更加充分,导致气体分子在较粗糙表面上散射后的平均切向动量大幅减小并接近于0,且气体分子在表面上经历的碰撞次数越多,其散射后的能量损失越严重.     

两个严格可解双原子分子势函数的散射态解与转动修正

文献［１］中提出了一个新的势函数，其Ｓｃｈｒｄｉｎｇｅｒ方程严格可解。同时提出一个新的变量变换关系，用超几何级数严格求解了双曲型ＰｓｃｈｌＴｅｌｅｒ分子势Ｓｃｈｒｄｉｎｇｅｒ方程的束缚态。文内进一步讨论这两个势函数Ｓｃｈｒｄｉｎｇｅｒ方程散射态的严格解，并求出了Ｓ波的散射相移。文献中关于修正ＰｓｃｈｌＴｅｌｅｒ势及无反射势散射态的结果均作为特例包含在这篇文章更一般的结论之中。此外还用一个简单的方法考虑了转动能修正，对ＨＦ基态转动谱作了具体计算     

Elastic and inelastic scattering of nucleons from 16O

Measurements of differential elastic and inelastic cross sections for neutron scattering from ¹⁶O at incident energies 18 to 26 MeV are presented. In addition to cross sections for neutron scattering differential cross sections for proton scattering up to 66 MeV are described in terms of phenomenological optical model potentials. At 24.5 MeV incident energy inelastic scattering up to 11.5 MeV excitation was measured. The elastic and inelastic compound nucleus contributions were examined. Direct inelastic scattering from the normal parity states was calculated using the DWBA and coupled-channel formalisms. The inelastic scattering cross section from non-normal parity state 2⁻ was calculated using the coupled-channel formalism via multi-step processes. Cross sections due to inelastic scattering from some of the states, which are thought to be members of an excited state rotational band were calculated using both vibrational and rotational approaches and were compared.     

Surface Corrugation in Rotational and Diffractive Scattering of O2 from LiF (001)

A quantum dynamic calculation on a five-dimensional O₂/LiF (001) model system is performed using the multi-configuration time-dependent Hartree method. The obtained results show that the mechanism of rotational and diffractive excitation in details: Comparison with the rotational excited state, the initially non-rotational state is seen to favor the inelastic scattering in the rotational excitation process. The surface corrugation can damp the quantum interferences and produce a greater amount of rotational inelastic scattering at the expense of the elastic process in the rotational excitation process. The diffraction process and the average energy transferred into the rotational and diffractive mode are also discussed.     

Angle-resolved desorption and removal of surface nitrogen in deNOx

This paper reports on recent progress on angle-resolved desorption leading to structure-sensitive desorption dynamics. The sensitivity is exemplified in NO and N₂O reduction on Pd and Rh surfaces. The energy partitioning in the repulsive desorption of hyper-thermal products into their rotational and translational modes is an indispensable concept to examine the structure of a reaction site from desorbing molecules because it connects the structure of a transition state with each energy of desorbed products. The extent of the energy partitioning will be derived from the desorption-angle dependences of both the rotational and translational energies at each vibrational state. Such energy analysis has never been completed for any thermal reactive desorption. A new type of measurement is thus proposed. Additionally, we discuss the inadequate use of the detailed balance principle in desorption dynamics, which has prevented desorption dynamics from being sensitive to surface structures.     

Scattering models for ultracold atoms

We present a review of scattering models that can be used to describe the low-energy behavior of identical bosonic atoms. In the simplest models, the only degrees of freedom are atoms in the same spin state. More elaborate models have other degrees of freedom, such as atoms in other spin states or diatomic molecules. The parameters of the scattering models are specified by giving the S-wave phase shifts for scattering of atoms in the spin state of primary interest. The models are formulated as local quantum field theories and the renormalization of their coupling constants is determined. Some of the parameters can be constrained by renormalizability or by the absence of negative-norm states. The Green’s functions that describe the evolution of two-atom states are determined analytically. They are used to determine the T-matrix elements for atom-atom scattering and the binding energies of diatomic molecules. The scattering models all exhibit universal behavior as the scattering length in a specific spin state becomes large.     

Dynamics of water molecules adsorbed by silica and resin SGK-7

This paper has presented neutron spectroscopy data on the dynamics of light water molecules adsorbed in the cation exchanger (ion-exchange resin) SGK-7 and on the surface of aerosils (highly dispersed pyrogenic silica) with different levels of hydration. The measurements have been performed on a DIN-2PI spectrometer (Frank Laboratory of Neutron Physics of the Joint Institute for Nuclear Research, Dubna, Russia). The characteristics of the diffusive and vibrational motions of adsorbed water molecules have been determined from the experimental neutron scattering spectra. The data obtained in the quasi-elastic neutron scattering region have been analyzed using a model accounting for the effects of restricted translational and rotational diffusion. The results have demonstrated a significant decrease in the diffusion mobility of adsorbed water molecules as compared to conventional (bulk) water. In particular, the self-diffusion coefficient decreases several times, and the diffusion rate is the lower, the smaller is the thickness of the hydration layer. The dependences of the intensity and half-width of the quasi-elastic scattering peak on the magnitude of the neutron momentum transfer q in the scattering process exhibit a nonmonotonic character. This indicates manifestation of the effects of restricted translational diffusion, rotational diffusion, and jump diffusion. The partial distributions of vibrational frequencies of hydrogen atoms of water molecules adsorbed by the cation exchanger and aerosils have been obtained from the inelastic neutron scattering data.     

利用单分子光学探针测量幂律分布的聚合物动力学

研究聚合物薄膜纳米尺度的动力学特性对于高性能材料的制备具有重要的意义.本文利用尼罗红单分子作为光学探针吸附在聚丙烯酸甲酯(PMA)聚合物链上,研究该聚合物薄膜的动力学特性.通过单分子散焦宽场荧光成像显微镜技术测量了单分子随PMA聚合物链转动弛豫的三维再取向特性,当环境温度高于PMA的玻璃点温度19 K时,发现处于PMA聚合物薄膜中的单分子光学探针的转动态和非转动态的持续时间概率密度服从指数截止的幂律分布.研究结果表明该温度下PMA聚合物薄膜的纳米环境动力学仍存在空间和时间异构性.     

Xe对NO(X)微分截面和非弹性散射中的碰撞诱导旋转准直

本文对NO(X)-Xe碰撞系统在碰撞能量为519 cm^-1,测量了完全?-双峰分解的微分截面和碰撞引起的旋转准直力矩. 同时结合初始量子态选择,使用六极杆的非均质电场,借助量子态分辨的测量,利用(1+1'')共振增强的多光子电离和速度离子成像. 结果显示,微分截面以及偏振相关的微分截面均显示与从头算势能面上进行的量子力学散射计算[J. K?os etal. J. Chem. Phys. 137, 014312 (2012)]一致. 通过与准经典轨迹、硬壳势能的量子力学散射以及运动近端模型的比较,评估了势能对所测微分截面和碰撞引起的旋转准直力矩的影响.