共查询到20条相似文献，搜索用时 15 毫秒
1.
Kitsopoulos TN Buntine MA Baldwin DP Zare RN Chandler DW 《Science (New York, N.Y.)》1993,260(5114):1605-1610
The differential cross section for the H + D(2) --> HD + D reaction has been measured using a technique called reaction product imaging. In this experiment, a photolytically produced beam of hydrogen (H) atoms crossed a beam of cold deuterium (D(2)) molecules. Product D atoms were ionized at the intersection of the two particle beams and accelerated toward a position-sensitive detector. The ion images appearing on the detector are two-dimensional projections of the three-dimensional velocity distribution of the D atom products. The reaction was studied at nominal center-of-mass collision energies of 0.54 and 1.29 electron volts. At the lower collision energy, the measured differential cross section for D atom production, summed over all final states of the HD(v,J) product, is in good agreement with recent quasi-classical trajectory calculations. At the higher collision energy, the agreement between the theoretical predictions and experimental results is less favorable. 相似文献
2.
We report the cooling of nitric oxide using a single collision between an argon atom and a molecule of NO. We have produced significant numbers (108 to 109 molecules per cubic centimeter per quantum state) of translationally cold NO molecules in a specific quantum state with an upper-limit root mean square laboratory velocity of 15 plus or minus 1 meters per second, corresponding to a 406 plus or minus 23 millikelvin upper limit of temperature, in a crossed molecular beam apparatus. The technique, which relies on a kinematic collapse of the velocity distributions of the molecular beams for the scattering events that produce cold molecules, is general and independent of the energy of the colliding partner. 相似文献
3.
Che L Ren Z Wang X Dong W Dai D Wang X Zhang DH Yang X Sheng L Li G Werner HJ Lique F Alexander MH 《Science (New York, N.Y.)》2007,317(5841):1061-1064
The reaction of F with H2 and its isotopomers is the paradigm for an exothermic triatomic abstraction reaction. In a crossed-beam scattering experiment, we determined relative integral and differential cross sections for reaction of the ground F(2P(3/2)) and excited F*(2P(1/2)) spin-orbit states with D2 for collision energies of 0.25 to 1.2 kilocalorie/mole. At the lowest collision energy, F* is approximately 1.6 times more reactive than F, although reaction of F* is forbidden within the Born-Oppenheimer (BO) approximation. As the collision energy increases, the BO-allowed reaction rapidly dominates. We found excellent agreement between multistate, quantum reactive scattering calculations and both the measured energy dependence of the F*/F reactivity ratio and the differential cross sections. This agreement confirms the fundamental understanding of the factors controlling electronic nonadiabaticity in abstraction reactions. 相似文献
4.
Mikosch J Trippel S Eichhorn C Otto R Lourderaj U Zhang JX Hase WL Weidemüller M Wester R 《Science (New York, N.Y.)》2008,319(5860):183-186
Anion-molecule nucleophilic substitution (S(N)2) reactions are known for their rich reaction dynamics, caused by a complex potential energy surface with a submerged barrier and by weak coupling of the relevant rotational-vibrational quantum states. The dynamics of the S(N)2 reaction of Cl- + CH3I were uncovered in detail by using crossed molecular beam imaging. As a function of the collision energy, the transition from a complex-mediated reaction mechanism to direct backward scattering of the I- product was observed experimentally. Chemical dynamics calculations were performed that explain the observed energy transfer and reveal an indirect roundabout reaction mechanism involving CH3 rotation. 相似文献
5.
The dispersion of longitudinal acoustic phonons was measured by inelastic x-ray scattering in the hexagonal closed-packed (hcp) structure of iron from 19 to 110 gigapascals. Phonon dispersion curves were recorded on polycrystalline iron compressed in a diamond anvil cell, revealing an increase of the longitudinal wave velocity (VP) from 7000 to 8800 meters per second. We show that hcp iron follows a Birch law for VP, which is used to extrapolate velocities to inner core conditions. Extrapolated longitudinal acoustic wave velocities compared with seismic data suggest an inner core that is 4 to 5% lighter than hcp iron. 相似文献
6.
Flynn GW 《Science (New York, N.Y.)》1989,246(4933):1009-1015
Very high resolution lasers allow spectroscopic pictures to be taken following a collision between two molecular reactants. The features of these "pictures" are the electronic, vibrational, rotational, and translational motions of the atomic particles, which relate the quantum states of the reactants to the quantum states of the products. Such state-to-state kinetic information can be used to test the shape and nature of the interaction potential that controls the collision process. The potential itself is akin to a map of the terrain through mountains and valleys where elevation is a measure of energy instead of height. Accurate mapping of this potential surface leads to an understanding of the forces which control rates and mechanisms of chemical reactions. The application of four different advanced laser techniques to the study of collisions between "hot" hydrogen(H) atoms and carbon dioxide(CO(2)) molecules has provided a wealth of information about both reactive and nonreactive collisions for this system. The availability of data for rotationally, vibrationally, and translationally inelastic excitation of CO(2) by H atoms, when compared with data for reactive events producing OH + CO, provides insights into the dynamics of collisions between H and CO(2), and illustrates the future promise of these powerful techniques for elucidating features of potential energy surfaces. 相似文献
7.
使用原子与双原子分子相互作用比较准确的Tang-Toennies势模型,通过密耦近似(Close-Coupling)方法计算了He原子与H2分子在不同碰撞能量(E=0.2,1.2,2.2 eV)时散射分波截面,首次对He-H2体系较高碰撞能量(大于2.0 eV)时的碰撞截面进行了研究,对计算结果进行了讨论,总结了该碰撞体系的分波散射截面在弹性散射00-00和非弹性散射00-04的变化规律. 相似文献
8.
Qiu M Ren Z Che L Dai D Harich SA Wang X Yang X Xu C Xie D Gustafsson M Skodje RT Sun Z Zhang DH 《Science (New York, N.Y.)》2006,311(5766):1440-1443
Reaction resonances, or transiently stabilized transition-state structures, have proven highly challenging to capture experimentally. Here, we used the highly sensitive H atom Rydberg tagging time-of-flight method to conduct a crossed molecular beam scattering study of the F + H2 --> HF + H reaction with full quantum-state resolution. Pronounced forward-scattered HF products in the v' = 2 vibrational state were clearly observed at a collision energy of 0.52 kcal/mol; this was attributed to both the ground and the first excited Feshbach resonances trapped in the peculiar HF(v' = 3)-H' vibrationally adiabatic potential, with substantial enhancement by constructive interference between the two resonances. 相似文献
9.
用ESR波谱术测定了室温下氮离子束辐照水稻、黄豆、玉米、绿豆等种子产生的各类自由基,计算了自由基的g因子值,观察了自由基的稳定性。实验结果表明,种子中自由基的浓度随辐照剂量增加,然后趋于饱和;种子的辐照损伤随自由基浓度增加而加重;自由基浓度随种子含水量增加而衰减。讨论了自由基与种子损伤关系。 相似文献
10.
Direct addition of O-H radicals at room temperature to the carbon No.5 of the uracil ring has been proved by measurement of the proton hyperfine structure in the electron spin resonance of the resulting uracil + O-H radicals. Upon addition of the OH, the H originally bound to carbon No. 5 shifts to No. 6, thus forming a methylene group at carbon No. 6, with the two protons having equivalent coupling of 28 gauss. The spin density on carbon No. 5 is O.64. The radicals were produced when powdered samples of uracil were subjected to a low-velocity beam of O-H radicals coming from either hydrogen peroxide or water vapor under reduced pressure and subjected to an electric discharge. 相似文献
11.
Phonon density of states of iron up to 153 gigapascals 总被引：4，自引：0，他引：4
Mao HK Xu J Struzhkin VV Shu J Hemley RJ Sturhahn W Hu MY Alp EE Vocadlo L Alfè D Price GD Gillan MJ Schwoerer-Böhning M Häusermann D Eng P Shen G Giefers H Lübbers R Wortmann G 《Science (New York, N.Y.)》2001,292(5518):914-916
We report phonon densities of states (DOS) of iron measured by nuclear resonant inelastic x-ray scattering to 153 gigapascals and calculated from ab initio theory. Qualitatively, they are in agreement, but the theory predicts density at higher energies. From the DOS, we derive elastic and thermodynamic parameters of iron, including shear modulus, compressional and shear velocities, heat capacity, entropy, kinetic energy, zero-point energy, and Debye temperature. In comparison to the compressional and shear velocities from the preliminary reference Earth model (PREM) seismic model, our results suggest that Earth's inner core has a mean atomic number equal to or higher than pure iron, which is consistent with an iron-nickel alloy. 相似文献
12.
Sinn H Glorieux B Hennet L Alatas A Hu M Alp EE Bermejo FJ Price DL Saboungi ML 《Science (New York, N.Y.)》2003,299(5615):2047-2049
Collective excitations have been observed in liquid aluminum oxide at high temperatures by combining a containerless sample environment with inelastic x-ray scattering. The excitation spectra show a well-defined triplet peak structure at lower wave vectors Q (1 to 6 nanometers-1) and a single quasi-elastic peak at higher Q. The high-Q spectra are well described by kinetic theory. The low-Q spectra require a frequency-dependent viscosity and provide previously unknown experimental constraints on the behavior of liquids at the interface between atomistic and continuum theory. 相似文献
13.
Talukdar RK Mellouki A Schmoltner AM Watson T Montzka S Ravishankara AR 《Science (New York, N.Y.)》1992,257(5067):227-230
The rate coefficients for the reaction of hydroxyl (OH) radicals with methyl chloroform (CH(3)CCI(3)) were measured between 243 and 379 kelvin with the pulsed photolysis-laserinduced fluorescence method. The measured rate coefficients at 298 and 277 kelvin were approximately 20 and approximately 15%, respectively, lower than earlier values. These results will increase the tropospheric OH concentrations derived from the CH(3)CCI(3) budget analysis by approximately 15%. The predicted atmospheric lifetimes of species whose main loss process is the reaction with OH in the troposphere will be lowered by 15% with consequent changes in their budgets, global warming potentials, and ozone depletion potentials. 相似文献
14.
Photochemical production rates and steady-state concentrations of hydroxyl radicals (.OH) were measured in sunlight-irradiated seawater. Values ranged from 110 nanomolar per hour and 12 x 10(-18) molar in coastal surface water to 10 nanomolar per hour and 1.1 x 10(-18) molar in open ocean surface water. The wavelengths responsible for this production are in the ultraviolet B region (280 to 320 nanometers) of the solar spectrum. Dissolved organic matter (DOM) appears to be the main source for .OH over most of the oceans, but in upwelling areas nitrite and nitrate photolysis may also be important. DOM in the deep sea is degraded more readily by .OH (and its daughter radicals), by a factor of 6 to 15, than is DOM in open-ocean surface water. This finding may in part bear on major discrepancies among current methods for measuring dissolved organic carbon in seawater. 相似文献
15.
Miller WH 《Science (New York, N.Y.)》1986,233(4760):171-177
16.
铁路客车结构大变形碰撞特性的仿真研究 总被引：4，自引：0，他引：4
应用动态大变形非线性有限元法对铁路客车整车的大变形正面碰撞过程进行了计算机模拟。在合理简化的基础上建立了整车有限元模型；运用PAM-CRASH软件的并行算法，通过计算机模拟，预测了某铁路客车在正面碰撞过程中的变形位置和变形形式，得到了车体碰撞过程的位移、速度、作用力、以及能量与时间的关系曲线。模拟结果表明，碰撞时间为0.15s。变形集中在车辆前部1/10内，车辆主体几乎不变形。采用多个CPU进行并行计算，结果表明，对于客车的正面碰撞问题采用并行计算非常必要。 相似文献
17.
探讨河蚬蛋白酶解液在体外的抗氧化特性,通过用普鲁士蓝反应法测定河蚬酶解液还原能力,在邻苯三酚自氧化体系和Fenton体系中分别检测河蚬酶解液清除超氧阴离子自由基(O_2~-·)和羟自由基(·OH)的能力,并考察河蚬酶解液在亚油酸过氧化体系中的抗氧化作用.河蚬酶解液具有显著的清除O_2~-·和·OH作用,且活性与剂量呈量效关系,并对亚油酸脂质过氧化也显示出较强的抑制作用. 相似文献
18.
Lorenz KT Chandler DW Barr JW Chen W Barnes GL Cline JI 《Science (New York, N.Y.)》2001,293(5537):2063-2066
The preferred sense of product molecule rotation (clockwise or counterclockwise) in a bimolecular collision system has been measured. Rotationally inelastic collisions of nitric oxide (NO) molecules with Ar atoms were studied by combining crossed molecular beams, circularly polarized resonant multiphoton ionization probing, and velocity-mapped ion imaging detection. The observed sense of NO product rotation varies with deflection angle and is a strong function of the NO final rotational state. The largest preferences for sense of rotation are observed at the highest kinematically allowed product rotational states; for lower rotational states, the variation with deflection angle becomes oscillatory. Quantum calculations on the most recently reported NO-Ar potential give good agreement with the observed oscillation patterns in the sense of rotation. 相似文献
19.
The phonon density of states of the geophysically important mineral forsterite has been calculated with a rigid-ion model, which gives good agreement with an experimental measurement by inelastic neutron scattering. The density of states has been used to calculate the specific heat as a function of temperature, the results of which are in excellent agreement with calorimetrically measured values. The rigid-ion model takes account of the interatomic interactions and normal modes of vibration on a detailed microscopic basis, and is therefore more realistic than the Debye and other empirical models used previously. 相似文献
20.
Jankunas J Zare RN Bouakline F Althorpe SC Herráez-Aguilar D Aoiz FJ 《Science (New York, N.Y.)》2012,336(6089):1687-1690
When a hydrogen (H) atom approaches a deuterium (D(2)) molecule, the minimum-energy path is for the three nuclei to line up. Consequently, nearly collinear collisions cause HD reaction products to be backscattered with low rotational excitation, whereas more glancing collisions yield sideways-scattered HD products with higher rotational excitation. Here we report that measured cross sections for the H + D(2) → HD(v' = 4, j') + D reaction at a collision energy of 1.97 electron volts contradict this behavior. The anomalous angular distributions match closely fully quantum mechanical calculations, and for the most part quasiclassical trajectory calculations. As the energy available in product recoil is reduced, a rotational barrier to reaction cuts off contributions from glancing collisions, causing high-j' HD products to become backward scattered. 相似文献