Schwingungsanregung von H2-Molekülen durch Zentralstoß mit Li+-Ionen

An apparatus is described for measuring the inelastic differential cross section for vibrational excitation in collisions of diatomic molecules with monoenergetic ions at laboratory energies between 10 and 50 eV. The method consists of measuring the time of flight of single ions with a time amplitude converter and displaying the results on a 100 channel pulse height analyzer. From the shift in the time of flight relative to that expected for elastic scattering the final state of the molecule excited in a single collision is identified. By studying only central collisions with almost zero impact parameter rotational excitation is strongly suppressed. Measured times of flight after collisions of monoenergetic Li⁺ ions with H₂ show that with increasing energy the most probable vibrational quantum jump increases from 0→1 to 0→2,0→3 etc. Contrary to the usual assumption of a small steric factor for vibrational excitation the results show that the inelastic cross section is larger than the elastic cross section. Using reported potential parameters the energy dependence of the most probable excited state is compared with the calculations of Secrest and Johnson for a one-dimensional collinear collision. The satisfactory agreement suggests that the steric factor is close to 1. From measurements at different scattering angles at 10 eV the integral inelastic cross section is found to be about 0.2 Ų corresponding to a differential cross section of 0.4 Ų/sr. Measured values of integral and differential total cross sections for Li⁺-He andLi⁺-H₂ are reported and compared with theory. Direct dissociation of D₂ by Li⁺ in the energy range from 25 to 55 eV was not observed, yielding an upper limit for the cross section of 4 · 10^?4 Ų/sr.     

Rotationsanregung in molekularen Stößen

In a crossed beam experiment K-atoms are velocity analysed before and after collisions with thermally distributed CO₂ molecules. The dependence of the scattering on CMS angle for 6 °???28 °, and on translational-rotational energy transfer from 10 to 50% of the initial relative collision energyE is measured forE=16.8 and 20.2 × 10^?14 erg. The energy transfer corresponds to high rotational excitation of CO₂, 6?Δj?22, if the average levelj=20 before the collision is taken to be the representative initial state. Independent of the amount of energy transfer the inelastic scattering increases towards small angles. Average partial cross sections, reduced to transitions between single levels, are estimated to be considerable fractions of the gas kinetic cross section at ?=20 °. The analysis strongly suggests the formation of a long-lived collision complex in the encounter.     

Broadening of a two-photon resonance in a zinc atom in collisions with CO<Subscript>2</Subscript>, CO,and NO molecules

The broadening of a two-photon resonance is studied experimentally at the 4s¹S₀?6s³S₁ transition in a zinc atom upon absorption of two waves with a small detuning from an intermediate state in collisions with CO₂, CO, and NO molecules. The measured absolute values of broadening cross sections greatly exceed gas-kinetic cross sections and are (9.4±2.4, 6.5±1.6, and 3.9±1.0)×10^{? 14}cm² for CO², CO, and NO, respectively. Anomalously large rate constants and cross sections obtained in experiments are explained by the efficient resonance quenching of the excited states of zinc atoms in collisions with molecules accompanied by transfer of the energy of excited atoms to vibrational-rotational degrees of freedom of molecules.     

Loss estimation of electrostatically trapped ND3 molecules

Losses of electrostatically trapped molecules are caused by the Majorana transition or the inelastic collisions between trapped molecules. The loss rate of electrostatically trapped ND₃ molecules in the (J=1,K=1,M=1, A) state was estimated: ND₃ molecules in this state have actually been trapped. When trapping cold molecules, using ¹⁵ND₃ molecules (fermion) with a minimum electric field higher than 10 kV/cm yields high stability.     

Collision dynamics between stretched states of spin-2 87Rb Bose–Einstein condensates

We experimentally observed the time dependence of the spin populations of spin-2 ⁸⁷Rb Bose–Einstein condensates confined in an optical trap. The condensed atoms were initially populated in the stretched states |F=2,m _F =+2〉 and |F=2,m _F =?2〉 with several varieties of population imbalances. No spin-exchange collisions were observed in a weak magnetic field of 45 mG. The atom loss rate depended on the observed relative population of spin-states. We calculated the loss rate due to two-body inelastic collisions with the population imbalance using an experimentally estimated rate of 17.0(±1.9)×10^?14 cm³?s^?1 under the population balance. The calculations were in good agreement with the measurements. Our results show that the dependence of inelastic collisions on spin channels plays an important role in the time-evolution of spin populations.     

Feshbach resonances in an ultracold mixture of 87Rb and 40K

We report the experimental preparations of the absolute ground states of ⁸⁷Rb and ⁴⁰K atoms (| F=1, m_F=1,〉+ |F=9/2, m_F=-9/2,〉) by means of the radio-frequency and microwave adiabatic rapid passages, and the observation of magnetic Feshbach resonances in an ultracold mixture of bosonic ⁸⁷Rb and fermionic ⁴⁰K atoms between 0 T and 6.0 × 10^-2 T, including 7 homonuclear and 4 heteronuclear Feshbach resonances. The resonances are identified by the abrupt trap loss of atoms induced by the strong inelastic three-body collisions. These Feshbach resonances should enable the experimental control of interspecies interactions.     

Polarization Modulation Effects in Infrared–Infrared Four-Level Double Resonance inCH3F andNH3

The elements of the Jones matrices for an optically pumped sample have been derived and used to predict four-level double resonance absorption coefficients that are functions of the velocity component of the molecules in the direction of the pump beam for different polarizations of the probe beam. When a saturating pump and weak probe are used in four-level double resonance experiments under population modulation conditions, these absorption coefficients are found to depend only on the first three statistical tensor ranks:n= 0 (population), 1 (orientation), and 2 (alignment). It is also found that for polarization modulation experiments with plane-polarized radiation, the absorption coefficient depends only on the alignment of them-state populations. Similarly, for polarization modulation with circularly polarized radiation, the absorption coefficient depends only on the orientation. The theory was used to interpret double-resonance polarization modulation experiments in¹³CH₃F and¹⁵NH₃in order to examine the effects of collisions on the initial anisotropy of the projection ofJon a space-fixedZaxis. The four-level double-resonance lineshapes were fit by least squares to absorption coefficients predicted by the theory. The collisional effects were modeled by a sum of Keilson–Storer collision kernels. The results of the fits were much improved when the value of the effective rate constant for the transfer of then= 0 tensor from the upper level of the pump to the lower level of the probe was larger than the values of the effective rate constants for the transfer of then= 1 and 2 populations. The best ratio of the rate constant forn> 0 to that forn= 0 is about 2/3 for¹³CH₃F and 1/3 for¹⁵NH₃. Additional analysis of the lineshapes showed the importance of long-range dipole–dipole interactions, elastic realignment and reorientation, and V–V mechanisms for collision-induced rotational energy transfer in¹³CH₃F and¹⁵NH₃.     

Charmonium suppression by gluon bremsstrahlung in p-A and A-B collisions

Prompt gluons are an additional source for charmonium suppression in nuclear collisions, in particular for nucleus-nucleus collisions. These gluons are radiated as bremsstrahlung in N-N collisions and interact inelastically with the charmonium states while the nuclei still overlap. The spectra and mean number <n _g> of the prompt gluons are calculated perturbatively and the inelastic cross section σ_abs ^Ψg is estimated. The integrated cross sections σ(A B →J/ψX) for p-A and A-B collisions and the dependence on transverse energy for S-U and Pb-Pb can be described quantitatively with some adjustment of one parameter <n _g>σ_abs ^Ψg. Received: 20 August 1999     

Precision measurement of structure function ratios for6Li,12C and40Ca

The structure function ratiosF ₂ ^C /F ₂ ^Li ,F ₂ ^Ca /F ₂ ^Li andF ₂ ^Ca /F ₂ ^C were measured in deep inelastic muonnucleus scattering at an incident muon energy of 90 GeV, covering the kinematic range 0.0085<x<0.6 and 0.8<Q ²<17GeV². The sensitivity of the nuclear structure functions to the size and mean density of the target nucleus is discussed.Supported by Bundesministerium für Forschung und Technologie     

Rb(62D)原子与基态Rb原子,H2分子碰撞转移截面

本文用荧光法测量Rb(6²D)原子与基态Rb原子,H₂分子碰撞转移截面。结果表明:Rb(6²D)-Rb(5²s)转移截面为σ_fs=67×10^-14cm²,σ_tr=4.3×10^-14cm²,Rb(6²D)—H₂转移截面为σ_fs 关键词：     

Methyl rotation in acetamide: the transition from quantum mechanical tunneling to classical reorientation studied by inelastic neutron scattering

Quasielastic and inelastic incoherent neutron scattering has been used to study in detail the transition from quantum mechanical tunneling motion to classical reorientation of the methyl groups in rhombohedral acetamide CH₃CONH₂. The temperature dependence of the low temperature quasielastic and inelastic scattering due toE ^a E ^b and AE transitions of the tunneling methyl groups has been investigated with eV resolution and — together with the higher temperature quasielastic scattering-compared with theoretical predictions. Microscopic theories are capable to describe most of the experimental observations at low temperatures. A heuristic theoretical approach accounts well for the high temperature results.     

Isotopeneffekt in der paraelastischen relaxation

The kinetics of the reorientation of the molecular impurities¹⁶O₂ ^? and¹⁸O₂ ^? in the host lattices KCl, KBr and KJ has been investigated by means of ESR as a function of uniaxial stress in the temperature range between 1,3°K and 4,5°K. Below 4°K the reorientation rate is proportional to the temperature, indicating that one-phonon processes dominate at low temperatures. ForT>4°K the reorientation through excited librational levels becomes significant. The large isotope effect in the reorientation rate of¹⁶O₂ ^? and¹⁸O₂ ^? confirms that tunneling plays an important role. The analogy between the diffusion of polarons and the reorientation of molecules in solids is discussed in detail. The evaluation and interpretation of the experimental results in terms of the theory of Gosar and Pire yields values for the tunneling matrix elements, the potential barrier between two equilibrium orientations, the libration frequencies and an effective moment of inertia. The theory accounts well for the contribution of the one-phonon processes to the relaxation rate for stress along [001]. For stress along [111] the agreement is only qualitative.     

Cold inelastic collisions between lithium and cesium in a two-species magneto-optical trap

We investigate collisional properties of lithium and cesium which are simultaneously confined in a combined magneto-optical trap. Trap-loss collisions between the two species are comprehensively studied. Different inelastic collision channels are identified, and inter-species rate coefficients as well as cross-sections are determined. It is found that loss rates are independent of the optical excitation of Li, as a consequence of the repulsive Li^*-Cs interaction. Li and Cs loss by inelastic inter-species collisions can completely be attributed to processes involving optically excited cesium (fine-structure changing collisions and radiative escape). By lowering the trap depth for Li, an additional loss channel of Li is observed which results from ground-state Li-Cs collisions changing the hyperfine state of cesium. Received 28 December 1998 and Received in final form 16 February 1999     

Experimental Study and Modelling of Formation and Decay of Active Species in an Oxygen Discharge

A microwave (2.45 GHz) oxygen discharge (3 hPa, 150 W, 50 mL.min^–1) is studied by optical emission spectroscopy of O(⁵P) (line 777.4 nm) and of the atmospheric system of O₂(head‐line 759.4 nm). Calibration of the spectral response of the optical setup is used to determine the concentrations of O(⁵P) and O₂(b). The concentration of the O(⁵P) atoms is in the range 108–109 cm^–3 and the concentration of the O₂(b) molecules is in the range 10¹⁴ – 2 × 10¹⁴ cm^–3 along the discharge tube. An attempt is made to simulate the experimental results by using coupling the Boltzmann equation, homogeneous energy transfer V‐V and V‐T, heterogeneous reactions on the walls (energy transfer and recombination of atoms) and a kinetic scheme (electronic transfer and chemical reactions). The Boltzmann equation includes momentum transfer, inelastic and superelastic processes and e‐e collisions. V‐V and V‐T transfer equations are obtained from the SSH theory and the kinetic scheme includes 65 reactions with 17 species [electrons e, ions O^– and O₂^–, fundamental electronic neutral species O(³P), O₂, O₂(X,v), O₃ and excited neutral species O₂(a), O₂(b), O₂(A), O(¹D), O(¹S), O(⁵P), O(4d ⁵D^o), O(5s ⁵S^o), O(3d ⁵D^o) and O(4s ⁵S^o)]. A fair agreement between experimental results and modelling is obtained with the following set of fitting values: – heterogeneous deactivation coefficient for O₂(b) γ = 2.6 × 10^–2; – rate constant of reaction [O(¹D) + O(³P) → 2 O(³P)] k₃₄ = 1.4 × 10^–11 cm³.s^–1; – electron concentration in the range 10¹⁰ – 10¹¹ cm^–3. Modelling shows that the recombination coefficient for oxygen atoms on the silica wall (range 1.4 × 10^–3 – 0.2 × 10^–3) is of the same order as the values obtained in a previous paper and that the ratio ([O] / 2 [O₂]initial) is about 33–50%. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Large angle jumps of small molecules in amorphous matrices analyzed by 2D exchange NMR

The reorientation dynamics of deuterated benzene and hexamethyl benzene as additives to the glass former oligostyrene is studied below the glass transition temperature T_g. By means of 2D ²H NMR, analyzed in the frequency and in the time domain, it is shown that the dynamics of the small molecules is governed by an isotropic large angle reorientation process, which is close to the random jump model. Furthermore, the dynamics is characterized by a broad distribution of correlation times. Even 65 K below T_g, a fraction of small molecules reorients on the timescale of 100 ms. In contrast, small angle reorientation dominates in the neat glass former polystyrene near T_g. As a consequence of the presence of large angle jumps, the 2D spectra can be described by an additive superposition of two sub-spectra—a ridge along the diagonal and a complete exchange pattern—where the weighting factor W(t_m) is directly given by the reorientational correlation function F₂(t_m). Additionally, for a sample with very low benzene concentration (c≈0.5%), the 1D spectra indicate that the same dynamic scenario is present in the single particle limit. Tentatively, we assume that the large angle reorientation of the small molecules is associated with a translational diffusion process of the small molecules within the amorphous matrix.     

Impact of humidity on quartz-enhanced photoacoustic spectroscopy based detection of HCN

The architecture and operation of a trace hydrogen cyanide (HCN) gas sensor based on quartz-enhanced photoacoustic spectroscopy and using a λ=1.53 μm telecommunication diode laser are described. The influence of humidity content in the analyzed gas on the sensor performance is investigated. A kinetic model describing the vibrational to translational (V–T) energy transfer following the laser excitation of a HCN molecule is developed. Based on this model and the experimental data, the V–T relaxation time of HCN was found to be (1.91±0.07)10^-3 s Torr in collisions with N₂ molecules and (2.1±0.2)10^-6 s Torr in collisions with H₂O molecules. The noise-equivalent concentration of HCN in air at normal indoor conditions was determined to be at the 155-ppbv level with a 1-s sensor time constant. PACS 82.80.Kq; 42.62.Fi