共查询到20条相似文献,搜索用时 31 毫秒
1.
G. Baum W. Raith H. Steidl 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1988,10(2-3):171-178
This beam was developed as a target for a crossed-beam electron-atom scattering experiment on the interaction of a polarized spin-1/2 electron with a polarized spin-1 atom. In the future this beam will be used in “Spin-Polarized Metastable Atom Deexcitation Spectroscopy” (SPMDS) for studying ferromagnetic surfaces without and with adsorbate layers. We use a discharge source for producing a beam of metastable helium atoms, a permanent sextupole magnet with a central stop at its exit for selecting He(23 S) atoms in the Zeeman substatem s =+1, a zero-field spin flipper for reversing the atomic beam polarization with respect to a magnetic guiding field, and a Stern-Gerlach magnet for analyzing the atomic polarization. At a distance of 90 cm beyond the exit of the sextupole, in the “interaction region” of an experiment, the polarized beam has a circular cross section of about 6 mm FWHM and a particle density of 1 · 107 atoms/cm3. The reversible spin polarization was determined asP=0.90±0.02. A possible contamination of the beam with metastable singlet atoms is included within this value; the ground-state He atoms are not considered to be part of the polarized beam. An observed contamination with long-lived Rydberg atoms can easily be destroyed by applying a high electric field. 相似文献
2.
The translational energy distribution of an atom can be calculated by differentiating the Doppler line shape of its emission line taken at a high optical resolution. The Balmer-β line of the excited hydrogen atom (n = 4) produced by electron impact on HCl has been measured at a high resolution (0.033Å) and at two angles (55° and 90°) with respect to the electron beam. The translation energy distribution depends on the electron energies and has almost two groups of components: ≈ 5 eV (fast) and ≈ eV (slow). Anisotropy is imporant for the slow component. The excitation function shows the corresponding structures. It is concluded that Rydberg states converging to the 2Π state of HCl+ produce the fast component and Rydberg states converging to the repulsive HCl+ states which cross the 2Σ+ state produce the slow component. 相似文献
3.
Spectroscopic signatures of Jahn-Teller (JT) coupling in then p x,y 2 E′ Rydberg series of H3 and D3 are analyzed within a simple multi-channel-quantum-defect-type model. The JT coupling constant has been inferred from existing ab initio calculations of the potential-energy surfaces of low Rydberg states of H3. The model predicts pronounced resonant perturbations in Rydberg members with intermediaten (n=5–7) as well as strong JT-induced autoionization in absorption and ion-yield spectra from vibrationally excited initial levels. The JT-induced effects are shown to exhibit a characteristic dependence on the vibrational angular momentum of the bending mode in the initial state. 相似文献
4.
5.
Pamela M. St. John Rainer D. Beck Robert L. Whetten 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):226-228
We review the processes which have been observed from collisions between alkali-halide clusters and solid surfaces. Soft impact of nanocrystalline NanF n?1 + clusters against solid surfaces causes them to cleave along the lowest energy (100) plane. At higher collision energies (Ei>1 eV/atom), an evaporative cascade occurs which is characteristic of a transformation of the nanocrystal to a molten state. Efficient F? transfer from the cluster to the surface can occur for the larger clusters (>60 atoms) scattering from Si(111), in direct competition with the cleaving channel at low energies. In this regime, strong bonds can form between the F? and silicon surface. The reaction probability increases with cluster size indicating that an impact-initiated shock wave is needed to enhance the reactive process. 相似文献
6.
We present pseudo-potential calculations of geometrical structures of stable isomers of LiAr
n
clusters with both an electronic ground state and excited states of the lithium atom. The Li atom is perturbed by argon atoms
in LiAr
n
clusters. Its electronic structure obtained as the eigenfunctions of a single-electron operator describing the electron in
the field of a Li+Ar
n
core, the Li+ and Ar atoms are replaced by pseudo-potentials. These pseudo-potentials include core-polarization operators to account for
the polarization and correlation of the inert core with the valence Lithium electron [J Chem Phys 116, 1839 1]. The geometry optimization of the ground and excited states of LiAr
n
(n = 1–12) clusters is carried out via the Basin-Hopping method of Wales et al. [J Phys Chem 101, 5111 2; J Chem Phys 285, 1368 3]. The geometries of the ground and ionic states of LiAr
n
clusters were used to determine the energy of the high excited states of the neutral LiAr
n
clusters. The variation of the excited state energies of LiAr
n
clusters as a function of the number of argon atoms shows an approximate Rydberg character, corresponding to the picture
of an excited electron surrounding an ionic cluster core, is already reached for the 3s state. The result of optical transitions calculations shows that the absorption spectral features are sensitive to isomer
structure. It is clearly the case for transitions close to the 2p levels of Li which are distorted by the cluster environment. 相似文献
7.
N.F. Verster H.C.W. Beijerinck J.M. Henrichs P.M.A. Van Der Kam 《Chemical physics》1981,55(2):169-176
Due to the forward peaked differential cross section for elastic atom—atom scattering the effect of multiple collisions has to be considered in the analysis of crossed beam measurements of the total cross section and especially of the small angle differential cross section at large values of the beam attenuation. At angles θ ≈ θ0, with θ0 the quantum mechanical scaling angle of the elastic differential cross section, the correction for the latter case amounts to 20% at beam attenuations I/I0 = exp(?1). Firstly, a careful analysis of the probabilities for single and multiple scattering is given, resulting in an expression for the measured beam signals which is correct for all values of the beam attenuation. The probability for multiple scattering is then calculated for an inverse power potential V(r) = ?Csr?s, with s = 4 through s = 7, which include both the case of ion—atom scattering (s = 4) and atom—atom scattering (s = 6). The results are given as effective differential cross sections σn(θ) for n-fold scattering. They are described by a single, simple analytical function with four free parameters that have been determined for n = 2, 3 and 4 by a least squares method. The σn(θ) are normalised to the total cross section Q. 相似文献
8.
Dong-Sheng Hao Jin-Rong Liu Wen-Guang Wu Ju-Cai Yang 《Theoretical chemistry accounts》2009,124(5-6):431-437
The neutral Si n K (n = 2–8) clusters and their anions have been systematically studied by means of the higher level of Gaussian-3 schemes. Equilibrium geometries and electron affinities have been calculated and are discussed for each considered size. For neutral Si n K clusters, the ground state structure is found to be “attaching structure”, in which the K atom is bound to Si n clusters. The most stable isomer for their anions, however, is found to be “substitutional structures”, which is derived from Si(n+1) by replacing the Si atom with a K. The dissociation energies of K atom from the lowest energy structures of Si n K have also been estimated to examine relative stabilities. 相似文献
9.
F. Balzer M. Hartmann M. Renger H. -G. Rubahn 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,28(4):321-329
Na clusters bound to mica surfaces have been irradiated with pulsed and cw visible laser light. Kinetic energy and angular distributions of the Na atoms desorbing from the clusters have been determined using cw two-photon laser-induced fluorescence detection. In addition the dependence of the desorption rate on laser power, wavelength and polarization has been measured. The most probable kinetic energyE kin of the photodesorbed atoms at the surface temperatureT S =300 K was found to beE kin=18±5 meV, independent of laser irradiance (3 µJ/cm2...20 mJ/cm2) and wavelength (450 nm, 505 nm, 658 nm). With increasing orientation angle between detection axis and surface normal (0°≦Θ≦90°)E kin was observed to decrease slightly, while it was nearly independent of surface temperature betweenT S =30 K andT S =300 K. Also, with increasing radius of the Na clusters the desorbing Na atoms slowed down. The angular distribution of the Na atoms was of cos2-type with respect to the surface normal. These observations suggest that laser-induced desorption of Na from Na clusters bound to mica surfaces involves an initial rate-limiting step of direct surface plasmon excitation followed by a final step of delayed thermal desorption. 相似文献
10.
F. Stienkemeier J. Higgins C. Callegari S. I. Kanorsky W. E. Ernst G. Scoles 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1996,38(3):253-263
Large liquid helium clusters (Hen, n ≈ 104) produced in a supersonic jet are doped with alkali atoms (Li, Na, K) and characterized by means of laser induced fluorescence. Each cluster contains, on average, less than one dopant atom. Both excitation and emission spectra have been recorded. The observed excitation spectra are analyzed, calculating the transitions within an approach based on the hypothesis that the chromophores are trapped in a dimple on the cluster’s surface as predicted by the theoretical calculations of Ancilotto et al. [9]. The results of the model calculations are in good qualitative agreement with the experimental findings. In spite of the very weak binding energy (a few cm?1), some of the excited atoms remain bound to the surface, provided the excitation occurs at frequencies not too far from the alkali’s gas phase absorptions. These bound-bound excitations produce very broad, red shifted emission spectra. At other, blue shifted frequencies, the excited atoms desorb from the cluster’s surface, giving rise to unshifted, free atom, emission spectra. The heavier alkali metals (Na, K) show, compared to the calculations, an additional broadening which is attributed to surface excitations on the helium droplet. 相似文献
11.
《Spectrochimica Acta Part B: Atomic Spectroscopy》1996,51(13):1679-1693
Laser excited atomic fluorescence-electrothermal atomizer (LEAFS-ETA) was used to study atomization and diffusion mechanisms in a novel diffusive graphite tube atomizer. The atomizer design included a hollow graphite cylinder mounted between two graphite rods which served as electrodes. One of the rods had a small graphite insert with a sampling hollow and could move backwards and forwards. After the sample was introduced into the hollow, the electrodes tightly sealed the graphite cylinder ensuring that the insert was directly in the center of the furnace. The furnace assembly was then heated and the vaporized sample diffused through the hot graphite wall. The atomic fraction of the sample vapor was excited by a laser beam which was directed along the graphite tube surface so that no gap remained between the beam and the tube surface. Fluorescence vs. time profiles for three elements — Cu, Ag and Ni — were obtained within the temperature range of 1400–2600 K. The rate constants of specific processes were measured from the decay portions of the fluorescence signals under the assumption of first-order kinetics. The Arrhenius plots were constructed and the activation energies, Ea were evaluated from their slopes. The plots obtained for Cu and Ag consisted of two linear parts, the corresponding values of Ea were: 195 kJ/mol and 77 kJ/mol for Cu (1550 K < T < 2600 K) and 238 kJ/mol and 97 kJ/mol for Ag (1430 K < T < 2280 K). The Arrhenius plot for Ni was linear within the temperature range of 1770–2530 K resulting in an Ea equal to 161 kJ/mol. The diffusion coefficients were evaluated on the basis of a steady-state diffusion model out of a hollow cylinder. The values for the diffusion coefficients were: 3.7·10−4−2.0·10−3 cm2/s (1750–2600 K) for Cu, 6.5·10−3−1.4·10−3 cm2/s (1750–2280 K) for Ag and 5.6·10−5−1.5·10−3 cm2/s (1770–2530 K) for Ni. 相似文献
12.
Reactive scattering of O atoms with CF3I molecules has been studied at an initial translational energy E = 32.1 kJ mol?1 using a He seeded O atom beam and at E = 13.9 kJ mol?1 using a Ne seeded O atom beam. Reactive scattering of IO product favours the backward hemisphere at low energy but becomes almost isotropic at high energy. The product translational energy distribution at low energy indicates substantial energy transfer with internal modes of the collision complex but at high energy the excess energy is disposed into product translation. 相似文献
13.
C. Menzel A. Knöner J. Kutzner H. Zacharias 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1996,38(2):179-184
The scattering behavior of neutral ammonia clusters off a LiF(100) surface is studied. Ammonia clusters are produced by a coexpansion of NH3 and Kr with an average kinetic energy of 48 meV per monomer molecule. Using single photon VUV laser ionization at λ = 118 nm (hv = 10.49 eV) the mass distribution of scattered particles is obtained in a reflecting time-of-flight mass spectrometer. Compared with the incoming cluster beam the average cluster size of the scattered particles is drastically decreased. The angular distribution of NH 3 + and NH 4 + after scattering reveals a strong inelastic interaction between the clusters and the LiF(100) surface which is described in the context of a thermokinetic model and a phonon excitation along the (001) azimuth of the LiF(100) surface. 相似文献
14.
SCF MO computations have been carried out on several excited states of CH and NH in which the excited MO 4σ is a Rydberg or near-Rydberg MO at internuclear distances R near that (Re) of equilibrium in the ground state, but becomes an antibonding valence-shell MO as R increases toward dissociation. For the lowest 3Πg state of H2 and the lowest 3Πg and 3Πu states of N2 the extent of 3dπ Rydberg character in the excited MO as a function of R for some R values ? Re is evaluated by SCF MO computations. 相似文献
15.
Infrared absorptions of the (CICF)?H—F? and HCF?2 anions have been identified in the spectrum of the products of the photolysis of HCCIF2 isolated in an argon matrix at 14 K using radiation in the 147—105 nm spectral range. These anions are also produced in matrix experiments in which the Ar:HCCIF2 sample is codeposited with an atomic beam of sodium and charge transfer processes are induced by mercury are radiation, as well as in experiments in which the sample is codeposited with a beam of argon that has been passed through a microwave discharge. Both the spectral data and molecular orbital calculations for HCF?2 indicate that this molecule has an exceptionally weak CH bond and that its apex angle is considerably more acute than that of the uncharged HCF2 radical. The high threshold for ion production from HCCIF2 dictates consideration of the role played by highly excited Rydberg states of argon in the discharge experiments. It appears probable that charge transfer occurs between these excited argon and HCCIF2. The high ion yield typical of the discharge experiments is also consistent with product formation resulting from collision of matrix-isolated HCCIF2 with free excitons formed by the impact of highly excited argon atoms at the surface of the solid deposit. 相似文献
16.
We report ab initio theoretical calculation on 32 excited states of H2 O found to lie below 11.7 eV. Of the eight states observed experimentally, the average discrepancy between theoretical and experimental excitation energies is 0.1 eV. We find that the excited states can each be characterized as arising from an excitation to a Rydberg orbital. Our results indicate that the ? and F? states are both 3d-like excited states rather than one 3d state and one 4s state as previously assumed and similarly for the two Rydberg series joining onto ? and F?. The nsa1 Rydberg series is found to have a quantum defect of 1.38. joining onto the Ã(1B1 state. We have assigned the 9.81 eV transition observed by electron impact as the 1b1 – 3pb1 excitation to a 3A1 state. 相似文献
17.
E. E. B. Campbell H. Hülser R. Witte I. V. Hertel 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1990,16(1):21-33
Rank 4 alignment has been observed in a quasi one electron system. Relative charge transfer cross sections of the Na 4dσ, 4dπ and 4dδ sublevels for the K++Na(4D)→K*+Na+ system have been measured. A strong energy dependence is observed. The results at energies less than 1 keV may be attributed to rotational coupling of the Na 4dπ state to the K 4fδ state. The Na atom is excited in a two-step process, the first step being excitation to the 32 P 3/2 level with a two-mode laser to pump from both hyperfine levels of the ground state simultaneously. This two-mode laser is described in detail. The optical pumping problem is solved using rate equations. The general formula for describing the scattering intensity for cylindrical symmetry, in terms of multipole moments, for atoms excited by two linearly polarised lasers is derived and used to evaluate the experimental results. 相似文献
18.
An all-valence-electron CI treatment is reported for the low-lying valence and Rydberg states of butadiene. All singly- and doubly-excited configurations relative to a series of the leading terms in a given CI expansion are taken into account, with resulting secular equation orders of as high as 150 000. The agreement between calculated and experimental transition energies is invariably better than 0.2 eV where comparison is possible, with all low-lying valence triplet and Rydberg singlet excited states being unambiguously assigned. The valence-shell excitation to the 2 1Ag species is concluded to correspond to the 7.06 eV band system, while the forbidden singlet—singlet transition reported by McDiarmid is assigned as x2 → 3s. The possibility of an avoided crossing between Rydberg valence 1Bu excited states having a determining influence on the appearance of the broad intense V1—N absorption is also discussed. 相似文献
19.
Rydberg states and clusters of Rydberg states have been reported in several cases from high temperature sources, so-called diffusion sources. The present study uses the same technique as the one used for the study of excited Cs clusters (Åmanet al., 1990), and is aimed at highly excited Rydberg cluster formation from hydrogen containing molecules. Ionized clusters from a diffusion source with the graphite foil emitter at 1400 K are studied by time-of-flight mass spectrometry. The excited clusters are shown to be ionized by field ionization. The best results are found using ethylene in the source, and the flux from the source probably contains both hydrogen and hydrocarbon species. Typical clusters have a mass of 10,000–200,000 a.u., assuming singly charged clusters. The formation and stabilization (cooling) of such large clusters under the present conditions is only possible since excited states can form a condensed phase, of so called Rydberg matter (Manykinet al., 1981, 1982, Petterssonet al., 1992, Svenssonet al., 1991, 1992). The importance of excited hydrogen containing clusters for the chemistry and physics of interstellar space is pointed out. 相似文献
20.
Eric J. Heller 《Chemical physics letters》1982,90(6):484-485
A simple analysis of the competition between the alternative channels of atom and excitation transfer in collisions of metastable rare-gas atoms with halogens is presented. The intervention of potential surfaces correlating with highly excited Rydberg states lying just below the limit of the molecular cation as well as those which correlate with the anion provides a qualitative rationale for much current experimental data. 相似文献