Wolfgang E. Ernst

Photoinduced Chemical Dynamics of High-Spin Alkali Trimers

Nanometer-sized helium droplets, each containing about 104 helium atoms, were used as an inert substrate on which to form previously unobserved, spin-3/2 (quartet state) alkali trimers. Dispersed fluorescence measurements reveal that, upon electronic excitation, the quartet trimers undergo intersystem crossing to the doublet manifold, followed by dissociation of the doublet trimer into an atom and a covalently bound singlet dimer. As shown by this work, aggregates of spin-polarized alkali metals represent ideal species for the optical study of fundamental chemical dynamics processes including nonadiabatic spin conversion, change of bonding nature, and unimolecular dissociation.

Dipole moments and potential energies of the alkaline earth monohalides from an ionic model

It is shown that the Rittner model is inadequate for the calculation of dipole moments for the alkaline earth monohalides. A modified model is proposed which takes into account explicitly the large charge shifts in the metal ions arising from the polarization. The new model is shown to give results consistent with available experimental data.

Energies and electric dipole moments of the low lying electronic states of the alkaline earth monohalides from an electrostatic polarization model

A simple electrostatic polarization model is applied to the low lying electronic states A 2 Π, B 2 ∑+, and A’ 2 Δ of the alkaline earth monohalides which correlate to the electronic d state of the free metal ion. The number of fit parameters can be greatly reduced using relations which are derived from the well known angular part of the free ion wave function. The model predicts energies and electric dipole moments for all Ca, Sr, and Ba monohalides in good agreement with experimental data. The model can also be applied to the C state confirming the highly ionic character of this state.

On the use of liquid helium cluster beams for the preparation and spectroscopy of the triplet states of alkali dimers and other weakly bound complexes

Sodium dimers in their lowest triplet state are prepared by pick‐up of sodium atoms by a beam of large (n≂104) liquid helium clusters when these cross a scattering box containing sodium vapor. Using laser induced fluorescence detection the vibrationally resolved laser excitation spectrum of the 1 3Σ+g←1 3Σ+u transitions of Na2 is obtained. The spectral shifts with respect to the corresponding gas phase values are surprisingly small. These data show the feasibility of using helium clusters as an almost ideal matrix for the synthesis and spectroscopic analysis of weakly bound complexes.

Rb–He exciplex formation on helium nanodroplets

Rubidium atoms have been deposited on helium nanodroplets and optical excitation and emission spectra of the 5 2P–5 2S transition have been measured and interpreted. After laser excitation of the 5 2P3/2 state, fluorescence from a Rb*He exciplex is observed while pumping the D1 (5 2P1/2–5 2S) line yields only emission from free rubidium atoms. This observation is in agreement with the predictions from a recent model by Reho et al. [J. Chem. Phys. 113, 9694 (2000)] about the extraction of an alkali–helium exciplex from the doped helium nanodroplet surface. A high barrier along the Hen–He–Rb* axis of the 1 2Π1/2 potential prevents desorption of Rb*He within the excited state lifetime, whereas the 1 2Π3/2 potential permits the exciplex extraction. The excitation spectrum, on the other hand, reflects the structure of the excited states 1 2Π1/2, 1 2Π3/2, and 2 2Σ1/2 of the HenRb complex whose potential surfaces will depend strongly on the alkali–He interaction dynamics near the droplet surface. For a heavy sur...

Mechanism and kinetics of color center formation on epitaxial thin films of MgO

Electron stimulated desorption of oxygen atoms from the surface of epitaxially grown MgO films on Ag(1 0 0) produces characteristic losses of color centers in electron energy loss spectroscopy. The formation of five distinct loss features at 1.0, 1.3, 2.4, 2.8 and 3.4 eV have been observed that are induced by electron bombardment of the MgO surface. The possibility to erase these losses completely by simple exposure to O2 gas at room temperature suggests that they are predominantly located at or near the surface. From the dependence of loss intensities on electron exposure we conclude that the defects are generated directly by the electron impingement. The observed losses are tentatively assigned to electronic transitions associated with surface F centers (Fs) located at differently coordinated sites (surface, steps) and to M-centers. The characteristic steps of the cross-section at electron energies of 55 and 86 eV, which are close to binding energies of the Mg 2p and 2s energy levels in MgO, suggest a desorption mechanism via a multielectron Auger decay process. 2002 Elsevier Science B.V. All rights reserved.

Imaging with neutral atoms—a new matter-wave microscope

Matter‐wave microscopy can be dated back to 1932 when Max Knoll and Ernst Ruska published the first image obtained with a beam of focussed electrons. In this paper a new step in the development of matter‐wave microscopy is presented. We have created an instrument where a focussed beam of neutral, ground‐state atoms (helium) is used to image a sample. We present the first 2D images obtained using this new technique. The imaged sample is a free‐standing hexagonal copper grating (with a period of about 36 μm and rod thickness of about 8 μm). The images were obtained in transmission mode by scanning the focussed beam, which had a minimum spot size of about 2.0 μm in diameter (full width at half maximum) across the sample. The smallest focus achieved was 1.9 ± 0.1 μm. The resolution for this experiment was limited by the speed ratio of the atomic beam through the chromatic aberrations of the zone plate that was used to focus. Ultimately the theoretical resolution limit is set by the wavelength of the probing particle. In praxis, the resolution is limited by the source and the focussing optics.

A triatomic Xe2Cl excimer laser in the visible

A new triatomic rare‐gas halide laser using the molecular exciplex Xe2Cl is described. Laser emission centered at 518 nm with a special bandwidth of 30 nm and peak power of about 2 kW was obtained from an electron‐beam‐excited high‐density Ar/Xe/CCl4 mixture.

High resolution electron microscopy of Ag-clusters in crystalline and non-crystalline morphologies grown inside superfluid helium nanodroplets

We present a first investigation of structural properties of Ag clusters with a diameter of up to 5.5 nm grown inside superfluid helium nanodroplets (He(N)) and deposited on an amorphous C surface. With high resolution transmission electron microscope images we are able to show that in addition to the crystalline face centered cubic (fcc) structure, noncrystalline icosahedral (Ih), and decahedral (Dh) morphologies are grown. Relative abundances (56% fcc, 31% Dh, and 13% Ih) as well as the size distribution of each morphology (mean diameters d(fcc)=2.62(5) nm, d(Dh)=3.34(7) nm, and d(Ih)=3.93(2) nm) do not reflect the situation expected from pure energetic considerations, where small Ihs should be followed by medium sized Dhs and large fccs. Instead, kinetic factors seem to play an important role in the formation of these structures, as it appears to be the case for clusters formed by inert gas aggregation. Considering the low temperatures (0.37 K) and extremely high cooling rates, we discuss basic ideas that might lead to a qualitative picture of the cluster formation process inside He(N).

DETERMINATION OF COPPER IN A533B STEEL FOR THE ASSESSMENT OF RADIATION EMBRITTLEMENT USING LASER-INDUCED BREAKDOWN SPECTROSCOPY

Determination of radiation embrittlement in nuclear reactor pressure vessels is crucial to assessing safe operative lifetimes for many aging nuclear power plants. Conservative nuclear fluence estimates and trace impurity diagnosis of the weldment material are the basis of radiation embrittlement analysis. Copper is thought to be a key impurity contributing to radiation embrittlement. In this paper, the application of laser-induced breakdown spectroscopy (LIBS) as a means to assess radiation embrittlement by the detection and quantification of copper in A553b steel was investigated. A LIBS configuration completely coupled by fiber optics was attempted, but because of low laser power and fiber losses, fiber-optic delivery of the laser beam was unsuccessful. Consequently, hard optics (lenses and mirrors) were employed for laser beam delivery. The plasma emission was delivered successfully via fiber optics to the detection apparatus. Copper measurements were made from custom-fabricated steel samples. Comparison of the LIBS results to an independent atomic absorption spectrophotometry (AAS) analysis showed LIBS to be of comparable accuracy, especially in low-level copper samples.