Particle Beam Experiments

Particle-beam experiments were carried out in the MAJESTIX device at IPP Garching. Different aspects of and modifications to the MAJESTIX set-up were described in several publications from our group [44-48], A thorough and detailed description of the experimental set-up and applied techniques will soon be published [49]. 

As has been pointed out already in Section 2.2, spectroscopy of atoms in the visible permits additional ways for the detection of signals as compared to experiments on molecules in the infrared. This results mainly from the fact that atoms usually emit fluorescence after excitation. Therefore, absorption measurements are not mandatory. If fluorescence quanta are detected, fewer particles are required for a sufficient signal-to-noise ratio, and atomic beam experiments with low particle densities but with all their advantages such as absence of collisional broadening become possible. 

An example where nonlinear phenomena in connection with laser-rf spectroscopy have been used in atomic beams, is the recent work on calcium isotopes carried out in our laboratory. The goal of these experiments was to determine nuclear electric quadrupole moments from precise hyperfine structure data of the atomic spectrum. This is of some importance in the case of calcium, since Ca as well as Ca are so-called double-magic nuclei, i.e., with closed proton and neutron shells. Radioactive Ca (t = 1.03 X 10 yr) and the stable isotope Ca have been investigated by laser-rf spectroscopy. The measurements allow to study the influence of a single neutron and three neutrons, respectively, on the double-magic °Ca core. 

F we 33. Setup for laser-rf spectroscopy in the 4s4p state of odd Ca isotopes. Owing to 

From the experiment the nuclear electric quadrupole moments have been derived  

These values make it possible to test theoretical calculations in connection with different nuclear models.  

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This article is organized as follows. In the next section, the properties of a C H layers are shortly summarized. The following section presents the experimental methods and set-ups applied in our experiments. Section 11.4.1 summarizes the knowledge about surface loss probabilities of different hydrocarbon radicals. The remainder of the article is dedicated to a review of the results from our particle-beam experiment MAJESTIX. 

W. Jacob, C. Hopf, A. von Keudell, M. Meier, and T. Schwarz-Selinger Particle-beam experiment to study heterogeneous surface reactions relevant to plasma-assisted thin film growth and etching. Rev. Sci. Instrum. 74, 5123 (2003)... 

They include investigations in linear plasma devices (i.e., PSI-2 at IPP Berlin and PISCES-B at UCSD), where plasma conditions are very similar to those expected in the ITER divertor, and simpler plasma devices or particle beam experiments. Here individual processes can be investigated and understood in isolation and, generally, conditions are better controlled and diagnosed... 

In principle, the photon-particle interaction environment can be of any shape and encompass any media parameter, depending on the chemical system under study. The interaction region can consist of a vacuum chamber, if particle beam experiments are conducted... 

Sample A contains eight H2 molecules and four Ne atoms, and Sample B contains four H2 molecules and eight Ne atoms. In a beam experiment, both samples would give two peaks in relative areas of 2 1. Which sample was used for this experiment Particles with small mass move faster than particles with large mass, so we expect HziM M — 2.02 g/mol) to reach the detector before Ne (M M = 20.2 g /mol). The data show that the first substance to reach the detector is present in the smaller amount. Consequently, the sample used in the beam experiment is the one with the smaller amount of the hydrogen molecules. Sample B. 

Figure 3.8. Kinetic data from molecular beam experiments with NO + CO mixtures on a Pd/MgO(100) model catalyst [70]. The upper panel displays raw steady-state C02 production rates from the conversion of Pco = PN0 = 3.75 x 10-8 mbar mixtures as a function of the sample temperature on three catalysts with different average particle size (2.8, 6.9, and 15.6 nm), while the bottom panel displays the effective steady-state NO consumption turnover rates estimated by accounting for the capture of molecules in the support. After this correction, which depends on particle size, the medium-sized particles appear to be the most active for the NO conversion. (Reproduced with permission from Elsevier, Copyright 2000).

The time-dependent Schrodinger equation governs the evolution of a quantum mechanical system from an initial wavepacket. In the case of a semiclassical simulation, this wavepacket must be translated into a set of initial positions and momenta for the pseudoparticles. What the initial wavepacket is depends on the process being studied. This may either be a physically defined situation, such as a molecular beam experiment in which the particles are defined in particular quantum states moving relative to one another, or a theoretically defined situation suitable for a mechanistic study of the type what would happen if. .. 

PAD (perturbed angular distribution) is a variation of PAC with nuclear excitation by a particle beam from an accelerator. QMS is quasielastic MdBbauer-spectroscopy, QNS is quasielastic neutron spectroscopy. For MOBbauer spectroscopy (MS), perturbed angular correlation (PAC), and /J-nuclear magnetic resonance (/3-NMR), the accessible SE jump frequencies are determined by the life time (rN) of the nuclear states involved in the spectroscopic process. Since NMR is a resonance method, the resonance frequency of the experiment sets the time window. With neutron scattering, the time window is determined by the possible energy resolution of the spectrometer as explained later. 

The technique for investigating scattering processes in crossed-beam experiments is well developed. For example, elastic scattering experiments with neutral particles at thermal energies are well understood,85 and the techniques for producing molecular and alkali atom beams and to detect them and interpret their kinematics has been reviewed on several occasions.86, 87. The new aspect of the present work is the technique for... 

Parent nuclides produced by the processes mentioned above can all be used for several half-lives. In contrast, one can also populate the Mossbauer excited state directly via Coulomb excitation (84). In this technique, a beam of high-energy ( 10 MeV) charged particles (e.g., O4+, Cl7 +) is directed onto the Mossbauer isotope and the electromagnetic field generated by these particles induces nuclear transitions, which can include transitions to the Mossbauer excited state. Subsequent decay to the nuclear ground state then provides the appropriate y radiation. The half-life of a source created in this manner is the half-life of the Mossbauer excited state (e.g., several nanoseconds), and thus Coulomb excitation is necessarily an in situ technique, i.e., the Mossbauer effect experiment must be performed at the location of the charged particle beam. 

We propose to divide the 79 Mossbauer isotopes into three classes according to the half-life of the parent nuclides since this determines the ease with which experiments can be carried out about 30 days (class A), 1-30 days (class B), less than 1 day (class C). Whereas experiments with class A isotopes can be carried out in most laboratories, experiments with class C isotopes require access to a nuclear facility or a charged particle beam. 

Our studies have involved the use of both neutron and charged-particle beams and Table I presents a partial list of the targets prepared. The Nobel gas targets were prepared for in-beam y ray studies which used an external thermal neutron beam. Large amounts of target material were required for these experiments and a special device - a cryostat - was constructed to isolate and contain a large quantity of gas (in the solid state) so that the measurements could be done. The details of the construction of this device,... 

All the experiments described in this chapter were performed on molecules in gas cells, and, unlike in beam experiments, the results obtained are averaged over all possible orientations of the molecules with respect to the relative velocity vector, and over the Maxwellian distribution of the relative velocities of the colliding particles. Nevertheless, the experimental data stimulated interest in the development of theoretical models of elastic... 

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