Atomic Beam Experiments

As is well known, the first-order Doppler effect can be eliminated by irradiating a well-collimated atomic beam perpendicular to a laser beam. Since tunable, narrow-band cw dye laser radiation of sufficiently short wavelength is not yet available, in most cases the excitation of atomic Rydberg states by a one-photon transition from the ground state is not feasible. The population of Rydberg states, however, might proceed from excited, metastable or even short-lived states which are continuously pumped by an additional laser beam. The latter approach was chosen by 

Coherent two-photon spectroscopy is a well-known technique that eliminates the Doppler effect, but suffers from the low cross sections o/g for the coherent absorption of two photons of equal frequency. For quasiresonant two-step excitation the overall absorption cross section is considerably larger, but the Doppler effect is only partially removed. For example, a residual Doppler width (FWHM) 

Quasiresonant two-step excitation was employed by Eliel and Hoger-vorst to study hyperfine structures and isotope shifts of 6sns and 

Quasiresonant Two-Step Excitation and Collisional (Penning) Ionization 

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The velocity distribution/(v) depends on the conditions of the experiment. In cell and trap experiments it is usually a Maxwell-Boltzmann distribution at some well defined temperature, but /(v) in atomic beam experiments, arising from optical excitation velocity selection, deviates radically from the nonnal thennal distribution [471. The actual signal count rate, relates to the rate coefficient through... 

The hfs experiments at ISOLDE may be divided into techniques employing atomic-beams and resonance-cells. Here, we will concentrate on the different atomic-beam experiments performed by the groups mentioned above. The present subject has been discussed in some detail in [EKS85], giving e.g. a full reference list on the atomic-beam works at ISOLDE. 

We want to outline such an experiment. The precision of the metastable atomic beam experiments is fundamentally determined by the short interaction... 

Although most suitable for use with lasers, Thermionic diodes have also been successfully applied to synchrotron radiation studies by using wiggler magnets to enhance the intensity of the beam [390]. Last but not least, one should mention the important category of atomic beam experiments, complemented by the techniques of photoelectron and photoion spectroscopy. All these techniques are suitable for the experimental study of interacting resonances. We turn now to their theoretical description, which will be illustrated by experimental examples. 

Fig. 10.9. The Ba spectrum in a high magnetic field with the motional Stark field compensated. The data are obtained in atomic beam experiments, and the relative intensities do not suffer from opacity or saturation effects. Both circular polarisations are separated experimentally, and are found to have the same structure (shown by presenting them as though reflected in the axis) when displaced in energy by the linear Zeeman splitting.

Wood s tubes have been used as a source of hydrogen atoms in atomic beam experiments [71]. A small slit in the middle of the long, glass section allows the atoms to diffuse into the beam chamber. A concentration of hydrogen atoms of 0 7 to 0-9 was reported. In recent microwave work [143], the use of a mixture of dimethyldichlorosilane and methyltrichlorosilane proved very effective as a wall-poisoner. 

Finally, we consider spin rotation interaction which is important only for small spherical molecules under conditions in which they are relatively free to rotate in the absence of other relaxation mechanisms. It is most common in gaseous samples (and it is indeed a major source of relaxation in molecular and atomic beam experiments) but exists also in liquids and even some solids. 

By symmetry arguments it can be shown that for nuclei the next multipole term possible is the hexadecapole. The magnitude of a hexadecapole term can be estimated to be of the order of 10" of the quadrupole term. Experimental evidence for the existence of hexadecapole moments for a number of nuclei has been obtained from atomic beam experiments as well as proton scattering studies. 

SR = calculated from measured spin rotation constants in molecular beam experiments. Aik.Hal. = from theoretical calculations of shieldings in alkali halide crystals. Aik.Hal. (P) = from pressure dependence of alkali halide shifts in combination with theoretical models for shielding values. T2 = calculated from experimental value of line width at infinite dilution. 6VS.T2 = calculated from concentration dependence of chemical shifts and line widths. H2O/D2O = calculated from solvent isotope effects on chemical shifts. NaVwa" = estimated from difference in chemical shift between Na and Na . Atomic beam = calculated from magnetic moment of free atom determined in atomic beam experiments. 

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. 

The first atomic beam experiments were performed between 1910 and 1920. In 1922 O. Stern and W. Gerlach performed their famous experiment, in which they showed that a beam of silver atoms was divided up into two components in an inhomogeneous magnetic field [7.4]. This was the first direct experimental demonstration of space quantization. In order to explain the results of this experiment we will first consider an electric dipole placed in an inhomogeneous electric field, as illustrated in Fig.7.3. The dipole is influenced by the force... 

Fig.9.38. Recording of a D - F caesium transition obtained in a collimated atomic beam experiment using stepwise excitations [9.134]... 

C. Ekstrdm, I. Lindgren Atomic beam experiments at the ISOLDE facility at CERN, in Atomic Physics 5, ed. by R. Marrus, M. Prior, H. Shugart (Plenum, New York 1977) p.201... 

Electron configurations of gaseous americium species as determined from spectroscopic and atomic-beam experiments show a Sf 7s ground state and (Sf 7 2 + 2 and Tables 14.3 and 15.6], Americium is the sixth member of the... 

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