Collimators, atomic spectroscopy

If a collimated atom beam expands into vacuum, the velocity component of the atoms in the beam direction is much greater than the thermal velocity component perpendicular to the beams axis, in accord with the low divergence of the beam. Consequently, the Doppler width of atom transitions, as seen by an intersecting laser field perpendicular to the axis of the atom beam, is reduced significantly. Isotopes, which have been excited selectively by the tunable laser, can then be detected by LIES, by ionization techniques, such as RIS and FILS, or by RIMS after photoionization. LEI spectroscopy cannot be applied since collisional ionization does not occur in the atom beam. 

With laser beams, the effect can be observed in absorption. This is the basis for collinear fast-beam laser spectroscopy. Among the Doppler-free techniques (described in Part A, Chapter 15 by W. Demtroder) it is the only one using linear absorption without velocity selection as in collimated atomic beams. 

We will first describe spectroscopy on collimated atomic beams and on kinematically compressed ion beams. Two groups of nonlinear spectroscopic techniques will be discussed saturation techniques and two-photon absorp-... 

Fig.9.36. Laser spectroscopy on a collimated atomic beam. Three different detection methods are illustrated...

We will first describe spectroscopy on collimated atomic beams and on kinematically compressed ion beams. Two groups of nonlinear spectroscopic tecliniques will be discussed saturation techniques and two-photon absorption techniques. We will also deal with the optical analogy to the Ramsey fringe technique (Sect. 7.1.2). In a subsequent section (Sect. 9.8) laser cooling and atom- and ion-trap techniques will be discussed. Here, the particles are basically brought to rest, ehminating the Doppler as well as the transit broadening effects. 

An example of laser spectrometers for Zeeman and Stark spectroscopy using a collimated atomic beam is shown schematically in Figure 2. It consists of a continuous-wave (CW) tunable dye-laser system, a frequency calibration system, a vacuum chamber with a fluorescence detector, and a data-acquisition system. The interaction point of the atomic beam with the laser is inside the vacuum chamber. 

The advent of lasers in spectroscopy has made possible highly precise measurements of spectroscopic as well as of fundamental interest, Particular emphasis has been put onto the elimination of the Doppler effect, which was one of the main obstacles in classical spectroscopy. This can be achieved using well collimated atomic beams or non-linear field/atom interactions, which, combined with quantum interference methods, are capable of yielding a resolution beyond the natural linewidth. In historical perspective, these methods were developed because of the problems associated with the Doppler effect, the possibilities offered by the high intensity and narrow spectral band width of lasers and, most important, an ever persistent wish to obtain very high optical resolution. 

Hyperfine structure measurements using on-line atomic-beam techniques are of great importance in the systematic study of spins and moments of nuclei far from beta-stability. We will discuss the atomic-beam magnetic resonance (ABMR) method, and laser spectroscopy methods based on crossed-beam geometry with a collimated thermal atomic-beam and collinear geometry with a fast atomic-beam. Selected results from the extensive measurements at the ISOLDE facility at CERN will be presented. 

Using modern techniques of laser cooling, high-intensity beams of metastable He (ls2s S) atoms can be prepared well suited for precision spectroscopy in high electric fields [7]. After transversal cooling, well collimated beams can be exposed... 

Emission spectroscopy has been used recently in an elegant attempt to elucidate the mechanism of the energy transfer process (10). Moulton and Herschbach have examined the emission from a triple molecul2ur beam experiment. Molecular beams of bromine and atomic potassium cross each other, and vibrationally excited KBr is formed, which is then collimated into a further beam. 

Thermal atomic beams have been used extensively to determine nuclear spins and moments by investigations of the atomic hyperfine structure. The atomic-beam magnetic resonance (ABMR) method has already become classical [2]. More recent efforts include laser spectroscopy in a crossed-beam geometry, in which a large supression of the Doppler width is obtained by collimation of the atomic beam. 

The dry samples of silica-coated alumina were analyzed by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Specific surface area was determined by nitrogen adsorption (BET). Particle-size determination was made by low-angle forward scattering of light from a laser beam (Leeds and Northrup s Microtrac particle sizer) and by monitoring sedimentation with a finely collimated beam of low-energy x-rays and a detector (Micromeritics Sedigraph 5100). 

The technique of reducing the Doppler width by the collimation of mo lecular beams was employed before the invention of lasers to produce light sources with narrow emission lines [389]. Atoms in a collimated beam were excited by electron impact. The fluorescence lines emitted by the excited atoms showed a reduced Doppler width if observed in a direction perpendicular to the atomic beam. However, the intensity of these atomic beam light sources was very weak and only the application of intense monochromatic, tunable lasers has allowed one to take full advantage of this method of Doppler-free spectroscopy. 

The Rabi technique of radio frequency or microwave spectroscopy in atomic or molecular beams [10.14-10.17] has made outstanding contributions to the accurate determination of ground state parameters, such as the hfs splittings in atoms and molecules, small Coriolis splitting in rotating and vibrating molecules, or the narrow rotational structures of weakly bound van der Waals complexes [10.18]. Its basic principle is illustrated in Fig. 10.9. A collimated beam of molecules with a permanent dipole moment is deflected in a static... 

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