Beam-foil method

Figure 1 shows that 8 terms of the B I quartet system are experimentally established, by means of absorption spectroscopy [8]. However, in emission spectra, studied by the beam-foil method, transitions between only 4 of these term have been reported. We now plan such measurements at higher resolution which should allow the separation of the quartet lines and those belonging to e.g. the B I doublet system or B III. Note also that only a theoretical value exists for the energy of the 2s2p3p "P term (dotted line in Fig. 1). Combinations between this even term and the adjacent odd-parity ones, 2s2p3s P and 2s2p3d P or should lie in the infrared region. Although this region has been studied in the region 1 - 4 pm [28] no quartet transitions seem to have been observed.

Beam-Foil Techniques. The beam-foil method has been discussed in Sect. 6.1. It is a very general method for measuring lifetimes of atoms and ions. However, the non-selective excitation, leading to cascading decays, places heavy demands on the data analysis and sometimes a detailed study of the different cascade channels is necessary for reliable lifetime evaluations. While the nonselective excitation frequently constitutes a problem, it is also an advantage of the method since a multitude of excited states are populated. For measurements of multiply charged ions in particular, the technique provides unique measurement possibilities where other techniques are not applicable. 

The fact that y e can now be studied in vacuum is very important for a number of fundamental experiments. One is the measurement of the Lamb shift in the first excited state of muonium (Fig.5). The beam-foil method produces not only the n=l state but excited states as well l with a probability roughly as l/n. We expect that 15% of the muonium formed is in the 2S state. 

We discuss first the beam-foil method for the measurement of lifetimes since conceptually and experimentally it is very simple. The technique is usually applied to the measurement of the lifetimes of excited states of atomic and molecular ions. The delayed-coincidence method which we treat in the second half of this chapter is experimentally more complex but it is perhaps the most accurate and widely applicable of the modern techniques. 

It also indicates that the beam-foil method permits the measurement of very short lifetimes which are at present beyond the range of most other techniques... 

Other Techniques. There exist other areas of potential application of ion beams. Ion beams can excite transitions in outer electron shells, with information in the optical wavelengths. Beam foil spectroscopy has used such methods for years, but no regular analytical use has been made. Nevertheless, samples irradiated under ion bombardment glow with characteristic radiation visible to the eye or television camera. Certain chemical or physical properties could be inferred by analyzing this radiation, perhaps including the physical condition of carbon atoms in graphitic or organic states. Certainly opportunities exist. 

Since the photoabsorption measurements of Madden and Codling autoionization processes have been investigated by various methods. Excita-tion has been initiated by electrons, " by heavy particles, or by beam-foil interaction. Whereas the number of states that can be excited by photon impact is limited by selection rules, this limitation is less stringent for electron collisions, especially at low impact energies. For ion-atom or atom-atom collisions it is possible to provoke or suppress the excitation of certain types of autoionization states by careful selection of the collision partners. ... 

There is a strong analogy between these interference structures and the so-called quantum beats that are observed in photon emission studies when atoms are coherently excited into different states by beam-foil, laser excitation, or other methods. Both phenomena are due to time-dependent interferences of transitions from different states. In quantum-beat studies... 

Also the autoionizing states of alkali atoms were studied by the various methods in the last years. Ross and co-workers performed a systematic study of all alkali atoms, in which alkali vapor was bombarded with electrons at various energies and the ejected electrons were analyzed. But also other methods such as light absorption from ground-state or excited state" atoms, heavy particle collisions, " and beam-foil excitation were used to excite the autoionizing states. 

In principle there are two techniques to process porous substrates with thermoforming. A novel method was shown by Giselbrecht et at [23], who processed pre-ion-beamed foils with thermoforming. Later the formed structures were etched to achieve porous microstructures. The advantage of this method is the easy process setup. 

C.L. Cocke Beam-Foil Spectroscopy, in Methods of Experimental Physics, Vol.13 (Academic, New York 1976)... 

S. Bashkin (ed.) Beam-Foil Spectroscopy. Proc. 3rd Int. Conf. on Beam-Foil Spectroscopy. Nucl. Instrmn. Methods 110 (1973)... 

Martinson Recent progi ess in the studies of atomic spectra and transition probabilities by beam-foil spectroscopy. Nucl. Instrum. Methods 202, 1 (1982)... 

The accuracy of the method is in principle limited only by the accuracy of distance measurement, and many lifetimes of highly excited atoms or ions have been measured this way (beam foil spectroscopy) [11.18b]. However, a severe drawback of this method with nonselective simultaneous excitation of many upper levels results from cascade effects. The level population excitation... 

We have shown in previous chapters that the -values of spectral lines are important fundamental data which must be known before detailed calculations of the behaviour of gas discharges, plasmas, or stellar atmospheres can be undertaken. Since it is difficult, in many cases, to make theoretical calculations of f-values to an accuracy of better than 20 per cent, experimental measurements of these quantities are essential. A considerable number of different techniques have been developed for this purpose, many of them involving the determination of radiative lifetimes. In this chapter we discuss two such techniques, namely the beam-foil and the delayed-coincidence methods. In Chapter 8 we shall discuss the determination of the f-values of resonance lines by studies of the profiles of spectral lines and in Chapters 15 and 16 the use of the Hanle effect and optical double resonance methods. 

In beam-foil experiments the velocities would be so great that no decay would be observed in any apparatus of convenient laboratory size. Similarly in the single-photon delayed-coincidence technique, the time required to obtain sufficient data would become quite prohibitive. The few reliable lifetime measurements that do exist have been made by the static afterglow technique. This was originally developed for experiments on the collisional destruction and diffusion of metastable atoms, which are discussed in detail in section 7.6. The difficulties encountered in the application of the afterglow and other methods to the experimental determination of the transition probabilities of forbidden lines have been reviewed by Corney (1973) and Corney and Williams (1972). 

This view is supported by the inconsistency which has been discovered between the abundances of elements determined from photospheric absorption lines and those obtained from the intensity of coronal emission lines (Pottasch (1963, 1964)). As shown in Table 10.3,- discrepancies of at least a factor of ten exist for all of the elements in the iron group. In the case of iron this difficulty has now been resolved. Recent measurements of f-values of iron by the beam-foil technique have shown conclusively that previous f-values for iron obtained by the emission method contained serious systematic errors and there has been a consequent revision of the abundance of iron in the Sun toivards the coronal value. 

The wave-front shearing interferometric method can be applied to transparent melts which do not attack cell windows made of silicate glass. The principle of this method has been given by Gustafsson. " A thin foil is placed vertically, a laser beam passing in the -direction along the plane. The heat is transferred in a melt in the direction of x. The slope of the... 

The need to be able to thin complex microelectronic devices, and to select and thin specific regions within them has resulted in ever-more sophisticated specimen preparation methods involving precision ion polishing. This requirement culminated in the development of the focused ion beam (FIB) technique, which is able to slice out electron-transparent foils from any multilayer, multiphase material with extreme precision. Overwijk et al. (1993) have described such a technique for producing cross-section TEM specimens from (e.g.) integrated circuits. 

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