Fast beam

All analytical techniques in the TEM are based on the inelastic scattering of the fast beam electrons by the electrons of the atoms in the material investigated. The primary event in each case is the transfer of energy and momentum from the fast electron to a sample atom, thereby exciting the... 

Three microbeam systems were developed at the TIARA facility for application to materials science and biotechnology. A heavy-ion microbeam system installed on a beam line of the 3-MV tandem accelerator is the first one developed to study single-event upset (SEU) of semiconductor devices used for space [36]. The microbeam system can focus heavy-ion beams such as a 15-MeV nickel ion with a spot size of less than 1 pm. In order to observe the SEU phenomena at a specific position of the microdevice, the microbeam system is equipped with a single-ion hit system, consisting of single-ion detectors and a fast beam switcher. 

While one might expect that the techniques developed for photodissociation studies of closed-shell molecules would be readily adaptable to free radicals, this is not the case. A successful photodissociation experiment often requires a very clean source for the radical of interest in order to minimize background problems associated with photodissociating other species in the experiment. Thus, molecular beam photofragment translation spectroscopy, which has been applied to innumerable closed-shell species, has been used successfully on only a handful of free radicals. With this problem in mind, we have developed a conceptually different experiment [4] in which a fast beam of radicals is generated by laser photodetachment of mass-selected negative ions. The radicals are photodissociated with a second laser, and the fragments are detected in coinci-... 

Fig. 3.2 The fast beam approach of Bayfield and Koch (ref. 13). H+ ions of roughly 10 keV energy pass through a charge exchange cell forming a fast beam of H Rydberg atoms. Down-stream from the charge exchange cell the ions are deflected from the beam and a band of n states is selected by a square wave modulated ionization field.

In fast beams optical excitation has proven to be most useful. Since the fast beams are low in intensity, but continuous, cw lasers have been used. Usually, fixed frequency lasers have been used since fine tuning can be done using the Stark shift or the Doppler shift of the fast beam. The Doppler shift can be used either by changing the angle at which the laser beam and fast beam cross, or by altering the velocity of the fast beam. An early example was the use of the uv line of an Ar laser to drive transitions from the metastable H 2s state to the 40 < n < 55 np states.27 In this particular case the velocity of the beam was changed to tune different np states into resonance. 

Fig. 10.1 Schematic drawing of a fast beam apparatus. A fast atomic beam enters from the left and is excited sequentially by two different C02 lasers in electric field regions Fj and F3, respectively. F2 avoids a zero field region between them. Ions produced by highly excited atoms being ionized in the biased microwave cavity are energy selected and detected by a Johnston particle multiplier (not shown). The output signal is detected in phase with the mechanically chopped Fj laser beam (from ref. 3).

The first measurements of microwave ionization in any atom were carried out with a fast beam of H by Bayfield and Koch1, who investigated the ionization of a band of approximately five n states centered at n = 65. Using microwave and rf fields with frequencies of 9.9 GHz, 1.5 GHz, and 30 MHz, to ionize the atoms they found that the same field was required at 30 MHz and 1.5 GHz to ionize the atoms, but that a smaller field was required at 9.9 GHz. The measurements showed that at n = 65 frequencies up to 1.5 GHz are identical to a static field. Later, more systematic measurements have confirmed the initial measurements and have allowed significant refinements of our understanding. In Fig. 10.16 we show the ionization threshold fields (in this case the field at which there is 10% ionization) of H in a 9.9 GHz field.21 The ionization fields are plotted as n4E vs n3a>, and they bring out two factors. First, at low frequencies the field required is l/9n4, the static field required to ionize the red n Stark state of m n. Second, as shown by the scaling of the horizontal axis, the required field drops below l/9n4 as the microwave frequency approaches the interval between adjacent n states, 1 In3. 

COLL INEAR FAST-BEAM LASER SPECTROSCOPY... 

As mentioned above, the radon and radium sequences have been investigated by collinear fast-beam laser spectroscopy, whereas in francium all three atomic-beam methods, ABMR, atomic-beam laser spectroscopy and collinear laser spectroscopy, have contributed. 

Prior to about 1955 much of the nuclear information was obtained from application of atomic physics. The nuclear spin, nuclear magnetic and electric moments and changes in mean-squared charge radii are derived from measurement of the atomic hyperfine structure (hfs) and Isotope Shift (IS) and are obtained in a nuclear model independent way. With the development of the tunable dye laser and its use with the online isotope separator this field has been rejuvenated. The scheme of collinear laser/fast-beam spectroscopy [KAU76] promised to be useful for a wide variety of elements, thus UNISOR began in 1980 to develop this type of facility. The present paper describes some of the first results from the UNISOR laser facility. 

In conclusion thre first half-life measurements of light neutron rich nuclei using the MSU Reaction Product Mass Separator has resulted in the measurement of eight half-lives,two of which represent first time measurements and three of which are second measurements.The RPMS coupled with fast beam switching has proven to provide a very clean environment in which to study the decays of neutron rich nuclei. 

Abstract. Laser spectroscopy of hydrogen-like and helium-like ions is reviewed. Emphasis is on the fast-beam laser resonance technique, measurements in moderate-/ ions which provide tests of relativistic and quantum-electrodynamic atomic theory, and future experimental directions. 

Fig. 1. (a) Energy levels of F8+ relevant to the measurement of the n = 2 Lamb shift (b) Schematic illustrating the fast-beam laser resonance technique... 

Use a fast beam of Ps atoms produced by ionization of an accelerated beam of negative positronium ions Ps- (e+e-e-). 

All previous 2S Lamb shift measurements for medium-Z hydrogen-like ions have been carried out using fast ion beams, and uncertainties associated with Doppler shifts form a significant source of error in all these experiments. Various methods have been employed or suggested for reducing the sensitivity of fast beam experiments to Doppler corrections [22]—[24]. A measurement of the 2S1/2-2P3/2 transition frequency in N6+ using a fast ion beam is currently under way at Florida State University [25]. Our approach, however, is to reduce such... 

Abstract. Using Doppler-tuned fast-beam laser spectroscopy the ls2p 3Po - 3Pi fine structure interval in 24Mg10+ has been measured to be 833.133(15) cm-1. The calibration procedure used the intercombination ls2s 1So - ls2p 3Pi transition in 14N5+. The result tests quantum-electrodynamic and relativistic corrections to high precision calculations, which will be used to obtain a new value for the fine structure constant from the fine structure of helium. 

For various ions the 2S /2 — 2P1/2 and 2,S - /2 — 2P3/2 transitions match the wavelengths of certain efficient lasers enabling spectroscopy with the fast-beam laser-resonance technique [9], The most precise measurement of this type, achieved after considerable experimental development, was of the 2Si/2 — 2P3/2 transition in P14+ [10], where an uncertainty equivalent to 0.15% of the 2Si/2 — 2Pi/2 interval was obtained. A high-power pulsed dye-laser producing intensities of over 10 MW/cm-2, but with a duty cycle of about 10-5, was required to obtain a good signal-to-background ratio. 

To reduce sensitivity to the Doppler shift further the experiment can be carried out using a transverse geometry on a fast beam, or on a slow beam, or... 

The fast beam separated oscillatory field technique (SOF) provides a method through which one can obtain a series of lines whose widths are less than the natural line width with a good understanding of the factors which determine the line shape and the line center. This paper summarizes a separated oscillatory field measurement of the Lamb shift in hydrogen.[1]... 

To eliminate the residual first order Doppler shift due to the failure of the direction of propagation of the rf field to be precisely perpendicular to the fast beam, measurements were taken with the rf drive on both the right and left sides of the beam. To eliminate the frequency shift due to phase errors in the rf drive system, measurements were made with the entire rf system, including the spectroscopy region, rotated 180° about an axis passing through the midpoint between the two... 

The fast beam separated oscillatory field method has been used to measure with high precision the Lamb shift in the n=2 state of hydrogen. The agreement between the measured value and the theoretical value is obscured by the discrepant values for... 

The basic NR mass spectrum contains information on the fraction of undissociated (survivor) ions and also allows one to identify dissociation products that are formed by purely unimolecular reactions. NRMS thus provides information on the intrinsic properties of isolated transient molecules that are not affected by interactions with solvent, matrix, surfaces, trace impurities, radical quenchers, etc. However, because collisional ionization is accompanied by ion excitation and dissociation, the products of neutral and post-reionization dissociations overlap in the NR mass spectra. Several methods have been developed to distinguish neutral and ion dissociations and to characterize further short lived neutral intermediates in the fast beam. Moreover, collisionally activated dissociation (CAD) spectra have been used to characterize the ions produced by collisional reionization of transient neutral intermediates [51]. This NR-CAD analysis adds another dimension to the characterization of neutral intermediates, because it allows one to uncover isomerizations that do not result in a change of mass and thus are not apparent from NR mass spectra alone. 

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