Laser Spectroscopy in Fast Ion Beams

Subtracting the first equation from the second yields 

Since = fnl2)vQ and v = 2eUwe obtain for the final velocity spread 

Example 4.4 AEth = 0.1 eV, eU = 10 keV Av = 3 x 10 Ano. This means that the Doppler width of the ions in the ion source has been decreased by acceleration cooling by a factor of 300 If the laser crosses the ion beam perpendicularly, the transverse velocity components for ions with n = 3 x 10 m/s at a collimation ratio e = 10 are = n, 3 x 10 m/s. This would result in a residual Doppler width of An 3 GHz, which illustrates that for fast beams the longitudinal arrangement is superior to the transverse one. 

This reduction of the velocity spread results from the fact that energies rather than velocities are added (Fig. 4.26). If the energy eU Eth, the velocity change is mainly determined by U, but is hardly affected by the fluctuations of the initial thermal velocity. This implies, however, that the acceleration voltage has to be extremely well stabilized to take advantage of this acceleration cooling. 

A voltage change AU results in a change An in the velocity v. From (ra/2)v = e G we obtain 

In the examples considered so far, the laser beam was crossed perpendicularly with the molecular beam, and the reduction of the Doppler width was achieved through the limitation of the maximum-velocity components v by geometrical apertures. One therefore often calls this reduction of the 

Assume that two ions start from the ion source (Fig.9.20) with different thermal velocities Vi(0) and V2(0). After being accelerated by the voltage U their kinetic energies are 

Subtracting the first from the second equation yields 

Laser spectroscopy in fast ion beams. The laser beam is oollimop with the ion beam 

This means that the initial velocity spread V2(0) has decreased to 

This allows one to use high-intensity fixed frequency lasers, such as the argon laser, which have a high gain and even allow one to place the interaction zone inside the laser cavity. 

This is demonstrated by the insert in Fig. 9.19. The linewidth of the Lamb dips in Fig. 9.19 is below 1 MHz and is mainly limited by frequency fluctuations of the cw single-mode dye laser [9.59]. 

Additionally, several experiments on saturation spectroscopy of molecules and radicals in molecular beams have been reported [9.60-9.61] where finer details of congested molecular spectra, such as hyperfine structure or A-doubling can be resolved. Another alternative is Doppler-free two-photon spectroscopy in molecular beams, where high-lying molecular levels with the same purity as the absorbing ground state levels are accessible [9.62]. 

The improvements in the sensitivity of CARS (Sect. 8.3) have made this nonlinear technique an attractive method for the investigation of molecular beams. Its spectral and spatial resolution allow the determination of the internal-state distributions of molecules in effusive or in supersonic beams, and their dependence on the location with respect to the nozzle (Sect. 8.5). An analysis of rotationally-resolved CARS spectra and their variation with increasing distance z from the nozzle allows the determination of rotational and vibrational temperatures Ti-ot(z), Tvib(z), from which the cooling rates can be obtained [9.63]. With cw CARS realized with focused cw laser beams the main contribution to the signal comes from the small focal volume, and a spatial resolution below 1 mm can be achieved [9.64]. 

Another example of the application of CARS is the investigation of cluster formation in a supersonic beam. The formation rate of clusters can be inferred from the increasing intensity I(z) of characteristic cluster bands in the CARS spectra. 

The advantage of CARS compared to infrared absorption ion spectroscopy is the higher sensitivity and the fact that nonpolar molecules, such as N2, can also be studied [9.65]. With pulsed CARS short-lived transient species produced by photo dissociation in molecular beams can also be investigated [9.66,9.67]. 

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The laser wavelength was kept at X = 5815 % and the velocity of the 0 ions was continuously tuned by controlling the acceleration voltage. More examples, including saturation spectroscopy in fast ion beams, can be found in the review by DUFAY and GAILLARD [10.22]. 

An absolute integrated intensity of 1880(290) cmatm" at STP was determined for the Vi band of N2H using the technique of direct laser absorption spectroscopy in fast ion beams. This value [37] supersedes a previous value determined in the same laboratory [38]. Higher values were predicted by high-quality ab initio calculations, CEPA-1 [36] and Ci-SDQ [39]. Integrated Intensities for the V2 [39] and V3 [36, 39] bands of N2H, for the Vi and V3 bands of N2D [36], and for bands from overtones and combinations of the v, and V3 fundamentals of N2H (N2D ) were also predicted in the ab initio calculations [36, 39]. The V3 band was predicted to be about 140 times weaker than the v, band [36], and the V2 band to be about five times weaker than the band [39]. 

We will conclude this section by describing the special conditions pertaining to fluorescence spectroscopy on fast ion beams. As we have noted in connection with beam-foil spectroscopy, the transverse Doppler broadening of an ion beam is substantial due to the high ion velocities. Thus, high-resolution spectroscopy cannot be accomplished by perpendicular laser beam radiation. However, it is possible to obtain a high resolu-... 

In order to obtain conclusive results one normally focuses on a single transition and detects the emitted fluorescence photons bearing the fine structure information. This is achievable by dye lasers or tunable laser diodes. In some setups the light travels collinearly to fast atomic beams which has some advantages with respect to spectral resolution [44]. The technique of fast ion beam spectroscopy has been applied to numerous measurements on rare earth ions, e.g. [45-49]). Some more recent high-resolution optical hfs measurements include Ta [50], [51] and the noble gas Xe [52] illustrate these advanced... 

The combination of fast ion beam photofragmentation with fleld-dissociation spectroscopy opens interesting new possibilities for studying long-range ion-atom interactions. This has been demonstrated by Bjerre and Keiding [477], who measured the 0+-0 potential in the internuclear distance range 1-2 nm from electric fleld-induced dissociation of selectively laser-excited Oj ions in a fast beam. 

S. Abed, M. Broyer, M. Carre, M.L. GaiUard, M. Larzilliere, High resolution spectroscopy of N2O+ in the near ultraviolet, using FIBLAS (Fast-Ion-Beam Laser Spectroscopy). Chem. Phys. 74,97 (1983)... 

Microwave techniques were combined with fast ion beam laser spectroscopy (see Figure 11) in order to investigate the hyperfine structure of A 50-keV, isotopically pure ion beam with a current... 

Many of the spectroscopic studies were performed to demonstrate the capability of the technique and of a number of variants which are specific for the combination of laser spectroscopy with fast beams of ions or atoms. An example has already been discussed in Section 3.3 Resonant two-photon exdtation becomes possible by adjusting the Doppler shifts for interaction with the direct and retroreflected laser beam to the atomic transition energies. Other features include the preparation of otherwise inaccessible atomic states, the separation of hyperfine structures from different isotopes by the Doppler shift, or the observation of time-resolved transient phenomena along the beam. The extensive research on nuclear moments and radii from the hyperfine structure and isotope shift constitutes a self-contained program, which will be discussed separately in Section 5. 

The development of fast ion beam laser spectroscopy techniques (for short FIBLAS) is not so unusual a case of simultaneous but independent technical evolution both in atomic and molecular physics. Although the concepts involved in both cases were quite similar, the apparatus used in the pioneering experiments were widely different, ranging from the table top mass spectrometer for the early molecular physics work to the largest tandem Van de Graaff accelerators for some of the atomic physics experiments. ... 

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. 

However, the first experimental achievement of a narrow Doppler width in fast-beam spectroscopy had already been reported independently by Wing et They performed the first precision measurements in the infrared vibrational-rotational spectrum of the simplest molecule by crossing the ion beam with the beam from a single-mode CO laser at a very small intersection angle. 

It may be surprising to find the most extensive application of collinear laser fast-beam spectroscopy in a field that a priori has little connection with the special features of this technique. Neither the Doppler shift nor the accessibility of ionic spectra plays a decisive role for the on-line experiments on radioactive isotopes from nuclear reactions. However, most of the problems encountered in the preparation of a sample of free atoms (cf. Part B, Chapter 17 by H.-J. Kluge) are solved by a combination of the fast-beam technique with the well-established concept of on-line isotope separation. The isotope separators (with ISOLDE at CERN as an outstanding example) provide the unstable species in the form of ion beams whose phase-space volume is well matched to the requirements of collinear spectroscopy. 

For all their useful properties, lasers are known to suffer from drastic limitations in power and/or wavelength range. A a result, their introduction in fast atomic ion beam spectroscopy has often been considered to result in a considerable reduction... 

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