Collinear laser spectroscopy resolution

Several groups at ISOLDE are planning further improvements of their techniques. For each element the most appropriate experimental scheme has to be found. Today, collinear laser spectroscopy is the most general high-resolution and sensitive method for optical spectroscopy on radioactive beams delivered by on-line mass separators. Its sensitivity ranges from 10 - 10 atoms/s depending on the strength and multiplicity of the optical transitions. 

When working on line with a mass separator, such as Isolde, the collinear laser spectroscopy is a method fully adapted. In a collaboration Orsay - Mainz, the second members of the principal series in francium have been located and studied at high resolution with this method. In table 1 the measured wavenumbers of the four lines are given. 

The first group comprises high-resolution laser spectroscopy of short-lived radioactive isotopes with lifetimes in the millisecond range. The ions are produced by nuelear reactions induced by bombardment of a thin foil with neutrons, protons, y-quanta, or other particles inside the ion source of a mass spectrometer. They are evaporated and enter after mass selection the interaction zone of the collinear laser [466]. 

The second volume of Laser Spectroscopy covers the different experimental techniques, necessary for the sensitive detection of small concentrations of atoms or molecules, for Doppler-free spectroscopy, laser-Raman-spectroscopy, doubleresonance techniques, multi-photon spectroscopy, coherent spectroscopy and time-resolved spectroscopy. In these fields the progress of the development of new techniques and improved experimental equipment is remarkable. Many new ideas have enabled spectroscopists to tackle problems which could not be solved before. Examples are the direct measurements of absolute frequencies and phases of optical waves with frequency combs, or time resolution within the attosecond range based on higher harmonics of visible femtosecond lasers. The development of femtosecond non-collinear optical parametric amplifiers (NOPA) has considerably improved time-resolved measurements of fast dynamical processes in excited molecules and has been essential for detailed investigations of important processes, such as the visual process in the retina of the eye or the photosynthesis in chlorophyl molecules. 

The inherent resolution of collinear-beam spectroscopy is still limited by the residual Doppler broadening. In beams with a broad velocity distribution the labeling of one velocity class by optical pumping, probed in a second Doppler-tuning zone, was exploited already before narrow Doppler widths were achieved. The complete elimination of the first-order Doppler effect in resonant two-photon absorption on Ne I has been discussed in Section 3.3, in connection with a precision measurement of the relativistic Doppler effect. A similar experiment was performed on In I, where the 29p Rydberg state was excited from 5p Pi/2 via 6s Si/2 and detected by field ionization. The linewidth caused by the laser jitter can be reduced to the transit-time limit of a few hundred kilohertz. 

It has been shown by means of examples that the collinear laser fast-beam technique has introduced many interesting aspects into the classical field of atomic spectroscopy. This discussion has not touched upon the promising applications to molecular ions including spectroscopy and reaction studies, as the physics involved is beyond the scope of this contribution. To date, it appears that a systematic application in atomic spectroscopy has been established in the work on radioactive nuclides, owing to the sensitivity and resolution, but even more the ideal adaptation of the spectroscopic method to the conditions of production. 

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... 

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