Isotope selective laser spectroscopy

V. Beutel, H.G. Kramer, G.L. Bahle, M. Kuhn, K. Weyers, W. Demtrdder, High resolution isotope selective laser spectroscopy of Aga molecules. J. Chem. Rhys. 98, 2699 (1997)... 

Snavely, B.B. Solarz, R. W. and Tuccio, S. A., "Separation of Uranium Isotopes by Selective Photoionization," in "Laser Spectroscopy, 43, Proceedings of the Second International Convcerence, Megere, June 23-27, 1975" Springer-Verlag Berlin, 1975, pp. 267-274. 

This chapter is concerned with experimental investigations of the dynamics of the dissociation of polyatomic neutral molecules carried out by the technique of laser Doppler spectroscopy, in bulk and under crossed-beam condition. Photodissociation is a basic process in the interaction of light with molecules, of interest in itself as an elementary molecular process and also with respect to a variety of applications in different fields. The interest has increased considerably in recent years, first, because the experimental investigation of photodissociation is rapidly advancing by the use of the laser, and second, because the laser makes possible to achieve photodissociation, state, and isotope selectively, by new excitation mechanisms. These are, aside from the common one-photon absorption, stepwise... 

Fig. 1.16 Isotope-selective intracavity absorption spectroscopy. The frequencies (Ok absorbed by the isotope inside the laser cavity are missing in the laser output, which therefore does not excite any fluorescence in the same isotope outside the laser resonator [20]...

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]. 

Isotope-Selective Elemental Analysis by High-Resolution Laser Spectroscopy... 

The isotope-selective analysis by optical detection methods is almost impossible unless transitions with sufficiently large isotope shifts as observed with light and heavy elements are available. In contrast to traditional emission or absorption techniques the high-resolution laser spectroscopy enables Doppler-free measurements since the spectral linewidth of tunable CW lasers is commonly less than the Doppler profile... 

Laser radiation is monochromatic and in many cases it also is tuneable these two characteristics together provide the basis for high-resolution laser spectroscopy. The interaction between laser radiation and molecules can be very selective (individual quantum states can be selected), permitting chemists to investigate whether energy in a particular type of molecular motion or excitation can influence its reactivity. Photochemical processes can be carried out with sufficient control that one can separate isotopes, or even write fine fines (of molecular dimensions) on surfaces. 

The narrow spectral line of a DL enables isotope selective analysis. For light and heavy elements (such as Li and U) the isotope shifts in spectral lines are often larger than the Doppler widths of the lines, in this case isotopically selective measurements are possible using simple Doppler-limited spectroseopy - DLAAS or laser induced fluorescence (LIF). For example, and ratios have been measured by Doppler-limited optogalvanic. spectroscopy in a hollow cathode discharge. DLAAS and LIF techniques have been combined with laser ablation for the selective detection of uranium isotopes in solid samples. This approach can be fruitful for development of a compact analytical instrument for rapid monitoring of nuclear wastes. 

There were also some remarkable meetings abroad during various scientific schools and conferences. I recall with pleasure my talks with Professor Ali Javan at the 1975 Les Houches School on Laser Spectroscopy (Fig. 14.12), and with Professor A. Siegman during my visit to Stanford University (Fig. 14.13), where we discussed animatedly the effect of isotope-selective multiple-photon dissociation of polyatomic molecules by IR laser pulses. At regular international conferences on laser spectroscopy and atomic... 

As the isotope shifts are often larger than the Doppler broadening, signals of different isotopes such as U and can often be differentiated, making isotopic analyses of enriched uranium possible [299]. Doppler-free spectroscopy with two laser beams in co- or counter-propagating directions can also be applied, which becomes even more isotopically selective, as isotope patterns hidden in the Doppler profile can then be visualized. 

Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

Resonant and non-resonant laser post-ionization of sputtered uranium atoms using SIRIS (sputtered initited resonance ionization spectroscopy) and SNMS (secondary neutral mass spectrometry) in one instrument for the characterization of sub-pm sized single microparticles was suggested by Erdmann et al.94 Resonant ionization mass spectrometry allows a selective and sensitive isotope analysis without isobaric interferences as demonstrated for the ultratrace analysis of plutonium from bulk samples.94 Unfortunately, no instrumental equipment combining both techniques is commercially available. 

When using two lasers and applying two-photon spectroscopy, only those atoms that do not have a velocity component in the observation direction will undergo LEI. Then the absorption signals become very narrow (Doppler-free spectroscopy). This enhances the selectivity and the power of detection, however, it also makes isotope detection possible. Uranium isotopic ratios can thus be detected, similarly to with atomic fluorescence [673] or diode laser AAS. Thus for dedicated applications a real alternative to isotope ratio measurements with mass spectrometry is available. 

In the columns identifying the experimental method, MW stands for any method studying the pure rotational spectrum of a molecule except for rotational Raman spectroscopy marked by the rot. Raman entry. FUR stands for Fourier transform infhired spectroscopy, IR laser for any infiured laser system (diode laser, difference frequency laser or other). LIF indicates laser induced fluorescence usually in the visible or ultraviolet region of the spectrum, joint marks a few selected cases where spectroscopic and diffraction data were used to determine the molecular structure. A method enclosed in parentheses means that the structure has been derived from data that were collected by this method in earlier publications. The type of structure determined is shown by the symbols identifying the various methods discussed in section II. V/ refers to determinations using the Kraitchman/Chutjian expressions or least squares methods fitting only isotopic differences of principal or planar moments (with or without first... 

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