Doppler-free spectroscopy

Doppler-free two-photon spectroscopy spect A version of Doppler free spectroscopy in which the wavelength of a transition Induced by the simultaneous absorption of two photons is measured by placing a sample In the path of a laser beam reflected on itself, so that the Doppler shifts of the Incident and reflected beams cancel. dap-lor fre tu fO,tan spek tras-ka-pe j... 

Multiphoton resonant processes with simplest fundamental quantum systems exposed to sufficiently strong laser fields attracted conspicuous attention over last years. Currently, this interest is being especially strongly stimulated by dramatic improvements in the precision of measurements presently attainable in spectroscopic experimental studies of hydrogenic and few-particle atoms. Using methods of ultra high precision Doppler-free spectroscopy, particularly impressive results have been recently obtained in studies of fundamental bounded systems such as hydrogen (H) and its natural isotopes deuterium (D) and tritium (T) [1,2,3,4,5,6,7], positronium [8,9], denoted Ps = (e+ — e ), muonium [10,11,9,12,13,14,15], denoted (M = — e ), and the helium atom (He) [16[... 

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. 

Molecular two-photon spectroscopy can also be applied in the infrared region to induce transitions between rotational-vibrational levels within the electronic ground state. One example is the Doppler-free spectroscopy of rotational lines in the V2 vibrational bands of NH3 [258]. This allows the study of the collisional properties of the V2 vibrational manifold from pressure broadening and shifts (Vol. 1, Sect. 3.3) and Stark shifts. 

The technique of reducing the Doppler width by the collimation of mo lecular beams was employed before the invention of lasers to produce light sources with narrow emission lines [389]. Atoms in a collimated beam were excited by electron impact. The fluorescence lines emitted by the excited atoms showed a reduced Doppler width if observed in a direction perpendicular to the atomic beam. However, the intensity of these atomic beam light sources was very weak and only the application of intense monochromatic, tunable lasers has allowed one to take full advantage of this method of Doppler-free spectroscopy. 

The techniques of coherent spectroscopy that are discussed below allow the elimination of the inhomogeneous contribution and therefore represent methods of Doppler-free spectroscopy, although the coherent excitation may use spectrally broad radiation. This is an advantage compared with the nonlinear Doppler-free techniques discussed in Chap. 2, where narrow-band single-mode lasers are required. 

F. Riehle, J. Ishikawa, J. Helmcke, Suppression of recoil component in nonlinear Doppler-free spectroscopy. Phys. Rev. Lett. 61, 2092 (1988)... 

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. 

FIGURE 34 The use of two-photon absorption with counterpropagating beams for Doppler-free spectroscopy, (a) Experimental arrangement, (b) Level diagram of transitions used. 

FIGURE 35 Example of high-resolution spectrum in sodium vapor obtained with two-photon Doppler-free spectroscopy. [Reproduced from Bloembergen, N., and Levenson, M. D. (1976). Doppler-free two-photon absorption spectroscopy. In High Resolution Laser Spectroscopy (K. Shimoda, ed.), p. 355, Springer, New York.l... 

The linewidth of the unstabilized single-mode laser has been measured to be smaller than 260 kHz, which was the resolution limit of the measuring system [5.144]. An estimated value for the overall linewidth Av is 25 kHz [5.146]. This extremely small linewidth is ideally suited to perform high-resolution Doppler-free spectroscopy (Chaps. 7-10). 

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