Doppler-free spectroscopy method

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[... 

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. 

While the laser is used essentially as a bright lamp in the above-mentioned contexts, a survey of the most important methods of Doppler-free spectroscopy, employing the extremely small linewidth of a single-mode laser, is made in Sect.9.5. 

This means that even in the presence of inhomogeneous line broadening, the homogeneous relaxation processes (i.e., the homogeneous part of the broadening) can be measured with the photon-echo method. This technique therefore allows Doppler-free spectroscopy. 

The methods of Doppler-free two-photon laser spectroscopy allow very precise comparison of the frequencies of the IS — 2S transitions in hydrogen and deuterium. The frequency difference... 

Most precision spectroscopy of medium Z ions has been conducted at accelerators or tokamak plasmas, but the recent development of the electron beam ion trap (EBIT) has offered a new spectroscopic source to experimenters. Our experimental method takes advantage of the Doppler free and relatively clean spectra produced by an EBIT and is coupled with an external calibration source to allow absolute measurement of highly charged ions. These are the first precision X-ray measurements conducted at the NIST EBIT [12],... 

The method we use is Doppler free two-photon laser spectroscopy, applied to the atomic hydrogen transitions from the metastable 2S state to the Rydberg nD states (n = 8, 10, 12) /8/. 

The main sources of data are microwave, inlfared and laser induced fluorescence spectroscopy and their related Doppler-free techniques. Results from magnetic and electric resonance methods are also considered. 

In the methods discussed in Sects. 2.3 and 2.4, the Doppler width had been reduced or even completely eliminated by proper selection of a velocity subgroup of molecules with the velocity components = 0 zb Ai , due to selective saturation. The technique of Doppler-free multiphoton spectroscopy does not need such a velocity selection because all molecules in the absorbing state, regardless of their velocities, can contribute to the Doppler-free transition. Therefore the sensitivity of Doppler-free multiphoton spectroscopy is comparable to that of saturation spectroscopy in spite of the smaller transition probabilities. 

Fortunately, several methods have been developed that overcome these difficulties and that allow ultranarrow Ramsey resonances to be obtained. One of these methods is based on Doppler-free two-photon spectroscopy, while another technique uses saturation spectroscopy but introduces a third interaction zone at the distance z = 2L downstream from the first zone to recover the Ramsey fringes [1257-1259]. We briefly discuss both methods. 

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... 

Really impressive progress toward higher spectral resolution has been achieved by the development of various Doppler-free techniques. They rely mainly on nonlinear spectroscopy, which is extensively discussed in Chap. 7. Besides the fundamentals of nonlinear absorption, the techniques of saturation spectroscopy, polarization spectroscopy, and multiphoton absorption are presented, together with various combinations of these methods. 

Of particular importance for the spectroscopy of highly excited states, such as Rydberg levels of atoms and molecules, and for the assignment of complex molecular spectra are various double-resonance techniques where atoms and molecules are exposed simultaneously to two radiation fields resonant with two transitions sharing a common level. In combination with Doppler-free techniques, these double resonance methods are powerful tools for spectroscopy. Some of these methods, representing modem versions of optical pumping techniques of the prelaser era, are introduced in Chap. 10. 

Using the dispersion profiles of Doppler-free molecular lines in polarization spectroscopy (Sect. 7.4), it is possible to stabilize a laser to the line center without frequency modulation. An interesting alternative for stabilizing a dye laser on atomic or molecular transitions is based on Doppler-free two-photon transitions (Sect. 7.5) [5.77]. This method has the additional advantage that the lifetime of the upper state can be very long, and the natural linewidth may become extremely small. The narrow Is —2s two-photon transition in the hydrogen atom with a natural linewidth of 1.3 Hz provides the best known optical frequency reference to date [5.76]. 

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