Doppler-free technique

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. 

P/S increases indefinitely, the two populations nQ and n tend toward the same limit N/2. the saturation of the interaction. The important point used in the Doppler-free technique is not the saturation itself but the nonlinearity the population ni of the excited state is less than doubled when the light intensity P/S is doubled from I to 21 this nonlinearity is stronger when one is approaching saturation. 

LASER DOPPLER-FREE TECHNIQUES IN SPECTROSCOPY, B. CAGNAC... 

Three-photon excitation may also be used for a Raman-type process, depicted in Fig. 2.42b, which proceeds via two virtual levels. This Doppler-free technique was demonstrated for the 3 5 i/2 3 Pi/2 transition of the Na atom, where the photons with the momentum hk and hk[ are absorbed while the photon with the momentum hk2 is emitted. The hyperfine structure of the upper and lower states could readily be resolved [266]. 

The residual Doppler width from the finite collimation ratio e of the molecular beam can be completely eliminated when nonlinear Doppler-free techniques are applied. Since collisions can generally be neglected at the crossing point of the molecular and laser beam, the lower molecular level /> depleted by absorption of laser photons can be only refilled by diffusion of new, unpumped molecules into the interaction zone and by the small fraction of the fluorescence terminating on the initial level i). The saturation intensity h is therefore lower in molecular beams than in gas cells (Example 2.3). 

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. 

From the measured halfwidth AB /2 the product gjTeff of Lande factor g/ of the excited level 2) times its effective lifetime teff can be derived. For atomic states the Lande factor gj is generally known, and the measured value of A i/2 determines the lifetime teff. Measurements of teff (p) as a function of pressure in the sample cell then yield by extrapolation p 0 the radiative lifetime (Sect. 6.3). The Hanle effect therefore offers, like other Doppler-free techniques, an alternative method for the measurement of atomic lifetimes from the width AB /2 of the signal [832]. 

Level-crossing spectroscopy with lasers has some definite experimental advantages. Compared with other Doppler-free techniques it demands a relatively simple experimental arrangement. Neither single-mode lasers and frequency-stabilization techniques nor collimated molecular beams are required. The experiments can be performed in simple vapor cells, and the experimental expenditure is modest. In many cases no monochromator is needed since sufficient selectivity in the excitation process can be achieved to avoid simultaneous excitation of different molecular levels with a resulting overlap of several level-crossing signals. 

Quantum-beat spectroscopy represents not only a beautiful demonstration of the fundamental principles of quantum mechanics, but this Doppler-free technique has also gained increasing importance in atomic and molecular spectroscopy. Whereas commonly used spectroscopy in the frequency domain yields information on the stationary states A ) of atoms and molecules, which are eigenstates of the total Hamiltonian... 

The first term represents the absorption frequency coq = Ek — Ei) of an atom at rest if the recoil of the absorbing atom is neglected. The second term describes the linear Doppler shift (first-order Doppler effect) caused by the motion of the atom at the time of absorption. The third term expresses the quadratic Doppler effect (second-order Doppler effect). Note that this term is independent of the direction of the velocity v. It is therefore not eliminated by the Doppler-free techniques described in Chaps. 2-5, which only overcome the linear Doppler effect. 

B. Cagnac, Laser Doppler-free techniques in spectroscopy, in Frontiers of Laser Spectroscopy of Gases, ed. by A.C.P. Alves, J.M. Brown, J.H. Hollas. Nato ASO Series C, vol. 234 (Kluwer, Dondrost, 1988)... 

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. 

Note The relations derived in this section are only valid for oscillators at rest in the observer s coordinate system. The thermal motion of real atoms in a gas introduces an additional broadening of the line profile, the Doppler broadening, which will be discussed in Sect. 3.2. The profiles (3.36,3.37) can therefore be observed only with Doppler-free techniques (Chaps. 7 and 9). 

With laser beams, the effect can be observed in absorption. This is the basis for collinear fast-beam laser spectroscopy. Among the Doppler-free techniques (described in Part A, Chapter 15 by W. Demtroder) it is the only one using linear absorption without velocity selection as in collimated atomic beams. 

It is not surprising that most results in atomic spectroscopy were obtained on singly charged ions which are difficult to prepare for the usual Doppler-free techniques on thermal samples. On the other hand, the fast-beam technique has certain advantages also on neutral atoms, such as the availability of metastable beams, the sensitivity, and the Doppler-tuning. 

Three-photon excitation may also be used for a Raman-type process, depicted in Fig. 7.37b, which proceeds via two virtual levels. This Doppler-free technique was demonstrated for the 3 81/2 transition of the... 

The energy difference between the Zeeman and Stark sublevels is usually far smaller than the line width of the optical transition in atoms at normal temperature due to Doppler broadening. Doppler-free techniques are necessary for obtaining the values of g-factors and polarizabilities in optical spectroscopy. 

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