Light Doppler narrowing

DOPPLER NARROWING AND COLLISION-INDUCED ZEEMAN COHERENCE IN FOUR-WAVE LIGHT MIXING... 

The spectrum emitted by the lamp corresponds to the superimposition of radiation emitted by the cathode and by the gaseous atmosphere within the lamp. The width of the emission lines, which depends upon different effects (Doppler, Stark (ionization) and Lorentz (pressure)), is narrower than the corresponding absorption band. The monochromator enables the elimination of a large part of the stray light due to the filling gas, and the selection of the most intense spectral line in order to obtain a better sensitivity (Figure 13.8), except for cases of interference caused by other elements. 

The three-photon absorption can be used for the excitation of high-lying molecular levels with the same parity as accessible to one-photon transitions. However, for a one-photon absorption, lasers with a wavelength A/3 have to be available in order to reach the same excitation energy. An example of Doppler-limited collinear three-photon spectroscopy is the excitation of high-lying levels of xenon and CO with a narrow-band pulsed dye laser at X = 440 nm (Fig. 2.40). For one-photon transitions light sources at A. = 146.7 nm in the VUV would have been necessary. 

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. 

For momentum transfer stimulated Raman transitions between the two hyperfine levels 1 and 2) of the Na(3 5 i/2) state are used, which are induced by two laser pulses with light frequencies coi and o)2 (o) —002 = < hfs) traveling into opposite directions (Fig. 9.71b). Each transition transfers the momentum Ap 2hk. The gravitational field causes a deceleration of the upwards moving atoms in the fountain. This changes their velocity, which can be detected as a Doppler shift of the Raman transitions. Because the Raman resonance a)i—a)2 = has an extremely narrow... 

In this contribution we present two laser spectroscopic methods that use coherent resonance Raman scattering to detect rf-or laser -induced Hertzian coherence phenomena in the gas phase these novel coherent double resonance techniques for optical heterodyne detection of sublevel coherence clearly extend the above mentioned previous methods using incoherent light sources. In the case of Doppler broadened optical transitions new signal features appear as a result of velocity-selective optical excitation caused by the narrow-bandwidth laser. We especially analyze the potential and the limitations of the new detection schemes for the study of collision effects in double resonance spectroscopy. In particular, the effect of collisional velocity changes on the Hertzian resonances will be investigated. 

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