Coherence collision-induced

The coherent fs time-resolved CARS method is highly sensitive for the investigation of collision induced (or pressure dependent) changes in optical line shapes especially when line mixing occurs and frequency resolved measurements come to their limits [7]. The fs-CARS spectroscopy is applied to various collision systems (N2-N2, N2-rare gas, C2H2-C2H2, CO-CO)... 

Y. Prior. Collision Induced Coherent Phenomena. In N. K. Rahman and C. Guidotti (ed.) Collisions and Half-Collisions with Lasers, Harwood Academic, New York, 1984, pp. 295-306. 

Collision-Induced Coherences in Optical Physics, G. S. Agarwal Muon-Catalyzed Fusion, Johann Rafelski and Helga E. Rafelski... 

While the previous chapter emphasized the high speotral resolution achievable with different sub-Doppler techniques, this chapter concentrates on some methods which allow high time resolution. The generation of extremely short and intense laser pulses has opened the way for the study of fast transient phenomena, such as molecular relaxation processes in gases or liquids due to spontaneous or collision-induced transitions. A new field of laser spectroscopy is the time-resolved detection of coherence and interference effects such as quantum beats or coherent transients monitored with pulse Fowoiev transform spectroscopy. 

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

During the past year we have replaced the Coherent Model 599-03 by the Coherent Model 599-21. With both dye lasers actively stabilized in frequency, the instrumental resolution could be increased to 1 or 2 MHz. Although the system has not yet operated according to specifications, we have obtained new quantitative data on the linewidth of the central components. It is the purpose of this paper to present these new data. They demonstrate the phenomena of colli-sional narrowing of residual Doppler broadening in four-wave light mixing, as discussed in section 2, and of collision-induced Zeeman coherences, treated in section 3. 

Fig, 6. Narrow resonances of collision-induced Zeeman and hyperfine structure coherences (FWHM is less than. Power... 

Fig. 7. Zeeman splitting, in an external magnetic field of 10 gauss, of the four-wave mixing resonance in the geometry of Fig. 1, caused by collision-induced Zeeman coherences with Am = 1.

The spectroscopic methods are based on time-resolved pump-probe schemes where the collision-free regime is usually attained by using low pressure conditions. Application of various linear and non-linear laser techniques, such as LIF (laser-induced fluorescence), REMPI (resonant-enhanced multiphoton ionization) and CARS (coherent antistokes Raman spectroscopy) have provided detailed information on the internal states of nascent reaction products [58]. Obviously, an essential prerequisite for the application of these techniques is the knowledge of the spectroscopic properties of the products. 

Collisions destroy the coherences associated with the initially prepared state and induce transitions between rotational levels. This results in redistribution of population between all interacting levels but the overall population (the trace of the density matrix) is not affected and decays only radiatively with the rate k Pss + Puu)- Collisions, thus, modify the form of the excited-state decay but the overall fluorescence yield remains equal to one unless the levels s, u, and/or /, m are subjected to additional nonradia-tive decay processes (e.g., in the case of predissociation). 

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