Resonance effects radiation pressure

We have above discussed certain mechanical actions on atoms induced by laser light. Using resonance-radiation pressure atomic beams can also be focused and manipulated in interesting ways [9.233,234]. Several other interesting mechanical effects exist and have been explored. Studied phenomena include light diffusive pulling [9.235], light-induced drift [9.236] and the optical piston [9.237]. 

As we have seen it has been known for more than 60 years that ultrasound can produce defoaming effects [62]. Despite this fact, authors are still forced to write in recent publications, for example, that the mechanisms of ultrasonic defoaming are not well known but it can be assured that they include the effects of the acoustic pressure, the radiation pressure, bubble resonance, streaming and liquid film cavitation [67]. This presumably reflects the difficult nature of the subject, although it may also reflect the limited extent of practical application of ultrasound in this context. It is even possible that these two factors mutually reinforce one another. 

The effects of the radiation pressure force and the gradient force on an atom are essentially different. The radiation pressure force (5.10) always accelerates the atom in the direction of the wave vector k. The gradient force (5.11) pulls the atom into the laser beam or pushes it out of the beam, depending on the sign of the Doppler shift detuning Z - k v. Both the radiation pressure force and the gradient force have a resonance at a velocity such that k v = Z, when the detuning A is compensated by the Doppler shift k v. 

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

It may also be mentioned here that in specific molecular actions a particularly marked influence of like molecules upon one another is often to be observed. This is encountered in various ways in spectroscopy, in the extinction of the polarization of mercury resonance radiation with increasing vapour pressure, in the damping of fluorescence in concentrated solutions, and in various chemical reactions. As an example of the latter the decomposition of acetaldehyde (p. 70) may be quoted, where collisions between two molecules of the aldehyde are much more effective than collisions of aldehyde molecules with those of other gases. 

An original method involves quadrupole oscillations of drops K The drop (a) in a host liquid (P) is acoustically levitated. This can be achieved by creating a standing acoustic wave the time-averaged second order effect of this wave gives rise to an acoustic radiation force. This drives the drop up or down in p, depending on the compressibilities of the two fluids, till gravity and acoustic forces balance. From then onwards the free droplet is, also acoustically, driven into quadrupole shape oscillations that are opposed by the capillary pressure. From the resonance frequency the interfacial tension can be computed. The authors describe the instrumentation and present some results for a number of oil-water interfaces. 

In this expression, r and ro are, respectively, the instantaneous and equilibrium (i.e., when no sound field is acting on the liquid) values of the bubble radius and f and r represent, respectively, the first and second order time derivatives of the instantaneous bubble radius p is the liquid density y is the polytropic exponent of the gas inside the bubble (i.e., the ratio of heat capacities, Cp/Cv) Pa is the acoustic pressure amplitude Poo is the hydrostatic (ambient) pressure b is the bubble pulsation damping term that accounts for thermal, viscous, and radiation effects cr is the liquid surface tension t is time and coj. is the resonance frequency of the bubble, which is defined by the equation below ... 

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