Focusing-Refocusing Events

Typical images of filamentation parallel to the laser propagation direction are shown in Fig. 5.5 for various values of incident power. 

Following the treatment given earlier [2,3,53], one can estimate the peak intensity, electron density, and the six-photon absorption cross section from experimental observation of filament radius. With the size of the single filament (Fig. 5.4) taken as 8.3pm (end-to-end diameter), the use of the above equations enable estimates to be obtained for the peak intensity (/max) and electron density within the crystal as 3.26 x 1013 Wcm-2 and 2.81 x 1019 cm-3, respectively [48]. Using these values, it also becomes possible to estimate the 6PA cross-section in BaF2 to be 0.33 x 10 70 cm12 W-6 s 1. 

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Societies concern with air quality has evolved from medieval times, when breathing smelting fumes was a major hazard, to where we are today (see Chapter 1). In modem society, a parallel effort has been under way to improve air quality in the outside or ambient air, which is the focus of this book, and in the industrial occupational setting in manufacturing and other traditional jobs. A combination of events is moving many countries to consider the quality of air in other locations where we live parts of our lives. Attention is now being refocused on "indoor" air quality. 

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