Light, absorption saturation

In both cases (i. e. emission or absorption saturation) the halfwidth of this/Lamb dip is slightly dependent on laser power but mainly determined by the interaction time of the individual molecules with the standing light wave in the cavity. This time may be limited by the finite lifetimes rb of upper or lower states, by the average time l/aup between two disturbing collisions, or by the transit time Tt of the gas molecules across the laser beam. This last limitation becomes important at low pressures of the absorbing gas and for transitions between long-lived states (see Section IV.3). 

It should be noted that the foregoing considerations apply to the major absorption bands. In some cases, weaker absorption bands of the major greenhouse gases can be sufficiently weak to fall in the linear region. This is the case, for example, for light absorption by 03 in the Chappius band, even if the strong Hartley-Huggins band (see Chapter 4.B) is saturated (e.g., see Lacis et al., 1990). 

Saturated three- and four-membered heterocyclics absorb little in the readily accessible regions of the UV spectrum. Sulfur-containing rings are an exception, as can be seen in Table 9, Despite the lack of absorption of most parent compounds, there is a wealth of photochemistry of small heterocyclics. Light absorption by substituents, and energy transfer from photoexcited molecules present in the photoreactive system make photoconversion of the heterocycles practical. On the other hand, the lack of substantial absorption of their own can be exploited in the preparation of small heterocycles, by designing the system to be unsuitable for destructive energy transfer. 

The case P < 0 corresponds, in the spectral domain of a resonant transition, to a saturation of absorption (SA) phenomenon. The origin of such a behaviour in metal/insulator nanocomposite media is not described the same way from authors to others. On the one hand, the simple and intuitive explanation generally given is that strong light absorption at the SPR causes the saturation (bleaching) of the... 

Quantitative measurements show that at low levels of illumination, where ATP synthesis is limited by light absorption, the quantum yield of ATP is about 15% (assuming approximately 50% of the light is absorbed by the scattering liposome solutions). Assuming that four protons must be transported out of the liposome by ATP synthase per molecule of ATP produced, the yield of the proton pump must be -0.6. The turnover number for the enzyme, measured under saturating illumination and optimal conditions, is ca. 40 molecules of ATP per CFoFj-ATP synthase per second. 

In the introduction to this chapter we gave an intuitive explanation of the origin of nonlinear optical effects and stressed the key role played by high power lasers and coherent light beams. These two concepts are defined here. We will describe one specific characteristic of laser light, namely the absorption saturation, and finally we will discuss susceptibility and frequency conversion of light. 

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