Optical absorption cross section

Three key issues must be addressed in the development of effective biomimetic solar energy conversion systems. First, the molecular system should possess a large optical absorption cross-section in the desired spectral region. Second, the system should possess appropriate characteristics to insure formation of a sufficiently long-lived, low-lying state which can initiate the primary ET efficiently. And third, the system should be able to effect the ET process irreversibility, that is electron-hole recombination should be substantially inhibited. 

The first issue can be addressed in two ways a primary ET species which has a large optical absorption cross-section can be chosen or arrays of molecules with large optical absorption cross-sections can be used as "antennas" that will efficiently collect and transport the electronic excitation energy to the primary ET species, in direct analogy to photosynthetic systems. While in the latter case it should be possible to develop systems with more efficient solar photon collection, the number of primary ET species will have to be reduced due to the spatial limitations, which will also reduce the potential electric current that can be produced by the system. Thus, questions related to the detailed molecular architecture of biomimetic solar energy conversion devices will have to address this issue, and it is quite likely that a number of compromises will have to be made before optimal design characteristics are obtained. 

In this case, a is the differential optical absorption cross section for the absorption band. In practice, of course, there are many different absorbers, t, present at different concentrations and absorbing at different wavelengths over the path length L. 

Regardless of the choice of the sample thickness, the total amount of sample particles in the x-ray probe beam under optimized conditions is directly proportional to the x-ray spot size and inversely proportional to the x-ray absorption cross section, whose photoinduced (small) changes we want to measure [12]. Typical x-ray foci at synchrotrons are in the 0.1 - 0.3 mm range. For the examples treated below, this means that we have between 1014 and 1016 molecules in the probed volume. In order to achieve a reasonable photoinduced signal we should excite as many solute molecules as possible. Neglecting the optical absorption cross sections for photoexcitation for the moment, this requires on the order of 1015 laser photons per pulse, or ca. 0.25 mJ of pulse energy (e.g., at 800 nm). In other words, one should aim to... 

In Chapter 3 we considered briefly the photoexcitation of Rydberg atoms, paying particular attention to the continuity of cross sections at the ionization limit. In this chapter we consider optical excitation in more detail. While the general behavior is similar in H and the alkali atoms, there are striking differences in the optical absorption cross sections and in the radiative decay rates. These differences can be traced to the variation in the radial matrix elements produced by nonzero quantum defects. The radiative properties of H are well known, and the radiative properties of alkali atoms can be calculated using quantum defect theory. 

FIGURE 4 The relative optical absorption cross section as a function of photon energy, hv. The solid curve corresponds to the fitting result with o0p, oc (hv - Eopt) 5/(hv)3, from which one obtains the optical ionisation energy Eopt = 2.69 eV. After [8],... 

The optical absorption cross-section will depend sensitively on the surface from which reflection takes place and the relative orientation of the surface and the electric vector of the incoming radiation. The effect of... 

To detennine the optical absorption cross-section of BrCl and its dissociation constant in carbon tetrachloride solution. 

Here, a and c are the optical absorption cross section of the sensitizer and its concentration in the mesoporous film, respectively. The value of <7 can be derived from the decadic extinction coefficient e of the sensitizer using the relation ... 

As seen from Figure 1.14, most of the optical absorption cross section of the charged species is shared between two excited states described as the Cl and C2 electronic transitions of Figure 1.13, namely the singly excited HOMO—>P1 and PI—>P2 configurations, respectively [63]. 

Fig. 4.15. Peak optical absorption cross sections vs cutoff wavelengths of impurity dopants in Si (dashed curve). Solid curve in inset isa typical shapeof the ff vs A curve of a dopant [4.65]...

The optical absorption cross sections of impurities in Si typically have a dependence on wavelength of the form shown in Fig. 4.15. Maximum or peak values of (7 for all the impurities in Si for which this parameter is known are also plotted in Fig, 4,15 [4.61],... 

The absorption coefficient aik [cm ] for a transition /) k) depends on the population densities Ni, Nk of the lower and upper levels, and on the optical absorption cross section anc [cm ] of each absorbing atom, see (2.42) ... 

Equation (2.73) allows the optical absorption cross section to be... 

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