Conversion, of photons

This problem asks if red and blue photons can cause potassium metal to lose electrons. We must analyze the energy requirements for ejection of an electron. No electrons will be ejected unless the energy of the photons exceeds some threshold value characteristic of the metal. If the photon energy exceeds this threshold value, electrons will be ejected with kinetic energy given by Equation. An important part of this problem is the conversion of photon frequency to photon energy. 

This conversion of photon energy into thermal energy is the underlying basis of the T-jump formulations forwarded by Ready and others. The transient temperature jump AT(t) induced by a laser pulse is... 

The energy content of the CS state should be as high as possible, thereby ensuring maximum conversion of photonic energy into chemical potential. Thus, AG should be as small as practicable while ensuring that requirement (1) is met. 

After excitation by visible or UV radiation, molecules relax back to the ground state by dissipating the excitation energy through molecular vibrations and collisions with solvent molecules. This process is called nonradiative decay and is typically a very fast set of processes for molecules in fluid solution at room temperature. Alas, this process, which is simply the conversion of photonic energy to heat, is rarely useful. 

It seems possible to amplify the photostimulated conformational changes in solution at the molecular level into shape changes of polymer gels or solids at the visible macro level. The first proposal to use the structural changes at the molecular level for direct conversion of photon energy into mechanical work has been made by Merian (13.) in 1966. Since then, many materials, most of which contained azobenzene chromophores, have been reported to show photostimulated deformation(JM). Till now, however, the reported deformations were limited to less than 10%. In addition, Matejka et. al. have pointed out that in many cases photo-heating effect instead of photochemical reaction plays a dominant role in the deformation(15,16). 

Photoeffects are also observed at metal electrodes, although the resulting photocurrents are much smaller. For example, the irradiation of a metal electrode can cause the photoejection of an electron into the solvent. If this electron is scavenged by some reactant in solution, a net cathodic photocurrent results (76, 77) (see Section 18.3 and Problem 8.10). These electron photoejection studies are of interest, because they can provide information about the nature of an electron at the instant of injection into a medium, as well as the energetics and kinetics of its relaxation to equilibrium solvation. Excitation of dyes adsorbed on metals can also lead to photocurrents, but they are usually much smaller than the photocurrents obtainable at semiconductor electrodes under comparable conditions (74). This low efficiency of net conversion of photons to external photocurrent is attributed to the ability of a metal to act as a quencher of excited states at or very near the surface by either electron or energy transfer (18, 19). 

Solar cells are photodiodes optimized for maximum conversion of photons to electrical current. These are optimized for either terrestrial-based sun or atmosphere-free exposure. Two basic types of ceils are the flat panel (large area) and the concentrator (few mm"). Concentrator cells employ mirrors or lenses to concentrate the sunlight by a factor of 100 to 500 X. 

Light-Induced Charge Separation and Conversion of Photons to Electric Current... 

The conversion of photonic to chemical energy is what powers nearly every ecosystem on Earth, because it drives primary production. And yet this ubiquitous process is, ct face value, not obviously one that warrants mimicry in solar-energy research. Even commercial solar cells based on polycrystalline silicon exceed the efficiency (at most 8-9%) of the chloroplast. the light-harvesting organelle ofplants. 

Fig. 6.13 Conversion of photon energy into chemical energy by semiconductors. Absorbed photons (hv) lead to charged ion pairs (e , h ) that can be separated and transferred to electron acceptors (A) and electron donors (D) at the surface. The figure is adapted from [15] with permission from Elsevier...

The reaction in this regenerative PEC is the conversion of photon energy into electrical energy without destroying chemical composition of the redox electrolyte or counter-electrode or semiconductor electrode, that is, hv (photon energy) electrical energy. 

Figure 3.22 Schematic representation of the working of organic-inorganic bUayer hybrid solar cell showing different steps involved in conversion of photon to electricity.

Developments in DSSCs depend on our understanding and control of the fundamental kinetic and thermochemical nanoscale phenomena that govern the conversion of photon energy into electron energy , namely on the interfacial electron transfer and charge transport dynamics. Efficient dye-sensitised solar cells depend on the fine-tuning of the energies of the states implicated and of the rates of the processes involved. 

Our observation of spin-polarized EPR spectra depends on achieving a delicate balance of structure dependent electronic interactions between the donors and acceptors within the supramolecular array. This strongly suggests that we now know how to precisely mimic the interactions found to date only in natural photosynthesis. With this knowledge we will be able to design efficient biomimetic supermolecules for the efficient conversion of photon energy into chemical energy in the solid state. 

When the laser beam is chopped at frequencies below 10 KHz, periodical pressure variations appear in the absorption cell which can be detected with a sensitive microphone placed at the inner side of the cell. The output signal from the microphone is proportional to the absorbed laser energy and therefore allows determination of the absorption coefficient. Because this method uses the conversion of photon energy into periodical pressure variations it is called photoacoustio spectroscopy and the device is named spectra-phone. 

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