Electronic excitation quantum yields

Addition of an inorganic sheet and a PVT-MV + layer results in efficient quenching of the porphyrin phosphorescence at 720 nm, due to electron transfer from excited porphyrin to MV +. Electron-transfer quantum yields were calculated as 0.58 and 0.76 for a-ZrP and HTiNbOs spacers, respectively. Figure 38 shows the steady-state emission spectrum of such a multilayer system. 

Figure 14. Dependence upon excitation photon energy hv of electron injection quantum yield 4>i obtained for various oxide semiconductors sensitized by cw-[Ru dcbpy)2(NCS)2].

The quantum yield calculated from laser data for solvated electron production by Aldrich HSX excited at 355 nm was extrapolated to a day averaged surface steady state concentration of solvated electron n sunlit waters. In direct sunlight an average rate of 1.62 X 10 photons per second Impinging on a square centimeter of surface water was calculated from solar photon fluxes (32-33). Using absorptlvltles of HS from 299 to 800 nm (with an O.D. of 0.1 at 355 nm) and a solvated electron production quantum yield of 0.014 (16), a rate of 3.50 x 10 moles of solvated electrons produced per second was calculated. Based on a lifetime of 0.25 microseconds for solvated electrons In air saturated natural waters, the surface steady state concentration the top coble centimeter of water was calculated to be about 10 M. 

Referenced to raei o-tetraphenylporphyrin Of = 0.11, excitation at 517 nm (free base excitation predominant) values in parentheses for excitation at 565 nm (zinc donor predominant). OeT is the electron transfer quantum yield from Zn to the free base electron acceptor. Ojjt is the hole transfer quantum yield (upper limit) from H2 to the Zn hole acceptor. 

The excitation quantum yield ( ex) is the product of the efficiencies of (1) the chemical reaction, (2) the conversion of chemical potential into electronic excitation energy and in the case of sensitized chemiluminescence, and (3) the energy transfer. As a consequence, most chemiluminescent reactions have relatively low quantum yields compared to those of photoluminescence the exception being the enzymatically mediated bioluminescent processes. In spite of this low quantum efficiency, chemiluminescence remains an attractive option for chemical analysis. This stems from three factors (1) improved... 

In this chapter, we have described the fundamental parameters that should be obtained when characterising an electronic, singlet or triplet, excited state and how to determine them experimentally including methodologies and required equipment. These characteristics include electronic energy, quantum yields, lifetimes and number and type of species in the excited state. Within this last context, i.e., when excited state reactions give rise to additional species in the excited state we have explored several excited state kinetic schemes, found to be present when excimers, exciplexes are formed and (intra and intermolecular) proton transfer occurs. This includes a complete formalism (with equations) for the steady-state and dynamic approaches for two and three-state systems, from where all the rate constants can be obtained. Additionally, we have explored additional recent developments in photophysics the competition between vibrational relaxation and photochemistry, and the non-discrete analysis (stretched-exponential) of fluorescence decays. 

Fig. 7. Electron carrier quantum yields at 255 nm as a function of applied field. Curve a is for a previously unirradiated or virgin crystal sample. Curve b illustrates the reduced low-field yield that occurs when trapped holes, left behind in the experiments shown in curve a are present in the excited volume of the crystal. Note that curves a and b actually superimpose at high fields but that curve b has been shifted arbitrarily for clarity of presentation.

One characteristic property of dyes is their colour due to absorption from the ground electronic state Sq to the first excited singlet state Sj lying in the visible region. Also typical of a dye is a high absorbing power characterized by a value of the oscillator strength/ (see Equation 2.18) close to 1, and also a value of the fluorescence quantum yield (see Equation 7.135) close to 1. 

The blue luminescence observed during cool flames is said to arise from electronically excited formaldehyde (60,69). The high energy required indicates radical— radical reactions are producing hot molecules. Quantum yields appear to be very low (10 to 10 ) (81). Cool flames never deposit carbon, in contrast to hot flames which emit much more intense, yellowish light and may deposit carbon (82). 

In principle, one molecule of a chemiluminescent reactant can react to form one electronically excited molecule, which in turn can emit one photon of light. Thus one mole of reactant can generate Avogadro s number of photons defined as one einstein (ein). Light yields can therefore be defined in the same terms as chemical product yields, in units of einsteins of light emitted per mole of chemiluminescent reactant. This is the chemiluminescence quantum yield which can be as high as 1 ein/mol or 100%. 

Figure 3. Energy diagram for 1064 nm excitation of PuFg(g). The 5f electron states of PuF6 are shown at the left. The solid arrows Indicate photon absorption or emission processes. The wavy arrows indicate nonradiative processes by which excited states of PuF6 are lost. Comparison of observed fluorescence photon yields versus the fluorescence quantum yield expected for the 4550 cm" state indicate that the PuFg state initially populated following 1064 nm excitation may dissociate as shown.

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Upon absorption of UV radiation from sunlight the bases can proceed through photochemical reactions that can lead to photodamage in the nucleic acids. Photochemical reactions do occur in the bases, with thymidine dimerization being a primary result, but at low rates. The bases are quite stable to photochemical damage, having efficient ways to dissipate the harmful electronic energy, as indicated by their ultrashort excited state lifetimes. It had been known for years that the excited states were short lived, and that fluorescence quantum yields are very low for all bases [4, 81, 82], Femtosecond laser spectroscopy has, in recent years, enabled a much... 

The overall OD vibrational distribution from the HOD photodissociation resembles that from the D2O photodissociation. Similarly, the OH vibrational distribution from the HOD photodissociation is similar to that from the H2O photodissociation. There are, however, notable differences for the OD products from HOD and D2O, similarly for the OH products from HOD and H2O. It is also clear that rotational temperatures are all quite cold for all OH (OD) products. From the above experimental results, the branching ratio of the H and D product channels from the HOD photodissociation can be estimated, since the mixed sample of H2O and D2O with 1 1 ratio can quickly reach equilibrium with the exact ratios of H2O, HOD and D2O known to be 1 2 1. Because the absorption spectrum of H2O at 157nm is a broadband transition, we can reasonably assume that the absorption cross-sections are the same for the three water isotopomer molecules. It is also quite obvious that the quantum yield of these molecules at 157 nm excitation should be unity since the A1B surface is purely repulsive and is not coupled to any other electronic surfaces. From the above measurement of the H-atom products from the mixed sample, the ratio of the H-atom products from HOD and H2O is determined to be 1.27. If we assume the quantum yield for H2O at 157 is unity, the quantum yield for the H production should be 0.64 (i.e. 1.27 divided by 2) since the HOD concentration is twice that of H2O in the mixed sample. Similarly, from the above measurement of the D-atom product from the mixed sample, we can actually determine the ratio of the D-atom products from HOD and D2O to be 0.52. Using the same assumption that the quantum yield of the D2O photodissociation at 157 nm is unity, the quantum yield of the D-atom production from the HOD photodissociation at 157 nm is determined to be 0.26. Therefore the total quantum yield for the H and D products from HOD is 0.64 + 0.26 = 0.90. This is a little bit smaller ( 10%) than 1 since the total quantum yield of the H and D productions from the HOD photodissociation should be unity because no other dissociation channel is present for the HOD photodissociation other than the H and D atom elimination processes. There are a couple of sources of error, however, in this estimation (a) the assumption that the absorption cross-sections of all three water isotopomers at 157 nm are exactly the same, and (b) the accuracy of the volume mixture in the... 

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