Photon processing time

Jent F, Paul H and Fischer H 1988 Two-photon processes in ketone photochemistry observed by time-resolved ESR spectroscopy Chem. Phys. Lett. 146 315-19... 

Using a perturbative analysis of the time-dependent signal, and focusing on the interference term between the one- and two-photon processes in Fig. 14, we consider first the limit of ultrashort pulses (in practice, short with respect to all time scales of the system). Approximating the laser pulse as a delta function of time, we have... 

Williams, S.O. and Imre, D.G. (1988b). Time evolution of single- and two-photon processes for a pulse-mode laser, J. Phys. Chem. 92, 6636-6647. 

Figure 1 Schematic representation of a time-resolved coherent Raman experiment, (a) The excitation of the vibrational level is accomplished by a two-photon process the laser (L) and Stokes (S) photons are represented by vertical arrows. The wave vectors of the two pump fields determine the wave vector of the coherent excitation, kv. (b) At a later time the coherent probing process involving again two photons takes place the probe pulse and the anti-Stokes scattering are denoted by subscripts P and A, respectively. The scattering signal emitted under phase-matching conditions is a measure of the coherent excitation at the probing time, (c) Four-photon interaction scheme for the generation of coherent anti-Stokes Raman scattering of the vibrational transition.

The electron excitation caused by the absorption of a photon can be processed by the chemical reactions leading to CO2 fixation about once every 5 x 10-3 s (5 ms). This processing time has important consequences for both... 

As we have just calculated, each chlorophyll molecule in an unshaded chloroplast can absorb a photon about once every 0.1 s. When there are 250 chlorophylls per reaction center, 12.5 of these molecules are excited every 5 ms (250 chlorophylls x 10 excitations per chlorophyll/1 s x 0.005 s). However, because the average processing time per reaction center is about 5 ms, only one of these 12.5 excitations can be used photochemically — the others are dissipated by nonphotochemical deexcitation reactions. Consequently, although the biochemical reactions leading to CO2 fixation operate at their maximum rates under such conditions of high PPF, over 90% of the electronic excitations caused by light absorption are not used for photosynthesis (Fig. 5-12). 

In Chapter 5 (Section 5.4B) we noted that the processing time per CO2 fixed is about 5 ms. Eight photons are required, which are absorbed by approximately 2000 chlorophyll molecules in chloroplasts having a chlorophyll concentration of about 30 mol m-3 (see Chapter 4, Section 4.4D). Hence, the photosynthetic rate per unit volume of chloroplasts is... 

Recently, Wong and coworkers reported an emissive Tb(III) complex [Tb(L73)(N03)3] (Tb-73, Figure 13.34) based on a ligand of 7V-[2-(bis 2-[(3-methoxybenzoyl)amino]-ethyl amino)ethyl]-3-methoxybenzamide) (L73) as a three-photon luminescence probe with low cytotoxicity [109]. The three-photon process of Tb-73 under excitation at a femtosecond 800 nm laser was confirmed by a power dependence experiment. The three-photon absorption cross section of Tb-73 is around 1.9 GM. Furthermore, the interaction of three selected cells with Tb-73 over different durations of time (from 0 to 60 min and 24 h) was investigated. As shown in Figure 13.34, intracellular luminescence increased as the exposure time increased. At an exposure time of 60 min, more than 95% of the cells under 800 nm excitation exhibited green luminescence, as was observed in the cytoplasmic foci around the cell nucleus. 

Time and temperature dependences of the delayed fluorescence in isotopi-cally mixed naphthalene crystals have been presented for various concentrations of traps. Coherent two-photon processes in naphthalene in the strong exciton-photon counting regime have also been investigated. Excited-state spectra of 1,5-naphthyridine in several solvents support those calculated using INDO molecular orbital formalism and show the lowest excited singlet state to... 

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