Two-photon mechanism

FIGURE 75 (A) Schematic illustration of the two-photon mechanism UC process of... 

C electrodes is reported to yield a photopotential of about 650 mV and a conversion efficiency of 1%. The reaction has been optimised and it is believed that a two photon mechanism... 

Other examples of generating free radicals utilized in our laboratory are the consecutive two-photon ionization of the aromatic pyrene residue in the benzo[a] pyrene derivative 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (B[a]PT) or the covalent adducts derived from the reactions of racemic anti-r7,t8-dihydroxy-t9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]PDE) with the N2-exocyclic amino groups of guanine or adenine bases in DNA [43-47]. The selective excitation of the pyrenyl residue of B[a]PT by intense 355-nm nanosecond laser pulses yields free radical products by a two-photon mechanism ... 

Irradiation of [Co(NHa)e]3+ in its LF 1Ala -> lT2g band with light ( = 337 nm) from a pulsed nitrogen laser causes photoreduction of the complex.102 A two-photon mechanism is presented in which a ligand field state LF absorbs a further photon to give the reactive CT state (steps 34—37). 

For two-photon memories, a number of media types and reading mechanisms have been used (165). Generally, media comprise two photon-absorbing chromophores dissolved within a soHd polymer matrix. Suitable reversible photochromic dyes are, for example, spiropyrans. Although photochromic materials often suffer from photobleaching, as well as from instability leading to self-erasure, new materials and host environments are under development (172). Bacteriorhodopsin (BR) also has been proposed as a two-photon memory material. 

When N,A/ -dimethyl-Af,N -diphenyl-p-phenylenediamine (231) underwent irradiation in die presence of oxygen, a remarkably regioselective reaction was observed, from which the /V,A/ -dimediylindolo[2,3-f ]carbazole (190) was obtained, without coformation of die other possible isomeric product, indolo[3,2-/)]carbazole (4). The expected intermediate 232, where only one ring closure had taken place, could not be detected. The authors attributed this fact to a two-photon autosensitization mechanism in which die conversion of 231 into 190 is sensitized by 232, permitting accumulation of 232 only in extremely small concentrations (83JA6268). 

The yield of the photocathodic dissolution of CdS in a solution containing 1 x 10" M SOi" is only 0.005 molecules dissolved per photon absorbed. In the presence of 5 X 10 M excess Cd " ions it amounts to 0.05. Sulfate and dithionate are formed in the ratio 2.2 to 1. The oxidation of SO3" is effected by the positive holes produced upon illumination, two holes being necessary to convert one SO ion into SO " or 1/2 SjOg . If the SOj anion captured the two holes from the same colloidal particle ( two-hole mechanism ), only sulfate would appear as the oxidation product. However, if SO3" captured only one hole to form the radical SOj", the final products would be formed bj reactions of two such radicals, and these two radicals could even originate from different colloidal particles ( one-hole mechanism )... 

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

A further striking finding by Stelmakh and Tsvirko is the S2 fluorescence following excitation to the S state by a low-energy photon. They also proposed possible excitation mechanisms to explain their results (24-26). Pumping can be achieved by two different ways of unimolecular successive two-photon absorption ... 

TWO-PHOTON EXCITATION TWO-PHOTON PROCESS TWO-PROTONIC-STATE ELECTROPHILES AFFINITY LABELING Two-site ping pong mechanism,... 

Atmospheric pressure laser ionization (APLl) is a complementary technique to existing API methods. It is based on resonant or near-resonant two-photon ionization of aromatic ring systems. APLI utilizes resonantly enhanced multiphotonionization (REMPI) as the primary ion-production mechanism, nevertheless it occurs at atmospheric pressure [80],... 

Two-photon excitation fluorescence is currently the most widely nsed nonlinear contrast mechanism for microscopic investigations. The first experimental demonstration of two-photon excitation fluorescence was provided in 1961 (Kaiser and Garrett 1961), even though the first theoretical description of two-photon excitation flnorescence stems back to 1931 (Goppert-Mayer 1931). Three-photon absorption was demonstrated a few years later by Singh and Bradley (1964). Two-photon absorption is a third-order nonlinear effect, whereas three-photon absorption is a fifth-order nonlinear effect. The transition rate for two-photon absorption, R, depends on the square of the intensity, /, as follows (see Boyd 1992) ... 

Two-photon absorption occurs when the energy of a molecular transition matches the combined energy of two photons. Quantum mechanically, the absorption probability is proportional to the two-photon transition moment from the ground state, g, to the excited state, n, via intermediate state, m, and can be expressed as follows (Boyd 1992 Abe 2001) ... 

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