Excitation-transfer systems cross-sections

Several investigations have been reported of excitation transfer from excited mercury atoms to ground-state atoms of silver, bismuth, cadmium, chromium, copper, indium, lead, and zinc. Most of these experiments which had been completed some time ago were surveyed by Seiwert [6,7]. Perhaps of particular interest is an investigation by Gough [105], who studied excitation transfer from mercury to cadmium and concluded that not only excitation energy but also coherence was transferred in the collisions. A similar conclusion was reached by Kraulinya, Sametis, and Bryukhovetskii [106] as the result of their study of the Hg-Tl system. Cross sections for Hg-Cd... 

Here t. is the intrinsic lifetime of tire excitation residing on molecule (i.e. tire fluorescence lifetime one would observe for tire isolated molecule), is tire pairwise energy transfer rate and F. is tire rate of excitation of tire molecule by the external source (tire photon flux multiplied by tire absorjDtion cross section). The master equation system (C3.4.4) allows one to calculate tire complete dynamics of energy migration between all molecules in an ensemble, but tire computation can become quite complicated if tire number of molecules is large. Moreover, it is commonly tire case that tire ensemble contains molecules of two, tliree or more spectral types, and experimentally it is practically impossible to distinguish tire contributions of individual molecules from each spectral pool. 

Reactions of Complex Ions. For reactions of systems containing H2 or HD the failure to observe an E 1/2 dependence of reaction cross-section was probably the result of the failure to include all products of ion-molecule reaction in the calculation of the experimental cross-sections. For reactions of complex molecule ions where electron impact ionization probably produces a distribution of vibrationally excited states, kinetic energy transfer can readily open channels which yield products obscured by primary ionization processes. In such cases an E n dependence of cross-section may be determined frequently n = 1 has been found. 

A little more complicated system is the de-excitation of He(2 P) by Ne, where the deexcitation is dominated by the excitation transfer and only a minor contribution from the Penning ionization is involved. The experimental cross section obtained by the pulse radiolysis method, together with the numerical calculation for the coupled-channel radial Schrodinger equation, has clearly provided the major contribution of the following excitation transfer processes to the absolute de-excitation cross sections [151] (Fig. 15) ... 

Several Other particle-transfer reactions have larger cross sections when the reactant ions are in electronically excited states. Two systems that have been studied extensively24 37,58,79a are... 

In many of these atomic systems, emissions are observed from the products of the charge-transfer reaction channel as well as from those formed via a direct excitation channel (see also Section II.B.2). For example, the spectra shown in Fig. 48 indicate the occurrence of transitions attributable to both Ar-I, and Ar II, that result from direct excitation and charge-transfer excitation, respectively, in the He + -Ar interaction.344 More than 50% of the total charge-transfer cross section for this reaction is attributable to the production of excited Ar+ states, rather than ground-state Ar +. 347,348 Similar results have been obtained for the other rare-gas... 

Figure 29. Ratio of cross section for excitation transfer followed by atomic autoionization (AAI), to total ionization cross section, as function of collision energy for systems He -Ar,Kr,Xe.77...

Note Differential elastic and excitation transfer cross sections have been measured for He(2 S) + Nc and for He(23S) + Ne for energies between 25 and 370 meV (1). Some of the data are shown in Fig. 52. It was possible to measure the differential excitation cross sections for the triplet system, too. A semiclassical two-state calculation was performed for the pumping transition of the red line of the HeNe-laser Hc(2 S)+ Nc— Hc + Ne(5S, lPt), which is the dominant transition for not too high energies (2). A satisfactory fit is obtained to the elastic and inelastic differential cross sections simultaneously, as well as to the known rate constant for excitation transfer. The Hc(215)+ Ne potential curve shows some mild structure, much less pronounced than those shown in Fig. 36. The excitation transfer for the triplet system goes almost certainly over two separate curve crossings. This explains easily the 80 meV threshold for this exothermic process as well as its small cross section, which is only 10% of that of the triplet system. 

The electronic quantum state ofthe pair H,ls> H+>= in> remains invariant at all distances. The electron transfer will not take place in a direct manner because the electronic parity is equal for both channels. The interconversion process requires aTS with parity -1. Among the states available to a system decomposable in one electron and two protons (or proton deuterium, etc) there are the hydrogen molecule ion species. The first electronic excited state (leu) ofthe molecular ion H2+ provides an "intermediate" (Q-state) for the interconversion once angular momentum conservation rules are fulfilled. The state (lau) is found above the in> and out> states leading to resonance in the cross section. This state may either relax to the (lrg) state yielding the hydrogen molecule ion and emitting a photon as this state is 2.8eV below dissociation, or it may take the product channels. This is a FC-like process. The reaction (27) is a prototype of electron transfer (ET). Thus, for any ET reaction whose in> and out> asymptotic electronic states share the same parity, the actual interconversion would require the mediation of a TS. 

While such a device has yet to be constructed, Debreczeny and co-workers have synthesized and studied a linear D-A, -A2 triad suitable for implementation in such a device.11641 In this system, compound 6, a 4-aminonaphthalene monoimide (AN I) electron donor is excited selectively with 400 nm laser pulses. Electron transfer from the excited state of ANI to Ai, naphthalene-1,8 4,5-diimide (NI), occurs across a 2,5-dimethylphenyl bridge with x = 420 ps and a quantum yield of 0.95. The dynamics of charge separation and recombination in these systems have been well characterized.11651 Spontaneous charge shift to A2, pyromellitimide (PI), is thermodynamically uphill and does not occur. The mechanism for switching makes use of the large absorption cross-section of the NI- anion radical at 480 nm, (e = 28,300). A second laser pulse at 480 nm can selectively excite this chromophore and provide the necessary energy to move the electron from NI- to PI. These systems do not rely on electrochemical oxidation-reduction reactions at an electrode. Thus, switching occurs on a subpicosecond time scale. 

The UV photon may be assumed to have an energy less than the band gap of the phosphor host so the absorption by A, S, and P takes place at the centers. Many activators, particularly Mn2+, have a low absorption cross-section so that direct pumping of the activator is inefficient. Sensitizers, such as Pb2+, are selected to have large absorption cross-sections at UV wavelengths. They then transfer the excitation to the activator which re-emits it as luminescent radiation. Most co-activators have a luminescent emission of their own so that the efficiency of the system depends on the kinetics of energy transfer compared with direct emission of the co-activator. In many mineral systems, one observes both and thus the activator radiation Ra and the sensitizer radiation Rg are... 

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