Intra-cluster reaction

In many cases it is not only the reagents depletion that can be measured, but also the product appearance, as a function of the excitation laser. The latter type of spectrum is termed an action spectrum. Figure 24.9 shows the CaCl action spectrum from the laser-induced Ca - HCl intra-cluster reaction. The Ca HCl complex is prepared in a supersonic expansion and excited by a laser pulse (at a frequency close to the atomic resonance) to a repulsive electronic state. The dissociation into CaCl and H products can be characterized either by tuning the laser frequency and recording the subsequent action spectmm, or by using LIF detection. 

In this subsection we describe, in more detail, the Ba FCH3 + liz/ —> product intra-cluster reaction, which has been extensively studied in frequency- and time-domain experiments. The weakly bound complex Ba - -FCH3 is produced in a laser vaporization source, followed by supersonic expansion. A gas pulse from a mixture of He with CH3F is generated, and the output of an Nd YAG laser is focused onto the surface of a rotating barium disk to produce a vapour of Ba that is injected into the gas pulse. This mixture expands supersonically into the vacuum chamber. 

Figure 24.15 Reaction mechanism suggested for the intra-cluster reaction Ba - FCHj. The related potential energy curves were shown in Figure 23.7...

Stert V, Farmanara P, Ritze HH, Radloff W, Gasmi K, Gonzalez-Urena A. 2001b. Femtosecond time-re-solved electron spectroscopy of the intra-cluster reaction in Ba- -FCHs . Chem. Phys. Lett. 337(4-6) 299-305. 

The reactions of vdW molecules and clusters can be divided into intra- and intercluster processes, and further into neutral and ionic cluster reactions. The latter were recently reviewed by Mark and Castleman. Therefore the scope of this contribution will be limited to neutral species only. We distinguish between intra- and intercluster reactions. In intracluster processes reactions are induced inside a cluster, usually by light. Examples of such reactions are the reaction of excited mercury atoms with various molecules attached to them, reactions that follow photodissociation in the cluster, and charge transfers inside a large cluster. In intercluster reactions the cross molecular beam technique is usually applied in order to monitor scattered products and their internal energy. The intercluster reactions may be divided into three major categories recombination processes, vdW exchange reactions, and reactions of clusters with metal atoms. 

Characterization of the intermolecular vibrations of molecular clusters in their neutral excited and cationic states, and the study of intra-cluster ion-molecule reactions, using the techniques of REMPI, mass-analysed threshold ionization (MATI) and zero kinetic energy (ZEKE) spectroscopy. 

Many transmembrane proteins that mediate intracellular signaling form complexes with both intra- and extracellular proteins. For example, neural cell adhesion molecules (NCAMs) are cell-surface glycoproteins (Ch. 7). The extracellular domains of NCAMs can activate fibroblast growth factor receptors when clustered by reaction with NCAM antibodies [4] or by homotypic binding to domains of adjacent cells (see Fig. 7-2). Activation was found to sequester a complex of NCAM, (31 spectrin and PKC(32 into rafts, as defined by the operational criteria discussed on p. 28. 

Poluektov et al. used high-frequency time-resolved spin-polarised EPR spectroscopy of radical pairs to characterise quantitatively isotopically labelled quinine exchange in the PS I reaction centre of proteins.91 Intra-subunit interactions in the Fe-S cluster of PS I of Synechocystis sp. PCC 6803 were studied by using mutations and following the changes in stabilisation of the cluster by EPR spectroscopy.92... 

This idea of the selective initiation of reactions can be further expanded to vibrationally initiating intra- and intercluster reactions. The absorption frequency in the infrared, of small clusters depends on the complex size. Hence if a reaction is endothermic, and is induced by vibrational excitation, one is able to label the reacting cluster by choosing a frequency corresponding to its infrared absorption. The difficulties in this method arise from the small cross section for the infrared absorption, which limits it to strong absorbers and to reactions with large probabilities. 

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