Ultrafast dissociation

To experimentally probe the CO trajectory after dissociation, ultrafast time-resolved polarized mid-IR spectra of photolyzed h-MbCO in G/W were recorded (34), the results of which are plotted in Fig. 8A. This study was performed in G/W primarily because the flatness of the solvent absorbance spectrum near 2100 cm-1 minimizes temporal distortion of the transmitted femtosecond IR probe pulse, thereby maximizing the effective time resolution of the measurement. Two features are already apparent at 0.2 ps, the earliest time shown, and these features rapidly develop into the docked states denoted Bi and B2. The development of the docked CO spectrum is further quantified by the time dependence of the polarization anisotropy, as defined in Equation (2). The B and B2 polarization anisotropies, plotted in Fig. 8B, evolve exponentially with time constants of 0.20 0.05 ps and 0.52 0.10 ps, respectively, and converge to the same anisotropy of approximately 0.2. According to Fig. 8C, ligand translocation is accompanied by a 1.6 0.3 ps growth of the integrated isotropic B-state absorbance. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Does the ultrafast dissociation process of FefCO) in solution, and that of related compounds in general, occur in a concerted or a rapid sequential way ... 

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

Subject the ultrafast dissociation process. Verb correct as is. 

Within the pulse duration, polarons and bipolarons dissociate into Drude-type electrons and the subsequent bleach recovery at various probe wavelengths indicates the reformation of those species. On the other hand, the equilibrated absorption spectrum of Drude-type electrons is substantially red-shifted [7] compared to the F-centre absorption band. According to our mechanism polarons and bipolarons form Drude-type electrons after ultrafast excitation which leads to an increase of the transient absorption in the NIR region. 

Solvation Ultrafast Dynamics of Reactions IX. Femtosecond Studies of Dissociation and Recombination of Iodine in Argon Clusters, J-K. Wang, Q. Liu, and A. H. Zewail, J. Phys. Chem. 99, 11309 (1995). 

Ultrafast dissociation, and reactive processes in general, can thus be... 

R. Kersting, et al., Ultrafast field-induced dissociation of excitons in conjugated polymers, Phys. Rev. Lett. 73 (1994) 1440-1443 ... 

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