Photodissociative-recombination

Monchick [36, 273] has used the diffusion equation and radiation boundary conditions [eqns. (122) and (127)] to discuss photodissociative recombination probabilities. His results are similar to those of Collins and Kimball [4] and Noyes [269]. However, Monchick extended the analysis to probe the effect of a time delay in the dissociation of the encounter pair. It was hoped that such an effect would mimic the caging of an encounter pair. Since the cage oscillations have periods < 1 ps, and the diffusion equation is hardly adequate over such times (see Chap. 11, Sect. 2), this is a doubtful improvement. Nor does using the telegraphers equation (Chap. 11, Sect. 3.3) help significantly as it is only valid for times longer than a few picoseconds. 

In the case of the Ag° isomer in solid Ar, the spectroscopic and photochemical results indicate the operation of an efficient Ag3 - Agj + Ag photodissociative-recombination process localized in the matrix cage.However>for the Ag3 isomer in solid Kr and Xe, the available data leans heavily in favour of a photoisomerization to the Ag° structural form within a deformable matrix cage. The observation of thermal and photolytic reversibility,amongst other things,argues in favour of a photoisomerization rather than a photodissociation or photoionization process for the Ag3 isomer in Kr and Xe matrices.These photoprocesses for Ag3 and Ag3 in rare gas solids are summerized in Scheme III. 

U. Schmitt, J. Brickmann, The photodissociation/recombination dynamics of I2 in an Ar matrix Wave packet dynamics in a mixed quantum classical picture, J. Mol. Model., 2 (1996) 300.,... 

Papanikolas J M, Gord J R, Levinger N E, Ray D, Versa V and Lineberger W C 1991 Photodissociation and geminate recombination dynamics of t in mass-seiected, (Cfjj j ciuster ions J. Phys. Chem. 90 8028-40... 

Harris A L, Berg M and Harris C B 1986 Studies of chemical reactivity in the condensed phase. I. The dynamics of iodine photodissociation and recombination on a picosecond time scale and comparison to theories for chemical reactions in solution J. Chem. Phys. 84 788... 

Batista V S and Coker D F 1996 Nonadiabatic molecular dynamics simulation of photodissociation and geminate recombination of liquid xenon J. Chem. Phys. 105 4033-54... 

Niv M Y, Krylov A I and Gerber R B 1997 Photodissociation, electronic relaxation and recombination of HCI in Ar-n(HCI) clusters—non-adiabatic molecular dynamics simulations Faraday Discuss. Chem. Soc. 108 243-54... 

Figure C3.5.4. Ensemble-averaged loss of energy from vibrationally excited I2 created by photodissociation and subsequent recombination in solid Kr, from 1811. The inset shows calculated transient absorjDtion (pump-probe) signals for inner turning points at 3.5, 3.4 or 3.3 A.

Photodissociation of a linear triatomic such as [85, 86] or Hgl2 [8] to produce a vibrationally excited diatomic, or cage recombination of a photodissociated diatomic such as I2 [78, 81] are classic model simple systems for reaction dynamics. Here we discuss tire Hgl2—>HgI + I reaction studied by Hochstrasser and co-workers [87, 88 and 89]. 

M. Wulff, S. Bratos, A. Plech, R. Vuilleumier, F. Mirloup, M. Eorenc, Q. Kong, and H. Ihee,. Recombination of photodissociated iodine a time-resolved X-ray diffraction study. J. Chem. Phys. 124(3), 034501 (2006). 

M. Odelius, M. Kadi, J. Davidsson, and A. N. Tarnovsky, Photodissociation of diiodomethane in acetonitrile solution and fragment recombination into iso-diiodomethane studied with ab initio molecular dynamics simulations. J. Chem. Phys. 121(5), 2208-2214 (2004). 

In the simple steady-state model of Thaddeus,117 bare carbon cluster seed molecules with 12 carbon atoms are used with reaction 28 to produce large linear carbon clusters with sizeable abundances since it is assumed that the C +l ions produced in reaction 28 do not dissociate when they recombine with electrons if n >12. Rather, neutral Cn+1 clusters are formed which either photodissociate (slowly) or recombine further with C+. In this limited system, cluster growth would be catastrophic were it not for photodissociation. The large abundances of carbon clusters with 20 < n < 40 suggests that such molecules may well be the carriers of the well-known DIBs.118... 

Thus the quantum yield for acid production from triphenylsulfonium salts is 0.8 in solution and about 0.3 in the polymer 2 matrix. The difference between acid generating efficiencies in solution and film may be due in part to the large component of resin absorption. Resin excited state energy may not be efficiently transferred to the sulfonium salt. Furthermore a reduction in quantum yield is generally expected for a radical process carried out in a polymer matrix due to cage effects which prevent the escape of initially formed radicals and result in recombination (IS). However there are cases where little or no difference in quantum efficiency is noted for radical reactions in various media. Photodissociation of diacylperoxides is nearly as efficient in polystyrene below the glass transition point as in fluid solution (12). This case is similar to that of the present study since the dissociation involves a small molecule dispersed in a glassy polymer. 

A new type of photodissociation for p-nitrobenzyl 9,10-dimethoxyanthracene-2-sulphonate 164 has been reported to give 9,10-dimethoxy-anthracene-2-sulphonic acid 165, 9,10-dimethoxy-2-(p-nitrobenzyl)-anthracene 166 and p,p -dinitrobibenzyl101 (equation 81). It is suggested to occur from excited intramolecular electron transfer followed by radical ion decompositions and recombinations. 

Experiments on 1 -CO using benzene in place of CTAB were also done to examine the effects solvent and environment on the photodissociation. None were found. The photointermediates arrived at the same time, had the same peak wavelengths, extinction coefficients and band shape. In so far as the dynamics observed in these experiments are independent of CO pressure and since there is no detectable geminate CO recombination, it is reasonable to expect effects on the photodissociation due to solvation to be minimal as diffusion has not yet occurred on the time scale studied. 

Pathway I was observed for all the 02 complexes studied, strained or unstrained, as well as for the unstrained CO-complexes. This particular pathway is the same one observed in the photodissociation of the natural heme complexes (3,4) (HbCO, MbCO, HbO and MbO ) with the exception that there is no detectable geminate recombination to the limit of our experiment, 50 ps. Pathway II, observed for the strained-CO complexes, reveals the presence of a fifth intermediate X found early in the dissociation that is either absent or undetectable in the natural or synthetic heme complexes following pathway I. The kinetics associated with the evolution of these intermediates will be discussed shortly. First, it is appropriate to examine in some detail the experimental AA difference spectra of two representative complexes, 1 -CO and 1-ET-CO. A discussion of 1-ST-CO and l-ET-O is also included for comparative purposes. ... 

FIGURE 1.13 Experimental results (points) obtained from pump-probe (left) and transient phase grating (right) measurements of MbCO recombination as a function of time, t, following photodissociation. Solid lines were computed from equations (26) and (27), and the best-fit parameter values are listed in Table 1.2. (From Walther, M., Raicu, V., Ogilvie, J. R, Phillips, R., Kluger, R., and Miller, R. J. D. 2005. J. Phys. Chem. B 109 20605-11. With permission.)... 

FIGURE 1.14 Distribution of relaxation times for the recombination of photodissociated Mb-CO. 

The first reaction hlmed by X-rays was the recombination of photodissociated iodine in a CCLt solution [18, 19, 49]. As this reaction is considered a prototype chemical reaction, a considerable effort was made to study it. Experimental techniques such as linear [50-52] and nonlinear [53-55] spectroscopy were used, as well as theoretical methods such as quantum chemistry [56] and molecular dynamics simulation [57]. A fair understanding of the dissociation and recombination dynamics resulted. However, a fascinating challenge remained to film atomic motions during the reaction. This was done in the following way. 

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