Anisotropic Photodissociation

It was at this stage that a family of new experimental methods burst upon the scene, developed by Pimentel, Berry, Bersohn, Zaie, Wilson, and their co-workers these were photodissociation laser spectroscopy, anisotropic photodissodation, and photofragment spectroscopy. Each of the methods can provide an insist into the molecular dynamics of photodissodation, and the experimental data can suggest the most appropriate model to adopt for any particular molecule. The literature survey be ns from the latter half of the 1960 s, the time of thdr advent. 

Anisotropic Photodissodation.—By its very nature a parallel beam of light is anisotropic, and whenever preferred orientations for dissociation of a target 

For the simplest case of a diatomic molecule dissociating instantaneously to products of high translational energy, the factor c — 2, but if the molecule has a finite average lifetime, r, then 

A much more sophisticated version of the photolysis mapping technique was developed later by Bersohn and co-workers, who replaced the thin film 

Igure ixmoratory angular aisiriouiion oj jmmujragmaun jrom (Reproduced by permission from /. Chem. Pkys., 1974, 61,4408) 

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Hatogns.—The techniques of anisotropic photodissociation ( photolysis mapping )... 

The studies of Hasinoff [53] on the recombination rate of carbon monoxide and the heme units after photodissociation of carboxy ferrous microperioxidase come close to satisfying the requirements for observing the effects of anisotropic reactivity and rotational diffusion on the rate of a translational diffusion-limited reaction. In Chap. 2, Sect. 5.6, the details of this study were briefly mentioned. Hasinoff found that the rate of recombination was substantially diffusion-limited in all three aqueous solvents used at 260 K, but at higher temperatures, the rate of reaction of the encounter pair, feact, was a significant factor in determining the overall rate of recombination (see Fig. 9). The observed rate coefficient of recombination, feobs, was separated into the rate coefficient of diffusive formation of encounter pairs, feD, and the rate coefficient of reaction of encounter pairs, fcact, with the Collins and Kimball expression, eqn. (26)... 

The A-state potential is, in general, quite anisotropic, especially for small angles where the two hydrogen atoms come close and repel each other. But near 104°, where the wavepacket in the photodissociation of the lowest bending state starts, it is almost isotropic (dV/da ss 0) with the result that no torque excites OH while the molecule breaks apart. As a consequence, the photodissociation of H2O via the A state yields... 

Up to now we have exclusively considered the scalar properties of the photodissociation products, namely the vibrational and rotational state distributions of diatomic fragments, i.e., the energy that goes into the various degrees of freedom. Although the complete analysis of final state distributions reveals a lot of information about the bond breaking and the forces in the exit channel, it does not completely specify the dissociation process. Photodissociation is by its very nature an anisotropic process — the polarization of the electric field Eo defines a unique direction relative to which all vectors describing both the parent molecule and the products can be measured. These are ... 

Several papers have been written on the subject of analyzing the anisotropic distribution of v and j vectors of molecules produced by a photodissociation reaction [17,36-42]. The most satisfying scheme is the renormalized bipolar moment analysis proposed by Dixon [18]. The basic premise is to use bipolar harmonics as the basis set to describe the angular distribution of a single recoil velocity v and fragment angular momentum j ... 

The rapid predissociation of the excited 082( 52) formed in the 193-nm excitation is believed to be the mechanism for the formation of CS + S. The Doppler spectroscopic experiment shows anisotropy in dissociation and provides a lifetime of 1-2ps for the excited 082( 52) state [94]. The TOF study of Frey and Felder [83] indicates that the 193-nm photodissociation C82 is anisotropic and that the parameters for processes 11 and 12 are different. 

Takayanagi, M., H. Okumura, et al. (2CX37). Anisotropic structural relaxation and its correlation with the excess energy diffusion in the incipient process of photodissociated MbCO High-resolution analysis via ensemble perturbation method. Journal of Physical Chemistry B 111(4) 864—869. 

Figure 1 Block diagram of experimental configuration used when obtaining angular distributions of photofragments. The polarizer is inserted into the light axis and the guatb upole is noted to the mass of interest. Rotation of the u.v. polarizer by a stepper motor produces a variation in the signal amplitude if the molecular photodissociation is anisotropic (Reproduced by permission from Rev. Sci. Instr., 1974, 45, 730)...

Aliphatic Carbonyl Compounds.—The photodissociation of formaldehyde, acetone, acetaldehyde, and propionaldehyde was studied by Solomon, Jonah, Chandra, and Bersohn, using the photolysis mapping technique. " For each molecule, broadband excitation in the (/> -> n ) band led to an anisotropic distribution of the... 

For example, photodissociation is an anisotropic process. The polarization of the electric field of the photolysis laser defines a spatial axis, to which the vectors describing both the parent molecule and the products can be correlated (see Figure 1.5). 

Figure 22.5 shows a pictorial representation of the so-called two-vector correlation, both in photodissociation (half-collisions) and in atom-exchange reactions (full collisions). The important point to consider is that photodissociation is an anisotropic process in which the polarization of the electric field Sp of the photolysis laser defines a direction with respect to which the vector describing both products and parent molecule can be correlated. As a consequence, one can measure and analyse the correlation between the parent transition dipole moment fi and the recoil photofragment velocity vector, i.e. the v correlation. Thus, the angular distribution of the photofragments I 6) can be described in the form (Zare, 1972)... 

Reaction gas-phase collisions should generate Newton spheres with isotropic distributions. However, very often these surface patterns are anisotropic (as the one shown in the figure) due to the existence of some directionality in the process. In photodissociation, this is often due to the use of a linearly polarized laser, which acts as the reference axis. In a bimolecular collision (see below), carried out in a crossed-beam experiment, the relative velocity vector introduces a reference axis to which the directional properties of the reaction products are referenced. 

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