Angular distribution, of photofragments

Many readers will have some familiarity with the standard expressions for the angular distribution of photofragments ejected from a randomly oriented (gas-phase) molecule by perfectly polarized light ... 

A scheme elucidating the interconnection between the linearly polarized light vector E which determines the laboratory 2-axis, and d, v/ is given in Fig. 6.1 for a parallel transition type in a parent diatomic molecule AB (see Fig. 1.5(a, c)), where r d leads to v/ d. Using (2.1) we obtain the desired probability W (6f) of angular distribution of photofragments (Fig. 6.1 shows only one of them on the right) as... 

The probability of angular distribution of photofragments has the same origin as the probability G(0, (p) in (2.8), but now it is a transition from a bonded state into a non-bonded one. 

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)...

Figure 3 Laboratory angular distribution of photofragments from 1-bromo-naphthaiene (Reproduced by permission from J. Chem. Phys., 1974,61,4408)...

Thus, by the late 1970 s the "basics" of reagent state selectivity and product state specificity in bimolecular reactions were well established, both experimentally and theoreticaUy [27]. In addition to the above-mentioned extensive work on reactive (as well as inelastic) collisions, the chemical dynamics of "half-collisions", i.e. laser-induced photofragmentation [28], had already reached a relatively mature state by the end of the 1970 s. The effect of laser polarization on the angular distribution of photofragments could be expressed by the simple equation... 

Another commonly encountered vector correlation is the angular distribution of photofragments. This correlation involves the correlation between the recoil velocity v of the photoffagment with respect to the electric vector E of a beam of light that causes the photolysis. Alrea in Chapter 7 we used the angular distribution of the general form... 

The angular distribution of photofragments depends particularly on the symmetry of the electronic transition and the direction of the photol) is laser polarization vector. The angular distribution of photofragments is usually described by the anisotropy parameter / . In particular, the recoil distribution, which is related to the velocity of the fragment v and the polarization vector of the electromagnetic field E, is given by... 

Yang, S.-C. and Bersohn, R. (1974). Theory of the angular distribution of molecular photofragments, J. Chem. Phys. 61, 4400-4407. 

Figure 2-1. A schematic diagram of the apparatus used to record photofragment angular distributions of complexes. An F-center laser is used to pump transitions in the parent complex which leads to dissociation. A second F-center laser is used as a probe to state selectively detect the fragments. The electrodes are used to orient the parent molecules prior to excitation.

Pe er, A., Shapiro, M. and Balint-Kurti, G.G. (1999) The breaking of forward-backvnrd symmetry in the angular distribution of m -selected photofragments, J. Chem. Phys. 110, 11928-11935. 

The properties measured in photofragmentation experiments (photodissociation or photoionization) may be divided into scalar quantities, such as photofragmentation cross sections and branching ratios, and vectorial quantities, such as angular distributions of the photofragments and their alignment or orientation. Moreover, these properties may be measured independently or in coincidence where two or more properties associated with either or both photofragmentation products are measured simultaneously, event-by-event. 

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