Alignment of the Photofragment

An atom (or a molecule) is said to be polarized with respect to a particular coordinate system (e.g., laboratory or parent molecule body fixed system) if any of its internal angular momenta are either aligned or oriented. The limit of a non-polarized fragment corresponds to equal populations in all the Mj sublevels of a given J. The system exhibits alignment when the populations are unequal in the Mj levels for a given J but cylindrical symmetry is preserved by equal populations in +Mj and —Mj levels. Orientation corresponds to the case where the +Mj and —Mj populations are not equal for any one of the Mj values (see Fig. 7.11). 

The polarization can be expressed in tensorial form (see Zare, 1988, p. 226-242). Since for any value of J, there are 2J + 1 Mj components, a complete description of the Mj populations and phases within a given J requires information about (2 J + 1) x (2 J + 1) components of the density matrix, pj (see Section 9.1.3), or independent observables. It is convenient to express these quantitites in terms of tensorial components (Section 3.4.5). For J = 0, there 

Measurement of the degree of polarization of the fluorescence from an excited atomic fragment can give more information than f3 about the relative absorption amplitudes for excitation to different dissociative states. The alignment of photofragment emission can give information above the relative phases of the transition moments for photoexcitation of the parent molecule (3 samples only the squared transition moments), due to interference effects (Vigue, 

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