Photofragment Branching Ratios for Photodissociation

To illustrate the significance of measurements of internal state branching ratios, we will turn once again to the example of the photodissociation of the hydrogen halides, HX. The fine structure ratio is the branching ratio of X /X populations 2Pi/2/2P3/2- In the non-relativistic adiabatic representation, this branching ratio would be predicted to be zero because the only case (a) basis state which has a non-zero transition moment from the X1E+ state is the 1IIi state which correlates adiabatically with the X(2P3/2) +H(2S) separated atom limit. However, in the more realistic relativistic adiabatic representation, Afl = 0 3E/, 1n3 3ni, and 3ni 3Ei spin-orbit matrix elements 

Le Roy, et al, (2002) have reviewed all of the different types of experimental observations and theoretical calculations for HI. By an empirical analysis, they have shown that, because in HI the spin-orbit interaction is especially important, the adiabatic relativistic potential curves can explain all of the experimental data without introducing residual nonadiabatic coupling. For the lighter halogen hydrides, the J = 1/2 J = 3/2 branching ratio can be obtained from the solution of inhomogeneous coupled equations with a source term representing the initial vibrational wavefunction multiplied by the electronic transition moment (Band, et al., 1981). These calculations are based on adiabatic electronic (or diabatic relativistic) potential curves (see, for example, for HC1, Alexander, et al., 1993 and for HBr, Peoux, et al., 1997). 

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