Example Photodissociation of H2O A

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 

Before concluding this section it is instructive to draw attention to the following point. It is common practice to compare measured rotational state distributions to the Boltzmann distribution 

T which is then compared to the temperature of the molecular sample. 

Inserting the appropriate parameters yields a temperature of approximately 400 K in reasonable accord with the average of the two measured values. A linear Boltzmann plot of the final rotational state distribution usually indicates that the torque in the exit channel is weak. 

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