Sources of rotational excitation

Three major sources contribute to the rotational excitation of the fragment molecule  

1) Overall rotation of the parent molecule in the electronic ground state. 

2) Bending or torsional vibration within the parent molecule. 

3) The torque generated by the anisotropy of the upper-state potential energy surface, i.e., the so-called exit channel dynamics. 

Rotational excitation as a consequence of overall rotation of the parent molecule before the photon is absorbed does not reveal much dynamical information about the fragmentation process. It generally increases with the magnitude of the total angular momentum J and thus increases with the temperature of the molecular sample. In order to minimize the thermal effect and to isolate the dynamical aspects of photodissociation, experiments are preferably performed in a supersonic molecular beam whose rotational temperature is less than 50 K or so. Broadening of final rotational state distributions as a result of initial rotation of the parent molecule will be discussed at the end of this chapter. 

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