Thermal broadening of rotational state distributions

As discussed in the introduction to this chapter the final rotation of the photofragment reflects three sources overall rotation of the parent molecule, which is preserved in the electronic excitation process, internal bending or torsional motion in the electronic ground state, and final state interaction. The latter two sources have been analyzed in Sections 10.1-10.4. The influence of overall rotation will be discussed in this section. Its relative contribution increases gradually with the magnitude of the total angular momentum J and therefore it increases with the temperature of the molecular sample. As we will demonstrate below, the conversion of overall rotation of the parent molecule in the electronic ground state into 

When the 0-0 bond breaks, the angular dependence of the upper-state PES generates a torque —dV/dp as discussed in Section 10.2.1. In analogy with (5.4f), the corresponding Hamilton equations for the evolution of ji and 32 are 

In the macroscopic picture we have to average over J, respectively j i and jj,2 according to the Boltzmann distribution 

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As in Chapter 9 we discuss first the elastic limit (no exit channel excitation) in Section 10.1 and subsequently the more interesting inelastic case in Section 10.2. In Section 10.3 we consider the decay of long-lived resonance states and the impact of exit channel dynamics on the product distributions. A simple approximation, the so-called impulsive model, which is frequently employed to analyze experimental distributions in the absence of a PES, is discussed critically in Section 10.4. The chapter ends with a more qualitative assessment of thermal broadening of rotational state distributions in Section 10.5... 

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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