Energy distribution, internal diatomic molecule

Energy in diatomic molecules is distributed among three kinds of internal excitations rotational, vibrational, and electronic. The rotational energy is described empirically by a truncated (denoted by. .. ) series in J( J + 1),... 

Even for molecules in the ground electronic state, our knowledge about cross sections is largely limited to the room-temperature condition, in which vibrational and rotational states are populated in a thermal distribution. Then, for a diatomic molecule, the ground vibrational state is predominantly populated. However, for a polyatomic molecule, normal modes with small quanta must be appreciably excited. For the full understanding of kinetics in plasma chemistry, it is important to assess the role of the internal energy of reactant molecules. 

Fig. 2.4. Illustration of the transitions from the neutral to the ionic state for a diatomic molecule. Electron ionization can be represented by a vertical line in this diagram. Thus, ions are formed in a vibrationally excited state if the intemuclear distance of the ionic state, ri, is longer than in the ground state, ro- Ions having internal energies below the dissociation energy D remain stable, whereas fragmentation wiU occur above. In few cases, ions are unstable, i.e., there is no minimum on their potential energy curve. The lower part schematically shows the distribution of Franck-Condon factors,/pc, for various transitions.

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