B Molecular Processes

The energy transferred by the photon or electron to the molecule may be lost by the latter in a number of different ways. If this energy is lower than the ionization threshold, the usual pathways are (1) reradiation or (2) dissociation into neutral fragments or charged fragments (from ion-pair processes) in other words  

If the energy is higher than the ionization threshold, the preceding processes may still occur, but we also have the possibility of two types of ionization process  

In energy transfers above the ionization threshold, it is usual for several different ionization processes to occur with probabilities depending on E. The measured optical oscillator strength for absorption is thus a sum corresponding to a variety of different processes. Denoting this total optical oscillator (ionization potential) strength by df(0)/dE, we have, for 

The individual partial oscillator strengths df (0)/dE for each ionization process are individual functions of E. It is usual to define the branching ratio bt as 

Characterization of the neutral processes is far more difficult, and little information is available at present. However, studies on some simple molecules (see Section V) have indicated that the ionization efficiency approaches unity quite rapidly as the energy loss increases above threshold, suggesting that, except where transitions to Rydberg states just below a new ionization threshold are significant, the dominant mode of energy loss in the far UV is by ionization often accompanied by molecular fragmentation. 

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Fig. la, b. Molecular processes of infrared absorption (a) and Raman scattering (b). m) and n) are two quantum states of a given vibration in the electronic ground state, and e) is an electronically excited state, which can be a virtual state... 

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