Ionization Efficiency Superexcited States

The distinction between photoabsorption and photoionization is important, particularly near threshold, where the probability that ionization will not occur upon photoabsorption is significant. Thus, the ionization efficiency is defined by TJ. = a. / T, where cr is the photoionization cross section and Ta, the photoabsorption cross section, is related to the absorption coefficient a by a = n(7, n being the absorber density. 

FIGURE 4.2 Ionization efficiency as a function of photon energy in the gas phase of water. Data from Haddad and Samson (1986), with permission of Am. Inst. Phys.  

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The above considerations need relativistic correction at v c, which may be performed in a straightforward manner. More importantly, Eq. (10) assumes that the ionization process is direct, i.e., once a state above the ionization potential is reached, ionization occurs with a certainty. Platzman [25] points out that in molecules, this is not necessarily so and superexcited states with energy exceeding the ionization potential may exist, which will dissociate into neutral fragments with a certain probability. For example, in water in the gas phase, ionization occurs with a sharp threshold at the ionization potential (I.P.) = 12.6 eV, but only with an efficiency of 0.4. Beyond the I.P., the ionization... 

Analyzing the data on molecular gases irradiated by vacuum UV emission,60 Platzman2 has noted that for certain gases the probability of ionization 77 (Eph) is smaller than unity when Eph exceeds Ix by 10 eV or more. This was confirmed in his subsequent study of molecule-noble-gas mixture,61 done in collaboration with Jesse. They have also observed an isotopic effect the substitution of deuterium for hydrogen increases the ionization probability. Platzman thus concluded that in such discrete states with E>lx the predissociation efficiently competes with autoionization. Platzman has named them the superexcitation states (SES). The SES were discussed in a special issue of Radiation Research62 (see also Refs. 25 and 63). 

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