Superexcitation states

The molecular time scale may be taken to start at 10 14 s following energy absorption (see Sect. 2.2.3). At this time, H atoms begin to vibrate and most OH in water radiolysis is formed through the ion-molecule reaction H20+ + H20 H30+ + OH. Dissociation of excited and superexcited states, including delayed ionization, also should occur in this time scale. The subexcitation electron has not yet thermalized, but it should have established a quasi-stationary spectrum its mean energy is expected to be around a few tenths of an eV. 

Platzman (1962a) has emphasized the implications of superexcited states in radiation chemistry. On the whole, his conjectures have been proved correct. Figure 4.2, using the data of Haddad and Samson (1986), shows the ionization efficiency in the gas phase of water. It shows that T] starts with a value of 0.4 at the... 

Excited states may be formed by (1) light absorption (photolysis) (2) direct excitation by the impact of charged particles (3) ion neutralization (4) dissociation from ionized or superexcited states and (5) energy transfer. Some of these have been alluded to in Sect. 3.2. Other mechanisms include thermal processes (flames) and chemical reaction (chemiluminescence). It is instructive to consider some of the processes generating excited states and their inverses. Figure 4.3 illustrates this following Brocklehurst (1970) luminescence (l— 2)... 

In Eq. (4.18), it is implicitly assumed that the ionization is a direct, one-electron process that is, the contribution of superexcited states to ionization is not included. The latter process is indirect and essentially of a two-electron nature. When the energy loss is much larger than the ionization potential, however, ionization is almost a certainty. For high energies of the secondary electron, Eq. (4.18) approaches the Rutherford cross section, or the Mott cross section if the incident particle is an electron. 

Following Platzman (1967), Magee and Mozumder (1973) estimate the total ionization yield in water vapor as 3.48. The yield of superexcited states that do not autoionize in the gas phase is 0.92. Assuming that all of these did autoion-ize in the liquid, we would get 4.4 as the total ionization yield. This figure is within the experimental limits of eh yield at 100 ps, but it is less than the total experimental ionization yield by about 1. The assumption of lower ionization potential in the liquid does not remove this difficulty, as the total yield of excited states in the gas phase below the ionization limit is only 0.54. 

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... 

Next we must consider the precise meaning to be attached to the term ionization in the condensed phase. Unlike the situation in an irradiated gas, the electron liberated by ionization of a molecule loses energy rapidly by colliding with other molecules and may have insufficient kinetic energy to escape the field of its parent ion. In this case we may justifiably speak of a superexcited state not to be found in gases. 

Superexcited states of water (32) would certainly provide as much energy for Reaction h as charge neutralization. Superexcited states are not required, however, for Reaction h to proceed, since charge neutralization undoubtedly produces HsO. Assume Reaction h to be reversible. 

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). 

Regarding the formation of subexcitation electrons, many authors consider only two possibilities they assume that each subexcitation electron either has been ejected during ionization (and happened to have the energy below fcco01), or it is one of the fast electrons that has slowed down to energies below hw(n. However, there may be other possible ways for subexcitation electrons to be formed. One of them is via a decay of a superexcitation state according to the ionization channel. Such a process may occur both in gaseous and in condensed media, and the majority of electrons produced in this case are the subexcitation ones. 

If the photon energy, hv, coincides with a superexcited state, AB, autoionization contributes extra intensity to the direct ionization signal. As a result, vibrational intensity distributions will not be described by Franck-Condon factors (see, for example, Caprace et at, 1976). These vibrational intensity anomalies may be explained similarly to the effect of perturbations on intensity borrowing (Section 6.2). 

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