Superexcited molecules

The recent stage for the study of the spectroscopy and dynamics of the superexcited molecules was comprehensively discussed in Ref. [5], and let us stress that the formation of the superexcited molecules and their decay processes produce a wide range of interesting structures in the c, c, and Cd curves as a function of the incident photon energy. 

Hatano, Y. Dissociation dynamics of superexcited molecules. In The Physics of Electronic and Atomic Collisions, Dube, L.J. Mitchell, J.B.A. McConkey, J.W. Brion, C.E. AIP Press New York, 1995 67 p. 

The spectroscopy and dynamics of superexcited molecules, including the hydrogen atom and molecule, are reviewed in the literature. Synchrotron radiation has been used in the study of photodissociation of a highly excited H2 molecules into two excited H atoms. ... 

The relative product distribution produced by photons of 10-e.v. energy in the krypton resonance radiation photolysis will also be taken as representative of excited neutral decomposition induced by electron impact at energies exceeding the ionization potential of ethyl chloride (10.9 e.v.). That is, the contribution to the radiolysis products from excited neutral molecule decomposition will be assumed to have the same relative distribution as that observed in the photolytic decomposition. While superexcited molecules will also be produced in the radiolysis, there is considerable evidence to support the view that their modes... 

In summary, both the conventional cascading (70) and the fcd measurement (94) techniques appear to hold promise for future unimolecular energy transfer experiments involving F-labeled superexcited molecules in a variety of host gases. 

Electrons with 70 to lOOeV energy are usually used in order to increase the sensitivity and to induce fragmentation. Following a proposal by Hurst, Platzman has shown that with electrons of such a high energy part of the molecules are not directly ionized, but are superexcited . These superexcited molecules either dissociate into two neutral fragments or pre-ionize to a molecular ion. Therefore there are at least two distinct mechanisms to form molecular ions. The simplified picture of a well-defined molecular ion, as it is often considered in mass spectrometry, is thus by no means justified. 

Optical emission is a result of electron impact excitation or dissociation, or ion impact. As an example, the SiH radical is formed by electron impact on silane, which yields an excited or superexcited silane molecule (e + SiHa SiH -t-e ). The excess energy in SiH is released into the fragments SiH SiH -I-H2 + H. The excited SiH fragments spontaneously release their excess energy by emitting a photon at a wavelength around 414 nm. the bluish color of the silane discharge. In addition, the emission lines from Si. H, and H have also been observed at 288, 656, and 602 nm, respectively. 

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. 

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. 

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). 

Cyclohexane (ionization energy, 9.88 e.v.) has been photolyzed at 1236 A. (10.0 e.v.) and 1048-1067 A. (11.6-11.8 e.v.). All major products have been determined in the absence of free radical scavengers and in the presence of added NO and 02. In addition, cyclo-C6Di2-H2S mixtures were irradiated to determine the free radicals formed in the decomposition of the superexcited cyclohexane molecule. Accurate quantum yield determinations have been made, both by use of saturation current measurements during photolysis and by chemical actinometry. It is seen that an increase in photon energy results in an increase in the relative importance of processes producing H atoms or alkyl radicals while the yields of products attributed to "molecular elimination processes, such as the formation of molecular hydrogen, diminish. Similar trends are seen in the solid-phase photolysis. The relative importances of the various primary processes are derived. The application of this information to the interpretation of the radiolysis of cyclohexane is discussed. 

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