Reactions of Electronically Excited Noble Gas Atoms

Electronically excited atoms and molecules occupy an imptntant position among reactive chemical spedes. They are the source of diemilumincsoence and bioluminescence and their reactions are crucial to the dimoical balance of the atmosphere. They are central intermediates in photodionistry and the active species in lasers. They are readily observed by the li t which they emit or the reactions whidi they induce in parting with their excitation enogy. 

Specific information is restricted at present to excited states of small molecules and, in particular, to atoms, and it is here, as evid )ced by recent reviews, - that the greatest advances towards understanding the chranistry of electronically excited species are taking place. 

These reviews are notable in that the chemistry of the noble gases is omitted completely or Is relegated to a brief final section. However, the contrast between the reactivity of ground and excited state species is nowhere more marked than in the noble gases for the excited states, reaction rates close to unit collision efficiency are the observed norm. 

The states of the noble gases, which have received by far the most study and which are the subject of this review, arise from excitation of an electron from the closed shell to the lowest vacant s orbital and thus comprise the IrZy S and states of He and the np (ji + l)i and Pi states of Ne, Ar, Kr, and Xe, for which 

Donovan and H. M. Gillespie, in Reaction Kinetics , ed. P. G. Ashmore (Specialist Periodical Reports), The Chemical Society, London 1975, Vol. 1, p. 14. 

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This alkalilike behavior of metastable noble-gas atoms effectively transforms the excitation energy of the metastable noble-gas atom into electronic energy of a rare-gas halide molecule with large reaction cross section. Because the electronically excited noble-gas halides have short radiative lifetimes and the ground-state noble-gas halides are not strongly bound, the process of formation of electronically excited noble-gas halides from metastable noble-gas atoms has been shown to be ideal for the operation of the electronic transition laser and has been successfully used in high-efficiency rare-gas halide lasers in recent years.21"23... 

The prime source of interest in these species is the fact that collisional electronic quenching of the excited noble gas atoms can, and is observed to, occur by a variety of channels, by virtue of the large excitation energies involved these channels include collisional excitation and deactivation, two- and three-body combination, chemi-ionization, electronic-to-electronic energy transfer, dissociation, dissociative excitation, and chemiluminescent reaction. 

Reaction Mechanisms in the Noble Gas Afleisfows.— The above survey of the electronic states of the noble gas molecules allows discussion of the kinetic data accumulated on the pure noble gas afterglows. These data comprise total quenching rate constants for excited atomic states, whidi are summarized in Table 3, and comparisons of the kinetics of atomic emissions with those of the first and second continua, from vdiich attempts have been made to elucidate the mechanisms for the formation of the emitting molecular states. 

Collisional-electron-release reactions of long-lived, high-lying states of noble-gas atoms and molecular hydrogen with a variety of molecules have been reported by Hotop " and Hotop and Niehaus/ In these experiments, apparatus similar to that shown schematically in Fig. 4 was used, with the intensity of the ions X being measured as a function of the energy of the exciting electrons. In all cases, the excitation... 

The c n a M fluorescence or c n<<-a excitation (LIF) spectra of NH or ND are presented in numerous publications which report on various formation processes of NH (ND) in the c 11 or a states, such as formation by VUV photolysis of NH3, HN3, HNCO, CH3NH2, or CH2NHCH2, by electron impact on NH3, HN3, HNCO, N2H4, CH3NH2, or CH2NHCH2, in collisions of metastable noble-gas atoms with NH3, HN3, or HNCO, and in chemical reactions of H atoms with NF2 or N3. The rotationally resolved c A spectra... 

A chemical reaction between an atom (X = He, Li+, Be2+, B3+, C4+) in various electronic states and a proton to form an XH+ molecule has been mimicked by a collision between X and H+. Maximum hardness and minimum polarizability values characterize a favorable dynamical process. Since a system becomes more reactive on electronic excitation, it becomes softer and more polarizable. A proton being a hard acid would prefer the most to react with X in its ground state according to HSAB principle in a dynamical context, and the preference will gradually decrease with electronic excitation. This fact is corroborated in this QFDFT [17,47] study on protonation [64] through a decrease in the maximum hardness value and an increase in the minimum polarizability value on excitation. Reactivity of noble gas elements toward protonation increases as He < Ne < Ar < Kr < Xe. 

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