Collisional Transfer of Electronic Energy

Collisions may also transfer electronic energy. For instance, the electronic energy of an excited atom A or molecule M can be converted in a collision with a partner B into translational energy E kin or, with higher probability, into internal energy of B [965]. 

For collisions at thermal energies the collision time Tcoii = d/vis long compared to the time for an electronic transition. The interaction V(A, B) or y(M, B) can then be described by a potential. Assume that the two potential curves V(A/, B) and V Ak, B) cross at the energy E Rq) (Fig. 8.11). If the relative kinetic energy of the collision partners is sufficiently high to reach the crossing point, the collision pair may jump over to the other potential curve [999]. In Fig. 8.11, for instance, a collision A/ -h B can lead to electronic excitation /) - k) if kin E2, while for a collisional deexcitation A ) /) only the kinetic energy E kin E is required. 

The cross sections of electronic energy transfer A +B A + B + A kin are particularly large in cases of energy resonance, which means AE(A — A) A (B — B) AE kin kT. A well-known example is the collisional excitation of Ne atoms by metastable He atoms, which represents the main excitation mechanism in the HeNe laser. 

The experimental proof for such electronic energy transfer (E E transfer) is based on the selective excitation of A by a laser and the spectrally resolved detection of the fluorescence from B [1000, 1001]. 

In collisions between excited atoms and molecules either the atom A or the molecule M may be electronically excited. Although the two cases 

For collisions at thermal energies the collision time Tcoll = d/v is long compared to the time for an electronic transition. The interaction V(A, B) or V(M, B) can then be described by a potential. Assume that the two potential curves V(Aj, B) and V(AjjB) cross at the energy E(Rg) (Fig. 13.11). 

If the relative kinetic energy of the collision partners is sufficiently high to reach the crossing point, the collision pair may jump over to the other potential curve [13.37]. In Fig.13.11, for instance, a collision Aj+B can lead to electronic excitation i) — k), if Ej, while for a collisional deexcitation k) — i) only the kinetic energy E j Ej is required. 

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As before, due to conservation of energy in the elastic Coulomb collisions, the total collisional transfer of energy between electrons and ions QT must fulfill ... 

Arrowsmith et al used the crossed beam reaction F+Na— NaF+Na (3 P) to study radiative transfer and electronic energy transfer (E — E, V) in the Na (3 P)-1-NajCX S ) system. Previous studies of the Na2 system have utilized high-pressure cells or heat pipes in which radiation trapping is strong and Na + Na2 collisional energy transfer dominates. Time-resolved emission, following pulsed dye-laser excitation, has been used by Husain and his coworkers in a systematic survey of the excited-state behaviour of Mg(3 Pj), Ca(4 P,), and Sr(5 Pj). Dye-laser excitation of Mg vapour at 457.1 nm resulted in the observation of slow spontaneous emission from Mg(3 P,) which... 

The shape of the decay profile of an excited donor is determined, amongst other parameters, by the distribution profile of the surrounding acceptors. Thus, the classical three dimensional Forster equation for non-collisional, one step electronic energy transfer (ET), had to be modified for the case of a two dimensional arrangement of donors and acceptors (35). This has been generalized recently, to include not only two and three dimensional acceptor distributions, but also D-dimensional distributions (36) ... 

The mechanism by which a vibrationally excited species relaxes to the nearest electronic state involves a t ransfer of its excess energy to other atoms in the system through a series of collisions. As noted, this process takes place at an enormous speed. Relaxation from one electronic state to another can also occur by collisional transfer of energy, but the rate of this process is slow enough that relaxation by photon release is favored. 

In this chapter we shall first outline the basic concepts of the various mechanisms for energy redistribution, followed by a very brief overview of collisional intennoleciilar energy transfer in chemical reaction systems. The main part of this chapter deals with true intramolecular energy transfer in polyatomic molecules, which is a topic of particular current importance. Stress is placed on basic ideas and concepts. It is not the aim of this chapter to review in detail the vast literature on this topic we refer to some of the key reviews and books [U, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, and 32] and the literature cited therein. These cover a variety of aspects of tire topic and fiirther, more detailed references will be given tliroiighoiit this review. We should mention here the energy transfer processes, which are of fiindamental importance but are beyond the scope of this review, such as electronic energy transfer by mechanisms of the Forster type [33, 34] and related processes. 

The next important phenomena that the result of supramolecular effect are the concentration and proximity effects concerning the components of analytical reaction, even through they are considerably different in hydrophobicity, charge of the species, complexing or collisional type of interaction. The concentration and proximity effects determine the equilibrium of analytical reaction, the efficiencies of intramolecular or intermolecular electronic energy or electron transfer and as a result the sensitivity of analytical reactions. 

Excited states can be formed by a variety of processes, of which the important ones are photolysis (light absorption), impact of electrons or heavy particles (radiolysis), and, especially in the condensed phase, ion neutralization. To these may be added processes such as energy transfer, dissociation from super-excited and ionized states, thermal processes, and chemical reaction. Following Brocklehurst [14], it is instructive to consider some of the direct processes giving excited states and their respective inverses. Thus luminescence is the inverse of light absorption, super-elastic collision is the inverse of charged particle impact excitation, and collisional deactivation is the inverse of the thermal process, etc. 

In the preceding we have reviewed collisional deactivation of ions in thermal or low-translational-energy collisions. An interesting phenomenon, however, is the observation of E-T and V-T transfers in ion-neutral collisions at translational energies of several hundred electron volts. These transfers cause the so-called superelastic peaks observed in translational-energy measurements of the scattered ions. Electronic to translational energy transfer was observed in collisions of 3.5-keV N+ with rare-gas atoms and with 02.255 256... 

COLLISIONAL ENERGY-TRANSFER SPECTROSCOPY WITH LASER-EXCITED ATOMS IN CROSSED ATOM BEAMS A NEW METHOD FOR INVESTIGATING THE QUENCHING OF ELECTRONICALLY EXCITED ATOMS BY MOLECULES... 

Collisional Energy-transfer Spectroscopy with Laser-excited Atoms in Crossed Atom Beams A New Method for Investigating the Quenching of Electronically Excited Atoms by Molecules... 

The increased cross sections for these three states are attributed to resonant electronic to vibrational energy transfer. Table 11.1 identifies the three atomic transitions and the resonant molecular transitions in CH4 and CD4. For example the rapid depopulation of the Na 7s state by CD4 is attributed to the Na 7s — 5d transition. To verify this assignment the cross section for the 7s — 5d transfer was measured for both CH4and CD4 by observing the 5d-3p fluorescence as well as the 7s-3p fluorescence. The 7s — 5d cross sections are 215 A2 for CD4 and 15 A2 for CH4. As shown by Fig. 11.16, the 7s CD4 cross sections is —240 A2 above the smooth dotted curve in good agreement with the 7s — 5d cross section. Similar confirmations were carried out for the other two resonant collisional transfers. 

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