Collisions of the second kind

A mechanism by which electronic energy is sometimes made available for chemical purposes is that which occurs in the so-called Stoss zweiter Art , or collision of the second kind . Cario and Franck found that on exposure of a mixture of mercury vapour and hydrogen to the mercury line 2536-7 A, the absorbed light stimulated the mercury atoms electronically, and that the excited mercury atoms were able on collision with hydrogen molecules to resolve them into atoms. The atomic hydrogen so produced was detected by its great chemical activity. A number of other examples of this phenomenon were found Z. Phyaik, 1922, 21, 161. 

Energy transfers between molecules are indeed very specific in fact the transfer of internal energy in a so-called collision of the second kind is something rather... 

The selection rules are not rigorously obeyed. In atoms that do not exhibit Russell-Saunders coupling, the quantum numbers L and S are not defined. Even in atoms that do have this type of coupling, forbidden transitions are merely of lower probability than allowed ones, and they may occur from a state from which no transitions are allowed by the rules, if conditions are such that collisions of the second kind do not remove the atom from the initial state before it radiates (e.g., at extremely low pressures). 

Astrophysical Observations. In spite of the suggestion that O( Z)) reacts with 02 to form ozone, there is no way either in the work of DeMore33 or in the older work of Groth or of Noyes to distinguish this reaction from a collision of the second kind, reaction (4), followed by... 

Further evidence relating to collisions of the second kind is to be found in astrophysical observations. The emission of a red doublet (6300.2 and 6363.9 A.) in the glow of the night sky corresponds to the transition... 

Figure 8. Energies of the initial (A + B) and the final (A + state in a collision of the second kind between atoms A and B, crossing one another with a change in interatomic distance.

The energy transfer back to the electrons by electron collisions of the second kind with excited atoms has been taken into account (Winkler eta/., 1983). These deexciting collisions can have a large impact on the population of the isotropic distribution at higher energies and become important at low electric fields and when electronically excited states are sufficiently populated. 

A collision of the second kind, illustrated by the general reaction... 

Collisions where excited neutrals lose energy through collisions without emission of radiation (collisions of the second kind)... 

K. Collisions of the second kind. Electronically excited (2.1 eV) Na atoms are quenched efficiently by O2, Na -I- O2 Na -I- O2 and much less so by the rare gases, (a) How would you demonstrate this difference in efficiency by a frequency domain experiment, (b) Sketch the relevant potentials for the quenching of Na by O2- The electron affinity of O2 is 0.44 eV and the ionization potential of Na is 5.8 eV... 

L. Collisions of the second kind. The one-dimensional potentials that you sketched in Problem K overlook the role of the vibration of 02- Examine this point in at least one of two ways, (a) Efraw potential energy surfaces that are functions of both the Na—O2 distance R and the 0—0 distance r. (b) Draw onedimensional potential energy curves where for each curve both the electronic state of Na and the vibrational state of O2 are fixed (such potentials are diabatic in two senses). Discuss the dynamics of the quenching and show why we expect that O2 will be vibrationally excited after a quenching collision. The final vibrational state distribution in processes in which an electronic state change occurs are often quite close to the prior limit. Can your considerations above rationalize why this will be so ... 

The field began in the 1920s when Cario and Franck discovered how these so-called collisions of the second kind could be put to practical use, e.g., the Hg -sensitized activation of paraffins. This is due to initiation of chain reactions by radical formation,... 

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