Charge transfer harpoon mechanism

This experiment shows that the harpoon mechanism can be observed through the spectroscopic observation of the charge transfer state Hg+Cl2. Moreover, it shows the interest of starting from a fixed geometry to understand the spectroscopy of the reactive collision complex. 

Alkaline earth metal atoms have fairly low ionization potentials, as have alkali metal atoms (e.g., 5.21 and 5.14 eV for barium and sodium, respectively [89]). Hence the reactions of alkaline earth metal atoms with oxidizing molecules are also expected to be initiated by an electron transfer and should follow the harpoon mechanism. However, alkali metal atoms are monovalent species, whereas alkaline earth metal atoms have two valence electrons. Hence peculiarities are to be expected in the alkaline earth metal reaction dynamics, especially when doubly charged products such as BaO are to be formed [90]. The second valence electron also opens up the possibility of chemiluminescent reactions, which are largely absent in alkali metal atom reactions [91, 92]. The second electron causes the existence of low-lying excited states in the product. 

This latter mechanism can be ruled out in a number of ways through judicious choice of experimental conditions, as most notably demonstrated by studies of the photo-induced harpoon reactions of ICl in liquid and solid xenon (Okada et al. 1989). Here it has been shown that Xel is produced even at wavelengths well beyond the threshold for atomic charge transfer between... 

Hernandez-Trujillo and Bader studied the evolution of the electron densities of two separated atoms into an equilibrium molecular distribution, and considered a range of interactions from closed-shell with and without charge transfer, through polar-shared, to equally shared interactions. The harpoon mechanism operative in the formation of LiF was found to exert dramatic effects on the electron density and on the atomic and molecular properties. The virial, the Hellmann-Feynman and the Ehrenfest force theorems provided an imderstanding of the similarities and differences in the bonding. 

For charge transfer leading to a fluorescent exciplex, aU rate constants can be evaluated from the fluorescence decays, but particular attention should be paid to the possibility of occurrence of (1) transient effects, (2) the harpoon mechanism [70] (the electron goes first and then the exciplex is formed) and (3) ground-state charge-transfer complexes. All these phenomena lead to deviations from doubleexponential decays and/or differences between Stem-Volmer plots obtained from time-resolved (tq/t vs [Q]) and steady-state (Iq// vs [Q]) measurements. 

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