Negative ion formation in alkali-atom-molecule collisions

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Abstract
Negative ion formation in alkali-atom-molecule collisions is a powerful method to study the dynamic properties of negative ions. By studying a collision with an electron donor such as an alkali atom one has access to the electron affinity of the molecule as it is manifested in the collision. The electron affinities obtained depend on the conditions of the collision and do not have to be identical to electron affinities obtained by other means. In this way adiabatic, vertical and reactive electron affinities can be discerned. Moreover if the collision energy is in the eV region one can probe the vibrational dynamics of a negative molecular ion on a 10 fs timescale. As a theoretical introduction to negative ion formation by electron transfer in atom-molecule collisions the curve crossing model and the collision dynamics of the atom-atom case are briefly reviewed. A short survey of the experimental methods used is presented. Measurements of total, differential and double-differential cross sections as a function of the collision energy and different collision partners are discussed. In atom-molecule collisions stretching of the bond of the molecular negative ion during the collision is among the most relevant consequences of the internal degrees of freedom of the molecule. The collision dynamics explored in this way not only gives insight into the properties of negative ions, but also into the dynamics of elementary processes occurring in chemical reactions.