Spin-polarized density functional theory is used to analyze chemical reactivity from a more general point of view, which distinguishes between the changes produced by charge transfer between the interacting species (changes in the total number of electrons, N = + TVj where f refers to spin-up or a and , to spin-down or ß) and the changes produced by the redistribution of the electronic density of each of the interacting species (changes in the spin number, Ns = Af -7V|). It is found that the response of the system to changes in N and the external potential is given in terms of the chemical potential, the hardness, the electronic density, and the Fukui function, while the response of the system to changes in 7VS and an external magnetic field is given in terms of a new set of parameters which we have named the spin potential, the spin hardness, the spin density, and the spin Fukui function. Making use of the Kohn-Sham approach to density functional theory, it is shown that the generalized Fukui functions can be reduced to a set of spin-polarized classical frontier orbitals by imposing frozen core approximations.

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The lifetime of a resonance state overlapping with a second resonance is shown to be strongly enhanced (by a factor of 3) by combining two pump pulses to simultaneously excite the two resonances. Such an enhancement is produced by interference effects occurring between the two coherently excited overlapping resonances. A high degree of control on the intensity of interference, and thus on the resonance lifetime enhancement, is found to be achieved by varying the delay time between the pulses and their relative intensities.

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