Example of the photoinduced reaction in the complex Ca (4s4p)1P -- CH3F prepared from its electronic ground state, Ca (4s2)1S -- CH3F. The right spectrum is obtained by tuning a laser across the resonances in the transition region, while observing the final product CaF*. We have only pictured here one reaction potential surface.
Metals play a crucial role in chemistry through their capability of transferring electrons. They are thus prone to oxidation reactions with electron acceptors. These reactions serve as model systems for reaction dynamics because they allow picturing the changes in electron distributions that drive the atoms in reactions, the goal of reaction dynamics. Excited state reactions are interesting in that respect, since they allow to access a variety of excited electronic configurations, especially in the case of metals.
Ground state reactions of metals have been extensively studied in gas phase reaction dynamics, while their excited states have received far less attention, though metals can provide a great wealth of excited configurations. These configurations relate to widely different distribution of the electrons around the atom. In this respect transition metals with their 3d or 4d orbitals are specifically interesting, given the catalytic implications of these metals.
We are interested on a fundamental standpoint for the above reasons in photoinduced reactions of metals of increased electronic complexity. We wish to characterise the intermediates in these reactions: when does charge transfer occur and under which movements. We also investigate the steps that lead to insertion reactions. First we have studied alkali earth metals such as calcium (4s2). This metal is interesting for its possibility of activating C-F bonds in fluorocarbons whose photochemistry is important in the stratosphere. The experimental method uses the direct excitation of a cooled reaction complex formed by the reactants, the metal and the molecule. This complex is inactive in the ground state and is activated by optical excitation. Variation of this excitation allows for a spectroscopy of the excited state transition region1). Spectroscopy can separate different electronic configurations generating different potential energy surfaces, while the movements within the complex are analysed in terms of damped vibrational modes. As an example an atom in a P state can produce Σ or Π surfaces, while perpendicular movements to the reaction coordinate will create vibrational levels broadened by their finite short lifetime (see figure).
Recently we investigate: