Transition state spectroscopy with electrons as studied by 3D-trajectory calculations of the reaction He+ + Br2− → He+Br− + Br

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Abstract
We investigate electron emission from the interaction of metastable helium (ls2s; 23S) with Br2 molecules. A small part of the electron energy spectrum is due to direct Penning ionization, giving rise to spectral features familiar from photoionization of Br2. However, the dominant contribution to electron emission originates from a collision complex that can be described as ion pair state He+–Br2−. In analogy to a collision between the alkali atom Li(2s) and Br2, which leads via Li+–Br2− to salt formation LiBr + Br, this system can be viewed as undergoing a chemical reaction to He+Br− Br. Of course, no stable products He+Br− can be produced in this way since the large recombination energy of He+ causes autoionization to He + Br+ + e−. On the other hand, this situation is ideal if one wishes to study spectroscopically the transition state of a chemical reaction, in our case the development of the system from He(1s2s) + Br2 to (He+Br−) + Br. Neither of the separate reaction partners is able to emit electrons. During the encounter the lifetime against autoionization is so short that decay into the ionized channel cannot occur long after the chemical reaction is complete. Thus, electron emission is confined to the lifetime of the reaction complex. Accordingly, the electron energy spectrum contains information exclusively on the transition state with no blending by emission from educts or products as could be the case in optical spectroscopy. As a case study we carry out 3D-trajectory calculations on the collision complex He+–Br2−, monitoring the energy distribution of emitted electrons throughout the lifetime of the complex. We find that the electron energy spectrum varies strongly from the initial to the final stages of the ongoing chemical reaction.

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