General treatment for stereo-dynamics of state-to-state chemi-ionization reactions

The investigation of chemi-ionization processes provides unique information on how the reaction dynamics depend on the energy and structure of the transition state which relate to the symmetry, relative orientation of reagent/product valence electron orbitals, and selectivity of electronic rearrangements. Here we propose a theoretical approach to formulate the optical potential for Ne*(3P2,0) noble gas atom chemi-ionizations as prototype oxidation processes. We include the selective role of atomic alignment and of the electron transfer mechanism. The state-to-state reaction probability is evaluated and a unifying description of the main experimental findings is obtained. Further, we reproduce the results of recent and advanced molecular beam experiments with a state selected Ne* beam. The selective role of electronic rearrangements within the transition state, quantified through the use of suitable operative relations, could cast light on many other chemical processes more difficult to characterize.

(1) where ( ) with 2 n( ) = + 4 ( ) 2 (3) Here  is the potential well depth and is its location, while n( ) defines the hardness of the repulsive wall and the radial modulation of the attraction. The switching function S( ), that accounts for the transition from the neutral-neutral to the ion-neutral representation of the interaction (see left upper panel of Supplementary Fig. 1), as previously 1 it has been defined as Here is the distance where the two combined limiting potential forms have the same weight, while d describes how fast the transition occurs.
The R dependence of the isotropic component of the interaction in the exit channel is represented again by an ILJ function. Adopted parameter values are reported in Supplementary   Table 1. However, for the complete representation of it is necessary to take into account all aspects discussed in the main text.

Additional Details on the Ne * -Kr phenomenology
The experiments performed with the molecular beam (MB) technique provided total and partial ionization cross sections (σ) 3-7 , associative to Penning (σas/σpe) ratios [8][9][10] and Penning Ionization Electron Spectra (PIES) 7,12-14 : crucially, all data have been measured under single collision condition and as a function of the collision energy.
It has to be noted that the theoretical treatment, presented in this paper and stimulated by the PIES measurements in our laboratory, is able to rationalize in a unifying picture all the observables quoted above, probing different features of the optical potential (see eq. (3) of the main text). In particular, as demonstrated below, the adopted methodology is able to reproduce, 3 in a state to state condition, all experimental data available for the Ne * -Kr system obtained from our and other laboratories, including total and partial ionization cross sections and branching ratios between selected channels. Therefore, the proposed theoretical approach, which also includes within the same framework exchange and radiative mechanisms proposed in the past 15 , is general and can be used to describe in a state to state condition the reactivity of all chemiionization reactions, including those involving molecules.

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Note that in the right panel of Supplementary Fig. 1 is shown a comparison between state to state cross section ratios, predicted by the treatment discussed in the main text and referred to specific spin orbit sublevels of reactants and products, with peak area ratios extracted from PIESs measured as a function of the collision energy 14,16 . For two of the four ratios such a comparison appears to be only in semi-quantitative agreement. However, considering the difficulties involved in the separation of individual spin orbit contributions from measured energy dependence of PIESs, also for such quantities the theoretical description appears to be consistent with the experimental observables.
The next Supplementary Fig. 2 shows the manifold of the potential energy curves associated to Vt, describing all adiabatic states for both entrance and exit channels. The same Figure, depicting half-filled and filled orbitals of reagents and products, justifies the origin of the configuration interaction by charge transfer which plays a crucial role in the control of the reaction dynamics.
6 Supplementary Fig. 2 The real part of W represented by adiabatic potential energy curves that are formulated in an internally consistent way for both entrance (a) and exit (b) channels, as described in detail in the main text and whose potential parameters are given in Supplementary Table 1  Supplementary Table 1 The potential parameters of the real part Vt of the optical potential for entrance and exit channels of Ne * -Kr system, used to calculate the adiabatic potential energy curves reported in Supplementary Fig. 2