Abstract
The entrance channel potentials of the prototypical polyatomic reaction family X + CH(4) → HX + CH(3) (X = F, Cl, Br, I) are investigated using anion photoelectron spectroscopy and high-level ab initio electronic structure computations. The pre-reactive van der Waals (vdW) wells of these reactions are probed for X = Cl, Br, I by photodetachment spectra of the corresponding X(-)-CH(4) anion complex. For F-CH(4), a spin-orbit splitting (∼1310 cm(-1)) much larger than that of the F atom (404 cm(-1)) was observed, in good agreement with theory. This showed that in the case of the F-CH(4) system the vertical transition from the anion ground state to the neutral potentials accesses a region between the vdW valley and transition state of the early-barrier F + CH(4) reaction. The doublet splittings observed in the other halogen complexes are close to the isolated atomic spin-orbit splittings, also in agreement with theory.
Highlights
Photodetachment spectroscopy of a stable anion to produce a neutral reactive system has been proven to be a powerful probe of key properties of the reactive potential energy surface, such as the pre-reactive van der Waals well and/or the reaction transition state (TS)
In this communication we report a joint experimental and theoretical study of the low-resolution photodetachment spectra of the anions X−–CH4 (X = F, Cl, Br, I) and determine the regions probed on the neutral reactive potentials
For Cl–CH4 system, previous work showed that the anion equilibrium geometry overlaps with that of the van der Waals (vdW) complex in the reactant valley,23,24,28 which was confirmed by the slow electron velocity-map imaging (SEVI) spectrum of Cl−–CH4 reported by Neumark
Summary
Photodetachment spectroscopy of a stable anion to produce a neutral reactive system has been proven to be a powerful probe of key properties of the reactive potential energy surface, such as the pre-reactive van der Waals (vdW) well and/or the reaction transition state (TS).1–7 The sensitivity of this technique to probe these aspects of the reactive potential depends on the position of the anion vibrational wave function with respect to the vdW well and/or TS. The VDE shift between F−–CH4 and F(2P3/2) (2420 cm−1) is larger than the D0 value of F−–CH4 (2337 cm−1); the experiment probes the neutral potential above the reactant asymptote by about 100 cm−1, which is less than the SO-corrected barrier height (∼270 cm−1) as expected considering the above mentioned structural parameters.
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