A velocity map imaging study of the photodissociation of the methyl iodide cation

Abstract

The photodissociation dynamics of the methyl iodide cation has been studied using the velocity map imaging technique. A first laser pulse is used to ionize methyl iodide via a (2 + 1) REMPI scheme through the 5pπ → 6p Rydberg state two-photon transition. The produced CHI(X[combining tilde]E) ions are subsequently excited at several wavelengths between 242 and 260 nm. The reported translational energy distributions for the methyl and iodine ions present a Boltzmann-type unstructured distribution at low excitation energies as well as a recoiled narrow structure at higher excitation energies highlighting two different dissociation processes. High level ab initio calculations have been performed in order to obtain a deeper understanding of the photodissociation dynamics of the CHI ion. Direct dissociation on a repulsive state from the manifold of states representing the B[combining tilde] excited state leads to CH(X[combining tilde]A') + I*(P), while the CH + I(P) channel is populated through an avoided crossing outside the Franck-Condon region. In contrast, an indirect process involving the transfer of energy from highly excited electronic states to the ground state of the ion is responsible for the observed Boltzmann-type distributions.

Photodissociation dynamics of methyl iodide probed using femtosecond extreme ultraviolet photoelectron spectroscopy

“…The ground state spectrum is plotted in blue with the intensity scale shown on the right-hand axis. Each feature is labelled with the asymptotic dissociation products as assigned from the calculations of Marggi-Poullain et al, 26 while the lower binding energy features are also identified with C 3v symmetry labels to aid comparison to earlier work. Of most importance to the time resolved analysis 13 for the neutral curves and from Marggi-Poullain et al 26 for the ion states, and have been calculated such that they allow for relaxation of the CH 3 pyramidal structure during dissociation.…”

“…The higher lying ion states accessible with the probe photon correlate with higher energy fragments and are in good agreement with the theoretical predictions of ref. 26. The assignments are based on the vertical ionisation energies obtained from ref.…”

“…The assignments are based on the vertical ionisation energies obtained from ref. 26 and are labelled with the dissociation limits as used in the reference. UV excitation drives an s*(C-I) ' n(I) transition and leads to an electronic configuration of (s) 2 (n) 3 (s*) 1 .…”

Femtosecond XUV induced dynamics of the methyl iodide cation

“…To complete the picture, it is necessary to know the exact probing mechanism induced by the IR. The potential energy surfaces of reference [5] can qualitatively describe the probing step. Further calculations are underway to obtain further insight into the role of the IR field on the coupling to dissociative states.…”

“…In the present contribution we investigate the dynamics of cationic states of the CH3I molecule. The dissociation of the CH3I + cation along the C-I axis is mainly accompanied by a geometry change in the methyl part from 一 Corresponding author: reitsma@mbi-berlin.de pyramid to planar and the spin-orbit splitting in the iodine atoms [5]. The latter induces avoided crossings in the manifold of potential energy surfaces which make CH3I + dissociation complex and interesting.…”