One- and Two-Dimensional
Translational Energy Distributions
in the Iodine-Loss Dissociation of 1,2-C<sub>2</sub>H<sub>4</sub>I<sub>2</sub><sup>+</sup> and 1,3-C<sub>3</sub>H<sub>6</sub>I<sub>2</sub><sup>+</sup>: What Does This Mean?
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Abstract
Threshold photoelectron photoion coincidence (TPEPICO)
has been
used to study the sequential photodissociation reaction of internal
energy selected 1,2-diiodoethane cations: C<sub>2</sub>H<sub>4</sub>I<sub>2</sub><sup>+</sup> → C<sub>2</sub>H<sub>4</sub>I<sup>+</sup> + I → C<sub>2</sub>H<sub>3</sub><sup>+</sup> + I +
HI. In the first I-loss reaction, the excess energy is partitioned
between the internal energy of the fragment ion C<sub>2</sub>H<sub>4</sub>I<sup>+</sup> and the translational energy. The breakdown
diagram of C<sub>2</sub>H<sub>4</sub>I<sup>+</sup> to C<sub>2</sub>H<sub>3</sub><sup>+</sup>, i.e., the fractional ion abundances below
and above the second dissociation barrier as a function of the photon
energy, yields the internal energy distribution of the first daughter,
whereas the time-of-flight peak widths yield the released translational
energy in the laboratory frame directly. Both methods indicate that
the kinetic energy release in the I-loss step is inconsistent with
the phase space theory (PST) predicted two translational degrees of
freedom, but is well-described assuming only one translational degree
of freedom. Reaction path calculations partly confirm this and show
that the reaction coordinate changes character in the dissociation,
and it is, thus, highly anisotropic. For comparison, data for the
dissociative photoionization of 1,3-diiodopropane are also presented
and discussed. Here, the reaction coordinate is expected to be more
isotropic, and indeed the two degrees of freedom assumption holds.
Characterizing kinetic energy release distributions beyond PST is
crucial in deriving accurate dissociative photoionization onset energies
in sequential reactions. On the basis of both experimental and theoretical
grounds, we also suggest a significant revision of the 298 K heat
of formation of 1,2-C<sub>2</sub>H<sub>4</sub>I<sub>2</sub>(g) to
64.5 ± 2.5 kJ mol<sup>–1</sup> and that of CH<sub>2</sub>I<sub>2</sub>(g) to 113.5 ± 2 kJ mol<sup>–1</sup> at
298 K