Comparing Femtosecond Multiphoton Dissociative Ionization
of Tetrathiafulvene with Imaging Photoelectron Photoion Coincidence
Spectroscopy
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Abstract
In this paper we describe femtosecond
photoionization and the imaging
photoelectron photoion coincidence spectroscopy of tetrathiafulvene,
TTF. Femtosecond photoionization of TTF results in the absorption
of up to twelve 808 nm photons leading to ion internal energies up
to 12.1 eV as deduced from the photoelectron spectrum. Within this
internal energy a variety of dissociation channels are accessible.
In order to disentangle the complex ionic dissociation, we utilized
the imaging photoelectron photoion coincidence (iPEPICO) technique.
Above the dissociation threshold, iPEPICO results show that the molecular
ion (<i>m</i>/<i>z</i> = 204) dissociates into
seven product ions, six of which compete in a 1.0 eV internal energy
window and are formed with the same appearance energy. Ab initio calculations
are reported on the possible fragment ion structures of five dissociation
channels as well as trajectories showing the loss of C<sub>2</sub>H<sub>2</sub> and C<sub>2</sub>H<sub>2</sub>S from high internal
energy TTF cations. A three-channel dissociation model is used to
fit the PEPICO data in which two dissociation channels are treated
as simple dissociations (one with a reverse barrier), while the rest
involve a shared barrier. The two lower energy dissociation channels, <i>m</i>/<i>z</i> = 146 and the channel leading to <i>m</i>/<i>z</i> = 178, 171, 159, 140, and 127, have <i>E</i><sub>0</sub> values of 2.77 ± 0.10 and 2.38 ±
0.10 eV, respectively, and are characterized by Δ<i>S</i><sup>‡</sup><sub>600 K</sub> values of −9 ±
6 and 1 ± 6 J K<sup>–1</sup> mol<sup>–1</sup>,
respectively. Competing with them at higher internal energy is the
cleavage of the central bond to form the <i>m</i>/<i>z</i> = 102 fragment ion, with an <i>E</i><sub>0</sub> value of 3.65 ± 0.10 eV and Δ<i>S</i><sup>‡</sup><sub>600 K</sub> = 83 ± 10 J K<sup>–1</sup> mol<sup>–1</sup>