Comparing Femtosecond Multiphoton Dissociative Ionization of Tetrathiafulvene with Imaging Photoelectron Photoion Coincidence Spectroscopy

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 C₂H₂ and C₂H₂S 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>₀ values of 2.77 ± 0.10 and 2.38 ± 0.10 eV, respectively, and are characterized by Δ<i>S</i>^‡_{600 K} values of −9 ± 6 and 1 ± 6 J K^–1 mol^–1, 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>₀ value of 3.65 ± 0.10 eV and Δ<i>S</i>^‡_{600 K} = 83 ± 10 J K^–1 mol<sup>–1