Photoexcitation of tetrathiafulvalene radical cations in a storage ring: Kinetics and energetics of the dissociation process

In this work, we report on the dissociation kinetics of tetrathiafulvalene radical cations (TTF+). In collisions with helium, the ions were first thermalised at room temperature or cooled to −65 °C in an ion trap before injection into an ion storage ring. Delayed dissociation of photoexcited ions was measured, and the kinetics were found to be non-trivial. The time spectrum obtained after 390 nm photoexcitation was not a single exponential decay normally observed for the dissociation of ions with a narrow energy distribution. We consider two interpretations of the time spectrum, either a multiple-exponential decay or a power-law decay. The physical implication of a multiple-exponential decay requires the presence of at least three isomers that interconvert slowly on the timescale for dissociation. In contrast, a power-law model implies a broad internal energy distribution of the excited ions. The origin of a broad distribution is, however, not obvious since we believe the width of the distribution to be narrow before excitation, about 0.1 eV, and TTF+ is non-fluorescent. We, therefore, discard the power-law model in favour of the multiple-isomer model. The dominant dissociation reaction at low internal energies is loss of SCH as measured in collision-induced dissociation experiments and fragmentation of metastable ions. We suggest that excited TTF+ rearranges via local minima on the potential energy surface to higher-energy isomers either before or after the photoexcitation. Isomerisation may be entropy-driven, which reduces the rate for reverse reactions to prevent equilibrium before the dissociation step. DFT calculations were applied in the search for possible structures of isomers.