Energy partitioning in the dissociation reaction Ar₃⁺ → Ar₂⁺ + Ar

Threshold-photoelectron photoion coincidence (TPEPICO) spectra of argon clusters ionized with synchrotron radiation have been observed, and the average kinetic energy of Ar₂⁺, which originates from Ar ₃⁺, has been estimated. All the line shapes of the Ar ₂⁺ ion observed in the TPEPICO spectra have been appropriately simulated in terms of superposition of a sharp and a broad Gaussian function, where the sharp Gaussian function corresponds to ionization of the argon dimer Ar₂. The observed line shapes agree very well with the simulated ones, implying that the excess energy in the Ar₃ ionization is thermally or quasithermally partitioned to translational energy of the fragment species. From the average kinetic energies of Ar₂ ⁺, it has been found that the percentage of excess energy in Ar ₃⁺, that is partitioned into the total translational energy of the fragment species, is dependent on the excitation energy. The percentage is slightly less than the value of 40% expected from statistical theory. It has also been found that complete dissociation of Ar₃ ⁺ takes place in the excess energy range 0.3-1.1 eV.