Dissociative recombination (DR) is a process in which a molecular ion recombines with a free electron and subsequently dissociates into neutral fragments. Previous experimental studies on the DR of simple triatomic dihydrides, XH₂⁺, have shown a strong propensity for three-body break-up, which constitutes 60-80% of the total reactivity for X=C, N and O [1-3]. However, these results disagree with the theoretical model developed by Bates predicting the two-body channel involving the loss of a single hydrogen atom to be dominant [4]. To aid understanding of this phenomenon, we are further extending investigation of the dynamics of three-body break-up channel for ions of the type XH₂⁺ to the study of PH₂⁺. The heavy-ion storage CRYRING located at the Manne Siegbahn Laboratory in Stockholm has been used, which enables together with a position-sensitive detector investigation into the internal excitation of atomic products, the distribution of kinetic energy between the fragments, and their angular distribution. In order to decrease hydrogen losses at the detector, the heavy hydrogen isotope, PD₂⁺, has been used instead.
The performed experiment has evidenced the DR of PD₂⁺ to be also dominated by three-body break-up with the ratio to two-body channels of about 3:1. Competition between the available three-body channels producing the ground state, P(⁴S), and the first two exited states, P(²D) and P(²P), is observed, in which the formation of the first excited state dominates over other two almost equal channels with about 70%. The results indicate that kinetic energy is randomly distributed between the two deuterium fragments. It is concluded that the DR three-body break-up of PD₂⁺ occurs via an asynchronous concerted mechanism. The intra-molecular angle on dissociation has been also investigated, and a preference to dissociate at both open and closed angles is observed.

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