Work

The reaction of P + with H 2 O (D 2 O): A pathway for the formation of PO and PO + in the interstellar medium and circumstellar envelopes

Year: 2026

Type: article

Abstract: Context. Interstellar phosphorus shows uncertain depletion patterns and an unidentified main reservoir, with only a few P-bearing molecules detected despite their key astrochemical and astrobiological relevance. In particular, recent observations of PO + with a high abundance with respect to PO suggest that ion–molecule chemistry may play a crucial role, motivating a reassessment of its formation pathways in the interstellar medium (ISM) and circumstellar envelopes. Aims. The role of the reaction between P + and water (D 2 O) for the synthesis of PO + and POH + is explored in a joint experimental and theoretical study. Furthermore, new possible routes leading to the conversion of i ) POH + into PO (via a non-dissociative proton transfer reaction with ammonia) and of ii ) PO into PN (via the adiabatic barrierless reaction of N( 4 S) with PO are proposed. Methods. The reaction P + plus D 2 O was studied experimentally by measuring absolute cross-sections (CSs) and branching ratios (BRs), as a function of collision energy. Experiments were supported by a theoretical investigation combining high-level electronic structure calculations of the multidimensional triplet and singlet potential energy surfaces with a kinetic investigation to derive BRs and channel-specific rate constants as a function of temperature in the 10–5000 K range. Results. The reaction P + + D 2 O leads mostly to POD + plus D (BR = 90%), with PO + plus D 2 being a minor channel (BR=10%). From the total reaction CS as a function of collision energy, estimates for the rate constant as a function of temperature have been obtained, with values ranging from 1.2 × 10 −9 cm 3 s −1 (at 10 K) to 7.5 × 10 −10 cm 3 s −1 (at 5000 K). The proton transfer reactions between POH + and NH 3 is found to be efficient with rate constants in the range of 1.1–2.7 × 10 −9 cm 3 s −1 molecule −1 . Conclusions. The reaction of P + with water should be considered in astrochemical models in which phosphorus can be released in the gas phase as a cation, from the energetic processing of icy interstellar grains due to shocks. The reaction leads to POH + as the main reaction product and relevant interstellar isomer, an important precursor for PO formation not only via dissociative recombination with electrons, but also by proton transfer to NH 3 . The direct formation of PO + as a secondary channel can explain the high PO + /PO ratio detected in the ISM.

Source: Astronomy and Astrophysics

DOI: 10.1051/0004-6361/202660442