Surface reactions of phosphine (PH3) with H or D atoms on icy surfaces atlow temperatures
Nguyen, T.,1 Oba, Y.,1* Sameera, W.M.C.,1,2,3 Kouchi, A.,1 Watanabe, N.1*presenter
1oba@lowtem.hokudai.ac.jp, Institute of Low Temperature Science, Hokkaido University,
Japan
2
University of Colombo, Sri Lanka
3Gothenburg University, Sweden

Phosphine (PH3) is one of the phosphorus (P)-bearing molecules identified in the interstellar
medium (ISM) whose fractional abundance is 10-8 relative to H2 in IRC +10216.1 Since theinterstellar PH3 is considered to primarily form by grain surface reactions, it may haveexperienced several processes after its formation on grains. In the present study, we performedexperimental and computational studies on the surface reactions of PH3 with H or D atoms
under the conditions in the ISM.
All experiments were performed using the Apparatus for Surface Reaction in Astrophysics(ASURA) system, which mainly consists of a stainless-steel vacuum chamber, multiple turbo
molecular pumps, an aluminum (Al) substrate attached to a closed-cycle helium cryostat, aquadrupole mass spectrometer, and an FTIR spectrometer. Gaseous PH3 was produced by thereaction of calcium phosphide (Ca3P2) with water in a vacuum vessel and was deposited onto
the reaction substrate covered with water ice (porous amorphous, compact amorphous, orcrystalline) with a thickness of 30 monolayers at 10 K. Atomic H or D was produced by thedissociation of H2 or D2 in the microwave-induced plasma and cooled to 100 K before collisions
with the substrate. The reaction was monitored in-situ by FTIR, and the desorbed products weredetected by the QMS. Details of the computational method for the calculation of the binding
energy of P-bearing species on ice and the reaction dynamics were shown elsewhere.2When solid PH3 was exposed to H atoms on ices at 10 K, the peak intensity of the P-H stretching
band at 2320 cm-1 decreased with atom exposure times, and ~80% of the initial PH3 was lostfrom the substrate after 20 min. Any other P-bearing products were not confirmed in the FTIR
analysis. We expect that the initial process for the reaction of solid PH3 with an H atom is theH-abstraction from PH3 (PH3 + H → PH2 + H2), followed by the H addition to the formed PH2,
resulting in the formation of PH3 again (PH2 + H → PH3). In that case, the net abundance ofsolid PH3 should not change even after the H-abstraction and addition reactions. The observed
phenomena may be explained by the loss of PH3 from the icy surface by chemical desorption,
which utilizes the heat of reaction for the desorption of a molecule from a substrate.3
When solid PH3 was exposed to D atoms, a new peak appeared at 1686 cm-1, which can beassigned to the P-D stretching band for deuterated phosphines such as PH2D, PHD2, and PD3.
Of note, with increasing D-exposure time, the peak intensity of the P-D stretching banddecreased. This is reasonably explained by the preferential H-D substitution of PH3 followedby the chemical desorption of its deuterated species. In the presentation, more detailed
quantitative experimental and computational results will be presented.
1

Agundez, M. et al. Astrophys. J. Lett. 2014, 790, L27.
Nguyen, T. et al. Astrophys. J. 2021, 918, 73.
3Oba, Y. et al. Nat. Astron. 2018, 2, 228.
2