Quantum–classical approach to the reaction dynamics in a superfluid helium nanodroplet. The Ne2 dimer and Ne–Ne adduct formation reaction Ne + Ne-doped nanodroplet

Abstract

The dynamics of the Ne₂ dimer and Ne–Ne adduct formation in a superfluid helium nanodroplet [(⁴He)_N; T = 0.37 K], Ne + Ne@(⁴He)_N → Ne₂@(⁴He)_N′/Ne–Ne@(⁴He)_N′ + (N–N′)⁴He with N = 500, has been investigated using a hybrid approach (quantum and classical mechanics (QM–CM) descriptions for helium and the Ne atoms, respectively) and taking into account the angular momentum of the attacking Ne atom, Ne₍₁₎. Comparison with zero angular momentum QM results of our own shows that the present results are similar to the quantum ones for the initial Ne₍₁₎ velocities (v₀) of 500 and 800 m s⁻¹ (the former one being the most probable velocity of Ne at 300 K), in all cases leading to the Ne₂ dimer (r_e = 3.09 Å). However, significant differences appear below v₀ = 500 m s⁻¹, because in the QM–CM dynamics, instead of the dimer, a Ne–Ne adduct is formed (r₀ = 5.45 Å). The formation of this adduct will probably dominate as the contribution to reactivity of angular momenta larger than zero is the leading one and angular momentum strongly acts against the Ne₂ production. Angular momentum adds further difficulties in producing the dimer, since it makes it more difficult to remove the helium density between both Ne atoms to lead, subsequently, to the Ne₂ molecule. Hence, the formation of the neon–neon adduct, Ne–Ne@(⁴He)_N′, clearly dominates the reactivity of the system, which results in the formation of a “quantum gel”/“quantum foam”, because the two Ne atoms essentially maintain their identity inside the nanodroplet. Large enough Ne₍₁₎ initial angular momentum values can induce the formation of vortex lines by the collapse of superficial excitations (ripplons), but they occur with greater difficulty than in the case of the capture of the Ne atom by a non doped helium nanodroplet, due to the wave interferences induced by the Ne induced by the solvation layers of the Ne atom originally placed inside the nanodroplet. We hope that this work will encourage other researchers to investigate the reaction dynamics in helium nanodroplets, an interesting topic on which there are few studies available.