Collision induced desorption of N₂ from Ru(001)

The dynamics of collision-induced desorption (CID) of N₂ from Ru(001) exposed to hyperthermal rare gas colliders generated in a supersonic atomic beam source have been studied. Low coverage of 0.01 ML ¹⁵N₂ at crystal temperature of 96 K was chosen to represent a CID process of a practically isolated molecule, neglecting the effect of lateral N₂-N₂ interactions. The cross sections for CID of nitrogen molecules, σ_des(E_i,θ_i), as a function of the kinetic energy and angle of incidence of Ar and Kr colliders have been measured. It was found that σ_des(E_i,θ_i=0°) changes monotonkally in the range 0-25 Ų for beam energy in the range of 0.5-5.5 eV and is insensitive to the type of collider (Ar, Kr) as well as to the adsorbate isotope (¹⁴N₂, ¹⁵N₂). The threshold energy for desorption has been determined to be 0.50±0.10 eV, which is twice the binding energy of N₂ to Ru(001). The cross section for CID at a fixed collider's energy rises approximately four times as the incidence angle θ_i increases from 0° to 70° relative to the surface normal. Neither normal nor total energy scaling of the cross section could describe the results. The σ_des(θ_i) scales reasonably well, however, with the tangential energy of the collider for angles above 30°. Classical molecular dynamics simulations were performed to gain better understanding of the CID process. Threshold energy and angular dependence of the cross section were reproduced very well. The predominant CID mechanism was concluded to originate from a direct rare gas-nitrogen collision, in which impulsive-bending and the motion along the surface are coupled to the adsorbate motion which leads to desorption.