Collision induced desorption of N2 from Ru(001)

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Abstract
The dynamics of collision-induced desorption (CID) of N 2 from Ru(001) exposed to hyperthermal rare gas colliders generated in a supersonic atomic beamsource have been studied. Low coverage of 0.01 ML 15 N 2 at crystal temperature of 96 K was chosen to represent a CID process of a practically isolated molecule, neglecting the effect of lateral N 2 – N 2 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 monotonically in the range 0–25 Å 2 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 adsorbateisotope ( 14 N 2 , 15 N 2 ). The threshold energy for desorption has been determined to be 0.50±0.10 eV, which is twice the binding energy of N 2 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.