Low-energy electron-stimulated desorption of cations and neutrals from Si(111)-(7x7):C2D2.

The interactions of low-energy (5-50 eV) electrons with acetylene-d(2) (C(2)D(2)) adsorbed on the Si(111)-(7x7) surface have been examined by monitoring the stimulated desorption products. These include primary cation desorbates, D(+) and C(2)D(2)(+) (C(2)HD(+)), the fragment ion C(2)D(+), smaller amounts of C(2)(+), CDH(+) (CH(3)(+)), and neutral D((2)S). The approximately 23-25 eV threshold energies for D(+) and hydrocarbon fragment ion detection indicate involvement of two-hole or two-hole one electron final states that Coulomb explode. These multihole states can be created via Auger decay of single holes in shallow core levels localized on C or Si surface atoms. The approximately 12 eV appearance threshold for the C(2)D(2)(+) molecular ion can be correlated with direct excitation of an adsorbate-induced surface state, which may initially possess character of the A(3) surface state of Si. The 18 eV threshold for C(2)D(+) correlates with decomposition of C(2)D(2)(+) with excess vibronic energy. C(2)D(+) desorption via direct excitation of the dissociative (2)Sigma(u)(+)-type state of the C(2)D(2)(+) ion is also possible. The approximately 8 eV threshold energy for production and desorption of neutral D((2)S) may correlate with excitation of the perturbed/mixed F (1)Sigma(u)(+)<--X (1)Sigma(g)(+) and E (1)Sigma(u)(+)<--X(1)Sigma(g)(+) dissociative transitions of adsorbed acetylene molecules. Time-of-flight distributions of D((2)S) indicate both nonthermal (557 and 116 meV; 4300 and 900 K) and thermal (17 meV; 130 K) components. The two fast components can be related to the geometry of di-sigma bonded acetylene on the Si(111)-(7x7) surface.