Vibrational Activation in Direct and Precursor-Mediated Chemisorption of SiH4 on Si(100)

The quantum state-resolved reactivity S0 of SiH4 on Si(100)-2x1 has been measured for the first time for two vibrationally excited Si-H stretch local mode states (|2000> and |1100>) as well the ground state S0 as a function of translational energy En and surface temperature Ts. We observe evidence for both direct and precursor-mediated chemisorption pathways. As expected, increasing En (or Ts) decreases S0 for the precursor-mediated reaction and increases S0 for the direct chemisorption. However, vibrational excitation of the incident SiH4 increases S0 for both the direct and the precursor-mediated pathway with a higher S0 for the |2000> state than for the |1100> state, indicating a non-statistical reaction mechanism.