BREAKING INDIVIDUAL CHEMICAL-BONDS VIA STM-INDUCED EXCITATIONS

We present experimental and theoretical results on the STM-induced Si- H bond-breaking on the Si(100)-(2 x 1):H surface. First, we examine th e character of the STM-induced excitations. Using density functional t heory we show that the strength of chemical bonds and their excitation energies can be decreased or increased depending on the strength and direction of the field. By shifting the excitation energy of an adsorb ate below the tip, energy transfer away from this excited site can be suppressed, and localized excited state chemistry can take place. Our experiments show that Si-H bonds can be broken when the STM electrons have an energy >6 eV, i.e. above the onset of the sigma-sigma transit ion of Si-H. The desorption yield is similar to 2.4 x 10(-6)-atoms/ele ctron and is independent of the current. We also find that D-atom deso rption is much less efficient than H-atom desorption. Using the isotop e effect and wavepacket dynamics simulations we deduce that a very fas t quenching process, similar to 10s(-1), competes with desorption. Mos t of the desorbing atoms originate from the ''hot'' ground state produ ced by the quenching process. Most interestingly, excitation at energi es below the electronic excitation threshold can still lead to H atom desorption, albeit with a much lower yield. The yield in this energy r ange is a strong function of the tunneling current. We propose that de sorption is now the result of the multiple-vibration excitation of the Si-H bond. Such excitation becomes possible because of the very high current densities in the STM, and the long Si-H stretch vibrational li fetime. The most important aspect of this mechanism is that it allows single atom resolution in the bond-breaking process - the ultimate lit hographic resolution.