sorb at about 600 K. The bonding configurations of molecular and atomic oxygen
on Pt(111) could be detected by low-temperature STM (8–350 K) [84, 86]. The
thermodynamically stable atomic O resides in face-centered cubic (fcc) 3-fold hollow sites; O atoms on the hexagonal close-packed (hcp) sites are metastable and
can only be created nonthermally by irradiation with electrons or photons [86]. No
molecules are observed at (100) steps. They then studied the dissociation of O 2 induced by tunneling electrons on the bridge and 3-fold hollow sites of Pt(111) [84].
The adjacent atoms of the underlying Pt atoms on the (111) surface are determined
to be 2.77 A ˚ ; the observed separation of the two O 2 molecules is 5.54 A ˚ . The STM
tip was then positioned about 6 A ˚ above one of the molecules and a voltage pulse
of 0.3 V and 100 ms duration was applied. The sudden change in the tunneling
current (12.5 nA) indicates the moment of dissociation. The image taken after the
dissociation showed the two O products, one on the hcp site and the other on the
fcc site, as well as the other O 2 molecule, which is not perturbed. The two O
products are found to be within two lattice constants of the original molecule and
one to three lattice constants apart. Approximately 1.1 eV of energy per O atom is
dissipated upon dissociation through substrate excitations. The rates of energy
dissipation and scattering with the corrugated potential of the surface limit the
range of motion of the products [87]. Ho et al. studied the mechanisms and the
dynamics of single-bond dissociation by measuring the dissociation rate as a function of bias voltage and tunneling current and by theoretical modeling of the data
[84]. The dissociation rate reveals a power law dependence on the tunneling current. They propose a mechanism based on inelastic vibrational excitations of an
adsorbed molecule by the tunneling electrons [88–90]. In this process tunneling
electrons inelastically scatter from an adsorbed molecule and transfer energy to the
vibrational modes of the molecule. For the dissociation of O 2 an energy barrier of
0.35–0.38 eV needs to be overcome via one or multiple vibrational excitations in
the ground electronic state. Such a vibrational ladder climbing process requires
one or multiple inelastic scatterings. For a 0.4 V bias used in the reported experiment, the O 2 molecule can be dissociated with a single inelastic scattering event,
leading to a linear dependence of the dissociation rate with the current. However,
at least two scatterings are required for 0.3 V bias, and the discrete nature of the
vibrational levels requires three scatterings for 0.2 V bias. As the vibrational relaxation rate is greater than the excitation rate, the most probable path to dissociation
is the one with the minimum relaxation probability, which is clearly the path with
the fewest transitions. For 0.4 V bias, the bound-to-continuum transition corresponds to a single excitation to overcome the dissociation barrier of 0.35–0.38 eV.
Finally it is interesting to calculate the electrical energy for this example to dissociate a single O 2 molecule. At a voltage of 0.3 V, a current of 12 nA and a pulse
duration of 30 ms an energy of 1.1 Â 10
À10 J is used. For the dissociation of 1 mol
O 2 this corresponds to 6.5 Â 10
10 kJ mol
À1 !
The dissociation of O 2 is the rate-determining step in the catalytic oxidation of
CO on metal surfaces, thus Ho et al. showed in another experiment [91] that bondselected chemistry can even be used to induce this catalytic reaction. After preparing oxygen atoms by dissociating single O 2 molecules with tunneling electrons, as
17.4 Chemical Reactions Induced by Confined Electrons 583
on Pt(111) could be detected by low-temperature STM (8–350 K) [84, 86]. The
thermodynamically stable atomic O resides in face-centered cubic (fcc) 3-fold hollow sites; O atoms on the hexagonal close-packed (hcp) sites are metastable and
can only be created nonthermally by irradiation with electrons or photons [86]. No
molecules are observed at (100) steps. They then studied the dissociation of O 2 induced by tunneling electrons on the bridge and 3-fold hollow sites of Pt(111) [84].
The adjacent atoms of the underlying Pt atoms on the (111) surface are determined
to be 2.77 A ˚ ; the observed separation of the two O 2 molecules is 5.54 A ˚ . The STM
tip was then positioned about 6 A ˚ above one of the molecules and a voltage pulse
of 0.3 V and 100 ms duration was applied. The sudden change in the tunneling
current (12.5 nA) indicates the moment of dissociation. The image taken after the
dissociation showed the two O products, one on the hcp site and the other on the
fcc site, as well as the other O 2 molecule, which is not perturbed. The two O
products are found to be within two lattice constants of the original molecule and
one to three lattice constants apart. Approximately 1.1 eV of energy per O atom is
dissipated upon dissociation through substrate excitations. The rates of energy
dissipation and scattering with the corrugated potential of the surface limit the
range of motion of the products [87]. Ho et al. studied the mechanisms and the
dynamics of single-bond dissociation by measuring the dissociation rate as a function of bias voltage and tunneling current and by theoretical modeling of the data
[84]. The dissociation rate reveals a power law dependence on the tunneling current. They propose a mechanism based on inelastic vibrational excitations of an
adsorbed molecule by the tunneling electrons [88–90]. In this process tunneling
electrons inelastically scatter from an adsorbed molecule and transfer energy to the
vibrational modes of the molecule. For the dissociation of O 2 an energy barrier of
0.35–0.38 eV needs to be overcome via one or multiple vibrational excitations in
the ground electronic state. Such a vibrational ladder climbing process requires
one or multiple inelastic scatterings. For a 0.4 V bias used in the reported experiment, the O 2 molecule can be dissociated with a single inelastic scattering event,
leading to a linear dependence of the dissociation rate with the current. However,
at least two scatterings are required for 0.3 V bias, and the discrete nature of the
vibrational levels requires three scatterings for 0.2 V bias. As the vibrational relaxation rate is greater than the excitation rate, the most probable path to dissociation
is the one with the minimum relaxation probability, which is clearly the path with
the fewest transitions. For 0.4 V bias, the bound-to-continuum transition corresponds to a single excitation to overcome the dissociation barrier of 0.35–0.38 eV.
Finally it is interesting to calculate the electrical energy for this example to dissociate a single O 2 molecule. At a voltage of 0.3 V, a current of 12 nA and a pulse
duration of 30 ms an energy of 1.1 Â 10
À10 J is used. For the dissociation of 1 mol
O 2 this corresponds to 6.5 Â 10
10 kJ mol
À1 !
The dissociation of O 2 is the rate-determining step in the catalytic oxidation of
CO on metal surfaces, thus Ho et al. showed in another experiment [91] that bondselected chemistry can even be used to induce this catalytic reaction. After preparing oxygen atoms by dissociating single O 2 molecules with tunneling electrons, as
17.4 Chemical Reactions Induced by Confined Electrons 583