Chemistry Reference
In-Depth Information
Fig. 2.11
Schematic illustration of the energy diagram of a direct and b indirect process
I
4.9±0.2
was found for the Xe switching, suggesting that the transfer of the Xe atom
takes place through over-barrier process and the stepwise climbing of a vibrational
ladder of the Xe-surface bond excitation is involved. The theoretical description of
the direct process was reported by Walkup et al. [
66
] and Gao et al. [
67
]. The
switching was described as the energy barrier climbing process between two
potential wells modeled by truncated harmonic oscillators, where the higher
vibrational states are populated by the excitation of tunneling electrons. On the
other hand, for O
2
molecule dissociation on a Pt(111) surface, it was revealed a
power low dependence of N *1, 2, and 3 for the applied voltage of 0.4, 0.3, and
0.2 V, respectively. The excitation of the O-O stretching (hX = 87 meV) is
associated with the dissociation and the potential barrier of 0.35-0.38 eV with 5
vibrational levels in the well was calculated using a truncated harmonic oscillator.
Thus, the coherent multiple jump of vibrational ladders in a one-electron scattering
dominates the dissociation at 0.4 V to overcome the barrier, while the step-by-step
ladder climbing through two (three)-electron process makes it possible to break the
O-O bond at 0.3 (0.2) V.
On the other hand, if the motion or reaction of adsorbates is triggered by the
mode that is not directly coupled with its coordinate (indirect process), the
intramolecular energy transfer have to be taken into account. This indirect process
was discovered in the rotation of oxygen and acetylene molecule [
42
,
43
]. For
instance, the threshold energy to induce the rotation of acetylene molecule on
Cu(100) coincides with the C-H stretch excitation [
43
]. Obviously the C-H stretch
mode is not the reaction coordinate of the rotation of acetylene. The mechanism
was rationalized by the intramolecular energy transfer via anharmonic coupling
between the high and low frequency modes. We now consider the fate of a
molecular vibration excited via IET process. After the excitation, the excited state
rapidly damps to the ground state via intramolecular energy transfer, surface
phonon excitation, or electron-hole-pair (EHP) excitations on a time scale of
femtosecond to picosecond. Among them the intramolecular energy transfer can
eventually causes the molecular motions and reactions. Figure
2.11
b illustrates the
concept of the intramolecular energy transfer. When a high frequency (HF) mode is
excited via IET process, its energy relaxes to a lower frequency modes associated
