10 Surface-Aligned Femtochemistry: Dynamics on Oxide Surfaces
247
The extended time needed for the liberation of the methyl thus reveals the effect
of the magnesia surface that manifests itself in the trapping of the CH 3 I ∗‡ transition state. The subsequent growth of the methyl signal with a time constant of
τ 3 = 680 fs (Fig. 10.9c) is consequently interpreted as the average lifetime of all
trajectories leading from the transition state to the release of the methyl fragment.
For the free molecule, due to the strongly repulsive nature of the dissociation and
thus well focused wave packet, this rise is instantaneous [58, 59].
At the surface, however, the wave packet spreads due to the inelastic interaction
with the surface. The corresponding trajectories therefore include the motion of
methyl toward the surface leading to the inelastic collision and the recoil away from
the surface followed potentially by a collision with the slowly moving, heavy iodine
atom. This interaction with both, the surface and the iodine atom, was predicted
theoretically and was termed as ‘chattering’ motion [78–80]. The scenario might
proceed until the methyl trajectory is rotated away from the iodine trajectory far
enough so that the fragment can escape.
In addition to the mass-resolved detection, the time-of-flight technique also enables the measurement of the relative initial velocity of the reaction products, i.e.
here, of the methyl fragments emerging from the methyl iodide photodissociation
at different reaction times [58, 94]. In this way, additional information concerning
the dynamic processes of the supported molecules can be obtained. In the present
work the velocity-resolved detection is accomplished by lowering the acceleration
voltage between the surface and the mass spectrometer entrance electrode by about
a factor of 4 compared to the settings for optimal mass resolution (cf. Fig. 10.5). Under these experimental conditions, the shape and the relative position of a mass peak
reflect the initial kinetic energy distribution of the corresponding reaction products
[58, 94].
Figure 10.10 displays the methyl ion time-of-flight mass signal recorded under
these ‘velocity-resolved’ conditions as a function of the pump-probe delay time in
a contour as well as a 3D-surface plot representation. A longer ion flight time (ToFscale in Fig. 10.10) corresponds to a smaller initial ion velocity or initial kinetic
energy, respectively. Clearly, the peak and the exponential rise structures originate
from methyl ions with different initial kinetic energy. This strongly supports the
above interpretation that the methyl transient signal reflects the concurrency of two
different processes occurring at different delay times ((10.1) and (10.2), respectively) after photoexcitation at surface.
The liberated methyl radicals in the two processes would, in addition, be expected
to appear with two different initial kinetic energies corresponding to the two dissociation channels along the 3 Q 0+ and the 1 Q 1 potentials, respectively (cf. Fig. 10.7c).
However, this difference is not resolved in the data of Fig. 10.10 which might be due
to the energy loss during the interaction with the surface. In contrast, the heavy iodine atoms are assumed to remain almost immobile during the dissociation time period and might be released in the spin-orbit excited as well as the electronic ground
state.
247
The extended time needed for the liberation of the methyl thus reveals the effect
of the magnesia surface that manifests itself in the trapping of the CH 3 I ∗‡ transition state. The subsequent growth of the methyl signal with a time constant of
τ 3 = 680 fs (Fig. 10.9c) is consequently interpreted as the average lifetime of all
trajectories leading from the transition state to the release of the methyl fragment.
For the free molecule, due to the strongly repulsive nature of the dissociation and
thus well focused wave packet, this rise is instantaneous [58, 59].
At the surface, however, the wave packet spreads due to the inelastic interaction
with the surface. The corresponding trajectories therefore include the motion of
methyl toward the surface leading to the inelastic collision and the recoil away from
the surface followed potentially by a collision with the slowly moving, heavy iodine
atom. This interaction with both, the surface and the iodine atom, was predicted
theoretically and was termed as ‘chattering’ motion [78–80]. The scenario might
proceed until the methyl trajectory is rotated away from the iodine trajectory far
enough so that the fragment can escape.
In addition to the mass-resolved detection, the time-of-flight technique also enables the measurement of the relative initial velocity of the reaction products, i.e.
here, of the methyl fragments emerging from the methyl iodide photodissociation
at different reaction times [58, 94]. In this way, additional information concerning
the dynamic processes of the supported molecules can be obtained. In the present
work the velocity-resolved detection is accomplished by lowering the acceleration
voltage between the surface and the mass spectrometer entrance electrode by about
a factor of 4 compared to the settings for optimal mass resolution (cf. Fig. 10.5). Under these experimental conditions, the shape and the relative position of a mass peak
reflect the initial kinetic energy distribution of the corresponding reaction products
[58, 94].
Figure 10.10 displays the methyl ion time-of-flight mass signal recorded under
these ‘velocity-resolved’ conditions as a function of the pump-probe delay time in
a contour as well as a 3D-surface plot representation. A longer ion flight time (ToFscale in Fig. 10.10) corresponds to a smaller initial ion velocity or initial kinetic
energy, respectively. Clearly, the peak and the exponential rise structures originate
from methyl ions with different initial kinetic energy. This strongly supports the
above interpretation that the methyl transient signal reflects the concurrency of two
different processes occurring at different delay times ((10.1) and (10.2), respectively) after photoexcitation at surface.
The liberated methyl radicals in the two processes would, in addition, be expected
to appear with two different initial kinetic energies corresponding to the two dissociation channels along the 3 Q 0+ and the 1 Q 1 potentials, respectively (cf. Fig. 10.7c).
However, this difference is not resolved in the data of Fig. 10.10 which might be due
to the energy loss during the interaction with the surface. In contrast, the heavy iodine atoms are assumed to remain almost immobile during the dissociation time period and might be released in the spin-orbit excited as well as the electronic ground
state.