10 Surface-Aligned Femtochemistry: Dynamics on Oxide Surfaces
251
The different appearance times can be connected to the different adsorption geometries of the methyl halide molecules on the magnesia surface. As discussed in
Sect. 10.1.2 methyl iodide molecules are proposed to be adsorbed with the methylend heading toward the surface. This leads to a trapping of the methyl group between
the MgO surface and the I atom subsequent to the photodissociation and the liberation of the methyl fragment from the surface takes a considerable time in the case
of methyl iodide. In contrast, according to Polanyi and coworkers [36], the methyl
bromide molecules are adsorbed with their C–Br axis almost parallel to the MgO
substrate. Therefore, the methyl photofragment can escape much easier and faster,
suffering in most cases only one collision with the adjacent molecule, which would
explain the faster appearance time of 320 ± 60 fs for CH 3 Br. This time constant
thus reflects the integral methyl desorption dynamics represented by the three reaction channels illustrated in Fig. 10.4b. The distinction of the DIR, IND(1), and
IND(2) mechanisms in the time-resolved experiment should now be possible due to
the clearly distinguishable translational energy of the emerging methyl fragments
(see Fig. 10.4a) and work is in progress to apply surface pump-probe fs-laser mass
spectrometry in the velocity-resolution mode (cf. Fig. 10.10) to accomplish this task.
10.3.2 Bimolecular Surface Reactions
The formation of a new chemical bond in, or between, molecules that are attached
to a substrate constitutes the elementary step of a bimolecular surface chemical reaction and also occurs on the ultrafast timescale of nuclear motion. The comprehension of the involved molecular dynamics is fundamental to the perception of
chemical reaction mechanisms on surfaces of, e.g., catalytic materials [1]. The key
to a molecular level insight of a bimolecular reactive encounter, however, is the
knowledge about the structure and the dynamics of the transition state of the reaction [3, 4].
Employing the model system of methyl iodide adsorbed at sub-monolayer coverages on a magnesia ultrathin film it could be demonstrated that with the technique
of surface pump-probe fs-laser mass spectrometry it is indeed possible to directly
probe the transition state and the product formation dynamics of a bimolecular surface reaction by time-, mass-, and velocity-resolved multi-photon ionization on the
surface [27]. The starting configuration is again obtained by a well defined reactant
adsorption geometry that represents the initial ‘collision complex’ of the reactive
bimolecular encounter as proposed by Polanyi (see Sect. 10.1.1 [4, 5, 10]). In the
following it will be shown that in this way unprecedented insight into the elementary steps of a complex bimolecular surface chemical reaction mechanism can be
obtained.
Methyl iodide adsorption layers were prepared under similar conditions as described in the previous section at which the mass spectrum displayed in Fig. 10.8
was obtained at a pump-probe delay time of 130 fs. If the delay time was changed
to 2 ps, the mass spectrum shown in Fig. 10.12a could be recorded (photoexcitation with 266 nm, 1 mW/cm 2 , and multi-photon ionization detection with 333 nm,
251
The different appearance times can be connected to the different adsorption geometries of the methyl halide molecules on the magnesia surface. As discussed in
Sect. 10.1.2 methyl iodide molecules are proposed to be adsorbed with the methylend heading toward the surface. This leads to a trapping of the methyl group between
the MgO surface and the I atom subsequent to the photodissociation and the liberation of the methyl fragment from the surface takes a considerable time in the case
of methyl iodide. In contrast, according to Polanyi and coworkers [36], the methyl
bromide molecules are adsorbed with their C–Br axis almost parallel to the MgO
substrate. Therefore, the methyl photofragment can escape much easier and faster,
suffering in most cases only one collision with the adjacent molecule, which would
explain the faster appearance time of 320 ± 60 fs for CH 3 Br. This time constant
thus reflects the integral methyl desorption dynamics represented by the three reaction channels illustrated in Fig. 10.4b. The distinction of the DIR, IND(1), and
IND(2) mechanisms in the time-resolved experiment should now be possible due to
the clearly distinguishable translational energy of the emerging methyl fragments
(see Fig. 10.4a) and work is in progress to apply surface pump-probe fs-laser mass
spectrometry in the velocity-resolution mode (cf. Fig. 10.10) to accomplish this task.
10.3.2 Bimolecular Surface Reactions
The formation of a new chemical bond in, or between, molecules that are attached
to a substrate constitutes the elementary step of a bimolecular surface chemical reaction and also occurs on the ultrafast timescale of nuclear motion. The comprehension of the involved molecular dynamics is fundamental to the perception of
chemical reaction mechanisms on surfaces of, e.g., catalytic materials [1]. The key
to a molecular level insight of a bimolecular reactive encounter, however, is the
knowledge about the structure and the dynamics of the transition state of the reaction [3, 4].
Employing the model system of methyl iodide adsorbed at sub-monolayer coverages on a magnesia ultrathin film it could be demonstrated that with the technique
of surface pump-probe fs-laser mass spectrometry it is indeed possible to directly
probe the transition state and the product formation dynamics of a bimolecular surface reaction by time-, mass-, and velocity-resolved multi-photon ionization on the
surface [27]. The starting configuration is again obtained by a well defined reactant
adsorption geometry that represents the initial ‘collision complex’ of the reactive
bimolecular encounter as proposed by Polanyi (see Sect. 10.1.1 [4, 5, 10]). In the
following it will be shown that in this way unprecedented insight into the elementary steps of a complex bimolecular surface chemical reaction mechanism can be
obtained.
Methyl iodide adsorption layers were prepared under similar conditions as described in the previous section at which the mass spectrum displayed in Fig. 10.8
was obtained at a pump-probe delay time of 130 fs. If the delay time was changed
to 2 ps, the mass spectrum shown in Fig. 10.12a could be recorded (photoexcitation with 266 nm, 1 mW/cm 2 , and multi-photon ionization detection with 333 nm,