10 Surface-Aligned Femtochemistry: Dynamics on Oxide Surfaces
241
that are adsorbed in a surface aligned geometry on the magnesium oxide substrate
(cf. Sect. 10.1.2). In the following, the employed laser excitation schema is first
discussed for the case of methyl iodide molecules on MgO. Time-delayed probing of the emerging methyl fragments enables the monitoring of the unimolecular decomposition dynamics of the adsorbed molecules. The thus obtained results
are then compared to the unimolecular decomposition dynamics of methyl bromide
molecules on the same substrate to elucidate the influence of the different initial
adsorption geometries. Furthermore, also light-induced bimolecular reactions are
detected in these model systems and in the final section the dynamics involved in
the formation of the halide molecules (I 2 and Br 2 ) on the surface will be discussed.
10.3.1 Unimolecular Photodissociation
10.3.1.1 Methyl Iodide on MgO(100)/Mo(100)
To monitor the real-time dissociation dynamics of the methyl iodide molecules
adsorbed at sub-monolayer coverage on the MgO/Mo(100) surface, the following
pump-probe schema is applied. The pump laser pulse is centered at a wavelength of
266 nm to electronically excite the adsorbed methyl iodide molecules to the A-band
by means of direct one photon adsorption. In Fig. 10.7a the measured spectrum of
the pump laser beam is presented. The one photon absorption spectrum of methyl
iodide in the A-band region [95] is also displayed in Fig. 10.7a.
The relevant potential energy curves of the methyl iodide molecule are depicted
in Fig. 10.7c. As the exact influence of the surface on the methyl iodide potentials
is not known, and because the methyl iodide molecules do only weakly bind to the
magnesia surface [78], the shown gas-phase potentials are assumed to be almost unperturbed by the surface to a first approximation. The A-band dissociation of CH 3 I
(see recent contributions [58, 59, 61, 96–103] and references therein) is prompt and
involves primarily the two dissociative excited electronic states denoted as 3 Q 0+
and 1 Q 1 by Mulliken [104]. One 266 nm photon (pump) excites the CH 3 I molecule
almost exclusively to the 3 Q 0+ state of the A-band and prepares the initial wave
packet at time zero. Close to the point of the initial excitation, at a C–I bond distance of 2.35 to 2.40 Å, the 3 Q 0+ potential leading to CH 3 and spin-orbit excited
I ∗ ( 2 P 1/2 ) crosses the 1 Q 1 potential that correlates diabatically with CH 3 and ground
state I( 2 P 3/2 ). At this point a bifurcation of the wave packet is possible and an experimental gas phase I( 2 P 3/2 ) yield of about 30 % [107] points toward an efficient
coupling between the 3 Q 0+ and 1 Q 1 states.
In previous photodissociation experiments with methyl iodide adsorbed at submonolayer coverage on MgO(100) single crystal surfaces the direct photoexcitation
(in contrast to a surface mediated excitation) has been shown to be the exclusive
excitation mechanism [70, 71]. In the present experiments, an ultrathin magnesia
film on Mo(100) has been employed instead of a magnesia single crystal. Still, all
experimental evidence indicates that the direct photoexcitation clearly dominates
over a surface mediated process [28].
241
that are adsorbed in a surface aligned geometry on the magnesium oxide substrate
(cf. Sect. 10.1.2). In the following, the employed laser excitation schema is first
discussed for the case of methyl iodide molecules on MgO. Time-delayed probing of the emerging methyl fragments enables the monitoring of the unimolecular decomposition dynamics of the adsorbed molecules. The thus obtained results
are then compared to the unimolecular decomposition dynamics of methyl bromide
molecules on the same substrate to elucidate the influence of the different initial
adsorption geometries. Furthermore, also light-induced bimolecular reactions are
detected in these model systems and in the final section the dynamics involved in
the formation of the halide molecules (I 2 and Br 2 ) on the surface will be discussed.
10.3.1 Unimolecular Photodissociation
10.3.1.1 Methyl Iodide on MgO(100)/Mo(100)
To monitor the real-time dissociation dynamics of the methyl iodide molecules
adsorbed at sub-monolayer coverage on the MgO/Mo(100) surface, the following
pump-probe schema is applied. The pump laser pulse is centered at a wavelength of
266 nm to electronically excite the adsorbed methyl iodide molecules to the A-band
by means of direct one photon adsorption. In Fig. 10.7a the measured spectrum of
the pump laser beam is presented. The one photon absorption spectrum of methyl
iodide in the A-band region [95] is also displayed in Fig. 10.7a.
The relevant potential energy curves of the methyl iodide molecule are depicted
in Fig. 10.7c. As the exact influence of the surface on the methyl iodide potentials
is not known, and because the methyl iodide molecules do only weakly bind to the
magnesia surface [78], the shown gas-phase potentials are assumed to be almost unperturbed by the surface to a first approximation. The A-band dissociation of CH 3 I
(see recent contributions [58, 59, 61, 96–103] and references therein) is prompt and
involves primarily the two dissociative excited electronic states denoted as 3 Q 0+
and 1 Q 1 by Mulliken [104]. One 266 nm photon (pump) excites the CH 3 I molecule
almost exclusively to the 3 Q 0+ state of the A-band and prepares the initial wave
packet at time zero. Close to the point of the initial excitation, at a C–I bond distance of 2.35 to 2.40 Å, the 3 Q 0+ potential leading to CH 3 and spin-orbit excited
I ∗ ( 2 P 1/2 ) crosses the 1 Q 1 potential that correlates diabatically with CH 3 and ground
state I( 2 P 3/2 ). At this point a bifurcation of the wave packet is possible and an experimental gas phase I( 2 P 3/2 ) yield of about 30 % [107] points toward an efficient
coupling between the 3 Q 0+ and 1 Q 1 states.
In previous photodissociation experiments with methyl iodide adsorbed at submonolayer coverage on MgO(100) single crystal surfaces the direct photoexcitation
(in contrast to a surface mediated excitation) has been shown to be the exclusive
excitation mechanism [70, 71]. In the present experiments, an ultrathin magnesia
film on Mo(100) has been employed instead of a magnesia single crystal. Still, all
experimental evidence indicates that the direct photoexcitation clearly dominates
over a surface mediated process [28].