10 Surface-Aligned Femtochemistry: Dynamics on Oxide Surfaces
233
10.1.1 Surface-Aligned Chemistry
The idea of surface-aligned chemistry (or surface-aligned reaction) originated, as
stated by Ning and Polanyi in a recent contribution [5], from an early investigation of
the light-induced dissociation of CH 3 Br molecules adsorbed on a LiF(001) surface
[29]. The recoiling methyl radical fragments exhibited 1.5 eV of excess translational
energy which was proposed to enable subsequent reactive encounters of CH 3 with
co-adsorbate molecules. Yet, due to the initial adsorbate alignment on the surface
a specific geometry of the internuclear collision would be defined, thus restricting
the ranges of collision energies, collision angles, and impact parameters and thus
selecting specified dynamical trajectories rather than averaging over a multitude of
collisional possibilities [5]. In this way, considerably more detailed insight into the
single reactive collisional events would be attainable.
Several groups have since demonstrated the viability of this approach employing
photoexcitation (e.g., Refs. [30–42]), electron injection with, e.g., the tip of a scanning tunneling microscope (STM) (e.g., Refs. [43–47]), electron beams [48, 49], or
even collisions with accelerated atom beams [50] to trigger the initial ejection of a
surface-aligned molecular fragment.
In particular the atomic resolution capabilities of the STM have impressively
revealed real-space views on the effect of molecular alignment on surface reactions. As an example, the electron-induced dissociation of CH 3 SSCH 3 molecules
adsorbed on a gold surface is illustrated in Fig. 10.1 [45]. The adsorption structure
of a CH 3 SSCH 3 molecule is depicted in Fig. 10.1a. Following a low-current voltage
pulse that is applied between the STM tip and the sample the CH 3 S fragments are
separated along the parent direction of the S–S bond by two lattice spacings (5.5 Å)
and the S–C bond directions in the CH 3 S species match closely those of the parent
CH 3 SSCH 3 molecule as can be seen from Fig. 10.1b. Therefore, the authors call the
dissociation of CH 3 SSCH 3 bond-aligned because the products are ejected along the
S–S bond direction as well as surface-aligned because the products are trapped on
the surface sites aligned with the breaking bond [45].
In a succeeding investigation also reactions of the emerging CH 3 S fragments
could be observed [51]. For this purpose, CH 3 SSCH 3 molecules were allowed to
self-assemble into linear chains on the gold surface. The electron-induced dissociation reaction of individual molecules in the self-assembled structures subsequently
led to a propagating chemical reaction along the molecular chain in which S–S
bonds were broken and then reformed to produce new CH 3 SSCH 3 molecules.
Comparable localized atomic reactions have been reported by the Polanyi group
as well [52–55]. In addition, it has been realized that the alignment effect can be
utilized to imprint self-assembled molecular patterns on surfaces [56, 57].
In their seminal contribution Zewail and Polanyi already proposed that such an
adsorbate surface alignment also presents an ideal starting structure for coherent
excitation and fs-time-resolved dynamical investigations. A prospective schema for
transition state spectroscopy by fs-laser excitation of the surface-aligned reaction
H + H 2 S → H 2 + HS induced in co-adsorbed H 2 S molecules on LiF(001) is illustrated in Fig. 10.2.
233
10.1.1 Surface-Aligned Chemistry
The idea of surface-aligned chemistry (or surface-aligned reaction) originated, as
stated by Ning and Polanyi in a recent contribution [5], from an early investigation of
the light-induced dissociation of CH 3 Br molecules adsorbed on a LiF(001) surface
[29]. The recoiling methyl radical fragments exhibited 1.5 eV of excess translational
energy which was proposed to enable subsequent reactive encounters of CH 3 with
co-adsorbate molecules. Yet, due to the initial adsorbate alignment on the surface
a specific geometry of the internuclear collision would be defined, thus restricting
the ranges of collision energies, collision angles, and impact parameters and thus
selecting specified dynamical trajectories rather than averaging over a multitude of
collisional possibilities [5]. In this way, considerably more detailed insight into the
single reactive collisional events would be attainable.
Several groups have since demonstrated the viability of this approach employing
photoexcitation (e.g., Refs. [30–42]), electron injection with, e.g., the tip of a scanning tunneling microscope (STM) (e.g., Refs. [43–47]), electron beams [48, 49], or
even collisions with accelerated atom beams [50] to trigger the initial ejection of a
surface-aligned molecular fragment.
In particular the atomic resolution capabilities of the STM have impressively
revealed real-space views on the effect of molecular alignment on surface reactions. As an example, the electron-induced dissociation of CH 3 SSCH 3 molecules
adsorbed on a gold surface is illustrated in Fig. 10.1 [45]. The adsorption structure
of a CH 3 SSCH 3 molecule is depicted in Fig. 10.1a. Following a low-current voltage
pulse that is applied between the STM tip and the sample the CH 3 S fragments are
separated along the parent direction of the S–S bond by two lattice spacings (5.5 Å)
and the S–C bond directions in the CH 3 S species match closely those of the parent
CH 3 SSCH 3 molecule as can be seen from Fig. 10.1b. Therefore, the authors call the
dissociation of CH 3 SSCH 3 bond-aligned because the products are ejected along the
S–S bond direction as well as surface-aligned because the products are trapped on
the surface sites aligned with the breaking bond [45].
In a succeeding investigation also reactions of the emerging CH 3 S fragments
could be observed [51]. For this purpose, CH 3 SSCH 3 molecules were allowed to
self-assemble into linear chains on the gold surface. The electron-induced dissociation reaction of individual molecules in the self-assembled structures subsequently
led to a propagating chemical reaction along the molecular chain in which S–S
bonds were broken and then reformed to produce new CH 3 SSCH 3 molecules.
Comparable localized atomic reactions have been reported by the Polanyi group
as well [52–55]. In addition, it has been realized that the alignment effect can be
utilized to imprint self-assembled molecular patterns on surfaces [56, 57].
In their seminal contribution Zewail and Polanyi already proposed that such an
adsorbate surface alignment also presents an ideal starting structure for coherent
excitation and fs-time-resolved dynamical investigations. A prospective schema for
transition state spectroscopy by fs-laser excitation of the surface-aligned reaction
H + H 2 S → H 2 + HS induced in co-adsorbed H 2 S molecules on LiF(001) is illustrated in Fig. 10.2.