232
M.E. Vaida and T.M. Bernhardt
ular beam [6–8]. On surfaces a complementary approach has been pioneered by
John C. Polanyi which was termed “surface-aligned reaction” and which relies on
an ordered adsorbate structure on a solid surface [5, 9, 10]. This approach will be
described in more detail below.
To detect the resulting dynamics subsequent to the excitation of a molecular complex in aligned geometry in the gas phase, pump-probe mass spectrometry employing ultrafast lasers in combination with molecular beams is a widely applied experimental technique that has considerably advanced the field of femtochemistry [11].
Different experimental approaches are employed to study molecular reaction dynamics in surface femtochemistry, such as two photon photoemission, time-resolved
sum frequency generation, or two pulse correlation spectroscopy [12]. In the present
contribution, we report results of a novel alternative experimental approach for direct mass resolved monitoring of surface transition states and products adapting the
gas phase pump-probe methodology.
The investigations presented here were motivated by a seminal contribution of
John C. Polanyi and Ahmed H. Zewail in 1995 in which they proposed the investigation of surface transition state dynamics via direct detection of reaction intermediates and products in a femtosecond (fs) pump-probe schema similar to gas-phase
femtochemistry investigations [4]. They termed this new approach that relies on a
well defined reactant adsorption geometry ‘surface-aligned femtosecond photoreaction’. A great deal of both, surface-aligned reaction studies (not time-resolved)
[5, 10] as well as real-time surface femtochemistry investigations (see, e.g., Refs.
[12–25]) have since provided insight into surface reaction dynamics. Nevertheless,
the direct time- and mass-resolved detection of the transition state and the product formation dynamics of a surface reaction as proposed by Polanyi and Zewail
has been realized only very recently employing the new approach of surface pumpprobe fs-laser mass spectrometry [26–28].
This technique, which will be described in detail in Sect. 10.2, relies on the combination of time-of-flight mass spectrometry with non-resonant or resonance enhanced multi-photon ionization (REMPI) detection and fast surface preparation by
a pulsed molecular beam [28]. Ultrathin oxide films serve as versatile substrates
that interact only weakly with adsorbed molecules. Yet, the molecular adsorption
structure on the substrate provides the geometrical alignment that determines the
reaction dynamics subsequent to photoexcitation. Escaping fragments might, e.g.,
directly desorb into the gas phase or inelastically exchange energy via collisional
interaction with the substrate surface depending on the adsorption geometry. Furthermore, collisional encounters with neighboring adsorbate molecules might lead
to the formation of new reaction products.
In the following, first, the concept of surface-aligned chemistry will be described
and the investigated methyl halide adsorbate systems as well as their photochemistry will be briefly presented. Second, the technique of surface pump-probe fs-laser
mass spectrometry will be explained. In the third section of this chapter, the data
obtained from the investigated methyl halide molecules on magnesia and the resulting insights into the photoinduced uni- and bimolecular surface reaction dynamics
will be discussed.
M.E. Vaida and T.M. Bernhardt
ular beam [6–8]. On surfaces a complementary approach has been pioneered by
John C. Polanyi which was termed “surface-aligned reaction” and which relies on
an ordered adsorbate structure on a solid surface [5, 9, 10]. This approach will be
described in more detail below.
To detect the resulting dynamics subsequent to the excitation of a molecular complex in aligned geometry in the gas phase, pump-probe mass spectrometry employing ultrafast lasers in combination with molecular beams is a widely applied experimental technique that has considerably advanced the field of femtochemistry [11].
Different experimental approaches are employed to study molecular reaction dynamics in surface femtochemistry, such as two photon photoemission, time-resolved
sum frequency generation, or two pulse correlation spectroscopy [12]. In the present
contribution, we report results of a novel alternative experimental approach for direct mass resolved monitoring of surface transition states and products adapting the
gas phase pump-probe methodology.
The investigations presented here were motivated by a seminal contribution of
John C. Polanyi and Ahmed H. Zewail in 1995 in which they proposed the investigation of surface transition state dynamics via direct detection of reaction intermediates and products in a femtosecond (fs) pump-probe schema similar to gas-phase
femtochemistry investigations [4]. They termed this new approach that relies on a
well defined reactant adsorption geometry ‘surface-aligned femtosecond photoreaction’. A great deal of both, surface-aligned reaction studies (not time-resolved)
[5, 10] as well as real-time surface femtochemistry investigations (see, e.g., Refs.
[12–25]) have since provided insight into surface reaction dynamics. Nevertheless,
the direct time- and mass-resolved detection of the transition state and the product formation dynamics of a surface reaction as proposed by Polanyi and Zewail
has been realized only very recently employing the new approach of surface pumpprobe fs-laser mass spectrometry [26–28].
This technique, which will be described in detail in Sect. 10.2, relies on the combination of time-of-flight mass spectrometry with non-resonant or resonance enhanced multi-photon ionization (REMPI) detection and fast surface preparation by
a pulsed molecular beam [28]. Ultrathin oxide films serve as versatile substrates
that interact only weakly with adsorbed molecules. Yet, the molecular adsorption
structure on the substrate provides the geometrical alignment that determines the
reaction dynamics subsequent to photoexcitation. Escaping fragments might, e.g.,
directly desorb into the gas phase or inelastically exchange energy via collisional
interaction with the substrate surface depending on the adsorption geometry. Furthermore, collisional encounters with neighboring adsorbate molecules might lead
to the formation of new reaction products.
In the following, first, the concept of surface-aligned chemistry will be described
and the investigated methyl halide adsorbate systems as well as their photochemistry will be briefly presented. Second, the technique of surface pump-probe fs-laser
mass spectrometry will be explained. In the third section of this chapter, the data
obtained from the investigated methyl halide molecules on magnesia and the resulting insights into the photoinduced uni- and bimolecular surface reaction dynamics
will be discussed.