236
M.E. Vaida and T.M. Bernhardt
desorption signal in Fig. 10.3a shifts with increasing coverage to lower temperatures indicating a desorption activation energy that further decreases with increasing coverage. On MgO single crystals [69–71] the similar temperature dependence
was interpreted in terms of a repulsive adsorbate-adsorbate interaction due to an
adsorbate structure with the CH 3 I symmetry axis and hence the permanent dipole
moment of the molecules aligned parallel to each other and parallel to or slightly
tilted from the surface normal. On bulk magnesia an adsorption structure with the
I-atom facing the surface was proposed earlier [70, 71]. On magnesia ultrathin films
on Mo(100), however, an orientation was reported previously with the methyl group
heading toward the substrate surface [26]. This adsorption geometry is in accordance with the time-resolved investigations and it is illustrated schematically in the
inset of Fig. 10.3a. However, it has to be noted that neither the adsorption site, nor
the exact tilt angle of the molecules with respect to the surface normal are known
[27].
In Fig. 10.3b a series of TPD spectra are shown that have been recorded after dosing different amounts of CH 3 Br onto an 8 ML MgO/Mo(100) film at 90 K
[68]. No evidence for methyl bromide dissociation subsequent to adsorption on the
MgO/Mo(100) substrate was found. No reaction products such as ethane or molecular bromine, which could be formed due to methyl bromide dissociation on the
surface, were observed. Hence, methyl bromide also adsorbs molecularly on the
MgO/Mo(100) surface and desorbs without decomposition. Also in the case of
methyl bromide the maximum of the desorption signal shifts to lower temperatures
with increasing coverage indicating a repulsive adsorbate-adsorbate interaction between the molecules. The same behavior had been observed previously for CH 3 Br
adsorbed at low coverages on bulk MgO(100) [36] and on LiF(100) [72]. However,
angular distributions of methyl fragments emerging from CH 3 Br photodissociation
on MgO(100) lead to the assignment of an adsorbate structure in which the C–Br
axes of the adsorbed CH 3 Br molecules lay close to the plane of the substrate surface
as indicated by the inset in Fig. 10.3b [36].
10.1.3 Laser-Induced Molecular Desorption and Reaction
on the Magnesium Oxide Surface
The adsorbate alignment on the surface that has been discussed in the previous section determines the photoinduced reaction dynamics of the methyl halide molecules
on magnesia. Light-induced fragmentation and photoreactions are commonly studied experimentally via angularly resolved time-of-flight quadrupole mass spectrometry. This technique has also been applied by several groups in the past to investigate
the photoreaction of methyl iodide molecules adsorbed on a magnesium oxide single crystal surface [70, 71, 73–77]. For sub-monolayer coverages [70, 71], the most
striking experimental observation was the considerably reduced I ∗ /I branching ratio as compared to the gas phase photodissociation, indicating a particular substrate
M.E. Vaida and T.M. Bernhardt
desorption signal in Fig. 10.3a shifts with increasing coverage to lower temperatures indicating a desorption activation energy that further decreases with increasing coverage. On MgO single crystals [69–71] the similar temperature dependence
was interpreted in terms of a repulsive adsorbate-adsorbate interaction due to an
adsorbate structure with the CH 3 I symmetry axis and hence the permanent dipole
moment of the molecules aligned parallel to each other and parallel to or slightly
tilted from the surface normal. On bulk magnesia an adsorption structure with the
I-atom facing the surface was proposed earlier [70, 71]. On magnesia ultrathin films
on Mo(100), however, an orientation was reported previously with the methyl group
heading toward the substrate surface [26]. This adsorption geometry is in accordance with the time-resolved investigations and it is illustrated schematically in the
inset of Fig. 10.3a. However, it has to be noted that neither the adsorption site, nor
the exact tilt angle of the molecules with respect to the surface normal are known
[27].
In Fig. 10.3b a series of TPD spectra are shown that have been recorded after dosing different amounts of CH 3 Br onto an 8 ML MgO/Mo(100) film at 90 K
[68]. No evidence for methyl bromide dissociation subsequent to adsorption on the
MgO/Mo(100) substrate was found. No reaction products such as ethane or molecular bromine, which could be formed due to methyl bromide dissociation on the
surface, were observed. Hence, methyl bromide also adsorbs molecularly on the
MgO/Mo(100) surface and desorbs without decomposition. Also in the case of
methyl bromide the maximum of the desorption signal shifts to lower temperatures
with increasing coverage indicating a repulsive adsorbate-adsorbate interaction between the molecules. The same behavior had been observed previously for CH 3 Br
adsorbed at low coverages on bulk MgO(100) [36] and on LiF(100) [72]. However,
angular distributions of methyl fragments emerging from CH 3 Br photodissociation
on MgO(100) lead to the assignment of an adsorbate structure in which the C–Br
axes of the adsorbed CH 3 Br molecules lay close to the plane of the substrate surface
as indicated by the inset in Fig. 10.3b [36].
10.1.3 Laser-Induced Molecular Desorption and Reaction
on the Magnesium Oxide Surface
The adsorbate alignment on the surface that has been discussed in the previous section determines the photoinduced reaction dynamics of the methyl halide molecules
on magnesia. Light-induced fragmentation and photoreactions are commonly studied experimentally via angularly resolved time-of-flight quadrupole mass spectrometry. This technique has also been applied by several groups in the past to investigate
the photoreaction of methyl iodide molecules adsorbed on a magnesium oxide single crystal surface [70, 71, 73–77]. For sub-monolayer coverages [70, 71], the most
striking experimental observation was the considerably reduced I ∗ /I branching ratio as compared to the gas phase photodissociation, indicating a particular substrate