254
M.E. Vaida and T.M. Bernhardt
constant for all trajectories leading from the bimolecular transition state to I 2 is
represented by the 310 fs rise of the molecular iodine signal.
Molecular iodine is bound in the B-state and the radiative lifetime of the molecule
in this state is on the order of microseconds [120]. However, the transient data in
Fig. 10.12d exhibit a decay of the I
+
2 signal starting after several ps. This decay is
attributed to a coupling between the B-state and a 1 Π u dissociative state induced by
the presence of the surface in the vicinity of the iodine molecule (see Fig. 10.13).
Thus, the breaking of the D ∞h symmetry of I 2 at the surface leads to molecular predissociation on the ps timescale [121]. As a result, only ground state iodine atoms
are released after several ps. Interestingly, this also provides a new mechanism for
the previously observed pronounced and puzzling I ∗ quenching in surface photodissociation experiments (cf. Sect. 10.1.3 [71]).
Thus, the mass spectrometric detection of intermediates and products in a pumpprobe scheme is able to reveal rich details of the bimolecular surface photoreaction
dynamics. In this respect the influence of the insulating surface on the reaction dynamics can be shown to manifests itself in the unimolecular decomposition by (1)
the alignment of the reactants prior to photoreaction, (2) the trapping of the dissociative transition state preventing direct decomposition as in the gas-phase and
in this way (3) enhancing the possibility for a non-adiabatic transition of the wave
packet through the conical intersection. Furthermore, the presence of the substrate
(4) provides an additional energy dissipation pathway for the bimolecular reaction to
proceed, and finally (5) leads to a breaking of the molecular symmetry of the I 2 (B)
state to enable predissociation and thus opening a new channel for I ∗ de-excitation
at the surface.
Also in the case of a methyl bromide adsorption layer on a magnesia surface the
bimolecular reaction of the emerging halide atoms could be detected [68] and a similar reaction dynamics as in the case of methyl iodide might be assumed. However,
the assignment of the bimolecular dynamics, i.e. of the actual detection excitation
pathways, remains so far to be confirmed by detailed laser power and wavelength
dependent measurements.
10.4 Conclusion and Prospects
In this contribution it was demonstrated that fs-laser induced excitation of a surfacealigned molecular adsorbate layer on an oxide surface in conjunction with time delayed direct ionization and mass spectrometric detection of reaction products on
the surface can provide detailed insight into the molecular surface reaction dynamics. As examples the photoreactions of methyl iodide and methyl bromide adsorbed
on ultrathin MgO(100) films on Mo(100) have been studied. A pronounced difference in the appearance times of the methyl radical fragments was observed for the
two different methyl halides. This observation was related to the different molecular adsorption structures on magnesia. Furthermore, the adsorption geometry on
the surface also defined the alignment of the encounter complex for the observed
bimolecular formation of the halide molecules I 2 and Br 2 .
M.E. Vaida and T.M. Bernhardt
constant for all trajectories leading from the bimolecular transition state to I 2 is
represented by the 310 fs rise of the molecular iodine signal.
Molecular iodine is bound in the B-state and the radiative lifetime of the molecule
in this state is on the order of microseconds [120]. However, the transient data in
Fig. 10.12d exhibit a decay of the I
+
2 signal starting after several ps. This decay is
attributed to a coupling between the B-state and a 1 Π u dissociative state induced by
the presence of the surface in the vicinity of the iodine molecule (see Fig. 10.13).
Thus, the breaking of the D ∞h symmetry of I 2 at the surface leads to molecular predissociation on the ps timescale [121]. As a result, only ground state iodine atoms
are released after several ps. Interestingly, this also provides a new mechanism for
the previously observed pronounced and puzzling I ∗ quenching in surface photodissociation experiments (cf. Sect. 10.1.3 [71]).
Thus, the mass spectrometric detection of intermediates and products in a pumpprobe scheme is able to reveal rich details of the bimolecular surface photoreaction
dynamics. In this respect the influence of the insulating surface on the reaction dynamics can be shown to manifests itself in the unimolecular decomposition by (1)
the alignment of the reactants prior to photoreaction, (2) the trapping of the dissociative transition state preventing direct decomposition as in the gas-phase and
in this way (3) enhancing the possibility for a non-adiabatic transition of the wave
packet through the conical intersection. Furthermore, the presence of the substrate
(4) provides an additional energy dissipation pathway for the bimolecular reaction to
proceed, and finally (5) leads to a breaking of the molecular symmetry of the I 2 (B)
state to enable predissociation and thus opening a new channel for I ∗ de-excitation
at the surface.
Also in the case of a methyl bromide adsorption layer on a magnesia surface the
bimolecular reaction of the emerging halide atoms could be detected [68] and a similar reaction dynamics as in the case of methyl iodide might be assumed. However,
the assignment of the bimolecular dynamics, i.e. of the actual detection excitation
pathways, remains so far to be confirmed by detailed laser power and wavelength
dependent measurements.
10.4 Conclusion and Prospects
In this contribution it was demonstrated that fs-laser induced excitation of a surfacealigned molecular adsorbate layer on an oxide surface in conjunction with time delayed direct ionization and mass spectrometric detection of reaction products on
the surface can provide detailed insight into the molecular surface reaction dynamics. As examples the photoreactions of methyl iodide and methyl bromide adsorbed
on ultrathin MgO(100) films on Mo(100) have been studied. A pronounced difference in the appearance times of the methyl radical fragments was observed for the
two different methyl halides. This observation was related to the different molecular adsorption structures on magnesia. Furthermore, the adsorption geometry on
the surface also defined the alignment of the encounter complex for the observed
bimolecular formation of the halide molecules I 2 and Br 2 .