250
M.E. Vaida and T.M. Bernhardt
pump photons. According to Ref. [113], a 3 A 1 dissociative A-band state can be
accessed near the C–Br equilibrium distance by two-photon excitation at 266 nm.
When spin-orbit interactions are considered, this 3 A 1 state gives rise to two states
corresponding to 3 Q 0 , and 3 Q 1 , respectively. As discussed in detail previously [68],
these states represent the most probable two-photon excitation channel of the CH 3 Br
molecule which lead to dissociation.
The emerging methyl radicals are subsequently detected via (2 + 1) REMPI at
a wavelength of 333 nm. This is confirmed by the probe power dependence measurements shown in the lower inset of Fig. 10.11a. Similar to CH 3 I/MgO, the initial
coherent delay of t 0 = 150 ± 50 fs of the methyl ion signal emerging from CH 3 Br
(Fig. 10.11b) is proposed to reflect the liberation of the methyl fragments from the
molecular force field and from the force field of the magnesia surface. The subsequent growth of the methyl signal with a time constant of τ = 320 ± 60 fs is
consequently interpreted as the average lifetime of all trajectories leading form the
initial excitation to the release of the methyl fragment.
Surprisingly, the time constant of τ = 320 ± 60 fs for the rise of the CH
+
3 signal
in Fig. 10.11b is considerably shorter compared to the one obtained for the exponential rise of the methyl cation signal in the case of CH 3 I photodissociation on MgO
(680 ± 50 fs, see previous section). As mentioned above, no direct measurement of
the CH 3 Br photodissociation time has been performed until now. However, based on
the method provided by Sander and Wilson [116] to calculate the lifetime of a dissociative state using the anisotropy parameter β which is deduced from the angular
distribution of the photofragments, Gougousi et al. [117] estimated that the lifetime
of the CH 3 Br A-band is 120 ± 40 fs. If this method is applied to CH 3 I, an upper
limit for the lifetime of the A-band of 70 fs can be estimated [118]. This means that
in the gas-phase the methyl bromide A-band photodissociation takes longer than the
methyl iodide A-band photodissociation. However, in the present investigation the
methyl bromide appears to photodissociate faster than methyl iodide, if adsorbed on
MgO.
In Sect. 10.1.3, the investigations of Polanyi and coworkers of the 193 nm photodissociation of CH 3 Br adsorbed on a bulk MgO(100) surface by means of angular resolved time-of-flight quadrupole mass spectrometry have been discussed [36].
This experiment indicates that the emerging methyl photofragments can be either
directly liberated into the gas-phase (DIR CH 3 fragments; see Fig. 10.4) or they
can lose a significant part of their total kinetic energy released from the A-band
photodissociation in a collision process with neighboring molecules (IND(1) and
IND(2) CH 3 fragments; see Fig. 10.4). These latter indirect mechanisms represent
the dominant methyl release channel in this system in which the escaping methyl
fragments have to experience a collision with adjacent molecules due to an adsorption geometry in which the C–Br axis is nearly parallel to the surface (cf. inset in
Fig. 10.3b and Fig. 10.4b). These results provide the basis for a reasonable explanation why the methyl photofragment appearance time on MgO, measured in our
experiment, is much faster for methyl bromide (320 ± 60 fs) compared to methyl
iodide (680 ± 50 fs) despite of the theoretical prediction of a longer dissociation
time for free CH 3 Br.
M.E. Vaida and T.M. Bernhardt
pump photons. According to Ref. [113], a 3 A 1 dissociative A-band state can be
accessed near the C–Br equilibrium distance by two-photon excitation at 266 nm.
When spin-orbit interactions are considered, this 3 A 1 state gives rise to two states
corresponding to 3 Q 0 , and 3 Q 1 , respectively. As discussed in detail previously [68],
these states represent the most probable two-photon excitation channel of the CH 3 Br
molecule which lead to dissociation.
The emerging methyl radicals are subsequently detected via (2 + 1) REMPI at
a wavelength of 333 nm. This is confirmed by the probe power dependence measurements shown in the lower inset of Fig. 10.11a. Similar to CH 3 I/MgO, the initial
coherent delay of t 0 = 150 ± 50 fs of the methyl ion signal emerging from CH 3 Br
(Fig. 10.11b) is proposed to reflect the liberation of the methyl fragments from the
molecular force field and from the force field of the magnesia surface. The subsequent growth of the methyl signal with a time constant of τ = 320 ± 60 fs is
consequently interpreted as the average lifetime of all trajectories leading form the
initial excitation to the release of the methyl fragment.
Surprisingly, the time constant of τ = 320 ± 60 fs for the rise of the CH
+
3 signal
in Fig. 10.11b is considerably shorter compared to the one obtained for the exponential rise of the methyl cation signal in the case of CH 3 I photodissociation on MgO
(680 ± 50 fs, see previous section). As mentioned above, no direct measurement of
the CH 3 Br photodissociation time has been performed until now. However, based on
the method provided by Sander and Wilson [116] to calculate the lifetime of a dissociative state using the anisotropy parameter β which is deduced from the angular
distribution of the photofragments, Gougousi et al. [117] estimated that the lifetime
of the CH 3 Br A-band is 120 ± 40 fs. If this method is applied to CH 3 I, an upper
limit for the lifetime of the A-band of 70 fs can be estimated [118]. This means that
in the gas-phase the methyl bromide A-band photodissociation takes longer than the
methyl iodide A-band photodissociation. However, in the present investigation the
methyl bromide appears to photodissociate faster than methyl iodide, if adsorbed on
MgO.
In Sect. 10.1.3, the investigations of Polanyi and coworkers of the 193 nm photodissociation of CH 3 Br adsorbed on a bulk MgO(100) surface by means of angular resolved time-of-flight quadrupole mass spectrometry have been discussed [36].
This experiment indicates that the emerging methyl photofragments can be either
directly liberated into the gas-phase (DIR CH 3 fragments; see Fig. 10.4) or they
can lose a significant part of their total kinetic energy released from the A-band
photodissociation in a collision process with neighboring molecules (IND(1) and
IND(2) CH 3 fragments; see Fig. 10.4). These latter indirect mechanisms represent
the dominant methyl release channel in this system in which the escaping methyl
fragments have to experience a collision with adjacent molecules due to an adsorption geometry in which the C–Br axis is nearly parallel to the surface (cf. inset in
Fig. 10.3b and Fig. 10.4b). These results provide the basis for a reasonable explanation why the methyl photofragment appearance time on MgO, measured in our
experiment, is much faster for methyl bromide (320 ± 60 fs) compared to methyl
iodide (680 ± 50 fs) despite of the theoretical prediction of a longer dissociation
time for free CH 3 Br.