240
M.E. Vaida and T.M. Bernhardt
Fig. 10.6 Transient recording routine: (1) Methyl iodide is admitted to the surface by the pulsed
valve. During the admission time the laser beam is blocked; (2) the non-adsorbed molecules are
pumped and the delay time t between the pump and the probe laser pulses is adjusted. The
laser beam remains blocked; (3) the surface is exposed to laser light and the methyl mass peak
intensity is integrated until all molecules are dissociated/desorbed from the irradiated area. The
initial surface coverage in this experiment was 0.5 ML and the laser exposure time 5 s (5000 laser
pulses) [28]
To monitor in real-time the methyl halide dissociation dynamics at submonolayer coverage on the magnesia ultrathin films, the following transient recording routine, which is exemplified here for the methyl iodide molecule (illustrated in
Fig. 10.6), is applied while the MgO/Mo(100) surface is positioned in the front of
the TOF-MS (as shown in Fig. 10.5) and cooled to 90 K. (1) An initial coverage
of 0.25 ML to 0.5 ML CH 3 I is dosed onto the surface with the pulsed valve while
the laser is blocked by a computer controlled mechanical shutter (see Fig. 10.5)
so that no methyl signal can be detected by the mass spectrometer as indicated in
Fig. 10.6. (2) A short pumping brake of a few seconds follows. (3) Subsequently, the
laser shutter is opened and the mass peak of interest (methyl cation in Fig. 10.6) is
detected and summed over 2500 to 5000 laser shots at one fixed pump-probe delay
time.
After 2500 to 5000 laser shots, depending on the initial coverage, essentially
all molecules are dissociated and desorbed from the irradiated surface area. Subsequently the laser beams are blocked again and the surface is covered with an
identical amount of molecules as before. During the following short pumping brake
the pump-probe delay time is adjusted and the described data acquisition procedure is repeated for a new pump-probe delay time [28]. Several of the thus obtained
transients were averaged to yield the data shown below.
10.3 Femtosecond Dynamics of Surface Aligned Reactions
The technique of surface pump-probe fs-laser mass spectrometry is now applied to
reveal in real-time the photoinduced reaction dynamics of methyl halide molecules
M.E. Vaida and T.M. Bernhardt
Fig. 10.6 Transient recording routine: (1) Methyl iodide is admitted to the surface by the pulsed
valve. During the admission time the laser beam is blocked; (2) the non-adsorbed molecules are
pumped and the delay time t between the pump and the probe laser pulses is adjusted. The
laser beam remains blocked; (3) the surface is exposed to laser light and the methyl mass peak
intensity is integrated until all molecules are dissociated/desorbed from the irradiated area. The
initial surface coverage in this experiment was 0.5 ML and the laser exposure time 5 s (5000 laser
pulses) [28]
To monitor in real-time the methyl halide dissociation dynamics at submonolayer coverage on the magnesia ultrathin films, the following transient recording routine, which is exemplified here for the methyl iodide molecule (illustrated in
Fig. 10.6), is applied while the MgO/Mo(100) surface is positioned in the front of
the TOF-MS (as shown in Fig. 10.5) and cooled to 90 K. (1) An initial coverage
of 0.25 ML to 0.5 ML CH 3 I is dosed onto the surface with the pulsed valve while
the laser is blocked by a computer controlled mechanical shutter (see Fig. 10.5)
so that no methyl signal can be detected by the mass spectrometer as indicated in
Fig. 10.6. (2) A short pumping brake of a few seconds follows. (3) Subsequently, the
laser shutter is opened and the mass peak of interest (methyl cation in Fig. 10.6) is
detected and summed over 2500 to 5000 laser shots at one fixed pump-probe delay
time.
After 2500 to 5000 laser shots, depending on the initial coverage, essentially
all molecules are dissociated and desorbed from the irradiated surface area. Subsequently the laser beams are blocked again and the surface is covered with an
identical amount of molecules as before. During the following short pumping brake
the pump-probe delay time is adjusted and the described data acquisition procedure is repeated for a new pump-probe delay time [28]. Several of the thus obtained
transients were averaged to yield the data shown below.
10.3 Femtosecond Dynamics of Surface Aligned Reactions
The technique of surface pump-probe fs-laser mass spectrometry is now applied to
reveal in real-time the photoinduced reaction dynamics of methyl halide molecules