4 Femtosecond Photodissociation Dynamics by Velocity Map Imaging
93
the lifetime of the excited state, and also to the degree of rotational excitation in the
parent molecule [9]. From semiclassical models, rotational temperatures of ∼ 40 K
are expected to account for the measured values [58, 59].
4.4.3 Time-Resolved Photoelectron Imaging
Time-resolved photoelectron velocity map imaging experiments provide additional
information on the process. In this case, a pump-probe scheme has been found that
allows the detection of photoelectrons originated by ionization of the parent in the
excited Rydberg state (at short delay times), or those originated by REMPI of the iodine atom born after the molecule breaks (for long delays). For intermediate delays,
both signals are present simultaneously and can be separated through their kinetic
energies. This was achieved through a pump-probe scheme where the pump beam
is centered at the desired B-band transition of the molecule, and the probe beam
causes REMPI ionization in the product. In this manner, photoelectrons originated
from ionization of the parent are created in a (1 + 1 ) process, and those originated
in the iodine fragment are caused by a (1 + 2 + 1 ) process, where the first photon
excites the parent molecule, and the subsequent (2 + 1 ) process occurs in the free
fragment. This is shown in Fig. 4.17, where the results corresponding to the 0 0
0 and
3 0
1 bands are shown in the left and right panels, respectively. In both cases, the photoelectron rings that appear most intense for short delays gradually become dimmer,
with the time constant of the molecular predissociation, and the higher-radius rings
due to ionization of the iodine product grow in intensity until they reach their final
value, with a time constant consistent with the former. It is interesting to note that
only photoelectrons due to the ionization of the iodine atom product, but not to the
methyl product, are detectable, since iodine ionization is strongly enhanced due to
the use of a REMPI scheme.
Photoelectron spectra obtained through angular integration of the rings in
Fig. 4.17 are shown in Fig. 4.18 for early (top panels) and late times (bottom panels)
for the 0 0
0 band (left panels) and the 3 0
1 band (right panels). In each case, they are
plotted as a function of the binding energy of the corresponding species. The photoelectron spectra provide information on the initial molecular excitation, and in that
sense, the results shown in panel (b) of Fig. 4.18 constitute a confirmation that the
initial excitation is almost exclusively to the ν 3 = 1 state. Since the potential energy
surface of the B 6s [2] Rydberg state is similar to the ground state surface of the ion,
transitions with ν = 0 are expected to dominate [56, 60] and, therefore, the main
contribution is related to the formation of CH 3 I + with one quantum of excitation in
the ν 3 mode due to initial excitation to the ν 3 = 1 level of the Rydberg state. Other
vibrational combinations are visible in the spectrum, but always with at least one
quantum of excitation in the ν 3 mode.
As mentioned before, when acquired with a long delay time between the pump
and probe laser pulses, the photoelectrons are originated from the iodine atoms resulting from dissociation. Due to the pulse bandwidth, the (2 + 1) REMPI schemes
93
the lifetime of the excited state, and also to the degree of rotational excitation in the
parent molecule [9]. From semiclassical models, rotational temperatures of ∼ 40 K
are expected to account for the measured values [58, 59].
4.4.3 Time-Resolved Photoelectron Imaging
Time-resolved photoelectron velocity map imaging experiments provide additional
information on the process. In this case, a pump-probe scheme has been found that
allows the detection of photoelectrons originated by ionization of the parent in the
excited Rydberg state (at short delay times), or those originated by REMPI of the iodine atom born after the molecule breaks (for long delays). For intermediate delays,
both signals are present simultaneously and can be separated through their kinetic
energies. This was achieved through a pump-probe scheme where the pump beam
is centered at the desired B-band transition of the molecule, and the probe beam
causes REMPI ionization in the product. In this manner, photoelectrons originated
from ionization of the parent are created in a (1 + 1 ) process, and those originated
in the iodine fragment are caused by a (1 + 2 + 1 ) process, where the first photon
excites the parent molecule, and the subsequent (2 + 1 ) process occurs in the free
fragment. This is shown in Fig. 4.17, where the results corresponding to the 0 0
0 and
3 0
1 bands are shown in the left and right panels, respectively. In both cases, the photoelectron rings that appear most intense for short delays gradually become dimmer,
with the time constant of the molecular predissociation, and the higher-radius rings
due to ionization of the iodine product grow in intensity until they reach their final
value, with a time constant consistent with the former. It is interesting to note that
only photoelectrons due to the ionization of the iodine atom product, but not to the
methyl product, are detectable, since iodine ionization is strongly enhanced due to
the use of a REMPI scheme.
Photoelectron spectra obtained through angular integration of the rings in
Fig. 4.17 are shown in Fig. 4.18 for early (top panels) and late times (bottom panels)
for the 0 0
0 band (left panels) and the 3 0
1 band (right panels). In each case, they are
plotted as a function of the binding energy of the corresponding species. The photoelectron spectra provide information on the initial molecular excitation, and in that
sense, the results shown in panel (b) of Fig. 4.18 constitute a confirmation that the
initial excitation is almost exclusively to the ν 3 = 1 state. Since the potential energy
surface of the B 6s [2] Rydberg state is similar to the ground state surface of the ion,
transitions with ν = 0 are expected to dominate [56, 60] and, therefore, the main
contribution is related to the formation of CH 3 I + with one quantum of excitation in
the ν 3 mode due to initial excitation to the ν 3 = 1 level of the Rydberg state. Other
vibrational combinations are visible in the spectrum, but always with at least one
quantum of excitation in the ν 3 mode.
As mentioned before, when acquired with a long delay time between the pump
and probe laser pulses, the photoelectrons are originated from the iodine atoms resulting from dissociation. Due to the pulse bandwidth, the (2 + 1) REMPI schemes