4 Femtosecond Photodissociation Dynamics by Velocity Map Imaging
83
produces crucial changes even in a prompt bond fission like A-band dissociation of
CH 3 I.
Clusters, or aggregates, offer a unique chance to study the influence of a weakly
bound environment on a photoinitiated process. The “solvent” effect does not necessarily involve subtle changes: even though, typically, inter-molecular distances
in van der Waals clusters are large, several examples exist in the literature of a
phenomenon known as “concerted” photochemistry, [50, 51] i.e. reaction routes
that are only possible in clusters, since they involve reactions between constituent
molecules. While concerted photochemistry is the most extreme case of clusterspecific chemistry, reactions that can take place in the isolated molecule can witness important differences when the molecule is immersed in such an environment.
It is this type of study that has been undertaken, where the ultrafast ejection of a
CH 3 group from a UV-irradiated methyl iodide dimer (CH 3 I) 2 in the A-band has
been analyzed and the changes with respect to the analogous process in the isolated
molecule have been identified.
The expansion conditions are similar to those employed in the investigation of
monomer photodissociation described above, to ensure a moderate degree of clustering, since the focus of the work is dimer dynamics, and the presence of larger
clusters is undesirable. The experiment was carried out, in this case, in the middle—
colder—part of the molecular beam, so that CH 3 I and (CH 3 I) 2 are the main species,
with number densities of the same order of magnitude. Aggregates up to (CH 3 I) 5
constitute a negligibly low fraction of the detected species [15].
The simultaneous presence of the monomer and the dimer has been turned to our
advantage in this experiment, since the photodissociation process is amenable to
study both species in only one experiment. Figure 4.10 shows the CH 3 images obtained using 267 nm pump (center of the CH 3 I A-band) and 333.5 nm probe photons
(for methyl detection), in monomer (Fig. 4.10a) and cluster conditions (Fig. 4.10b),
acquired at asymptotic time delays. Figures 4.10c and 4.10d show the corresponding
center-of-mass kinetic energy distributions. As was said above, contributions related
to A-band dissociation of both the monomer and the dimer appear on the same image, due to the simultaneous presence of both species. Comparison of Figs. 4.10a
and 4.10b indicates that the presence of clusters introduces three new contributions
to the image. One is a broad, structureless component that can be seen approximately from 0 to ∼ 3 eV and that can be fitted as the sum of two Gaussian distributions on the velocity axis, peaking at ∼ 1.2 eV and ∼ 3 eV. The other two are two
new rings (peaks 1 and 2 in Fig. 4.10d), which appear at lower kinetic energies
compared with those of the monomer (peaks 1 and 2 in Fig. 4.10d), broadened in
energy and with a larger ratio between the high kinetic energy component (I( 2 P 3/2 )
channel in the monomer) and the low kinetic energy component (I ∗ ( 2 P 1/2 ) channel
in the monomer). For the low kinetic energy channel, the contributions from the
monomer and dimer are clearly distinguishable, but this is no longer the case for the
high kinetic energy channel, which appears very significantly broadened and more
intense, so much that the monomer I( 2 P 3/2 ) channel only appears as a shoulder in
the high kinetic energy area of the dimer peak. The weaker CH 3 (ν 1 = 1) + I( 2 P 3/2 )
channel (see section on A-band photodissociation) can no longer be observed in
these conditions.
83
produces crucial changes even in a prompt bond fission like A-band dissociation of
CH 3 I.
Clusters, or aggregates, offer a unique chance to study the influence of a weakly
bound environment on a photoinitiated process. The “solvent” effect does not necessarily involve subtle changes: even though, typically, inter-molecular distances
in van der Waals clusters are large, several examples exist in the literature of a
phenomenon known as “concerted” photochemistry, [50, 51] i.e. reaction routes
that are only possible in clusters, since they involve reactions between constituent
molecules. While concerted photochemistry is the most extreme case of clusterspecific chemistry, reactions that can take place in the isolated molecule can witness important differences when the molecule is immersed in such an environment.
It is this type of study that has been undertaken, where the ultrafast ejection of a
CH 3 group from a UV-irradiated methyl iodide dimer (CH 3 I) 2 in the A-band has
been analyzed and the changes with respect to the analogous process in the isolated
molecule have been identified.
The expansion conditions are similar to those employed in the investigation of
monomer photodissociation described above, to ensure a moderate degree of clustering, since the focus of the work is dimer dynamics, and the presence of larger
clusters is undesirable. The experiment was carried out, in this case, in the middle—
colder—part of the molecular beam, so that CH 3 I and (CH 3 I) 2 are the main species,
with number densities of the same order of magnitude. Aggregates up to (CH 3 I) 5
constitute a negligibly low fraction of the detected species [15].
The simultaneous presence of the monomer and the dimer has been turned to our
advantage in this experiment, since the photodissociation process is amenable to
study both species in only one experiment. Figure 4.10 shows the CH 3 images obtained using 267 nm pump (center of the CH 3 I A-band) and 333.5 nm probe photons
(for methyl detection), in monomer (Fig. 4.10a) and cluster conditions (Fig. 4.10b),
acquired at asymptotic time delays. Figures 4.10c and 4.10d show the corresponding
center-of-mass kinetic energy distributions. As was said above, contributions related
to A-band dissociation of both the monomer and the dimer appear on the same image, due to the simultaneous presence of both species. Comparison of Figs. 4.10a
and 4.10b indicates that the presence of clusters introduces three new contributions
to the image. One is a broad, structureless component that can be seen approximately from 0 to ∼ 3 eV and that can be fitted as the sum of two Gaussian distributions on the velocity axis, peaking at ∼ 1.2 eV and ∼ 3 eV. The other two are two
new rings (peaks 1 and 2 in Fig. 4.10d), which appear at lower kinetic energies
compared with those of the monomer (peaks 1 and 2 in Fig. 4.10d), broadened in
energy and with a larger ratio between the high kinetic energy component (I( 2 P 3/2 )
channel in the monomer) and the low kinetic energy component (I ∗ ( 2 P 1/2 ) channel
in the monomer). For the low kinetic energy channel, the contributions from the
monomer and dimer are clearly distinguishable, but this is no longer the case for the
high kinetic energy channel, which appears very significantly broadened and more
intense, so much that the monomer I( 2 P 3/2 ) channel only appears as a shoulder in
the high kinetic energy area of the dimer peak. The weaker CH 3 (ν 1 = 1) + I( 2 P 3/2 )
channel (see section on A-band photodissociation) can no longer be observed in
these conditions.