88
R. de Nalda et al.
does not occur. Even though this is somewhat shorter than the experimental delay
(∼ 140 fs), it is clearly of the same order of magnitude, and we believe this is the
main mechanism producing a delay in the apparent dissociation times for the dimer.
We consider that this is a clear illustration of the fact that this effect, where a substantial delay occurs in the time opening of the optical detection window, needs to
be taken into account whenever resonant probing is employed, especially for large
molecules, and in particular clusters, where intermediate orbitals used for the resonant transition can be substantially modified by the nearby presence of a similar
moiety.
The time-resolved work performed on the (CH 3 I) 2 species shows that even the
influence of a weakly bound environment can have profound influences on a molecular dissociation process. The phenomenon that was chosen (fast ejection of a CH 3
fragment from a (CH 3 I) 2 dimer) would not seem prone to showing this effect, due
to the extremely fast nature of the dissociation and the weak bond between the two
CH 3 I molecules. Nevertheless, dramatic changes both in the absorbing states and
in the strength of non-adiabatic couplings between them have been detected. It is
expected that these cluster-related effects will be even more important in slower
processes and more tightly bound systems.
4.4 The B Band
The second absorption band in methyl iodide, the B band, has been by far much
less studied than the more accessible A-band. It starts at around 200 nm and hence,
the possibility of performing detailed spectroscopic studies with conventional lasers
is restricted to the red edge of the band. The B band results from the excitation
of a 5pπ electron of the I atom to a 6s molecular Rydberg orbital. The remaining
three 5pπ electrons are subject to strong spin-orbit coupling, so that, in C 3v symmetry, the ionic core can be in the 2 Π 3/2 ( 2 E 3/2 ) or the 2 Π 1/2 ( 2 E 1/2 ) states. From
the J –j coupling between the ionic core states and the Rydberg electron several
bound states are formed. Transitions to those related with the 2 Π 3/2 ( 2 E 3/2 ) core,
from the ground state, constitute the B band, of perpendicular character. Discrete
vibrational structure appears in the spectrum [52–56], but the lines are considerably
lifetime broadened due to interaction with dissociative surfaces that belong to the
A band, which causes electronic predissociation. Lifetimes of these states are in the
picosecond regime and are strongly dependent on vibrational excitation in a non
monotonic manner [53].
The characteristics of B-band predissociation in CH 3 I, including lifetimes,
anisotropy, branching ratio, and vibrational activity of the methyl fragments, have
proven to be strongly dependent on the vibrational level of initial excitation in the
parent molecule. The extreme sensitivity to the vibrational excitation is related to
the details of the crossings between the potential energy surfaces and the spatial
distributions of the wave functions at each vibrational level. Therefore, systematic measurements of the properties of this predissociation process provide challenging constraints for the theoretical description of this system. Here, we will
R. de Nalda et al.
does not occur. Even though this is somewhat shorter than the experimental delay
(∼ 140 fs), it is clearly of the same order of magnitude, and we believe this is the
main mechanism producing a delay in the apparent dissociation times for the dimer.
We consider that this is a clear illustration of the fact that this effect, where a substantial delay occurs in the time opening of the optical detection window, needs to
be taken into account whenever resonant probing is employed, especially for large
molecules, and in particular clusters, where intermediate orbitals used for the resonant transition can be substantially modified by the nearby presence of a similar
moiety.
The time-resolved work performed on the (CH 3 I) 2 species shows that even the
influence of a weakly bound environment can have profound influences on a molecular dissociation process. The phenomenon that was chosen (fast ejection of a CH 3
fragment from a (CH 3 I) 2 dimer) would not seem prone to showing this effect, due
to the extremely fast nature of the dissociation and the weak bond between the two
CH 3 I molecules. Nevertheless, dramatic changes both in the absorbing states and
in the strength of non-adiabatic couplings between them have been detected. It is
expected that these cluster-related effects will be even more important in slower
processes and more tightly bound systems.
4.4 The B Band
The second absorption band in methyl iodide, the B band, has been by far much
less studied than the more accessible A-band. It starts at around 200 nm and hence,
the possibility of performing detailed spectroscopic studies with conventional lasers
is restricted to the red edge of the band. The B band results from the excitation
of a 5pπ electron of the I atom to a 6s molecular Rydberg orbital. The remaining
three 5pπ electrons are subject to strong spin-orbit coupling, so that, in C 3v symmetry, the ionic core can be in the 2 Π 3/2 ( 2 E 3/2 ) or the 2 Π 1/2 ( 2 E 1/2 ) states. From
the J –j coupling between the ionic core states and the Rydberg electron several
bound states are formed. Transitions to those related with the 2 Π 3/2 ( 2 E 3/2 ) core,
from the ground state, constitute the B band, of perpendicular character. Discrete
vibrational structure appears in the spectrum [52–56], but the lines are considerably
lifetime broadened due to interaction with dissociative surfaces that belong to the
A band, which causes electronic predissociation. Lifetimes of these states are in the
picosecond regime and are strongly dependent on vibrational excitation in a non
monotonic manner [53].
The characteristics of B-band predissociation in CH 3 I, including lifetimes,
anisotropy, branching ratio, and vibrational activity of the methyl fragments, have
proven to be strongly dependent on the vibrational level of initial excitation in the
parent molecule. The extreme sensitivity to the vibrational excitation is related to
the details of the crossings between the potential energy surfaces and the spatial
distributions of the wave functions at each vibrational level. Therefore, systematic measurements of the properties of this predissociation process provide challenging constraints for the theoretical description of this system. Here, we will