i.e., via statistical fragmentation schemes. It should be noticed that in the case of
photo-fragmentation, the energy content of an ion is relatively well defined as the
thermal energy of the ion before excitation plus the photon energy.
Primary fragments can still contain a lot of energy and can fragment further.
However, the primary fragment does not contain a well-defined internal energy.
Indeed in the first fragmentation process, the initial energy is shared between the
neutral fragments, the ionic fragments and the translational kinetic energy. Thus,
among the primary fragments, the ones that have more internal energy will fragment rapidly while those that have an internal energy just above the fragmentation
barrier will take a very long time to break up. The entire succession of events occurs
on different time scales ranging from the hundred of femtoseconds [28] to seconds
[30], i.e., over more than 12 orders of magnitude and thus different experimental
techniques have to be used to characterise the complete process.
9.4
Fragmentation Pathways and Time Evolution
To disentangle the fragmentation processes induced by optical excitation, a lot of
information is required: the fragmentation time, the nature and the number of
fragments, and the energy distribution (internal and kinetic) of the fragments.
In most experiments only fragment ions are detected, whereas a complete
analysis of the fragmentation mechanism requires the detection of both ionic and
neutral species. In a few experiments, the neutral fragments have been detected
through electron-impact ionisation or electron capture from an alkali gas [1], but
there is no link between a given ion and a neutral product since these fragments are
not detected in coincidence, and the neutral fragment may undergo additional
fragmentation in the re-ionisation process, complicating the analysis. In particular,
it is important to know how many neutral fragments are produced together with a
fragment ion: if more than one neutral fragment is produced, are they produced
together or sequentially and in which order?
Understanding the fragmentation mechanisms also involves analysing the energetics and kinetics of the reaction. For the energetics, this means that one has to
control the initial energy of the system before the fragmentation; then a UV photon
is well suited for exciting the system as long as the thermal energy is not too high,
i.e., small compared to the photon energy. As we will see, information on the
fragment kinetic energies can be obtained, but the internal energy (vibrational and
rotational energy distribution) has not yet been obtained in such large systems.
The fragmentation time is also a very important information. It can help to
differentiate fragmentation occurring in the excited states (very fast sub nanosecond)
from processes occurring after internal conversion in the ground state or secondary
fragmentation (i.e., fragmentation of hot fragments).
160
C. Dedonder et al.
photo-fragmentation, the energy content of an ion is relatively well defined as the
thermal energy of the ion before excitation plus the photon energy.
Primary fragments can still contain a lot of energy and can fragment further.
However, the primary fragment does not contain a well-defined internal energy.
Indeed in the first fragmentation process, the initial energy is shared between the
neutral fragments, the ionic fragments and the translational kinetic energy. Thus,
among the primary fragments, the ones that have more internal energy will fragment rapidly while those that have an internal energy just above the fragmentation
barrier will take a very long time to break up. The entire succession of events occurs
on different time scales ranging from the hundred of femtoseconds [28] to seconds
[30], i.e., over more than 12 orders of magnitude and thus different experimental
techniques have to be used to characterise the complete process.
9.4
Fragmentation Pathways and Time Evolution
To disentangle the fragmentation processes induced by optical excitation, a lot of
information is required: the fragmentation time, the nature and the number of
fragments, and the energy distribution (internal and kinetic) of the fragments.
In most experiments only fragment ions are detected, whereas a complete
analysis of the fragmentation mechanism requires the detection of both ionic and
neutral species. In a few experiments, the neutral fragments have been detected
through electron-impact ionisation or electron capture from an alkali gas [1], but
there is no link between a given ion and a neutral product since these fragments are
not detected in coincidence, and the neutral fragment may undergo additional
fragmentation in the re-ionisation process, complicating the analysis. In particular,
it is important to know how many neutral fragments are produced together with a
fragment ion: if more than one neutral fragment is produced, are they produced
together or sequentially and in which order?
Understanding the fragmentation mechanisms also involves analysing the energetics and kinetics of the reaction. For the energetics, this means that one has to
control the initial energy of the system before the fragmentation; then a UV photon
is well suited for exciting the system as long as the thermal energy is not too high,
i.e., small compared to the photon energy. As we will see, information on the
fragment kinetic energies can be obtained, but the internal energy (vibrational and
rotational energy distribution) has not yet been obtained in such large systems.
The fragmentation time is also a very important information. It can help to
differentiate fragmentation occurring in the excited states (very fast sub nanosecond)
from processes occurring after internal conversion in the ground state or secondary
fragmentation (i.e., fragmentation of hot fragments).
160
C. Dedonder et al.