induces an important modification of the charge distribution between the ground
and excited states.
9.6
Excited-State Dynamics
The excited-state dynamics at very short times have been obtained for ions directly
issued from an electrospray source, thus from hot ions (how hot nobody knows!).
The method to obtain information on the excited-state dynamics is to perform
pump/probe femtosecond spectroscopy. This is a standard technique in which the
first photon promotes the system to the excited state while the second photon probes
the evolution of the excited state by re-exciting the parent and monitoring the
fragments. In the case of ions for which the fragmentation pattern is the only
observable, the second laser should change the fragmentation pathways. It is in
fact a complex phenomenon to disentangle since fragmentation can be affected in
two ways, either by a change in the fragmentation rate or by a change in the
fragmentation branching ratio.
To a first approximation, one can assume that the probe laser is bringing some
additional energy to the system, and that this will change (increase) the reaction
rates and the branching ratio when there are many fragments. Indeed, the observed
signal depends strongly upon what the experiment is measuring:
(a) The experiment is measuring fragmentation times.
Fragmentation-time sensitive experiments can be achieved by photofragmenting a fast ion in an electric field parallel to the propagation axis and
by selecting the observed fragment ions produced at a given position in this
field, which is equivalent to selecting the ion produced at a given time [52, 53].
If one selects experimentally fragment ions produced in a time interval [0–20 ns
for example] and if these photo-fragment ions are produced with a time constant
T [as an example 100 ns], the absorption of the probe-laser photon will increase
the internal energy and thus increase the fragmentation rate k
0
¼ 1/T
0 [as an
example to T
0 ~1 ns], so that more fragment ions are produced in the observation window. The absorption of the probe photon will be seen as an increase of
the ion signal, and the excited-state lifetime is seen as a decay of this ion signal
when the delay between the pump and the probe is increased. At the opposite, if
the observation temporal window selects ions produced at long time delay, the
probe laser will produce a depopulation of the ion signal. This is one of the
reasons why the pump/probe signals recorded on the TrpH
+ fragments in
Fig. 9.10 are so complex.
(b) If the experiment is not set up for such a time sensitive detection scheme, i.e., if
the analysis of the fragmentation process occurs a long time after the fragmentation has occurred (in an ion trap for example), then only the variation of the
branching ratio between the fragment ions can be detected. This implies that no
pump/probe signal can be detected when only one fragment is produced since
only the rate is changing, which the experiment is not measuring. It may be
interesting to revise the above deduction. If one ion (TrpH
+ for example) gives
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
169
and excited states.
9.6
Excited-State Dynamics
The excited-state dynamics at very short times have been obtained for ions directly
issued from an electrospray source, thus from hot ions (how hot nobody knows!).
The method to obtain information on the excited-state dynamics is to perform
pump/probe femtosecond spectroscopy. This is a standard technique in which the
first photon promotes the system to the excited state while the second photon probes
the evolution of the excited state by re-exciting the parent and monitoring the
fragments. In the case of ions for which the fragmentation pattern is the only
observable, the second laser should change the fragmentation pathways. It is in
fact a complex phenomenon to disentangle since fragmentation can be affected in
two ways, either by a change in the fragmentation rate or by a change in the
fragmentation branching ratio.
To a first approximation, one can assume that the probe laser is bringing some
additional energy to the system, and that this will change (increase) the reaction
rates and the branching ratio when there are many fragments. Indeed, the observed
signal depends strongly upon what the experiment is measuring:
(a) The experiment is measuring fragmentation times.
Fragmentation-time sensitive experiments can be achieved by photofragmenting a fast ion in an electric field parallel to the propagation axis and
by selecting the observed fragment ions produced at a given position in this
field, which is equivalent to selecting the ion produced at a given time [52, 53].
If one selects experimentally fragment ions produced in a time interval [0–20 ns
for example] and if these photo-fragment ions are produced with a time constant
T [as an example 100 ns], the absorption of the probe-laser photon will increase
the internal energy and thus increase the fragmentation rate k
0
¼ 1/T
0 [as an
example to T
0 ~1 ns], so that more fragment ions are produced in the observation window. The absorption of the probe photon will be seen as an increase of
the ion signal, and the excited-state lifetime is seen as a decay of this ion signal
when the delay between the pump and the probe is increased. At the opposite, if
the observation temporal window selects ions produced at long time delay, the
probe laser will produce a depopulation of the ion signal. This is one of the
reasons why the pump/probe signals recorded on the TrpH
+ fragments in
Fig. 9.10 are so complex.
(b) If the experiment is not set up for such a time sensitive detection scheme, i.e., if
the analysis of the fragmentation process occurs a long time after the fragmentation has occurred (in an ion trap for example), then only the variation of the
branching ratio between the fragment ions can be detected. This implies that no
pump/probe signal can be detected when only one fragment is produced since
only the rate is changing, which the experiment is not measuring. It may be
interesting to revise the above deduction. If one ion (TrpH
+ for example) gives
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
169