fragmentation was purely statistical, all the decay curves should be the same
whatever the fragment, which clearly is not the case. The fastest decay time
observed in the excited-state dynamics is a few hundreds of femtoseconds, which
is the fastest that could be measured with the laser. This is in line with the absence
of well-defined vibronic bands in the spectroscopic experiment in a cold ion trap,
which suggests even faster dynamics (in the order of 10 fs) to wipe out the
vibrational spectrum [35]. This was rather surprising since the first excited-state
lifetime of neutral Trp is in the ns range [5]. The presence of an excess proton does
not change the energetics of the transition much but it strongly affects the nonradiative processes in the excited state.
One should also note that the probe photon is changing the branching ratio
between fragments, in particular increasing the signal of m/z ¼ 130 (C α –C β bond
rupture) at very short times. This experimental result is a clear evidence that one can
control the fragmentation channels and branching ratios using appropriate femtosecond laser pulses with the right delay between the pump and the probe laser.
To be able to understand such complex time-dependent signals, more information is necessary, in particular the fragmentation mechanism and the parent/daughter ion relationship as well as the fragmentation time (from coincidence
experiments).
9.6.2 Bi-exponential Decays
For all the fragments (except m/z ¼ 132), the observed time-dependent signals
could be fitted by a bi-exponential decay. It is a good opportunity to discuss the
danger of over-interpreting experimental results, and we want to spend a few lines
considering the case of bi-exponential decays. As already mentioned, the pumpprobe signals for TrpH
+ fragmentation were obtained with hot ions directly
extracted from the electrospray source, and thus the UV photon is exciting many
conformers. Theoretical calculations indicate that some conformers should have a
very short lifetime and others should have long lifetimes since the energies of the
dissociative states (πσ*) and stable state (ππ*) are changing quite a lot upon rotation
around the C α –C β bond. Since the excited ions are hot, the signal observed should
be the sum of all these lifetimes ranging from a few femtoseconds to some
nanoseconds. Although not mathematically exact, the signal produced by such an
ensemble of molecules can be fitted nicely with a bi-exponential decay function
[55]. However, the two time constants obtained have no physical meaning, and in
particular they should not be assigned to the excitation of two species nor to two
excited states as is so often done in the literature. Basically, if the only information
on a system is a bi-exponential decay, one can only deduce that there are some
species that have a short lifetime and some with longer lifetimes, that’s all!
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
171
whatever the fragment, which clearly is not the case. The fastest decay time
observed in the excited-state dynamics is a few hundreds of femtoseconds, which
is the fastest that could be measured with the laser. This is in line with the absence
of well-defined vibronic bands in the spectroscopic experiment in a cold ion trap,
which suggests even faster dynamics (in the order of 10 fs) to wipe out the
vibrational spectrum [35]. This was rather surprising since the first excited-state
lifetime of neutral Trp is in the ns range [5]. The presence of an excess proton does
not change the energetics of the transition much but it strongly affects the nonradiative processes in the excited state.
One should also note that the probe photon is changing the branching ratio
between fragments, in particular increasing the signal of m/z ¼ 130 (C α –C β bond
rupture) at very short times. This experimental result is a clear evidence that one can
control the fragmentation channels and branching ratios using appropriate femtosecond laser pulses with the right delay between the pump and the probe laser.
To be able to understand such complex time-dependent signals, more information is necessary, in particular the fragmentation mechanism and the parent/daughter ion relationship as well as the fragmentation time (from coincidence
experiments).
9.6.2 Bi-exponential Decays
For all the fragments (except m/z ¼ 132), the observed time-dependent signals
could be fitted by a bi-exponential decay. It is a good opportunity to discuss the
danger of over-interpreting experimental results, and we want to spend a few lines
considering the case of bi-exponential decays. As already mentioned, the pumpprobe signals for TrpH
+ fragmentation were obtained with hot ions directly
extracted from the electrospray source, and thus the UV photon is exciting many
conformers. Theoretical calculations indicate that some conformers should have a
very short lifetime and others should have long lifetimes since the energies of the
dissociative states (πσ*) and stable state (ππ*) are changing quite a lot upon rotation
around the C α –C β bond. Since the excited ions are hot, the signal observed should
be the sum of all these lifetimes ranging from a few femtoseconds to some
nanoseconds. Although not mathematically exact, the signal produced by such an
ensemble of molecules can be fitted nicely with a bi-exponential decay function
[55]. However, the two time constants obtained have no physical meaning, and in
particular they should not be assigned to the excitation of two species nor to two
excited states as is so often done in the literature. Basically, if the only information
on a system is a bi-exponential decay, one can only deduce that there are some
species that have a short lifetime and some with longer lifetimes, that’s all!
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
171