9.4.1 The Method: Coincidence Experiments
Photo-fragmentation of protonated Trp and Tyr has been investigated in a unique
experimental setup (Arc en Ciel setup in Orsay), in which ionic and neutral
fragments from photo-fragmentation are detected in coincidence, in time and in
position. From these data the kinetic energy, the number of neutral fragments
associated with a fragment ion, their masses, and the order of the fragmentation
steps are extracted. Moreover the fragmentation time scale ranging from tens of
nanoseconds to milliseconds is obtained. Longer time scales (in the hundred of
millisecond range) have been studied at the ELISA setup in Aarhus [30]. From all
these data a comprehensive fragmentation mechanism has been proposed.
A detailed description of the experimental set-up and data analysis can be found
in [31–33]. In order to get the number of neutral fragment(s) produced with one
ionic fragment, coincidence techniques have to be used, which means that one and
only one parent ion is photo-dissociated each time the laser is fired, and that the
ionic and neutral fragments are detected individually. To ensure that no more than
one ion is excited for each laser shot, the number of ions detected per laser pulse has
to be a lot smaller than the repetition rate: for example if one ion is detected every
100 laser shots, the probability of exciting two ions (false coincidences) is 10
À4
(assuming a detection efficiency of 100 %). Experimentally, the ions produced by
an electrospray ion source (ESI) are bunched in packets of 50-ns duration,
containing about 1,000 ions at a repetition rate of 1 kHz. The ion bunch is
accelerated up to 2.5 kV. When one parent ion is photo-dissociated, the neutral
species reach a position sensitive detector (PSD), while the ionic fragment is
deflected in a 45
electrostatic parallel-plate analyser before reaching another
PSD. For each laser shot, the arrival times and positions of the neutral and ionic
fragments are recorded. This experimental set-up allows recording the fragmentation time in the range of ns to ms, the number of neutral fragments produced from
each ion detected, and the order of the fragmentation steps.
9.4.2 Results: The Tryptophan Case
The photo-dissociation mass spectrum of protonated tryptophan (Fig. 9.4) has been
recorded by many groups [20, 34–36]. Some fragments are the same as those
obtained from Collision Induced Dissociation (CID) [37] and some are specific to
the photo-dissociation process, namely the loss of H (m/z ¼ 204) and the C α –C β
bond rupture (m/z ¼ 130). This is already an indication that the optical excitation
induces some dynamics in the excited state, and that not everything is governed by
internal conversion followed by statistical fragmentation.
The coincidence experiments allow to go further, the main results being: [32]
(1) Among the fragmentation channels, the ion at m/z ¼ 204 reveals the H-loss
reaction is specific to UV photo-excitation.
(2) Except for the m/z ¼ 130 fragments due to secondary fragmentations after
H-loss ion, all fragmentation processes are finished in 10 μs after 266-nm
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
161
Photo-fragmentation of protonated Trp and Tyr has been investigated in a unique
experimental setup (Arc en Ciel setup in Orsay), in which ionic and neutral
fragments from photo-fragmentation are detected in coincidence, in time and in
position. From these data the kinetic energy, the number of neutral fragments
associated with a fragment ion, their masses, and the order of the fragmentation
steps are extracted. Moreover the fragmentation time scale ranging from tens of
nanoseconds to milliseconds is obtained. Longer time scales (in the hundred of
millisecond range) have been studied at the ELISA setup in Aarhus [30]. From all
these data a comprehensive fragmentation mechanism has been proposed.
A detailed description of the experimental set-up and data analysis can be found
in [31–33]. In order to get the number of neutral fragment(s) produced with one
ionic fragment, coincidence techniques have to be used, which means that one and
only one parent ion is photo-dissociated each time the laser is fired, and that the
ionic and neutral fragments are detected individually. To ensure that no more than
one ion is excited for each laser shot, the number of ions detected per laser pulse has
to be a lot smaller than the repetition rate: for example if one ion is detected every
100 laser shots, the probability of exciting two ions (false coincidences) is 10
À4
(assuming a detection efficiency of 100 %). Experimentally, the ions produced by
an electrospray ion source (ESI) are bunched in packets of 50-ns duration,
containing about 1,000 ions at a repetition rate of 1 kHz. The ion bunch is
accelerated up to 2.5 kV. When one parent ion is photo-dissociated, the neutral
species reach a position sensitive detector (PSD), while the ionic fragment is
deflected in a 45
electrostatic parallel-plate analyser before reaching another
PSD. For each laser shot, the arrival times and positions of the neutral and ionic
fragments are recorded. This experimental set-up allows recording the fragmentation time in the range of ns to ms, the number of neutral fragments produced from
each ion detected, and the order of the fragmentation steps.
9.4.2 Results: The Tryptophan Case
The photo-dissociation mass spectrum of protonated tryptophan (Fig. 9.4) has been
recorded by many groups [20, 34–36]. Some fragments are the same as those
obtained from Collision Induced Dissociation (CID) [37] and some are specific to
the photo-dissociation process, namely the loss of H (m/z ¼ 204) and the C α –C β
bond rupture (m/z ¼ 130). This is already an indication that the optical excitation
induces some dynamics in the excited state, and that not everything is governed by
internal conversion followed by statistical fragmentation.
The coincidence experiments allow to go further, the main results being: [32]
(1) Among the fragmentation channels, the ion at m/z ¼ 204 reveals the H-loss
reaction is specific to UV photo-excitation.
(2) Except for the m/z ¼ 130 fragments due to secondary fragmentations after
H-loss ion, all fragmentation processes are finished in 10 μs after 266-nm
9 Excited-State Dynamics of Protonated Aromatic Amino Acids
161