8 Ultrafast Ionization and Fragmentation: From Small Molecules
175
(MS) has been hailed as the most promising technology for laser control of chemical reactions. However, despite all of the high hopes, there are only a handful of
groups across the world that have conducted these types of experiments. The pioneers in this field are Gustav Gerber, who published a series of papers in 1998–2003
[22–26], Robert Levis and Hershel Rabitz [27–29], Ludger Wöste [30, 31], Thomas
Weinacht [32–37], and Robert Jones [38]. All of their experiments are based on a
closed-loop approach using learning algorithms to control the laser fields with feedback from the experimental signal [39]. A different strategy that has worked well
on diatomic molecules is to exploit the influence of the laser field on the potential
energy surfaces, through the dynamic Stark shift [40, 41]. Our group has followed
a different approach, often called open-loop, in which sets of differently shaped but
predefined pulses are evaluated for their ability to control chemistry [42–46].
In 2008, we published an article that reviewed the latest work in which shaped
near-IR pulses were used to control molecular fragmentation [2]. In that work, we
studied the laser fragmentation of 16 different molecules as a function of different phase functions, including linear chirp. We found that for several molecules the
relative yield of certain fragment ions could be changed by almost two orders of
magnitude. Interestingly, we found that linear chirping of the laser pulses was sufficient to cause these large changes. In those experiments, we found that vibrational
coherence seemed to play a relatively small role (less than 30 % change in fragment
abundance) while the pulse duration could change some fragment abundances by an
order of magnitude. This observation led us to the conclusion that near-IR femtosecond laser pulses could play a very important role in the development of powerful
analytical methods; however, the reproducibility of the results would depend on the
implementation of methods to ensure that the pulse duration (< 40 fs) would always
be the same. This goal became possible with the development of automated pulse
compression by multiphoton intrapulse interference phase scan (MIIPS) [47, 48].
The combination of ultra-short intense femtosecond pulses with an ion-trap
mass spectrometer led to the development of femtosecond laser-induced ionization/dissociation (fs-LID) by the Dantus and Reid groups [49]. By coupling ultrafast
near-IR laser pulses with the MS n capability of an ion trap mass spectrometer, extensive dissociation of peptides is achieved. The fs-LID instrumentation and method
were first described in 2009, and the fs-LID spectra of four singly, doubly, and triply
protonated peptides allowed for complete sequence determination [49]. We’ve found
that, in positive ion mode, fs-LID is most efficient for singly protonated precursor
ions, which is consistent with the estimate that ionization energy of peptides increases approximately 1.1 eV with each additional positive charge [50]. Fs-LID
is also useful for the mapping of labile post-translational modifications along the
peptide chain, such as phosphorylation, which was demonstrated on two synthetic
phosphothreonine containing peptides. The non-ergodic dissociation patterns observed were due to the femtosecond time-scale of activation, which resulted in the
ultrafast creation of a radical cation and for the ultrafast cleavage of chemical bonds
faster than intramolecular energy redistribution. The applicability of fs-LID to phosphopeptide analysis was investigated further for singly protonated phosphopeptides
175
(MS) has been hailed as the most promising technology for laser control of chemical reactions. However, despite all of the high hopes, there are only a handful of
groups across the world that have conducted these types of experiments. The pioneers in this field are Gustav Gerber, who published a series of papers in 1998–2003
[22–26], Robert Levis and Hershel Rabitz [27–29], Ludger Wöste [30, 31], Thomas
Weinacht [32–37], and Robert Jones [38]. All of their experiments are based on a
closed-loop approach using learning algorithms to control the laser fields with feedback from the experimental signal [39]. A different strategy that has worked well
on diatomic molecules is to exploit the influence of the laser field on the potential
energy surfaces, through the dynamic Stark shift [40, 41]. Our group has followed
a different approach, often called open-loop, in which sets of differently shaped but
predefined pulses are evaluated for their ability to control chemistry [42–46].
In 2008, we published an article that reviewed the latest work in which shaped
near-IR pulses were used to control molecular fragmentation [2]. In that work, we
studied the laser fragmentation of 16 different molecules as a function of different phase functions, including linear chirp. We found that for several molecules the
relative yield of certain fragment ions could be changed by almost two orders of
magnitude. Interestingly, we found that linear chirping of the laser pulses was sufficient to cause these large changes. In those experiments, we found that vibrational
coherence seemed to play a relatively small role (less than 30 % change in fragment
abundance) while the pulse duration could change some fragment abundances by an
order of magnitude. This observation led us to the conclusion that near-IR femtosecond laser pulses could play a very important role in the development of powerful
analytical methods; however, the reproducibility of the results would depend on the
implementation of methods to ensure that the pulse duration (< 40 fs) would always
be the same. This goal became possible with the development of automated pulse
compression by multiphoton intrapulse interference phase scan (MIIPS) [47, 48].
The combination of ultra-short intense femtosecond pulses with an ion-trap
mass spectrometer led to the development of femtosecond laser-induced ionization/dissociation (fs-LID) by the Dantus and Reid groups [49]. By coupling ultrafast
near-IR laser pulses with the MS n capability of an ion trap mass spectrometer, extensive dissociation of peptides is achieved. The fs-LID instrumentation and method
were first described in 2009, and the fs-LID spectra of four singly, doubly, and triply
protonated peptides allowed for complete sequence determination [49]. We’ve found
that, in positive ion mode, fs-LID is most efficient for singly protonated precursor
ions, which is consistent with the estimate that ionization energy of peptides increases approximately 1.1 eV with each additional positive charge [50]. Fs-LID
is also useful for the mapping of labile post-translational modifications along the
peptide chain, such as phosphorylation, which was demonstrated on two synthetic
phosphothreonine containing peptides. The non-ergodic dissociation patterns observed were due to the femtosecond time-scale of activation, which resulted in the
ultrafast creation of a radical cation and for the ultrafast cleavage of chemical bonds
faster than intramolecular energy redistribution. The applicability of fs-LID to phosphopeptide analysis was investigated further for singly protonated phosphopeptides