8 Ultrafast Ionization and Fragmentation: From Small Molecules
187
dissociate into smaller and smaller pieces; however, a helium bath gas maintains a
pressure around 1 mTorr in the ion trap, so as product ions are formed, they undergo
collisional cooling and are stored until the detection period.
While it was known that pulse shaping led to changes in the relative yields of
different fragments, it was yet to be determined if these changes were related to
coherently controlled quantum mechanical interference or if they were related to
ladder-switching mechanisms that were first identified when picosecond pulses were
compared with nanosecond pulses. In other words, longer pulses can be absorbed
by the molecule at different times, and thereby access different pathways.
In order to explore evidence of coherence in the fragmentation of p-NT using
shaped laser pulses we first determined that laser intensity and central wavelength
has little or no effect on the relative ion yields. We then evaluated the relative yield of
several ions as a function of different families of shaped pulses (sinusoidal function,
chirp, binary phases, cubic phases, etc.). The results from thousands of experiments
were analyzed and there seemed to be no evidence of a vibrational or electronic
coherence that had been selectively excited by one of the different shaped pulses.
A selection of those results is shown in Fig. 8.10, where results are compared for
four different types of shaped pulses and four different laser intensities. Note that the
relative yields of C 7 H
+
7 and C 3 H
+
3 , track closely, and show no intensity dependence.
When the study was submitted for publication the reviewer asked if p-NT was
a particularly different molecule, and perhaps an exception. We then studied 16
other molecules, and our findings are best summarized in Fig. 8.11. The yield of
the strongest peaks in the mass spectrum of the different molecules was tracked as a
function of pulse shaping. Results are shown as a function of chirp (continuous line)
and as a function of sinusoidal shaping (dotted lines). Correlation between chirp and
sinusoidal shaping was achieved by matching the integrated (all masses) ion yield
produced by the laser pulses. The close agreement shown in the data, evidenced as
coincidence between chirped and sinusoidal shaped pulses, suggests to us that the
spectral-temporal details of the pulse were not as important as the average duration
of the pulses.
We concluded that pulse duration, in fact, was the most important predictor of
fragmentation. One explanation for the observed effect on pulse duration is that ions
are able to undergo further fragmentation through the absorption of additional photons of the incident field. Deviations from this conclusion are possible, especially
when vibrational and electronic coherence survive the initial tunnel ionization process. We believe that coherent control of photofragmentation under tunnel ionization
conditions should be possible, provided the excitation fields used are shorter than
∼ 5 optical cycles.
8.4 Results from Peptides [5]
Here we analyze the fragmentation mechanism involved in fs-LID MS/MS of
trapped peptides. We start with the protonated amino acids and several derivatives
187
dissociate into smaller and smaller pieces; however, a helium bath gas maintains a
pressure around 1 mTorr in the ion trap, so as product ions are formed, they undergo
collisional cooling and are stored until the detection period.
While it was known that pulse shaping led to changes in the relative yields of
different fragments, it was yet to be determined if these changes were related to
coherently controlled quantum mechanical interference or if they were related to
ladder-switching mechanisms that were first identified when picosecond pulses were
compared with nanosecond pulses. In other words, longer pulses can be absorbed
by the molecule at different times, and thereby access different pathways.
In order to explore evidence of coherence in the fragmentation of p-NT using
shaped laser pulses we first determined that laser intensity and central wavelength
has little or no effect on the relative ion yields. We then evaluated the relative yield of
several ions as a function of different families of shaped pulses (sinusoidal function,
chirp, binary phases, cubic phases, etc.). The results from thousands of experiments
were analyzed and there seemed to be no evidence of a vibrational or electronic
coherence that had been selectively excited by one of the different shaped pulses.
A selection of those results is shown in Fig. 8.10, where results are compared for
four different types of shaped pulses and four different laser intensities. Note that the
relative yields of C 7 H
+
7 and C 3 H
+
3 , track closely, and show no intensity dependence.
When the study was submitted for publication the reviewer asked if p-NT was
a particularly different molecule, and perhaps an exception. We then studied 16
other molecules, and our findings are best summarized in Fig. 8.11. The yield of
the strongest peaks in the mass spectrum of the different molecules was tracked as a
function of pulse shaping. Results are shown as a function of chirp (continuous line)
and as a function of sinusoidal shaping (dotted lines). Correlation between chirp and
sinusoidal shaping was achieved by matching the integrated (all masses) ion yield
produced by the laser pulses. The close agreement shown in the data, evidenced as
coincidence between chirped and sinusoidal shaped pulses, suggests to us that the
spectral-temporal details of the pulse were not as important as the average duration
of the pulses.
We concluded that pulse duration, in fact, was the most important predictor of
fragmentation. One explanation for the observed effect on pulse duration is that ions
are able to undergo further fragmentation through the absorption of additional photons of the incident field. Deviations from this conclusion are possible, especially
when vibrational and electronic coherence survive the initial tunnel ionization process. We believe that coherent control of photofragmentation under tunnel ionization
conditions should be possible, provided the excitation fields used are shorter than
∼ 5 optical cycles.
8.4 Results from Peptides [5]
Here we analyze the fragmentation mechanism involved in fs-LID MS/MS of
trapped peptides. We start with the protonated amino acids and several derivatives