184
M. Dantus and C.L. Kalcic
ganic molecules is experimentally observed around 1–2 × 10 14 W/cm 2 [3, 90]. Additionally, it was observed that shaping of the femtosecond laser pulses resulted in
different product ion distributions, sparking several studies in laser control of molecular fragmentation [22]. Our group has reviewed these studies and followed up with
a systematic evaluation of which field parameters are responsible for the changes observed in the fragmentation pathways [2]. We observed that the mass spectrum observed for polyatomic molecules under near-threshold ionization by transform limited sub-40 fs pulses was very similar to that obtained by electron-impact ionization
(EIMS). In general, mechanisms for fragmentation following EIMS are well understood as radical-cation chemical reactions that proceed in the absence of molecular
coherence. In order to better understand the system, the interaction of near-IR femtosecond laser pulses with small isolated molecules and cations was investigated
through fs-TOF and fs-LID experiments on para-nitrotoluene (pNT) [54]. The different fragmentation pathways for the neutral and protonated forms of pNT were of
particular interest.
Mass spectra of pNT obtained by electron ionization (EI), fs-TOF, CID, and fsLID are shown in Fig. 8.8. The EI spectrum is obtained from the NIST database.
Transform limited pulses with 35 fs time duration are used for fs-TOF and fsLID experiments, with the peak power density of 4.0 × 10 15 W/cm 2 and 1.6 ×
10 14 W/cm 2 , respectively.
The photochemistry of pNT has been studied intensively. Although some controversy remains, the major pathways of dissociation are well established and are
adopted here to explain our data. In the fs-TOF experiments, pNT molecules are
tunnel ionized instantaneously. The molecular ions undergo isomerization of CH 3 -
f -NO 2 to CH 3 -f -O-NO, as observed by the CH 3 -f -O + product ion and the competitive production of NO 2 and NO through a common transition state [91]. In the
presence of a strong field, the elimination of NO 2 is the predominant step following isomerization. The resulting C 7 H
+
7 ions exist as an equilibrium of benzyl and
tropylium ions, which was confirmed by their absorption bands at 263 and 353 nm
respectively [92]. Sequential absorption of photons releases C 2 H 2 fragments, giving
rise to C 5 H
+
5 , C 3 H
+
3 , and C + ions.
The fragmentation pattern of pNT by electron ionization (Fig. 8.8) is very similar
to that seen by fs-TOF with transform limited pulses. However, less molecular ion
and more small fragments (C + , H + ) are observed in the fs-TOF spectrum. This is
due to the absorption of additional photons by larger fragments including molecular
ions, resulting in sequential fragmentation
The fs-LID ion trap experiments on protonated pNT reveal that the even-electron,
protonated molecule (MH + ) follows a slightly different fragmentation pathway than
the ionized neutral (M +• ). The fs-LID spectrum (Fig. 8.8) shows a combination of
even- and odd-electron product ions, and is dominated by losses of heteroatoms
from the nitro group. While the peak at 121 (C 7 H 7 NO +• ), resulting from the loss of
OH, is absent in the fs-TOF spectrum, all other fs-LID product ions match up with
fs-TOF product ions, with an occasional shift of 1 m/z due to the retention of an
extra proton. The tropylium ion is a minor product in fs-LID and therefore the benzene ring fragment ions (such as C 5 H
+
5 ) are also in low abundance. While smaller
M. Dantus and C.L. Kalcic
ganic molecules is experimentally observed around 1–2 × 10 14 W/cm 2 [3, 90]. Additionally, it was observed that shaping of the femtosecond laser pulses resulted in
different product ion distributions, sparking several studies in laser control of molecular fragmentation [22]. Our group has reviewed these studies and followed up with
a systematic evaluation of which field parameters are responsible for the changes observed in the fragmentation pathways [2]. We observed that the mass spectrum observed for polyatomic molecules under near-threshold ionization by transform limited sub-40 fs pulses was very similar to that obtained by electron-impact ionization
(EIMS). In general, mechanisms for fragmentation following EIMS are well understood as radical-cation chemical reactions that proceed in the absence of molecular
coherence. In order to better understand the system, the interaction of near-IR femtosecond laser pulses with small isolated molecules and cations was investigated
through fs-TOF and fs-LID experiments on para-nitrotoluene (pNT) [54]. The different fragmentation pathways for the neutral and protonated forms of pNT were of
particular interest.
Mass spectra of pNT obtained by electron ionization (EI), fs-TOF, CID, and fsLID are shown in Fig. 8.8. The EI spectrum is obtained from the NIST database.
Transform limited pulses with 35 fs time duration are used for fs-TOF and fsLID experiments, with the peak power density of 4.0 × 10 15 W/cm 2 and 1.6 ×
10 14 W/cm 2 , respectively.
The photochemistry of pNT has been studied intensively. Although some controversy remains, the major pathways of dissociation are well established and are
adopted here to explain our data. In the fs-TOF experiments, pNT molecules are
tunnel ionized instantaneously. The molecular ions undergo isomerization of CH 3 -
f -NO 2 to CH 3 -f -O-NO, as observed by the CH 3 -f -O + product ion and the competitive production of NO 2 and NO through a common transition state [91]. In the
presence of a strong field, the elimination of NO 2 is the predominant step following isomerization. The resulting C 7 H
+
7 ions exist as an equilibrium of benzyl and
tropylium ions, which was confirmed by their absorption bands at 263 and 353 nm
respectively [92]. Sequential absorption of photons releases C 2 H 2 fragments, giving
rise to C 5 H
+
5 , C 3 H
+
3 , and C + ions.
The fragmentation pattern of pNT by electron ionization (Fig. 8.8) is very similar
to that seen by fs-TOF with transform limited pulses. However, less molecular ion
and more small fragments (C + , H + ) are observed in the fs-TOF spectrum. This is
due to the absorption of additional photons by larger fragments including molecular
ions, resulting in sequential fragmentation
The fs-LID ion trap experiments on protonated pNT reveal that the even-electron,
protonated molecule (MH + ) follows a slightly different fragmentation pathway than
the ionized neutral (M +• ). The fs-LID spectrum (Fig. 8.8) shows a combination of
even- and odd-electron product ions, and is dominated by losses of heteroatoms
from the nitro group. While the peak at 121 (C 7 H 7 NO +• ), resulting from the loss of
OH, is absent in the fs-TOF spectrum, all other fs-LID product ions match up with
fs-TOF product ions, with an occasional shift of 1 m/z due to the retention of an
extra proton. The tropylium ion is a minor product in fs-LID and therefore the benzene ring fragment ions (such as C 5 H
+
5 ) are also in low abundance. While smaller