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M. Dantus and C.L. Kalcic
8.3 Results from Small Polyatomic Molecules
8.3.1 Vibrational and Electronic Coherence
For over three decades short pulses have been used to create coherent superpositions of states and to observe quantum beats as a function of time delay. As shorter
laser pulses have become available, it has become possible to create these vibrational wave packets involving vibrational modes of even the lightest atoms. Shorter
pulses have also accessed the creation of superpositions of electronic states. When
the vibrational or electronic wave packets are formed through excitation using a
field that is one-photon resonant with the states, the formation and time evolution
are well understood. Here, however, we focus on an application involving femtosecond lasers that are in the near-IR, and therefore not resonant with the dissociative
or ionic states of the molecules being studied. If a typical organic molecule has an
ionization-energy of 9 eV and the photon energy is ∼1.5 eV, then at least 6 photons
are needed to create the ion and several more are required to yield fragment ions.
The focus of this section is to provide information about the vibrational and electronic coherence in small polyatomic molecules soon after they have been subjected
to strong-field ionization by a near-IR femtosecond laser pulse. The extent of vibrational and electronic coherence that survives the strong-field ionization is relevant
because it can be harvested in order to achieve coherent control using shaped pulses.
Strong field experiments in diatomic molecules have revealed that it is possible to form coherent vibrational and rotational wave packets. An example of such
observations is the strong field ionization of deuterium to form D
+
2 , and the observation of vibrational oscillations (∼ 25 fs period) and rotational recurrences (∼550 fs)
[86, 87]. The vibrational oscillations were clearly visible when 12 fs pulses were
used; however, when longer pulses were used the vibrational oscillations were no
longer observed. More recently, the creation of superpositions of electronic and vibrational states has been observed following tunnel ionization of N 2 , O 2 , and CO
with few-cycle pulses [88]. Results from the strong field ionization of CH 2 I 2 show
evidence of I–C–I bending coherent wave packet motion considered to arise through
the formation of “multihole” wave packets. Interest in using tunnel ionization as a
method for activating macromolecules being studied by mass spectrometry requires
us to consider if vibrational and electronic coherence survive in larger molecules.
Results from acetophenone and substituted acetophenones from our group showed
evidence of coherent wave packet motion [89]. Of particular interest in that research
was the effect of substituents in the aromatic ring. For example, Fig. 8.7 shows portions of the transients obtained for acetophenone (red) partially deuterated (CD 3 )
acetophenone (black), ortho (cyan), meta (indigo) and para (green)-methyl acetophenone. Ortho substitution increases the oscillation period from 0.7 ps to 1 ps,
while meta substitution results in no oscillations. while meta substitution results in
no oscillations. The observations are related to the preferred electronic configuration
of the differently substituted compounds and the torsion of the carbonyl group.
Although it is suspected that the tunnel ionization process is capable of producing superpositions of electronic states, the evolution of the electronic wave packets
M. Dantus and C.L. Kalcic
8.3 Results from Small Polyatomic Molecules
8.3.1 Vibrational and Electronic Coherence
For over three decades short pulses have been used to create coherent superpositions of states and to observe quantum beats as a function of time delay. As shorter
laser pulses have become available, it has become possible to create these vibrational wave packets involving vibrational modes of even the lightest atoms. Shorter
pulses have also accessed the creation of superpositions of electronic states. When
the vibrational or electronic wave packets are formed through excitation using a
field that is one-photon resonant with the states, the formation and time evolution
are well understood. Here, however, we focus on an application involving femtosecond lasers that are in the near-IR, and therefore not resonant with the dissociative
or ionic states of the molecules being studied. If a typical organic molecule has an
ionization-energy of 9 eV and the photon energy is ∼1.5 eV, then at least 6 photons
are needed to create the ion and several more are required to yield fragment ions.
The focus of this section is to provide information about the vibrational and electronic coherence in small polyatomic molecules soon after they have been subjected
to strong-field ionization by a near-IR femtosecond laser pulse. The extent of vibrational and electronic coherence that survives the strong-field ionization is relevant
because it can be harvested in order to achieve coherent control using shaped pulses.
Strong field experiments in diatomic molecules have revealed that it is possible to form coherent vibrational and rotational wave packets. An example of such
observations is the strong field ionization of deuterium to form D
+
2 , and the observation of vibrational oscillations (∼ 25 fs period) and rotational recurrences (∼550 fs)
[86, 87]. The vibrational oscillations were clearly visible when 12 fs pulses were
used; however, when longer pulses were used the vibrational oscillations were no
longer observed. More recently, the creation of superpositions of electronic and vibrational states has been observed following tunnel ionization of N 2 , O 2 , and CO
with few-cycle pulses [88]. Results from the strong field ionization of CH 2 I 2 show
evidence of I–C–I bending coherent wave packet motion considered to arise through
the formation of “multihole” wave packets. Interest in using tunnel ionization as a
method for activating macromolecules being studied by mass spectrometry requires
us to consider if vibrational and electronic coherence survive in larger molecules.
Results from acetophenone and substituted acetophenones from our group showed
evidence of coherent wave packet motion [89]. Of particular interest in that research
was the effect of substituents in the aromatic ring. For example, Fig. 8.7 shows portions of the transients obtained for acetophenone (red) partially deuterated (CD 3 )
acetophenone (black), ortho (cyan), meta (indigo) and para (green)-methyl acetophenone. Ortho substitution increases the oscillation period from 0.7 ps to 1 ps,
while meta substitution results in no oscillations. while meta substitution results in
no oscillations. The observations are related to the preferred electronic configuration
of the differently substituted compounds and the torsion of the carbonyl group.
Although it is suspected that the tunnel ionization process is capable of producing superpositions of electronic states, the evolution of the electronic wave packets