4 Femtosecond Photodissociation Dynamics by Velocity Map Imaging
81
surface of rapidly changing potential, and therefore, parts of the wave packet with a
range of kinetic energies will contribute for a given delay time.
The above discussion shows that a strategy based on employing an off-resonant
laser probe, can go beyond the idea of finding resonances in transient species, and
can profit from universal ionization at sufficiently high laser intensities, coupled to
a detection technique that is sensitive to the energy content of the fragments.
4.3.3 Observation of Transient Molecular Alignment
A few words will be said here about the contribution of laser-induced molecular
alignment to the fragment angular distributions observed in photodissociation processes. In general, any molecular system whose polarizability is not isotropic will
show some tendency to align in the presence of an alternating electric field [44, 45],
in a process where the axis of highest polarizability will tend to align along the
light polarization axis. For long pulses (pulses of longer duration than the rotational
period of the molecule), molecular alignment adiabatically follows the laser pulse
intensity envelope, whereas for short pulses, the molecule cannot follow the envelope and, instead, coherences are created between the rotational components that
give rise to recurrences, or revivals of the molecular preferential alignment [46, 47].
The two types of alignment are often termed adiabatic and impulsive alignment,
respectively.
Laser-induced molecular alignment must be taken into account in all situations
where the target molecule has anisotropic polarizability (as most do) and the laser
intensity is high enough. In practice, typical photodissociation processes induced
in the UV with moderate pulse energies are not carried out at intensities that are
sufficiently high to induce an important degree of alignment. However, whenever
infrared fields are added, with the purpose of inducing nonlinear effects (multiphoton absorption, Coulomb explosion, etc.), the contribution of molecular alignment
to the observed angular distributions cannot be disregarded as long as the intensities
are of the order of 10 12 W/cm 2 or above.
An example of the considerations above will be shown for the case of CH 3 I Aband dissociation, through the inspection of the angular distributions of the CH
+
3
images shown in Fig. 4.7. It is obvious simply through visual inspection that the
most important contribution at times near temporal overlap between the 266 nm UV
pulse and the 800 nm IR pulse is angularly narrower than the main asymptotic contribution observed at 450 fs delay. Two effects may contribute to this narrowing of
the angular distribution near time zero: one is a multiphoton pump step, and the other
is a dynamic alignment process of the parent molecule due to the intense 800 nm
pulse [48, 49]. In order to separate these effects, a further three laser pulse experiment was performed where impulsive molecular alignment was induced using an
equivalent—same energy per pulse and focusing conditions—800 nm pulse. After
a controlled delay, a photodissociation experiment was performed at 266 nm, followed by an on-resonance, long delay time REMPI probing of the resulting methyl
81
surface of rapidly changing potential, and therefore, parts of the wave packet with a
range of kinetic energies will contribute for a given delay time.
The above discussion shows that a strategy based on employing an off-resonant
laser probe, can go beyond the idea of finding resonances in transient species, and
can profit from universal ionization at sufficiently high laser intensities, coupled to
a detection technique that is sensitive to the energy content of the fragments.
4.3.3 Observation of Transient Molecular Alignment
A few words will be said here about the contribution of laser-induced molecular
alignment to the fragment angular distributions observed in photodissociation processes. In general, any molecular system whose polarizability is not isotropic will
show some tendency to align in the presence of an alternating electric field [44, 45],
in a process where the axis of highest polarizability will tend to align along the
light polarization axis. For long pulses (pulses of longer duration than the rotational
period of the molecule), molecular alignment adiabatically follows the laser pulse
intensity envelope, whereas for short pulses, the molecule cannot follow the envelope and, instead, coherences are created between the rotational components that
give rise to recurrences, or revivals of the molecular preferential alignment [46, 47].
The two types of alignment are often termed adiabatic and impulsive alignment,
respectively.
Laser-induced molecular alignment must be taken into account in all situations
where the target molecule has anisotropic polarizability (as most do) and the laser
intensity is high enough. In practice, typical photodissociation processes induced
in the UV with moderate pulse energies are not carried out at intensities that are
sufficiently high to induce an important degree of alignment. However, whenever
infrared fields are added, with the purpose of inducing nonlinear effects (multiphoton absorption, Coulomb explosion, etc.), the contribution of molecular alignment
to the observed angular distributions cannot be disregarded as long as the intensities
are of the order of 10 12 W/cm 2 or above.
An example of the considerations above will be shown for the case of CH 3 I Aband dissociation, through the inspection of the angular distributions of the CH
+
3
images shown in Fig. 4.7. It is obvious simply through visual inspection that the
most important contribution at times near temporal overlap between the 266 nm UV
pulse and the 800 nm IR pulse is angularly narrower than the main asymptotic contribution observed at 450 fs delay. Two effects may contribute to this narrowing of
the angular distribution near time zero: one is a multiphoton pump step, and the other
is a dynamic alignment process of the parent molecule due to the intense 800 nm
pulse [48, 49]. In order to separate these effects, a further three laser pulse experiment was performed where impulsive molecular alignment was induced using an
equivalent—same energy per pulse and focusing conditions—800 nm pulse. After
a controlled delay, a photodissociation experiment was performed at 266 nm, followed by an on-resonance, long delay time REMPI probing of the resulting methyl