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R. de Nalda et al.
4.3.2 Transition-State Imaging: The Non-resonant Experiment
It is interesting to discuss at this point the relevance of performing resonant ionization (versus non-resonant) in the probe step, given the broad spectral width and
relatively high intensity of femtosecond pulses. To shed light on this issue, we will
show an example for the A-band dissociation of CH 3 I, described above. The methyl
product formed upon the C–I bond fission is probed through ionization via two
methods: resonant, (2 + 1) REMPI, through the Q branch of the ground-to-Rydberg
3p z ( 2 A
2 ← 2 A
2 )0 0
0 transition with a 333.5 nm, as was described in the previous section, and non-resonant ionization with a moderate intensity (≈ 10 12 W/cm 2 )
800 nm pulse.
Figure 4.6 shows the Abel-inverted velocity maps of CH 3 for the two experiments (non-resonant ionization in (a) and resonant (2 + 1) ionization in (b)) in a
situation where the probe laser is located at a sufficient delay so that dissociation
is complete in both cases. As can be seen in the figure, the two images bear strong
resemblances, with a strong inner ring that is a reflection of the CH 3 + I ∗ ( 2 P 1/2 )
channel, an external, weaker ring for the CH 3 + I( 2 P 3/2 ) channel, and some intermediate structures corresponding to vibrationally excited methyl in the C–H stretch
mode. It has to be noted that the overall signal intensity is considerably lower for the
non-resonant experiment, even for notably higher laser intensities than in the resonant case. This is expected, due to the relatively low photon energy of the 800 nm
beam, and the absence of ionization-enhancing resonances. Another important difference is the more pronounced contribution of ions with low kinetic energies that
appear near the central part of the image and that are the result of competing dissociative ionization pathways. This contribution causes a decrease in contrast for
the channels under study and in certain cases it may mask them beyond detection.
Finally, an important consequence of the use of a non-resonant probe pulse, and
one that may often be desirable, is the absence of selectivity with regards to the
rovibrational components of the nascent fragments. In the case of the methyl fragment, it may quite safely be presumed that practically no selection is performed
in the ionization step. Since the velocity map imaging technique allows to distinguish the degree of internal excitation through the measurement of velocities and
energy conservation arguments, non-resonant probing can then be used to estimate
the global energy distribution in the internal degrees of freedom of the fragments.
The lack of selectivity is shown in the red curve of panel (c) of the figure, which
contains kinetic energy distributions, as broadened features when compared to the
resonant experiment. The difference with the resonant case, where only vibrationless methyl fragments are observed, is particularly marked in correlation with the
I channel, where vibrational population inversion occurs [40]. Under the assumption that all vibrational states of methyl have the same ionization probability by the
non-resonant probe laser, this result provides a direct measurement of the nascent
vibrational populations. In general, this type of experiments can be employed as a
measurement of the complete internal energy content of the fragment if resolution
allows it.
R. de Nalda et al.
4.3.2 Transition-State Imaging: The Non-resonant Experiment
It is interesting to discuss at this point the relevance of performing resonant ionization (versus non-resonant) in the probe step, given the broad spectral width and
relatively high intensity of femtosecond pulses. To shed light on this issue, we will
show an example for the A-band dissociation of CH 3 I, described above. The methyl
product formed upon the C–I bond fission is probed through ionization via two
methods: resonant, (2 + 1) REMPI, through the Q branch of the ground-to-Rydberg
3p z ( 2 A
2 ← 2 A
2 )0 0
0 transition with a 333.5 nm, as was described in the previous section, and non-resonant ionization with a moderate intensity (≈ 10 12 W/cm 2 )
800 nm pulse.
Figure 4.6 shows the Abel-inverted velocity maps of CH 3 for the two experiments (non-resonant ionization in (a) and resonant (2 + 1) ionization in (b)) in a
situation where the probe laser is located at a sufficient delay so that dissociation
is complete in both cases. As can be seen in the figure, the two images bear strong
resemblances, with a strong inner ring that is a reflection of the CH 3 + I ∗ ( 2 P 1/2 )
channel, an external, weaker ring for the CH 3 + I( 2 P 3/2 ) channel, and some intermediate structures corresponding to vibrationally excited methyl in the C–H stretch
mode. It has to be noted that the overall signal intensity is considerably lower for the
non-resonant experiment, even for notably higher laser intensities than in the resonant case. This is expected, due to the relatively low photon energy of the 800 nm
beam, and the absence of ionization-enhancing resonances. Another important difference is the more pronounced contribution of ions with low kinetic energies that
appear near the central part of the image and that are the result of competing dissociative ionization pathways. This contribution causes a decrease in contrast for
the channels under study and in certain cases it may mask them beyond detection.
Finally, an important consequence of the use of a non-resonant probe pulse, and
one that may often be desirable, is the absence of selectivity with regards to the
rovibrational components of the nascent fragments. In the case of the methyl fragment, it may quite safely be presumed that practically no selection is performed
in the ionization step. Since the velocity map imaging technique allows to distinguish the degree of internal excitation through the measurement of velocities and
energy conservation arguments, non-resonant probing can then be used to estimate
the global energy distribution in the internal degrees of freedom of the fragments.
The lack of selectivity is shown in the red curve of panel (c) of the figure, which
contains kinetic energy distributions, as broadened features when compared to the
resonant experiment. The difference with the resonant case, where only vibrationless methyl fragments are observed, is particularly marked in correlation with the
I channel, where vibrational population inversion occurs [40]. Under the assumption that all vibrational states of methyl have the same ionization probability by the
non-resonant probe laser, this result provides a direct measurement of the nascent
vibrational populations. In general, this type of experiments can be employed as a
measurement of the complete internal energy content of the fragment if resolution
allows it.