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R. de Nalda et al.
in conventional experiments, performed with a large number of molecules, each
event is characterized by its own impact parameter, relative velocities and positions,
so that, for the ensemble, the origin of time is particularly ill-defined in the scale
characteristic of the reaction itself.
On the contrary, photo-induced processes (photoionization, photoisomerization,
photodissociation) are often termed “half reactions” [1], with an origin of time that
is set by the laser pulse initiating the process. Some degree of uncertainty remains,
however, due to the temporal width of the laser pulse, and thus time zero is best defined for the shortest pulse available. In practice, it is common to assign the origin
of time to the maximum of the intensity envelope of the laser pulse used to induce
a given process. It is the temporal width of the laser pulse that sets the time resolution and hence, the time scales that can be explored with such pulse. Even for half
reactions, however, the end of the process remains considerably less well defined,
and in practice, the most useful definition depends on the technique to be used as a
probe of the process. If a short non-resonant laser pulse is employed as an ionization probe, transition state species can be subjected to ionization at all times, and
one would have to define the “end” of the process as the time delay for which some
parameter characteristic of the process (i.e. the kinetic energy distribution) reaches
its asymptotic value. The situation is different if a short laser pulse, resonant with
an intermediate level of a product fragment, is employed as probe in a stepwise
ionization process (REMPI), or in a transition to a fluorescence emitting state. The
presence of the resonance enhances the magnitude of the observable, be it fluorescence or ionization, by orders of magnitude with respect to the non-resonant case.
In this instance, only if the co-fragment is distant enough, so that the targeted resonance is not shifted beyond the bandwidth of the probe laser, is the product fragment
detected with high efficiency. Typically, the probe laser central wavelength is tuned
to the free radical resonance, so that detection only starts when the fragments are
far from each other so that the above condition is fulfilled. This is normally referred
to as “the opening of the optical window”. This type of measurement provides a
natural definition of the “end” of the process, allowing “clocking”, although it has
to be noted that it is a definition that is dependent on the bandwidth of the probe
laser [2]. In any case, comparison of “clocking” times in multichannel processes
allows to extract information on the energy flow processes between electronic and
nuclear degrees of freedom in a molecular species, and can provide valuable information on the dynamics at special regions of the potential energy surfaces like
conical intersections.
The study of these fast energy distribution processes in molecules has been at the
core of the discipline that has been termed Femtochemistry for the last decades [3].
In the heart of the gear of such progress, several molecular systems, which possess the valuable characteristic of being complex and yet theoretically accessible, can be found. Among them, methyl iodide, CH 3 I, constitutes the five-atom
paradigm [4]. Due to the high electronegativity of the halogen atom, methyl halides
can be viewed as pseudo-diatomic systems (where the methyl moiety plays the role
of a pseudo-atom), pseudo-triatomic (the pseudo-atom consists of the three hydrogen atoms) or full five-atom molecules, depending on the theoretical framework.
R. de Nalda et al.
in conventional experiments, performed with a large number of molecules, each
event is characterized by its own impact parameter, relative velocities and positions,
so that, for the ensemble, the origin of time is particularly ill-defined in the scale
characteristic of the reaction itself.
On the contrary, photo-induced processes (photoionization, photoisomerization,
photodissociation) are often termed “half reactions” [1], with an origin of time that
is set by the laser pulse initiating the process. Some degree of uncertainty remains,
however, due to the temporal width of the laser pulse, and thus time zero is best defined for the shortest pulse available. In practice, it is common to assign the origin
of time to the maximum of the intensity envelope of the laser pulse used to induce
a given process. It is the temporal width of the laser pulse that sets the time resolution and hence, the time scales that can be explored with such pulse. Even for half
reactions, however, the end of the process remains considerably less well defined,
and in practice, the most useful definition depends on the technique to be used as a
probe of the process. If a short non-resonant laser pulse is employed as an ionization probe, transition state species can be subjected to ionization at all times, and
one would have to define the “end” of the process as the time delay for which some
parameter characteristic of the process (i.e. the kinetic energy distribution) reaches
its asymptotic value. The situation is different if a short laser pulse, resonant with
an intermediate level of a product fragment, is employed as probe in a stepwise
ionization process (REMPI), or in a transition to a fluorescence emitting state. The
presence of the resonance enhances the magnitude of the observable, be it fluorescence or ionization, by orders of magnitude with respect to the non-resonant case.
In this instance, only if the co-fragment is distant enough, so that the targeted resonance is not shifted beyond the bandwidth of the probe laser, is the product fragment
detected with high efficiency. Typically, the probe laser central wavelength is tuned
to the free radical resonance, so that detection only starts when the fragments are
far from each other so that the above condition is fulfilled. This is normally referred
to as “the opening of the optical window”. This type of measurement provides a
natural definition of the “end” of the process, allowing “clocking”, although it has
to be noted that it is a definition that is dependent on the bandwidth of the probe
laser [2]. In any case, comparison of “clocking” times in multichannel processes
allows to extract information on the energy flow processes between electronic and
nuclear degrees of freedom in a molecular species, and can provide valuable information on the dynamics at special regions of the potential energy surfaces like
conical intersections.
The study of these fast energy distribution processes in molecules has been at the
core of the discipline that has been termed Femtochemistry for the last decades [3].
In the heart of the gear of such progress, several molecular systems, which possess the valuable characteristic of being complex and yet theoretically accessible, can be found. Among them, methyl iodide, CH 3 I, constitutes the five-atom
paradigm [4]. Due to the high electronegativity of the halogen atom, methyl halides
can be viewed as pseudo-diatomic systems (where the methyl moiety plays the role
of a pseudo-atom), pseudo-triatomic (the pseudo-atom consists of the three hydrogen atoms) or full five-atom molecules, depending on the theoretical framework.