4 Femtosecond Photodissociation Dynamics by Velocity Map Imaging
79
the measurements reflect early-time dynamics of the molecular system in the Aband, as will be discussed below [32, 43].
The strongly enhanced signal obtained at early delay times showing a global
structure with strong similarities to the asymptotic case is the main observable of
this experiment. For such short times, the mechanism involves a 267 nm photon absorption in the early part of the pulse, which triggers the CH 3 I A-band dissociation.
In a fraction of molecules, the wave packet will evolve undisturbed and give rise
to the neutral CH 3 and I fragments, which are amenable to be probed by a delayed
802 nm pulse at long delay times (asymptotic component). In part of the excited
molecules, however, subsequent absorption of additional 267 nm photons is possible; in fact it is likely to be favoured by the presence of high-lying Rydberg states
just below the ionization continuum, and only one additional 802 nm photon would
be sufficient to produce CH 3 I + in its ground state. Such (2 + 1 ) process would have
a strong probability due to the resonant enhancement for the 267 nm photons at oneand two-photon level, and the strong absorption probability of the 802 nm, which
would produce the ion with little excess energy above the ionization potential.
Given the very rapid dissociation in the A-band (∼ 100 fs), the transient species
[CH 3 · · · I] ‡ can still absorb further 267 nm photons coming from the trailing edge
of the pulse even for considerably elongated internuclear distances. If sufficient time
has passed before absorption takes place, the wave packet on the A-band will have
already split due to the early non-adiabatic crossing between the 3 Q 0 and 1 Q 1 surfaces, acquiring different excess kinetic energies. Upon simultaneous 267 nm and
802 nm absorption, it would be expected that the part of the wave packet evolving on
the 1 Q 1 surface could be promoted to the A excited state of CH 3 I + , which is known
to undergo a fast internal conversion to the CH
+
3 + I( 2 P 3/2 ) asymptote (ground state
of the ion), and the part evolving on 3 Q 0 to the B excited state of CH 3 I + , which
would dissociate to the CH
+
3 + I ∗ ( 2 P 1/2 ) asymptote (see Fig. 13 of Ref. [32]).
Considering this mechanism, which is very much in the spirit of the methodology
presented by Zhong and Zewail in Ref. [43], the kinetic energy finally present in the
methyl ion fragment should contain two contributions. The first contribution arises
from the available energy of the neutral A-band dissociation at a given intermediate
C–I internuclear distance (E
‡
av ), which will be smaller than the asymptotic available
energy (E final
av ), since the wave packet cannot have reached a very long internuclear
distance at the time of the second absorption (see Fig. 13 of Ref. [32]). Since the
excess energy upon the ionization step will be taken by the ejected electron, the other
contribution comes only from the available energy on the ionic repulsive potential
surface once the wave packet has initially evolved in the neutral potential surface.
The CH 3 kinetic energy shifts observed in the kinetic energy distributions shown in
Fig. 4.8 indicate that the methyl fragment carries less energy close to the temporal
overlap of the pump and probe pulses than in the asymptotic region (long delay
time), which can be explained if the ionic dissociative surface is flatter than the
neutral surface. The fact that we observe a larger shift as the delay time is shortened
is an indication that the 802 nm pulse provides the time when the wave packet is
frozen in the neutral dissociation surface and taken to the flatter ionic dissociative
surface, where the total energy gained is bound to be lower. The broadened kinetic
79
the measurements reflect early-time dynamics of the molecular system in the Aband, as will be discussed below [32, 43].
The strongly enhanced signal obtained at early delay times showing a global
structure with strong similarities to the asymptotic case is the main observable of
this experiment. For such short times, the mechanism involves a 267 nm photon absorption in the early part of the pulse, which triggers the CH 3 I A-band dissociation.
In a fraction of molecules, the wave packet will evolve undisturbed and give rise
to the neutral CH 3 and I fragments, which are amenable to be probed by a delayed
802 nm pulse at long delay times (asymptotic component). In part of the excited
molecules, however, subsequent absorption of additional 267 nm photons is possible; in fact it is likely to be favoured by the presence of high-lying Rydberg states
just below the ionization continuum, and only one additional 802 nm photon would
be sufficient to produce CH 3 I + in its ground state. Such (2 + 1 ) process would have
a strong probability due to the resonant enhancement for the 267 nm photons at oneand two-photon level, and the strong absorption probability of the 802 nm, which
would produce the ion with little excess energy above the ionization potential.
Given the very rapid dissociation in the A-band (∼ 100 fs), the transient species
[CH 3 · · · I] ‡ can still absorb further 267 nm photons coming from the trailing edge
of the pulse even for considerably elongated internuclear distances. If sufficient time
has passed before absorption takes place, the wave packet on the A-band will have
already split due to the early non-adiabatic crossing between the 3 Q 0 and 1 Q 1 surfaces, acquiring different excess kinetic energies. Upon simultaneous 267 nm and
802 nm absorption, it would be expected that the part of the wave packet evolving on
the 1 Q 1 surface could be promoted to the A excited state of CH 3 I + , which is known
to undergo a fast internal conversion to the CH
+
3 + I( 2 P 3/2 ) asymptote (ground state
of the ion), and the part evolving on 3 Q 0 to the B excited state of CH 3 I + , which
would dissociate to the CH
+
3 + I ∗ ( 2 P 1/2 ) asymptote (see Fig. 13 of Ref. [32]).
Considering this mechanism, which is very much in the spirit of the methodology
presented by Zhong and Zewail in Ref. [43], the kinetic energy finally present in the
methyl ion fragment should contain two contributions. The first contribution arises
from the available energy of the neutral A-band dissociation at a given intermediate
C–I internuclear distance (E
‡
av ), which will be smaller than the asymptotic available
energy (E final
av ), since the wave packet cannot have reached a very long internuclear
distance at the time of the second absorption (see Fig. 13 of Ref. [32]). Since the
excess energy upon the ionization step will be taken by the ejected electron, the other
contribution comes only from the available energy on the ionic repulsive potential
surface once the wave packet has initially evolved in the neutral potential surface.
The CH 3 kinetic energy shifts observed in the kinetic energy distributions shown in
Fig. 4.8 indicate that the methyl fragment carries less energy close to the temporal
overlap of the pump and probe pulses than in the asymptotic region (long delay
time), which can be explained if the ionic dissociative surface is flatter than the
neutral surface. The fact that we observe a larger shift as the delay time is shortened
is an indication that the 802 nm pulse provides the time when the wave packet is
frozen in the neutral dissociation surface and taken to the flatter ionic dissociative
surface, where the total energy gained is bound to be lower. The broadened kinetic