4 Femtosecond Photodissociation Dynamics by Velocity Map Imaging
69
Finally, the Levenberg–Marquardt nonlinear regression method is applied to fit
the parameters in the “constructed” images for least discrepancy with the experimental set of data. This methodology has proven extremely efficient for the extraction of meaningful values for physical parameters (temperatures, anisotropy parameters, population level distributions, lifetimes, cross-correlations) from the complex
data provided by extended sets of images acquired in experiments.
4.3 The A Band
The decomposition dynamics of CH 3 I upon UV photon absorption in the A band
constitutes one of the most extensively documented cases of the consequences of
non-adiabatic surface-crossing in molecular dynamics. Electronic non-adiabatic interactions, which involve the breakdown of the Born–Oppenheimer approximation,
are ubiquitous and considerable theoretical and experimental efforts have been made
to characterize the broad variety of possibilities. In particular, conical intersections
in polyatomic molecules attract special interest [19, 20], partly because they have
been identified as candidates for control under strong laser fields [21]. The effect of
a conical intersection can be quite complex since it does not need to be energetically
accessible to affect the molecular dynamics [22]. When the conical intersection lies
close to the Franck–Condon region, as in the case of the alkyl halides in general and
in CH 3 I in particular, [23] the strong interaction between the involved states plays a
major role on the dissociation dynamics.
The CH 3 I A-band constitutes the lowest-energy absorption feature of the
molecule and consists of a broad featureless continuum ranging from 210 to 350 nm
with a maximum at about 260 nm [24]. As was first demonstrated by Mulliken and
Teller [25], the lowest energy electronic excitation in CH 3 I corresponds to an n–σ
transition, where a non-bonding p electron of iodine is promoted to the lowest energy available anti-bonding molecular orbital [25]. The spin-orbit (SO) coupling is
large, due to the presence of the heavy iodine atom, and the SO configuration can
be used for the first excited electronic states [26]. Three SO states are accessible
through dipole allowed transitions from the ground state: the 3 Q 1 and 1 Q 1 states
(in Mulliken’s notation) [27] through weak perpendicular transitions and the 3 Q 0
state through a strong parallel transition [28]. The 3 Q 0 state correlates adiabatically
with CH 3 (X 2 A 2 ) + I ∗ ( 2 P 1/2 ) products, while the 3 Q 1 and 1 Q 1 states correlate with
CH 3 (X 2 A 2 ) + I( 2 P 3/2 ). From now on we will use I ∗ and I to refer to I ∗ ( 2 P 1/2 ) and
I( 2 P 3/2 ), respectively, and just CH 3 to refer to CH 3 (X 2 A 2 ). At the curve maximum,
around 260 nm, the absorption is dominated by the 3 Q 0 state, while transitions to the
3 Q 1 and 1 Q 1 states become more important towards the low energy (red) and high
energy (blue) regions of the absorption band, respectively [27, 29]. The asymptotic
correlation between excited surfaces and photoproducts implies that a curve crossing between the 3 Q 0 and 1 Q 1 states must take place close to the Franck–Condon
region.
Structurally, the non-adiabatic curve crossing implies a reduction of the molecular symmetry from C 3v to C s caused by e-type vibrations during the absorption
69
Finally, the Levenberg–Marquardt nonlinear regression method is applied to fit
the parameters in the “constructed” images for least discrepancy with the experimental set of data. This methodology has proven extremely efficient for the extraction of meaningful values for physical parameters (temperatures, anisotropy parameters, population level distributions, lifetimes, cross-correlations) from the complex
data provided by extended sets of images acquired in experiments.
4.3 The A Band
The decomposition dynamics of CH 3 I upon UV photon absorption in the A band
constitutes one of the most extensively documented cases of the consequences of
non-adiabatic surface-crossing in molecular dynamics. Electronic non-adiabatic interactions, which involve the breakdown of the Born–Oppenheimer approximation,
are ubiquitous and considerable theoretical and experimental efforts have been made
to characterize the broad variety of possibilities. In particular, conical intersections
in polyatomic molecules attract special interest [19, 20], partly because they have
been identified as candidates for control under strong laser fields [21]. The effect of
a conical intersection can be quite complex since it does not need to be energetically
accessible to affect the molecular dynamics [22]. When the conical intersection lies
close to the Franck–Condon region, as in the case of the alkyl halides in general and
in CH 3 I in particular, [23] the strong interaction between the involved states plays a
major role on the dissociation dynamics.
The CH 3 I A-band constitutes the lowest-energy absorption feature of the
molecule and consists of a broad featureless continuum ranging from 210 to 350 nm
with a maximum at about 260 nm [24]. As was first demonstrated by Mulliken and
Teller [25], the lowest energy electronic excitation in CH 3 I corresponds to an n–σ
transition, where a non-bonding p electron of iodine is promoted to the lowest energy available anti-bonding molecular orbital [25]. The spin-orbit (SO) coupling is
large, due to the presence of the heavy iodine atom, and the SO configuration can
be used for the first excited electronic states [26]. Three SO states are accessible
through dipole allowed transitions from the ground state: the 3 Q 1 and 1 Q 1 states
(in Mulliken’s notation) [27] through weak perpendicular transitions and the 3 Q 0
state through a strong parallel transition [28]. The 3 Q 0 state correlates adiabatically
with CH 3 (X 2 A 2 ) + I ∗ ( 2 P 1/2 ) products, while the 3 Q 1 and 1 Q 1 states correlate with
CH 3 (X 2 A 2 ) + I( 2 P 3/2 ). From now on we will use I ∗ and I to refer to I ∗ ( 2 P 1/2 ) and
I( 2 P 3/2 ), respectively, and just CH 3 to refer to CH 3 (X 2 A 2 ). At the curve maximum,
around 260 nm, the absorption is dominated by the 3 Q 0 state, while transitions to the
3 Q 1 and 1 Q 1 states become more important towards the low energy (red) and high
energy (blue) regions of the absorption band, respectively [27, 29]. The asymptotic
correlation between excited surfaces and photoproducts implies that a curve crossing between the 3 Q 0 and 1 Q 1 states must take place close to the Franck–Condon
region.
Structurally, the non-adiabatic curve crossing implies a reduction of the molecular symmetry from C 3v to C s caused by e-type vibrations during the absorption