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R. de Nalda et al.
ratio of approximately 2:1 is extracted for those two routes. This is an important
result, since it indicates that the participation of absorption of the perpendicular
states is not negligible, as was the case for the monomer at 267 nm excitation; instead, it accounts for approximately 1/3 of the total absorption at this wavelength.
However, this alone cannot account for an increase in the I( 2 P 3/2 )/I ∗ ( 2 P 1/2 ) ratio from 0.14 ± 0.05 for the monomer to 0.72 ± 0.05 for the dimer. From the
combined values of ratios and anisotropy, we conclude that the efficiency of the
non-adiabatic crossing approximately doubles in the dimer with respect to the
monomer.
The experimental results presented above show the richness of the information
that can be obtained in relatively simple experiments. In this case, through a global
approach where velocities and anisotropies are examined simultaneously, detailed
information on the stabilization energy, the degree of internal energy content, the
contribution of several electronic excited states to the overall absorption, and finally, the effect of dimerization on the population transfer through the non-adiabatic
crossing, has been obtained.
4.3.5 Resonant Probing: The Role of the Optical Coupling Window
(CH 3 I) 2 dimer photodissociation will be explored temporally in this section to illustrate an important feature of resonantly probed ”clocking” experiments. Figure 4.11
shows the time evolution in the detection of CH 3 coming from the monomer in the
I ∗ ( 2 P 1/2 ) channel, and also from the dimer in both the I( 2 P 3/2 ) and I ∗ ( 2 P 1/2 ) channels. The time delay for free CH 3 observation is still short in the dimer (∼ 220 fs),
but clearly delayed with respect to monomer dissociation (∼ 80 fs). If the decrease
in available energy, due to the blue shift of the band, is taken into account, calculated delays of only ∼ 10 fs are obtained classically, indicating that this is clearly
insufficient to explain the experimental measurement.
As is described in detail in Ref. [15], this case is a particularly illuminating example of the role of the probing step. In particular, it must be noted that resonant CH 3
probing is done here through a (2 + 1) REMPI process via a Rydberg state. This is a
state of spatially extended character, and during the first phases of dissociation, it is
situated in the vicinity of the [I · · · ICH 3 ] moiety, where close-lying Rydberg states
can be present. In this situation, the resonance can be significantly perturbed by the
nearby presence of [I · · · ICH 3 ], so that the CH 3 radical would only appear as “free”
after an elongated distance (i.e. time).
A simplified theoretical simulation was performed in order to assess that the
delay in the measured clocking time was not due to an intrinsic difference in the
photodissociation dynamics in the dimer with respect to the monomer, but to an
important difference in the probing step (through the late opening of the optical
coupling window) due to the extended character of the orbitals. A CASSCF calculation was performed to describe the effect of the nearby presence of the [I · · · ICH 3 ]
species on the Rydberg electronic states of the ejected CH 3 group. A reduced model
R. de Nalda et al.
ratio of approximately 2:1 is extracted for those two routes. This is an important
result, since it indicates that the participation of absorption of the perpendicular
states is not negligible, as was the case for the monomer at 267 nm excitation; instead, it accounts for approximately 1/3 of the total absorption at this wavelength.
However, this alone cannot account for an increase in the I( 2 P 3/2 )/I ∗ ( 2 P 1/2 ) ratio from 0.14 ± 0.05 for the monomer to 0.72 ± 0.05 for the dimer. From the
combined values of ratios and anisotropy, we conclude that the efficiency of the
non-adiabatic crossing approximately doubles in the dimer with respect to the
monomer.
The experimental results presented above show the richness of the information
that can be obtained in relatively simple experiments. In this case, through a global
approach where velocities and anisotropies are examined simultaneously, detailed
information on the stabilization energy, the degree of internal energy content, the
contribution of several electronic excited states to the overall absorption, and finally, the effect of dimerization on the population transfer through the non-adiabatic
crossing, has been obtained.
4.3.5 Resonant Probing: The Role of the Optical Coupling Window
(CH 3 I) 2 dimer photodissociation will be explored temporally in this section to illustrate an important feature of resonantly probed ”clocking” experiments. Figure 4.11
shows the time evolution in the detection of CH 3 coming from the monomer in the
I ∗ ( 2 P 1/2 ) channel, and also from the dimer in both the I( 2 P 3/2 ) and I ∗ ( 2 P 1/2 ) channels. The time delay for free CH 3 observation is still short in the dimer (∼ 220 fs),
but clearly delayed with respect to monomer dissociation (∼ 80 fs). If the decrease
in available energy, due to the blue shift of the band, is taken into account, calculated delays of only ∼ 10 fs are obtained classically, indicating that this is clearly
insufficient to explain the experimental measurement.
As is described in detail in Ref. [15], this case is a particularly illuminating example of the role of the probing step. In particular, it must be noted that resonant CH 3
probing is done here through a (2 + 1) REMPI process via a Rydberg state. This is a
state of spatially extended character, and during the first phases of dissociation, it is
situated in the vicinity of the [I · · · ICH 3 ] moiety, where close-lying Rydberg states
can be present. In this situation, the resonance can be significantly perturbed by the
nearby presence of [I · · · ICH 3 ], so that the CH 3 radical would only appear as “free”
after an elongated distance (i.e. time).
A simplified theoretical simulation was performed in order to assess that the
delay in the measured clocking time was not due to an intrinsic difference in the
photodissociation dynamics in the dimer with respect to the monomer, but to an
important difference in the probing step (through the late opening of the optical
coupling window) due to the extended character of the orbitals. A CASSCF calculation was performed to describe the effect of the nearby presence of the [I · · · ICH 3 ]
species on the Rydberg electronic states of the ejected CH 3 group. A reduced model