4 Femtosecond Photodissociation Dynamics by Velocity Map Imaging
73
Fig. 4.5 CH 3 transients
corresponding to the
dissociation channels yielding
CH 3 (ν) in different
vibrational states (ν = 0,
ν 1 = 1, ν 2 = 1 and ν 2 = 2) in
correlation with I and I ∗ . The
reaction times (with statistical
uncertainties) are indicated in
each transient. A transient
corresponding to the parent
molecule CH 3 I + defining the
time zero is also depicted
(Color figure online)
tion, θ is the angle between the polarization axis of the photolysis laser and the
fragment velocity vector, β is the anisotropy parameter, and P 2 (cos θ) is the second order Legendre polynomial. From least-squares fits to this function, asymptotic values (i.e., for a long pump-probe delay time) obtained for the β parameter
are 1.89 ± 0.05, 1.69 ± 0.05, and 1.84 ± 0.08 for the CH 3 (ν = 0) + I ∗ ( 2 P 1/2 ),
CH 3 (ν = 0) + I( 2 P 3/2 ), and CH 3 (ν 1 = 1) + I( 2 P 3/2 ) channels, respectively, in good
agreement with previous values reported in the literature. No significant changes
in the anisotropy parameter are observed as a function of time with respect to that
of the asymptotic region. These values could be affected by fragment alignment
effects, but those are very weak for the CH 3 0 0
0 Q branch.
Of course the most appealing possibility of the time-resolved experiment with
respect to the well understood nanosecond experiments is to watch the appearance
of the fragments in the temporal window where they appear after parent molecule
excitation. In order to determine the reaction times for the different channels, integration of each of the peaks in the kinetic energy distribution is performed at each
time delay. Figure 4.5 shows the results obtained for the channels under study, with
a more complete collection in Table 4.1 [39] at the end of this section. The figure
shows that the main channel of this fragmentation reaction, yielding methyl and
spin-orbit excited iodine, CH 3 (ν = 0) + I ∗ ( 2 P 1/2 ), takes place in approximately
73
Fig. 4.5 CH 3 transients
corresponding to the
dissociation channels yielding
CH 3 (ν) in different
vibrational states (ν = 0,
ν 1 = 1, ν 2 = 1 and ν 2 = 2) in
correlation with I and I ∗ . The
reaction times (with statistical
uncertainties) are indicated in
each transient. A transient
corresponding to the parent
molecule CH 3 I + defining the
time zero is also depicted
(Color figure online)
tion, θ is the angle between the polarization axis of the photolysis laser and the
fragment velocity vector, β is the anisotropy parameter, and P 2 (cos θ) is the second order Legendre polynomial. From least-squares fits to this function, asymptotic values (i.e., for a long pump-probe delay time) obtained for the β parameter
are 1.89 ± 0.05, 1.69 ± 0.05, and 1.84 ± 0.08 for the CH 3 (ν = 0) + I ∗ ( 2 P 1/2 ),
CH 3 (ν = 0) + I( 2 P 3/2 ), and CH 3 (ν 1 = 1) + I( 2 P 3/2 ) channels, respectively, in good
agreement with previous values reported in the literature. No significant changes
in the anisotropy parameter are observed as a function of time with respect to that
of the asymptotic region. These values could be affected by fragment alignment
effects, but those are very weak for the CH 3 0 0
0 Q branch.
Of course the most appealing possibility of the time-resolved experiment with
respect to the well understood nanosecond experiments is to watch the appearance
of the fragments in the temporal window where they appear after parent molecule
excitation. In order to determine the reaction times for the different channels, integration of each of the peaks in the kinetic energy distribution is performed at each
time delay. Figure 4.5 shows the results obtained for the channels under study, with
a more complete collection in Table 4.1 [39] at the end of this section. The figure
shows that the main channel of this fragmentation reaction, yielding methyl and
spin-orbit excited iodine, CH 3 (ν = 0) + I ∗ ( 2 P 1/2 ), takes place in approximately