Wave packet calculations on the effect of the femtosecond pulse
width in the time-resolved photodissociation of CH3I in the A-band
A.
1
García-Vela
and L.
2
Bañares
1Instituto
de Física Fundamental, Consejo Superior de Investigaciones Científicas, Serrano 123, 28006 Madrid, Spain
2Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
Introduction: The effect of varying the temporal width of the pump and probe pulses in femtosecond
experiments of the CH3I photodissociation in the A band has been studied by means of multisurface nonadiabatic
wave packet simulations [1,2] using the best ab initio potential surfaces currently available [3,4]. Specifically, the
effect of decreasing the temporal width of the pulses on magnitudes like the photodissociation reaction time in the
different dissociation channels and the CH3 fragment vibrational and rotational distributions has been explored. Four
widths (FWHM) for the pump and probe pulses have been used, namely 100, 50, 20, and 10 fs. In addition, in order
to investigate a possible effect of the excitation wavelength used, the simulations are carried out for two excitation
wavelengths, λ=295 and 230 nm, which are located to the red and to the blue of the maximum of the absorption
spectrum, respectively.
Reaction times for the different dissociation channels
Reaction times are found to decrease
significantly as the temporal width of the
pump and probe pulses decreases, for all
the dissociation channels and excitation
wavelengths. The effect is more
pronounced for the CH3(ν) +I* channels,
and for the 295 nm wavelength.
The decrease of the reaction times is
larger as the excitation in the umbrella
mode ν increases, which causes the
relative time τ1-τ2 between the I* and I
dissociation channels to diminish rapidly
with decreasing temporal width.
These results indicate that by changing
the width of the pump and probe pulses
some control can be exerted on the
reaction times.
Time and energy profiles of the different
pulses used in the simulations
Absorption spectrum of CH3I
Vibrational distributions of the CH3I fragment
Vibrational distributions of the CH3I fragment are affected
very little by decreasing the width of the pulses. Thus, in
contrast with the behavior of the reaction times, the CH3
vibrational distributions are magnitudes predicted to remain
very stable upon substantial changes in the temporal width
of the pump and probe pulses.
The simulations show that the variations found in the
vibrational distributions are almost entirely due to the
changes in the pump pulse width.
Rotational distributions of the CH3 fragment
Similarly as the vibrational distributions, the CH3 fragment rotational
distributions are also little affected by changing substantially the width
of the pulses. The shape of both vibrational and rotational distributions
is originated in the CH3-I interaction region where the wave packet is
created, and that shape is retained as the CH3 and I*(I) fragments
separate. Thus the shape of these distributions is essentially unaffected
by the increasing extent of the detection energy window produced by
the probe pulse spectral width.
The largest changes are found for the distributions of the CH3(ν) + I*
dissociation channels, and for the 295 nm excitation wavelength.
Conclusions: The reaction time of the CH3I photodissociation is found to decrease significantly with decreasing temporal width of the pump and
probe pulses. An effect of the excitation wavelength is also found, being the changes in the time smaller for wavelengths located to the blue of the
maximum of the absorption spectrum than the changes found for wavelengths to the red of this maximum. These results are explained in terms of
the shape and slope of the absorption spectrum at the different excitation wavelengths. In contrast, the vibrational and rotational distributions of
the CH3 fragment are found to remain very stable upon significant changes of the width of the pulses. The trends found here for CH3I seem to be
general to other photodissociation processes of similar characteristics
References
[1] R. de Nalda, J. Durá, A. García-Vela, J. G. Izquierdo, J. González-Vázquez, and L. Bañares, J. Chem. Phys. 128, 244309 (2008).
[2] D. Xie, H. Guo, Y. Amatatsu, and R. Kosloff, J. Phys. Chem. A 104, 1009 (2000).
[3] A. B. Alekseyev, H.-P. Liebermann, R. J. Buenker, and S. N. Yurchenko, J. Chem. Phys. 126, 234102 (2007).
Acknowledgements: Ministerio de Educación y Ciencia, Spain, Grants No. CTQ 2005-08493-C02-01 and FIS-2007-62002, Consolider program
SAUUL, No. CSD2007-00013, and the Centro de Supercomputación de Galicia (CESGA).