A 4D wave packet study of the CH3I photodissociation in the A
band. Comparison with femtosecond velocity map imaging
experiments
A.
1
García-Vela ,
R. de
2
Nalda ,
J.
3
Durá ,
J.
2,3
González-Vázquez ,
and L.
3
Bañares
1Instituto
de Física Fundamental, Consejo Superior de Investigaciones Científicas, Serrano 123, 28006 Madrid, Spain
2Instituto de Química Física Rocasolano, Consejo Superior de Investigaciones Científicas, Serrano 123, 28006 Madrid, Spain
3Departamento de Química Física I, Facultad de Ciencias Químicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
Introduction: The photodissociation of methyl iodide in the A band (λ=267 nm) has been studied both
experimentally and theoretically [1]. Femtosecond pump-probe experiments in combination with velocity map imaging
were carried out to measure the relative reaction times (clocking) of the different photodissociation channels.
Multisurface nonadiabatic wave packet calculations in 3D were also previously performed using the best ab initio
potential surfaces, nonadiabatic coupling, and transition dipole moments currently available [2-4]. Good general
agreement was found between experiment and theory except for the channels involving excitation of the umbrella
mode of the CH3 fragment [1]. New, more accurate absolute reaction times have been recently measured for the
different dissociation channels, and improved multisurface wave packet calculations including 4 degrees of freedom
have been recently carried out. Both the new experiments and simulations are reported in the present work.
CH3I A-band: clocking experiment
Abel inverted CH3+ images
Femtosecond laser pump + resonant probe
combined with velocity-map ion imaging
 Initial excitation to A-band with 267 nm
 Resonant probing of CH3 fragment causing a
2+1 REMPI process
 Velocity-map ion imaging of CH3+. The speed
and angular distributions contain detailed
information of the multichannel bond-breaking
 Ultrashort pump and probe pulses
process
velocity-map imaging
(ion lens + 2D detection)
radius
kinetic energy
Multisurface wave packet treatment in 4D
A.T. J.B. Eppink and D.H.
Parker, Rev. Sci. Instrum.
68, 3477 (1997)
De Nalda et al., J. Chem. Phys. 128, 244309 (2008)
Differences between previous 3D and present
4D simulations
4D:
-- Inclusion of the C-H stretch mode
-- Calculated magnitudes are averaged over the spectral
bandwidth of both the pump and the probe pulses
-- Initial pumping to both the 3Q0 and 1Q1 excited states
3D:
-- Calculated magnitudes are averaged over the spectral
bandwidth of only the probe pulse
-- Initial pumping only to the 3Q0 excited state
Good agreement is found between experiment and the 4D
theoretical model for the properties Involving the I* channel and
the ground vibrational level of the umbrella mode in the I
dissociation channel. Such a good agreement (which improves
that of the previous 3D description) is mainly due to the
averaging of the calculated magnitudes over the spectral
bandwidth of the pump pulse (in addition to the probe pulse)
carried out in the present model, since it describes better the
experimental conditions. In this sense, 3D simulations carried out
in the same conditions give very similar results to the 4D ones,
indicating a small influence of the symmetric stretch mode in the
dynamics. Disagreement between theory and experiment is still
found, however, for the properties corresponding to the
vibrational excitations of the CH3 fragment (for both the umbrella
and the symmetric stretch mode) in the I dissociation channel.
Conclusions: After applying a 4D quantum dynamical model to describe the CH3I photodissociation the discrepancies with the experimental
data still remain regarding the properties related to the I dissociation channel when the CH3 fragment is vibrationally excited. The results indicate
that such a discrepancy is not due to a dynamical effect or to poor description of the model because of its reduced dimensionality nature. We
suspect that the discrepancy is rather due to an effect caused by the experimental fragment detection process, which is not modeled in the theory.
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).
[4] A.B. Alekseyev, H.-P. Liebermann, and R.J. Buenker, J.Chem. Phys. 126, 234102 (2007).
Acknowledgements: Ministerio de Ciencia e Innovación, Spain, Grants No. CTQ 2005-08493-C02-01 and FIS2010-18132, the Consolider
program, Grant No. CSD2007-00013, and the Centro de Supercomputación de Galicia (CESGA).