Orbital alignment in atomic photofragments

Musahid Ahmed, Darcy S. Peterka, Allan S. Bracker, Oleg S. Vasyutinskii, and Arthur G. Suits, "Coherence in polyatomic photodissociation: Aligned O (3P) from photodissociation of NO2 at 212.8 nm," J. Chem. Phys. 110, 4115 (1999).

A.S. Bracker, E. R. Wouters, O. S. Vasyutinskii and A.G. Suits, "Imaging the alignment angular distribution:  State symmetries, coherence,  and nonadiabatic dynamics in photodissociation," J. Chem. Phys., 110, 6749 (1999).

A.S. Bracker, E. Wouters, A.G. Suits, Y.T. Lee and O. Vasyutinskii, "Observation of coherent and incoherent dissociation mechanisms in the angular distribution of photofragment alignment," Phys. Rev. Lett., 80, 1626 (1997).

E.R. Wouters, M. Ahmed, D.S. Peterka, A.S. Bracker, A.G. Suits and O.S. Vasyutinskii, "Imaging the Atomic Orientation and Alignment in Photodissociation," in Imaging in Chemical Dynamics, A. G. Suits and R. E. Continetti, eds.,  ACS Symposium Series 770, (American Chemical Society, Washington DC, 2000).

New techniques such as ion imaging can provide information on the angular distribution of angular momentum polarization, which in turn affords insight into the dynamics of a photochemical event in the frame of the molecule. This can be used to reveal the symmetries of the excited states involved, to probe nonadiabatic processes in the photodissociation dynamics and to reveal coherence effects. In conjunction with Prof. O. Vasyutinskii of the Ioffe Institute, St. Petersburg, we have recently developed the theoretical machinery making the connection between ion imaging experiments and anisotropy parameters having explicit physical significance.  These tools have allowed us to show the contribution of nonadiabatic processes in Cl 2dissociation at 355nm, and to show directly the presence of coherence in the ground state O atom photofragment from NO 2dissociation at 212.8nm.

Above left are shown the ion images for the O( 3P1) product of NO 2photodissociation at 212.8 nm for indicated photolysis and probe laser polarizations.  The results reveal a significant contribution from a coherence between parallel and perpendicular contributions to the excitation process, indicating that the electron cloud in the recoiling atom (right) remembers the original molecular plane.

New results obtained using DC slice imaging for ethylene sulfide photodissociation below show our ability to obtain the speed dependence of these anisotropy parameters directly from the images.

The results above may be used to illustrate the anisotropy of the charge cloud, even though the alignemnt effect in this case is rather slamm. The charge cloud anisotropy is shown for three different recoil speeds and three recoil angles below. The coherent contributions dominate, and give rise to azimuthal anisotropy that is a maximum at 45 degrees.

D. Townsend, S.K. Lee, M. P. Minitti and A. G. Suits, Manuscript in preparation.

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