Control of photofragment velocity anisotropy by optical alignment of CH3I

A molecule placed in an electric field can be aligned because of the anisotropy of its polarizability tensor. As an alternative to high static electric fields, the electric field of a pulsed laser can be used for alignment. The alignment can be demonstrated by measurement of the control of the anisotropy of photodissociated fragments generated by polarized light. Sakai have recently reported such an experiment on aligned iodine molecules [J. Chem. Phys. 110, 10235 (1999)]. This paper reports an independent finding of the same phenomenon with CH3I. General equations are presented for the degree of alignment and its control of the photodissociation amplitude. The control parameter for a symmetric top or linear molecule is -(alpha-alpha(zz))(E-2/2kT)P-2(cos chi), where alpha is the mean polarizability, alpha(zz) is the element of the polarizability tensor along the symmetry axis, and chi is the angle between the E vectors of the aligning and dissociating lasers. The fragment velocity anisotropy can be increased or decreased by varying this control parameter. (C) 2000 American Institute of Physics. [S0021-9606(00)00604-8].