Control of photofragment velocity anisotropy by optical alignment of CH3I

A molecule placed in an electric field can be aligned because of the anisot ropy of its polarizability tensor. As an alternative to high static electri c fields, the electric field of a pulsed laser can be used for alignment. T he alignment can be demonstrated by measurement of the control of the aniso tropy of photodissociated fragments generated by polarized light. Sakai hav e recently reported such an experiment on aligned iodine molecules [J. Chem . Phys. 110, 10235 (1999)]. This paper reports an independent finding of th e same phenomenon with CH3I. General equations are presented for the degree of alignment and its control of the photodissociation amplitude. The contr ol 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 t he element of the polarizability tensor along the symmetry axis, and chi is the angle between the E vectors of the aligning and dissociating lasers. T he fragment velocity anisotropy can be increased or decreased by varying th is control parameter. (C) 2000 American Institute of Physics. [S0021-9606(0 0)00604-8].