Evidence of a perpendicular component in the photodissociation of BrCN at 213 nm

A strong, uniform electric field was used to achieve brute force orientatio n of supersonically cooled cyanogen bromide. Laser-induced fluorescence of the CN fragment from photolysis of the sample at 213 nm was detected. A 20% enhancement in signal was observed when the dissociation laser was polariz ed parallel, rather than perpendicular, to the orientation field. This indi cates the dominance of a transition dipole moment parallel to the permanent dipole of the molecule. However, the degree of enhancement falls well belo w that predicted for a pure parallel transition at a rotational temperature of 1.5 K. Thus, evidence of contribution from a perpendicular transition i s seen. Based on quantitative analysis, the direction of the effective tran sition dipole, and thereby the amount of contribution from a transition dip ole perpendicular to the permanent dipole of the molecule, can be determine d. Using this method, approximately 33% perpendicular character was found i n the dissociation process of BrCN at 213 nm. (C) 1999 American Institute o f Physics. [S0021-9606(99)01029-6].