We present quantitative results on photodissociation of CH2((X) over tilde B-3(1)) through the coupled 2A '' and 3A '' states. A three-dimensional, hybrid quantum dynamical method was used, employing hyperspherical coordinates. The diabatic potential energy surfaces (PES's) used in the dynamics were derived from ab initio calculations. A small product fraction (2.7%) was computed for the CH(A (2) Delta)+H channel, in agreement with experiment and approximate dynamical calculations. The dissociation proceeds mostly on a A(2)-like diabatic surface, into CH(a (4) Sigma(-))+H(93.3%) and C(P-3)+ H-2(4.0%). Resonances of widths in the range 0.1-10 meV affect the photodissociation. Pre-exciting a vibrational mode of CH2((X) over tilde B-3(1)) prior to photodissociation does not alter the picture, except if the antisymmetric stretch mode is excited: In this case the product fractions for the C(P-3)+H-2 and CH(A (2) Delta)+H channels collapse to values of 1% or lower, and the resonances disappear. Model calculations show that the large product fraction found for CH(a (4) Sigma(-))+H is due to the initial motion on the ''bright'' B-1-like surface, which biases the outcome of the dissociation in favor of CH(a (4) Sigma(-))+H. (C) 1997 American Institute of Physics.